The purpose of this document, a rather lenghty white papers on Nuclear Reactor Physics is the non-technical introduction of some of the most important concepts of physics required for a general understanding of the generation of energy through the use of nuclear power.
Inherent in any discussion on the structure of matter and certainly in the discussion of the production of electrical energy through the fission of nuclear material are some of the topics of modern physics: Quantum Mechanics and Special Relativity.
In this introduction to nuclear physics we shall consider only those laws of mechanics, thermodynamics and hydraulics which can appropriately be discussed in the guise of Rational Mechanics and we will use the concepts which form the basis of modern Physics: Quantum Mechanics and Special Relativity only when absolutely necessary to impart the information needed to the reader to continue the study in this white paper.
The discovery of fission in 1939 was and event of epochal significance in the annals of physics because it ushered in the age of the atom.This discovery opened up the prospect of an entirely new source of power utilizing the internal binding energy of the atom.
The operation of a nuclear reactor depends upon various interactions of neutrons with atomic nuclei. In order to appreciate the complexities of a nuclear reactor it is desirable to consider briefly some of the fundamental of atomic and nuclear physics. This paper was produced to provide such an introduction.
CHAPTER ONE: INTRODUCTION TO NUCLEAR PHYSICS
SECTION ONE: BASIC CONCEPTS
1.1 History of Structure of Matter
Early Greek philosophers speculated that the earth was made up of different combinations of basic substances, or elements. They considered these basic elements to be earth, air,water, and fire. Modern science shows that the early Greeks held the correct concept that matter consists of a combination of basic elements, but they incorrectly identified the elements.
In 1661 the English chemist Robert Boyle published the modern criterion for an element. He defined an element to be a basic substance that cannot be broken down into any simpler substance after it is isolated from a compound, but can be combined with other elements to form compounds. To date, 105 different elements have been confirmed to exist, and researchers claim to have discovered three additional elements. Of the 105 confirmed elements, 90 exist in nature and 15 are man-made.
Another basic concept of matter that the Greeks debated was whether matter was continuous or discrete. That is, whether matter could be continuously divided and subdivided into ever smaller particles or whether eventually an indivisible particle would be encountered. Democritus in about 450 B.C. argued that substances were ultimately composed of small, indivisible particles that he labeled atoma. He further suggested that different substances were composed of different atoms or combinations of atoms, and that one substance could be converted into another by rearranging the atoms. It was impossible to conclusively prove or disprove this proposal for more than 2000 years.
The modern proof for the atomic nature of matter was first proposed by the English chemist John Dalton in 1803. Dalton stated that each chemical element possesses a particular kind of atom, and any quantity of the element is made up of identical atoms of this kind. What distinguishes one element from another element is the kind of atom of which it consists, and the basic physical difference between kinds of atoms is their weight.
1.2 Subatomic Particles
For almost 100 years after Dalton established the atomic nature of atoms, it was considered impossible to divide the atom into even smaller parts. All of the results of chemical experiments during this time indicated that the atom was indivisible. Eventually, experimentation into electricity and radioactivity indicated that particles of matter smaller than the atom did indeed exist.
In 1906, J. J. Thompson won the Nobel Prize in physics for establishing the existence of electrons. Electrons are negatively-charged particles that have 1/1835 the mass of the hydrogen atom. Soon after the discovery of electrons, protons were discovered. Protons are relatively large particles that have almost the same mass as a hydrogen atom and a positive charge equal in magnitude (but opposite in sign) to that of the electron. The third subatomic particle to be discovered, the neutron, was not found until 1932. The neutron has almost the same mass as the proton, but it is electrically neutral.
1.3 Bohr Model of the Atom
The British physicist Ernest Rutherford postulated that the positive charge in an atom is concentrated in a small region called a nucleus at the center of the atom with electrons existing in orbits around it.
Niels Bohr, coupling Rutherford's postulation with the newly minted theories of quantum mechanics introduced by Max Planck, proposed that the atom consists of a dense nucleus of protons surrounded by electronstraveling in discrete orbits at fixed distances from the nucleus.
An electron in one of these stationary orbits or shells has a specific or discrete quantity of energy (quantum). When an electron moves from one allowed orbit to another allowed orbit, the energy difference between the two states is emitted or absorbed in the form of a single quantum of radiant energy called a photon.
The Quantum of energy emitted from this jump from one stationary state to another is given by the so called Plank formula:
E= hn
Where h = Planck's constant = 6.63 x 10-34 J-s
n= frequency of the photon.
Bohr's theory was the first to successfully account for the discrete energy levels of this radiation as measured in the laboratory. Although Bohr's atomic model was designed specifically to explain the hydrogen atom, his theories apply generally to the structure of all atoms. Additional information on electron shell theory can be found in any introductory book on Quantum Mechanics.
1.4 Measuring Units on the Atomic Scale
The size and mass of atoms are so small that the use of normal measuring units, while possible, is often inconvenient. Units of measure have been defined for mass and energy on the atomic scale to make measurements more convenient to express.
The unit of measure for mass is the atomic mass unit (amu).
One atomic mass unit is equal to 1.66x10-24 grams. The reason for this particular value for the atomic mass unit will be made clear later in this introduction. Note that the mass of a neutron and a proton are both about 1 amu.
The unit for energy is the electron volt (eV). The electron volt is the amount of energy acquired by a single electron when it falls through a potential difference of one volt. One electron volt is equivalent to 1.602x10-19 joules or 1.18 x10-19 foot-pounds.
1.5 Nuclides
The total number of protons in the nucleus of an atom is called the atomic number of the atom and is given the symbol Z. The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus.
The number of neutrons in a nucleus is known as the neutron number and is given the symbol N. The mass number of the nucleus is the total number of nucleons, that is, protons and neutrons in the nucleus. The mass number is given the symbol A and can be found by the equation
A= Z + N.
Each of the chemical elements has a unique atomic number because the atoms of different elements contain a different number of protons. The atomic number of an atom identifies the particular element.
Each type of atom that contains a unique combination of protons and neutrons is called a nuclide
Because each element has a unique name, chemical symbol, and atomic number, only one of the three is necessary to identify the element. For this reason nuclides can also be identified by either the chemical name or the chemical symbol followed by the mass number (for example, U-235 or uranium-235).
Another common format is to use the abbreviation of the chemical element with the mass number superscripted (for example, U235). In this white paper the format used will usually be the element's name followed
by the mass number as a superscript.
1.6 Isotopes
Isotopes are nuclides that have the same atomic number and are therefore the same element, but differ in the number of neutrons. Most elements have a few stable isotopes and several unstable,radioactive isotopes. For example, oxygen has three stable isotopes that can be found in nature (oxygen16, oxygen17, oxygen18, and eight radioactive isotopes. Another example is hydrogen, which has two stable isotopes (hydrogen-1, hydrogen1 and hydrogen-2) and a single radioactive isotope (hydrogen-3, hydrogen3).
The isotopes of hydrogen are unique in that they are each commonly referred to by a unique name instead of the common chemical element name. Hydrogen-1 is almost always referred to as hydrogen, but the term protium is infrequently used also. Hydrogen-2 is commonly called deuterium and Hydrogen-3 is commonly called tritium.
1.7 Atomic and Nuclear Radii
The size of an atom is difficult to define exactly due to the fact that the electron cloud, formed by the electrons moving in their various orbitals, does not have a distinct outer edge. A reasonable measure of atomic size is given by the average distance of the outermost electron from the nucleus.
Except for a few of the lightest atoms, the average atomic radii are approximately the same for all atoms, about 2 x 10-8 cm.Like the atom the nucleus does not have a sharp outer boundary. Experiments have shown that the nucleus is shaped like a sphere with a radius that depends on the atomic mass number of the atom.
1.8 Nuclear Forces
In the Bohr model of the atom, the nucleus consists of positively-charged protons and electrically neutral neutrons. Since both protons and neutrons exist in the nucleus, they are both referred to as nucleons. One problem that the Bohr model of the atom presented was accounting for an attractive force to overcome the repulsive force between protons.
Two of the four forces present in the nucleus are;
1. Electrostatic forces between charged particles
2. Gravitational forces between any two objects that have mass.
It is possible to calculate the magnitude of the gravitational force and electrostatic force based upon principles from classical physics.Newton stated that the gravitational force between two bodies is directly proportional to the masses of the two bodies and inversely proportional to the square of the distance between the bodies. This relationship is shown in the equation below.
Fg = (G x m1 x m2)/ r12
where:
Fg = gravitational force (newtons)
m1 = mass of first body (kilograms)
m2 = mass of second body (kilograms)
G = gravitational constant (6.67 x 10 -11 N-m2/kg2)
r = distance between particles (meters)
The equation illustrates that the larger the masses of the objects or the smaller the distance between the objects, the greater the gravitational force. So even though the masses of nucleons are very small, the fact that the distance between nucleons is extremely short may make the gravitational force significant. It is necessary to calculate the value for the gravitational force and compare it to the value for other forces to determine the significance of the gravitational force in the nucleus. The gravitational force between two protons that are separated by a distance of 10-20 meters is about 10-24 newtons.
Coulomb's Law can be used to calculate the force between two protons. The electrostatic force is directly proportional to the electrical charges of the two particles and inversely proportional to the square of the distance between the particles.
Coulomb's Law is stated as the following equation:
Fe = (K x Q1 x Q2 )
r12
where:
Fe = Electrostatic force (newtons)
K = electrostatic constant (9.0 x 109 N-m2/C2)
Q1 = charge of first particle (coulombs)
Q2 = charge of second particle (coulombs)
r12 = Distance between particles (meters)
Using this equation, the electrostatic force between two protons that are separated by a distance of 10-20 meters is about 1012 newtons. Comparing this result with the calculation of the gravitational force (10-24 newtons) shows that the gravitational force is so small that it can be neglected.
If only the electrostatic and gravitational forces existed in the nucleus, then it would be impossible to have stable nuclei composed of protons and neutrons. The gravitational forces are much too small to hold the nucleons together compared to the electrostatic forces repelling the protons. Since stable atoms of neutrons and protons do exist, there must be another attractive force acting within the nucleus. This force is called the nuclear force.
The nuclear force is a strong attractive force that is independent of charge. It acts equally only between pairs of neutrons, pairs of protons, or a neutron and a proton. The nuclear force has a very short range; it acts only over distances approximately equal to the diameter of the nucleus (10-13 cm). The attractive nuclear force between all nucleons drops off with distance much faster than the repulsive electrostatic force between protons.
1.9 Atomic Nature of Matter Summary
Structure of the Atom
An atom consists of a positively charged Nucleus surrounded by a number of negatively charged particles, called electrons, so that the atom as a whole is electrically neutral. The atomic nuclei are built up of two kinds of primary particles, namely protons and neutrons,which can be ordinarily referred to as Nucleons. The masses of the protons and neutrons are similar and much heavier than an electron, by a factor of 1840. As the nucleus contains all the protons and neutrons, it follows that the mass of the atoms are concentrated in the nucleus.
The proton carries a single unit of charge and the neutron is electrically neutral. The unit of charge carried by the proton is equal in magnitude and opposite in sign, to the charge on the electron. This charge is often referred to in physics as the fundamental charge.
Each electron carries a unit of negative charge equal to the charge on the proton. The number of orbital electrons is equal to the number of protons in the nucleus so that the their charge balances and overall the atom is electrically neutral. If the atom loses or gains a planetary electron, it is left with a residual electrical charge and the atom is said to be ionized.
The number of protons in the nucleus determines the number of orbital electrons. It is the electronic structure of the atom, in particular the outermost orbiting electrons, that give the atoms its chemical properties. Chemical reactions are due to electronic interactions.
In this document we will only mention that the electron, in a stable orbit around the nucleus occupies a stationary state, often referred to in Physics as an Eigenstate or an Eigunfunction of the orbital decomposition of the central field problem. The solution of the electronic motion of an electron in a central field of force will not be addressed here. The force which holds the electron to the nucleus is the electrostatic force of electromagnetic theory and is accurately describe by an inverse square law of force, similar the Newtons law of gravitation.
SECTION TWO: STRUCTURE OF THE NUCLEUS
2.1 Atomic Number and Chart of Nuclides
As has been previously stated the nucleus consists of protons and neutrons. For a given element, the number of protons present in the atomic nucleus, which is the same as the number of positive charges it carries, is called the atomic number.This number is identical with the ordinal number of the element which is used in the familiar Periodic table of the elements. Thus the atomic number of hydrogen is 1, of helium 2, and lithium 3, up to 92 for Uranium, the element of highest atomic number existing in nature to any extent. A large number of heavier elements have been produced artificially, of these elements, plutonium, atomic number 94, is the most important because of its connection with nuclear weapons.
The total number of Nucleons, i.e. of protons and neutrons, in an atomic nucleus is referred to as the mass number.Since the masses of neutrons and protons and very nearly identical, it is evident that the mass number is the integer nearest to the atomic weight of the species under considerations.
It is the atomic number, i.e. the number of protons in the nucleus, which determines the chemical nature of an element. This is because the chemical properties depend on the (orbital ) electrons, surrounding the nucleus, and their number must be equal to the number of protons in the nucleus since the atom must be electrically neutral. Consequently, atoms with nuclei containing the same numbers of protons, i.e. with the same atomic number, but with different numbers of neutrons, i.e. with different mass number, are essentially identical chemically. Such species having the same atomic number but different mass numbers are called Isotopes.Isotopes are, in general, chemically identical but have different atomic weight. They are, in general, indistinguishable chemically, but have different
atomic weights. As of January 1, 1962, all atomic weights are expressed on a single scale which assigns a value of 12 to the common isotope of C12.
In Nuclear physics, and related fields, the masses of atoms, of nuclei, and of nuclear particles are invariably expressed on the so-called physical scale. The Atomic Mass unit (amu) is then defined as exactly one-twelfth of the mass of the C12 atom.
2.2 Chart of the Nuclides
A tabulated chart called the Chart of the Nuclides lists the stable and unstable nuclides in addition to pertinent information about each one. This chart plots a box for each individual nuclide, with the number of protons (Z) on the vertical axis and the number of neutrons (N = A - Z) on the horizontal axis. The chart indicates stable isotopes.
Some isotopes are artificially radioactive, meaning that they are produced by artificial techniques and do not occur naturally.
A. Information for Stable Nuclides
For the stable isotopes, in addition to the symbol and the atomic mass number, the number percentage of each isotope in the naturally occurring element is listed, as well as the thermal neutron activation cross section and the mass in atomic mass units (amu).
B. Information for Unstable Nuclides
For unstable isotopes the additional information includes the halflife, the mode of decay (for example, b-, a), the total disintegration energy in MeV (million electron volts), and the mass in amu when available
C. Neutron - Proton Ratios
If you plot the number of protons on the x-axis and the number of neutrons on the y axis, then you will see that as the mass numbers become higher, the ratio of neutrons to protons in the nucleus becomes larger. For helium-4 (2 protons and 2 neutrons) and oxygen-16 (8 protons and 8 neutrons) this ratio is unity. For indium-115 (49 protons and 66 neutrons) the ratio of neutrons to protons has increased to 1.35, and for uranium-238 (92 protons and 146 neutrons) the neutron to-proton ratio is 1.59.
D. Natural Abundance of Isotopes
The relative abundance of an isotope in nature compared to other isotopes of the same element is relatively constant. The Chart of the Nuclides presents the relative abundance of the naturally occurring isotopes of an element in units of atom percent. Atom percent is the percentage of the atoms of an element that are of a particular isotope.
Atom percent is abbreviated as a/o. For example, if a cup of water contains 8.23 x 1024 atoms of oxygen, and the isotopic abundance of oxygen-18 is 0.20%, then there are 1.65 x 1022 atoms of oxygen-18 in the cup.
E. Atomic Weight
The atomic weight for an element is defined as the average atomic weight of the isotopes of the element. The atomic weight for an element can be calculated by summing the products of the isotopic abundance of the isotope with the atomic mass of the isotope.
SECTION THREE: MASS DEFECT AND BINDING ENERGY
3.1 MASS DEFECT
Careful measurements have shown that the mass of a particular atom is always slightly less than the sum of the masses of the individual neutrons,protons, and electrons of which the atom consists. The difference between the mass of the atom and the sum of the masses of its parts is called the mass defect (Dm ).
The mass defect can be calculated using the equation show below. In calculating the mass defect it is important to use the full accuracy of mass measurements because the difference in mass is small compared to the mass of the atom. Rounding off the masses of atoms and particles to three or four significant digits prior to the calculation will result in a calculated mass defect of zero.
Dm= (Z(mp + me) + (A-Z)mn)matom
Dm = mass defect (amu)
mp = mass of a proton (1.007277 amu)
mn = mass of a neutron (1.008665 amu)
me = mass of an electron (0.000548597 amu)
matom = mass of nuclide (amu)
Z = atomic number (number of protons)
A = mass number (number of nucleons)
3.2 Binding Energy
The loss in mass, or mass defect, is due to the conversion of mass to binding energy when the nucleus is formed. Binding energy is defined as the amount of energy that must be supplied to a nucleus to completely separate its nuclear particles (nucleons). It can also be understood as the amount of energy that would be released if the nucleus was formed from the separate particles.
Binding energy is the energy equivalent of the mass defect. Since the mass defect was converted to binding energy (BE) when the nucleus was formed, it is possible to calculate the binding energy using a conversion factor derived by the mass-energy relationship from Einstein's Theory of Relativity.
Einstein's famous equation relating mass and energy is;
E = mc2
E = Energy in Joules
m = mass in kilograms
c = is the velocity of light (c = 3 x 108 meters/sec).
The energy equivalent of 1 amu can be determined by inserting this quantity of mass into Einstein's equation and applying conversion factors.
1 amu = 1.6606 x 10-27 kg
3.3. Energy Levels of Atoms
The electrons that circle the nucleus move in fairly well-defined orbits. Some of these electrons are more tightly bound in the atom than others. For example, only 7.38 eV is required to remove the outermost electron from a lead atom, while 88,000 eV is required to remove the innermost electron.
The process of removing an electron from an atom is called ionization, and the energy required to remove the electron is called the ionization energy. In a neutral atom (number of electrons = Z) it is possible for the electrons to be in a variety of different orbits, each with a different energy level. The state of lowest energy is the one in which the atom is normally found and is called the ground state. When the atom possesses more energy than its ground state energy, it is said to be in an excited state.
An atom cannot stay in the excited state for an indefinite period of time. An excited atom will eventually transition to either a lower-energy excited state, or directly to its ground state, by emitting a discrete bundle of electromagnetic energy called an x-ray. The energy of the x-ray will be equal to the difference between the energy levels of the atom and will typically range from several eV to 100,000 eV in magnitude.
3.4. Energy Levels of the Nucleus
The nucleons in the nucleus of an atom, like the electrons that circle the nucleus, exist in shells that correspond to energy states. The energy shells of the nucleus are less defined and less understood than those of the electrons.
There is a state of lowest energy (the ground state) and discrete possible excited states for a nucleus. Where the discrete energy states for the electrons of an atom are measured in eV or keV, the energy levels of the nucleus are considerably greater and typically measured in MeV.
A nucleus that is in the excited state will not remain at that energy level for an indefinite period. Like the electrons in an excited atom, the nucleons in an excited nucleus will transition towards their lowest energy configuration and in doing so emit a discrete bundle of electromagnetic radiation called a gamma ray (g-ray). The only differences between x-rays and g-rays are their energy levels and whether they are emitted from the electron shell or from the nucleus.
SECTION FOUR: MODES OF RADIOACTIVE DECAY
Most atoms found in nature are stable and do not emit particles or energy that change form over time. Some atoms, however, do not have stable nuclei. These atoms emit radiation in order to achieve a more stable configuration.
4.1 Stability of Nuclei
As mass numbers become larger, the ratio of neutrons to protons in the nucleus becomes larger for the stable nuclei. Non-stable nuclei may have an excess or deficiency of neutrons and undergo a transformation process known as beta (b) decay.
Non-stable nuclei can also undergo a variety of other processes
such as alpha (a) or neutron (n)
decay. As a result of these decay processes, the final nucleus is in a more
stable or more tightly bound configuration.
4.2 Natural Radioactivity
In 1896, the French physicist Becquerel discovered that crystals of a uranium salt emitted rays that were similar to x-rays in that they were highly penetrating, could affect a photographic plate, and induced electrical conductivity in gases. Becquerel's discovery was followed in 1898 by the identification of two other radioactive elements, polonium and radium, by Pierre and Marie Curie.
Heavy elements, such as uranium or thorium, and their unstable decay chain elements emit radiation in their naturally occurring state. Uranium and thorium, present since their creation at the beginning of geological time, have an extremely slow rate of decay. All naturally occurring nuclides with atomic numbers greater than 82 are radioactive.
4.3 Nuclear Decay
Whenever a nucleus can attain a more stable (i.e., more tightly bound) configuration by emitting radiation, a spontaneous disintegration process known as radioactive decay or nuclear decay may occur. In practice, this "radiation" may be electromagnetic radiation, particles, or both. Detailed studies of radioactive decay and nuclear reaction processes have led to the formulation of useful conservation principles.
The four principles of most interest in this white paper are discussed below.
1. Conservation of electric charge:Conservation of electric charge implies that sum of the charges in the beginning of a process is equal to the sum of the charges after the interaction has occurred.
2. Conservation of mass number:Conservation of mass number does not allow a net change in the number of nucleons. However, the conversion of a proton to a neutron and vice versa is allowed.
3. Conservation of mass and energy:Implies that the total of the kinetic energy and the energy equivalent of the mass in a system must be conserved in all decays and reactions. Mass can be converted to energy and energy can be converted to mass, but the sum of mass and energy must be constant.
4. Conservation of momentum:is responsible for the distribution of the available kinetic energy among product nuclei, particles, and/or radiation. The total amount is the same before and after the reaction even though it may be distributed differently among entirely different nuclides and/or particles.
4.4 Alpha Decay (a)
Alpha decay is the emission of alpha particles (helium nuclei) which may be represented as either He4 or (a).When an unstable nucleus ejects an alpha particle, the atomic number is reduced by 2 and the mass number decreased by 4. An example is uranium-234, (U234) which decays by the ejection of an alpha particle accompanied by the emission of a 0.068 MeV gamma ray photon.
The combined kinetic energy of the new nucleus (Thorium-230, Th 230) and the a particle is designated as KE. The sum of the KE and the gamma energy is equal to the difference in mass between the original nucleus U234 (Uranium-234) and the final particles (equivalent to the binding energy released, since Dm = BE). The alpha particle will carry off as much as 98% of the kinetic energy and, in most cases, can be considered to carry off all the kinetic energy.
4.5 Beta Decay ( b)
Beta decay is
the emission of electrons of nuclear rather than orbital origin. These
particles are electrons that have been expelled by excited nuclei and may have
a charge of either sign. If both energy and momentum are to be conserved, a
third type of particle, the neutrino must be involved.
The neutrino is associated with positive
electron emission, and its antiparticle, the antineutrino, , is emitted
with a negative electron. These uncharged particles have only the weakest
interaction with matter, no mass, and travel at the speed of light. For all
practical purposes, they pass through all materials with so few interactions
that the energy they possess cannot be recovered.
The neutrinos and antineutrinos are included here only because they carry a portion of the kinetic energy that would otherwise belong to the beta particle, and therefore, must be considered for energy and momentum to be conserved. They are normally ignored since they are not significant in the context of nuclear reactor applications.
Negative electron emission, effectively converts a neutron to a proton, thus increasing the atomic number by one and leaving the mass number unchanged. This is a common mode of decay for nuclei with an excess of neutrons.
Positively charged electrons (beta-plus) are known as positrons. Except for sign, they are nearly identical to their negatively charged cousins. When a positron is ejected from the nucleus, the atomic number is decreased by one and the mass number remains unchanged. A proton has been converted to a neutron.
4.6 Electron Capture (EC, K-capture)
Nuclei having an excess of protons may capture an electron from one of the inner orbits which immediately combines with a proton in the nucleus to form a neutron. This process is called electron capture (EC). The electron is normally captured from the innermost orbit (the K-shell), and, consequently, this process is sometimes called K-capture.
A neutrino is formed at the same time that the
neutron is formed, and energy carried off by it serves to conserve momentum.
Any energy that is available due to the atomic mass of the product being
appreciably less than that of the parent will appear as gamma radiation. Also,
there will always be characteristic x-rays given off when an electron from one
of the higher energy shells moves in to fill the vacancy in the K-shell.
Electron capture and positron emission result in the production of the same
daughter product, and they exist as competing processes.
For positron emission to occur, however, the
mass of the daughter product must be less than the mass of the parent by an
amount equal to at least twice the mass of an electron. This mass difference
between the parent and daughter is necessary to account for two items present
in the parent but not in the daughter. One item is the positron ejected from
the nucleus of the parent. The other item is that the daughter product has one
less orbital electron than the parent. If this requirement is not met, then
orbital electron capture takes place exclusively.
4.7 Gamma Emission ( g)
Gamma radiation is a high-energy
electromagnetic radiation that originates in the nucleus. It is emitted in the
form of photons, discrete bundles of energy that have both wave and
particle properties. Often a daughter nuclide is left in an excited state after
a radioactive parent nucleus undergoes a transformation by alpha decay, beta
decay, or electron capture. The nucleus will drop to the ground
state by the emission of gamma radiation.
4.8 Internal Conversion
The usual method for an excited nucleus to go
from the excited state to the ground state is by emission of gamma radiation.
However, in some cases the gamma ray (photon) emerges from the nucleus only to
interact with one of the innermost orbital electrons and, as a result, the
energy of the photon is transferred to the electron. The gamma ray is then said
to have undergone internal conversion.
The conversion electron is ejected from the
atom with kinetic energy equal to the gamma energy minus the binding energy of
the orbital electron. An orbital electron then drops to a lower energy state to
fill the vacancy, and this is accompanied by the emission of characteristic
x-rays.
4.9 Isomers and Isomeric Transition
Isomeric transition commonly occurs immediately
after particle emission; however, the nucleus may remain in an excited state
for a measurable period of time before dropping to the ground state at its own
characteristic rate. A nucleus that remains in such an excited state is known
as a nuclear isomer because it differs in energy and behavior
from other nuclei with the same atomic number and mass number.
The decay of an excited nuclear isomer to a lower energy level is called an isomeric transition. It is also possible for the excited isomer to decay by some alternate means, for example, by beta emission.
4.10 Decay Chains
When an unstable nucleus decays, the resulting daughter nucleus is not
necessarily stable. The nucleus resulting from the decay of a parent is often
itself unstable, and will undergo an additional decay. This is especially
common among the larger nuclides.
It is possible to trace the steps of an
unstable atom as it goes through multiple decays trying to achieve stability.
The list of the original unstable nuclide, the nuclides that are involved as
intermediate steps in the decay, and the final stable nuclide is known as the decay
chain.
4.11 Predicting Type of Decay
Radioactive nuclides tend to decay in a way that results in a daughter
nuclide that lies closer to the line of stability. Due to this, it is possible
to predict the type of decay that a nuclide will undergo based on its location
relative to the line of stability.
SECTION FIVE: RADIOACTIVITY
The rate at which a sample of radioactive material decays is not
constant. As individual atoms of the material decay, there are fewer of those
types of atoms remaining. Since the rate of decay is directly proportional to
the number of atoms, the rate of decay will decrease as the number of atoms
decreases.
5.1 Definition Radioactivity
Radioactivity is the property of certain
nuclides of spontaneously emitting particles or gamma radiation. The decay of
radioactive nuclides occurs in a random manner, and the precise time at which a
single nucleus will decay cannot be determined. However, the average behavior
of a very large sample can be predicted accurately by using statistical
methods. These studies have revealed that there is a certain probability that
in a given time interval a certain fraction of the nuclei within a sample of a
particular nuclide will decay.
This probability per unit time that an atom of a nuclide will
decay is known as the radioactive decay constant. The units for the decay
constant are inverse time such as 1/second, 1/minute, 1/hour, or 1/year.
5.2 Activity
The activity (A) of a sample is the rate of decay of
that sample. This rate of decay is usually measured in the number of
disintegrations that occur per second. For a sample containing millions of
atoms, the activity is the product of the decay constant and the number of atoms
present in the sample.
The relationship between the activity, number of atoms, and decay
constant is shown below;
A = l N
where:
A = Activity of the nuclide
(disintegrations/second)
l = decay constant of the nuclide (second-1)
N = Number of atoms of the
nuclide in the sample
Since l is a constant, the activity and the number of atoms are always proportional.
5.3 Units of Measurement for Radioactivity
Two common units to measure the activity of a substance are the curie
(Ci) and becquerel (Bq). A curie is a unit
of measure of the rate of radioactive decay equal to 3.7 x 1010 disintegrations per second.
This is approximately equivalent to the number of disintegrations that one gram
of radium-226 will undergo in one second. A becquerel is a more
fundamental unit of measure of radioactive decay that is equal to 1
disintegration per second.
Currently, the curie is more widely used in the United
States, but usage of the becquerel can be expected to broaden as the
metric system slowly comes into wider use. The conversion between curies and
becquerels is shown below.
1 curie = 3.7 x 1010 becquerels
5.4 Variation of Radioactivity Over Time
The rate at which a given radionuclide sample decays is stated in section 5.2 as being equal to the product of the number of atoms and the decay constant.
From this basic relationship it is possible to use calculus to derive an expression which can be used to calculate how the number of atoms present will change over time. The derivation is beyond the scope of this white paper but the following equation is the useful result of the solution of this important differential equation:
Nt = No e-lt
where:
Nt = number of atoms present
at time t
No = number of atoms initially
present o
l= decay constant (time-1)
t = time
5.5 Radioactive Half-Life
One of the most useful terms for estimating how quickly a nuclide
will decay is the radioactive half-life. The radioactive half-life
is defined as the amount of time required for the activity to decrease to
one-half of its original value.
A relationship between the half-life and decay constant can be developed from the equation developed in section 5.4.
Assuming an initial number of atoms No the
population, and consequently, the activity may be noted to decrease by one-half
of this value in a time of one half-life. Additional decreases occur so that
whenever one half-life elapses, the number of atoms drops to one-half of what
its value was at the beginning of that time interval. After five half-lives
have elapsed, only 1/32, or 3.1%, of the original number of atoms remains.
After seven half-lives, only 1/128, or 0.78%, of the atoms remains. The number
of atoms existing after 5 to 7 half-lives can usually be assumed to be
negligible.
5.6 Plotting Radioactive Decay
It is useful to plot the activity of a nuclide as it changes over time. Plots of this type can be used to determine when the activity will fall below a certain level. This plot is usually done showing activity on either a linear or a logarithmic scale. The decay of the activity of a single nuclide on a logarithmic scale will plot as a straight line because the decay is exponential. If a substance contains more than one radioactive nuclide, the total activity is the sum of the individual activities of each nuclide. The initial activity of each of the nuclides would be the product of the number of atoms and the decay constant.
5.7 Radioactive Equilibrium
A. Radioactive equilibrium exists when a radioactive nuclide is decaying at the same rate at which it is being produced. Since the production rate and decay rate are equal, the number of atoms present remains constant over time.
B. Transient radioactive equilibrium occurs when the parent nuclide and the daughter nuclide decay at essentially the same rate. For transient equilibrium to occur, the parent must have a long half-life when compared to the daughter. An example of this type of compound decay process is barium-140, which decays by beta emission to lanthanum-140, which in turn decays by beta emission to stable cerium-140.
The decay constant for barium-140 is considerably smaller
than the decay constant for lanthanum-140. Remember that the rate of
decay of both the parent and daughter can be represented as lN. Although the decay
constant for barium-140 is smaller, the actual rate of decay (lN) is initially larger than
that of lanthanum-140 because of the great difference in their initial
concentrations. As the concentration of the daughter increases, the rate of
decay of the daughter will approach and eventually match the decay rate of the
parent. When this occurs, they are said to be in transient equilibrium.
C. Secular equilibrium occurs when the parent has
an extremely long half-life. In the long decay chain for a naturally
radioactive element, such as thorium-232, where all of the elements in the
chain are in secular equilibrium, each of the descendants has built up to an
equilibrium amount and all decay at the rate set by the original parent. The
only exception is the final stable element on the end of the chain. Its number
of atoms is constantly increasing.
Neutrons can cause many different types of interactions. The
neutron may simply scatter off the nucleus in two different ways, or it may
actually be absorbed into the nucleus. If a neutron is absorbed into the
nucleus, it may result in the emission of a gamma ray or a subatomic particle,
or it may cause the nucleus to fission.
6.1 Scattering
A neutron scattering reaction occurs when a
nucleus, after having been struck by a neutron, emits a single neutron. Despite
the fact that the initial and final neutrons do not need to be (and often are
not) the same, the net effect of the reaction is as if the projectile neutron
had merely "bounced off," or scattered from, the nucleus. The two
categories of scattering reactions, elastic and inelastic scattering,
are described below:
A. Elastic Scattering
In an elastic
scattering reaction between a neutron and a target nucleus, there is no
energy transferred into nuclear excitation. Momentum and kinetic energy of the
"system" are conserved although there is usually some transfer of
kinetic energy from the neutron to the target nucleus. The target nucleus gains
the amount of kinetic energy that the neutron loses. Elastic scattering of
neutrons by nuclei can occur in two ways:
1.
The more unusual of the two interactions is the absorption of the
neutron, forming a compound nucleus, followed by the re-emission of a neutron
in such a way that the total kinetic energy is conserved and the nucleus
returns to its ground state. This is known as resonance elastic scattering and is very dependent upon
the initial kinetic energy possessed by the neutron. Due to formation of the
compound nucleus, it is also referred to as compound elastic scattering.
2. The second, more usual method, is termed potential elastic scattering and can be understood by visualizing the neutrons and nuclei to be much like billiard balls with impenetrable surfaces. Potential scattering takes place with incident neutrons that have an energy of up to about 1 MeV. In potential scattering, the neutron does not actually touch the nucleus and a compound nucleus is not formed. Instead, the neutron is acted on and scattered by the short range nuclear forces when it approaches close enough to the nucleus.
B. Inelastic Scattering
In inelastic scattering, the incident neutron is
absorbed by the target nucleus, forming a compound nucleus. The compound
nucleus will then emit a neutron of lower kinetic energy which leaves the
original nucleus in an excited state. The nucleus will usually, by one or more
gamma emissions, emit this excess energy to reach its ground state.
For the nucleus that has reached its ground state, the sum of the kinetic energy of the exit Inelastic Scattering neutron, the target nucleus, and the total gamma energy emitted is equal to the initial kinetic energy of the incident neutron.
6.2 Absorption Reactions
Most absorption reactions result in the loss of a
neutron coupled with the production of a charged particle or gamma ray. When
the product nucleus is radioactive, additional radiation is emitted at some
later time. Radiative capture, particle ejection, and fission
are all categorized as absorption reactions and are briefly described
below.
B. Particle Ejection:In a particle ejection reaction the incident particle enters the target nucleus forming a compound nucleus. The newly formed compound nucleus has been excited to a high enough energy level to cause it to eject a new particle while the incident neutron remains in the nucleus. After the new particle is ejected, the remaining nucleus may or may not exist in an excited state depending upon the mass-energy balance of the reaction.
C. Fission:One of the most important interactions that neutrons can cause is fission, in which the nucleus that absorbs the neutron actually splits into two similarly sized parts. Fission will be discussed in detail in the next chapter.
SECTION SEVEN: NUCLEAR FISSION
Nuclear fission is a process in which an atom splits and releases energy, fission products, and neutrons. The neutrons released by fission can, in turn, cause the fission of other atoms.
7.1 Fission
In the fission reaction the incident neutron enters the heavy target nucleus, forming a compound nucleus that is excited to such a high energy level (E > E ) that the nucleus "splits" (fissions) into two large fragments plus some neutrons. A large amount of energy is released in the form of radiation and fragment kinetic energy.
7.2. Liquid Drop Model of a Nucleus
The nucleus is held together by the attractive nuclear force
between nucleons. The characteristics of the nuclear force are listed below:
1.
Very short range, with essentially no effect beyond nuclear
dimensions (10-13 cm)
2. Stronger than the repulsive electrostatic forces within the nucleus
3. Independent of nucleon pairing, in that the attractive forces between pairs of neutrons are no different than those between pairs of protons or a neutron and a proton
4. Saturable, that is, a nucleon can attract only a few of its nearest neighbors
One theory of
fission considers the fissioning of a nucleus similar in some respects to the
splitting of a liquid drop. This analogy is justifiable to some extent by the
fact that a liquid drop is held together by molecular forces that tend to make
the drop spherical in shape and that try to resist any deformation in the same
manner as nuclear forces are assumed to hold the nucleus together.
By considering the nucleus as a liquid drop, the fission process can be described. the nucleus in the ground state is undistorted, and its attractive nuclear forces are greater than the repulsive electrostatic forces between the protons within the nucleus. When an incident particle (in this instance a neutron) is absorbed by the target nucleus, a compound nucleus is formed. The compound nucleus temporarily contains all the charge and mass involved in the reaction and exists in an excited state. The excitation energy added to the compound nucleus is equal to the binding energy contributed by the incident particle plus the kinetic energy possessed by that particle.
The excitation energy thus imparted to the compound nucleus, which
may cause it to oscillate and become distorted. If the excitation energy is
greater than a certain critical energy, the oscillations may cause the compound
nucleus to become dumbbell-shaped. When this happens, the attractive nuclear
forces (short-range) in the neck area are small due to saturation, while the
repulsive electrostatic forces (long-range) are only slightly less than before.
When the repulsive electrostatic forces exceed the attractive nuclear forces,
nuclear fission occurs,
7.3. Critical Energy
The measure of how far the energy level of a nucleus is above its
ground state is called the excitation energy. For fission to occur, the
excitation energy must be above a particular value for that nuclide. The critical
energy (Ecrit)
is the minimum excitation energy required for fission to occur.
7.4. Fissionable, Fissile and Fertile Materials
Theoretically, all nuclei heavier than iron have the potential to undergo fission, however the energy barrier that needs to be exceeded before fission can occur is impossibly high for all but the heavier elements. It is only for mass numbers greater than about 230 that the fission activation energy may be less than 10 MeV.
A. Fissionable Materials
Consider the compound nucleus Uranium 239 formed by the absorption of a neutron by U238.For the neutron to induce fission the sum of the binding and kinetic energy transferred to the U239 compound nucleus must exceed its fission activation energy. The activation energy of U239 is 7 MeV; the difference between the binding energy of U238 and U239 is 5.5 MeV. Thus the kinetic energy of the incoming neutron must be at least 1.5 MeV.
Materials,
such as U238, which may undergo fission following absorption
of fast neutrons of a few MeV kinetic energy are called Fissionable
Materials.
B. Fissile Materials
A fissile material is composed of nuclides for
which fission is possible with neutrons of any energy level. What is especially
significant about these nuclides is their ability to be fissioned with zero
kinetic energy neutrons (thermal neutrons). Thermal neutrons have very low
kinetic energy levels (essentially zero) because they are roughly in
equilibrium with the thermal motion of surrounding materials.
Therefore, in order to be classified as fissile, a material must
be capable of fissioning after absorbing a thermal neutron. Consequently, they
impart essentially no kinetic energy to the reaction. Fission is possible in
these materials with thermal neutrons, since the change in binding energy
supplied by the neutron addition alone is high enough to exceed the critical
energy. Some examples of fissile nuclides are U235 (uranium-235), U233 (uranium-233), and PU239 (plutonium-239).
Consider the compound nucleus U236 formed from the absorption of a neutron by U235.In this case the U236 fission activation energy is 6.5 MeV whereas the difference in binding energy between U235 and U236 is 6.8 MeV. Thus neutrons of any kinetic energy can induce fission following absorption in U235.U235 is the only naturally occurring fissile material.
C. Fertile Materials
All of the neutron absorption reactions that do not result in
fission lead to the production of new nuclides through the process known as transmutation. These nuclides can, in
turn, be transmuted again or may undergo radioactive decay to produce still
different nuclides. The nuclides that are produced by this process are referred
to as transmutation products. Because several of the fissile nuclides do not
exist in nature, they can only be produced by nuclear reactions
(transmutation).
The target nuclei for such reactions are said to be fertile. Fertile
materials are materials that can undergo transmutation to become fissile
materials. The fertile nuclides, thorium-232 and uranium-238 can be bombarded
with neutrons to produce uranium-233 and plutonium-239, respectively.
If a reactor contains fertile material in addition to its fissile
fuel, some new fuel will be produced as the original fuel is burned up. This is
called conversion. Reactors that are specifically designed to produce fissionable
fuel are called "breeder" reactors. In such reactors, the amount of
fissionable fuel produced is greater than the amount of fuel burnup. If less
fuel is produced than used, the process is called conversion, and the reactor
is termed a "converter."
A fissionable material is composed of nuclides for
which fission with neutrons is possible. All fissile nuclides fall into this
category. However, also included are those nuclides that can be fissioned only
with high energy neutrons. The change in binding energy that occurs as the
result of neutron absorption results in a nuclear excitation energy level that
is less than the required critical energy. Therefore, the additional excitation
energy must be supplied by the kinetic energy of the incident neutron.
The reason for this difference between fissile and fissionable
materials is the so-called odd-even effect for nuclei. It has been observed
that nuclei with even numbers of neutrons and/or protons are more stable than
those with odd numbers. Therefore, adding a neutron to change a nucleus with an
odd number of neutrons to a nucleus with an even number of neutrons produces an
appreciably higher binding energy than adding a neutron to a nucleus already
possessing an even number of neutrons. Some examples of nuclides requiring high
energy neutrons to cause fission are
Th232 (thorium-232), U238 (uranium-238), and Pu240 (plutonium-240).
Uranium 239, which may be formed as the result of U238 absorbing a neutron, is radioactive and decays by b emission, with a half-life of 23-1/2 minutes,
to Neptunium 239. This neptunium 239 decays by b emission, with half-life of 2.3 days, to Plutonium 239, an a emitter of half life of 24,000 years. It turns out that plutonium 239 is a fissile material; that is, as in the case of U235, it readily undergoes fission on absorption of neutrons of any energy including slow neutrons of very low energies. The uranium 238 is called a Fertile Material because the absorption of the neutrons, which we have seen previously it most readily does in the resonance capture mode, leads to the formation of the fissile material Pu239.
Similarly thorium 232 is also a fertile material because neutron absorption leads, via Protactinium 233, to the fissile material Uranium 233.
Thus the fertile materials U238 and Th232 yield the fissile materials Pu239 and U233, respectively.
7.5 Natural Uranium
Natural Uranium is found in ore deposits in many places around the world. It is predominantly a mixture of the two isotopes 238, 234 and 235, in the proportions mentioned at the beginning of this white paper. All three isotopes are radioactive.
Therefore of the three fissile materials mentioned above, natural uranium is a direct source for one, U235,and an indirect source for a second, Pu239 via the fertile U238. These facts underscore the importance of natural uranium in the production of Nuclear Power.
The
third fissile material, U233, is of little significance at
present, although of possibly important potential because of large ore reserves
of the fertile thorium.
Before returning to our discussion on fission it will be useful to summarize some of the properties of natural uranium and its isotopes.Natural uranium consists of:
99.3% U238 a emitter with half life of 4.5 x 109 years
0.7% U235aemitter half life7.1 x 108 years
0.1% U234 a5 years
U238 is a fissionable material; it can undergo fission provided the absorbed neutron has an incident kinetic energy of at least 1.1 MeV.
·
U238 is
a fertile material, forming fissile Pu239 following capture
of a neutron. Neutrons of intermediate
energy are readily captured in the resonance capture peaks of U238.
·
U235 is
a fissile material; it can undergo fission with neutrons of any energy but is
much more likely to do so the less energetic, or slower, the neutron.
The isotope uranium-235
is usually the desired material for use in reactors. A vast amount of equipment
and energy are expended in processes that separate the isotopes of uranium (and
other elements). The details of these processes are beyond the scope of this
white paper. These processes are called enrichment processes because they
selectively increase the proportion of a particular isotope. The enrichment
process typically starts with feed material that has the proportion of isotopes
that occur naturally.
In the case of uranium, the natural uranium ore is 0.72 a/o uranium-235. The desired outcome of the enrichment process is to produce enriched uranium.
·
Enriched uranium is defined as uranium in
which the isotope uranium-235 has a concentration greater than its natural
value. The enrichment process will also result in the byproduct of depleted
uranium.
·
Depleted uranium is defined as uranium in
which the isotope uranium-235 has a concentration less than its natural value.
Although depleted uranium is referred to as a by-product of the enrichment
process, it does have uses in the nuclear field and in commercial and defense
industries.
7.6 Critical Energies Compared to Binding Energy
of Last Neutron
Uranium-235 fissions with thermal neutrons because the binding energy released by the absorption of a neutron is greater than the critical energy for fission; therefore uranium-235 is a fissile material. The binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so additional energy must be possessed by the neutron for fission to be possible. Consequently, uranium-238 is a fissionable material.
7.7
Binding Energy Per Nucleon (BE/A)
As the number of particles in a nucleus increases, the total
binding energy also increases. The rate of increase, however, is not uniform.
This lack of uniformity results in a variation in the amount of binding energy
associated with each nucleon within the nucleus. This variation in the binding
energy per nucleon (BE/A) is easily seen when the average BE/A is plotted
versus atomic mass number (A).
This plot illustrates that as the atomic mass number increases,
the binding energy per nucleon decreases for A > 60. The BE/A curve reaches
a maximum value of 8.79 MeV at A = 56 and decreases to about 7.6 MeV for A =
238. The general shape of the BE/A curve can be explained using the general
properties of nuclear forces. The nucleus is held together by very short-range
attractive forces that exist between nucleons. On the other hand, the nucleus
is being forced apart by long range repulsive electrostatic (coulomb) forces
that exist between all the protons in the nucleus.
As the atomic number and the atomic mass number increase, the
repulsive electrostatic forces within the nucleus increase due to the greater
number of protons in the heavy elements. To overcome this increased repulsion,
the proportion of neutrons in the nucleus must increase to maintain stability.
This increase in the neutron-to-proton ratio only partially compensates for the
growing proton-proton repulsive force in the heavier, naturally occurring
elements. Because the repulsive forces are increasing, less energy must be
supplied, on the average, to remove a nucleon from the nucleus. The BE/A has
decreased. The BE/A of a nucleus is an indication of its degree of stability.
Generally, the more stable nuclides have higher BE/A than the less stable ones.
The increase in the BE/A as the atomic mass number decreases from 260 to 60 is
the primary reason for the energy liberation in the fission process. In
addition, the increase in the BE/A as the atomic mass number increases from 1
to 60 is the reason for the energy liberation in the fusion process, which is
the opposite reaction of fission.
The heaviest nuclei require only a small distortion from a spherical shape (small energy addition) for the relatively large coulomb forces forcing the two halves of the nucleus apart to overcome the attractive nuclear forces holding the two halves together. Consequently, the heaviest nuclei are easily fissionable compared to lighter nuclei.
SECTION EIGHT:
ENERGY RELEASE FROM FISSION
Fission of heavy nuclides converts a small amount of mass into an enormous amount of energy. The amount of energy released by fission can be determined based on either the change in mass that occurs during the reaction or by the difference in binding energy per nucleon between the fissile nuclide and the fission products.
8.1.
Calculation of Fission Energy
Nuclear fission results in the release of enormous quantities of
energy. It is necessary to be able to calculate the amount of energy that will
be produced. The logical manner in which to pursue this is to first investigate
a typical fission reaction. When the compound nucleus splits, it breaks into
two fission fragments, rubidium-93, cesium-140, and some neutrons. Both fission
products then decay by multiple - emissions as a result of the high
neutron-to-proton ratio possessed by these nuclides.
In most cases, the resultant fission fragments have masses that
vary widely. The most probable pair of fission fragments for the thermal
fission of the fuel uranium-235 have masses of about 95 and 140.
Referring now to the binding energy per nucleon curve (Figure 20), we can estimate the amount of energy released by our "typical" fission by plotting this reaction on the curve and calculating the change in binding energy (DBE) between the reactants on the left-hand side of the fission equation and the products on the right-hand side. Plotting the reactant and product nuclides on the curve shows that the total binding energy of the system after fission is greater than the total binding energy of the system before fission. When there is an increase in the total binding energy of a system, the system has become more stable by releasing an amount of energy equal to the increase in total binding energy of the system. Therefore, in the fission process, the energy liberated is equal to the increase in the total binding energy of the system.
8.2
Binding Energy per Nucleon Curve
The energy released will be equivalent to the difference in
binding energy ( BE) between the reactants and the products. The energy
liberation during the fission process can also be explained from the standpoint
of the conservation of mass-energy. During the fission process, there is a
decrease in the mass of the system. There must, therefore, be energy liberated
equal to the energy equivalent of the mass lost in the process.
Again, referring to the "typical" fission reaction. E ,
the instantaneous energy, is the energy released immediately after the fission
process. It is equal to the energy equivalent of the mass lost in the fission
process. The total energy released per fission will vary from the fission to
the next depending on what fission products are formed, but the average total
energy released per fission of uranium-235 with a thermal neutron is 200
MeV.
The majority of the energy liberated in the fission process is
released immediately after the fission occurs and appears as the kinetic energy
of the fission fragments, kinetic energy of the fission neutrons, and
instantaneous gamma rays. The remaining energy is released over a period of
time after the fission occurs and appears as kinetic energy of the beta,
neutrino, and decay gamma rays.
8.3.
Estimation of Decay Energy
In addition to this instantaneous energy release during the actual
fission reaction, there is additional energy released when the fission
fragments decay by - emission. This additional energy is called decay energy, E . The
energy released during the decay for each chain will be equivalent to the mass
difference between the original fission product and the sum of the final stable
nuclide and the beta particles emitted.
8.4. Distribution of Fission Energy
The average energy distribution for the energy released per fission with a thermal neutron in uranium-235 is shown below:
A.
Instantaneous Energy from Fission
Kinetic Energy of Fission Products 167 Mev
Energy of Fission Neutrons 5 MeV
Instantaneous Gamma-ray Energy 5 MeV
Capture Gamma-ray Energy 10 MeV
Total Instantaneous Energy 187 MeV
B.
Delayed Energy from Fission
Beta Particles From Fission Products 7 Mev
Gamma-rays from Fission Products 6 MeV
Neutrinos 10 MeV
Total Delayed Energy 23 MeV
All of the energy released, with the exception of the neutrino energy, is ultimately transformed into heat through a number of processes. The fission fragments, with their high positive charge and kinetic energy, cause ionization directly as they rip orbital electrons from the surrounding atoms. In this ionization process, kinetic energy is transferred to the surrounding atoms of the fuel material, resulting in an increase in temperature. The beta particles and gamma rays also give up their energy through ionization, and the fission neutrons interact and lose their energy through elastic scattering.
Of the 200 MeV released per fission, about seven percent (13 MeV)
is released at some time after the instant of fission. When a reactor is shut
down, fissions essentially cease, but energy is still being released from the
decay of fission products. The heat produced by this decay energy is referred
to as "decay heat." Although decay energy represents about seven
percent of reactor heat production during reactor operation, once the reactor
is shut down the decay heat production drops off quickly to a small fraction of
its value while operating. The decay heat produced is significant, however, and
systems must be provided to keep the reactor cool even after shutdown.
SECTION NINE:
INTERACTION OF RADIATION WITH MATTER
Different types of radiation interact with matter in widely
different ways. A large, massive, charged alpha particle cannot penetrate a
piece of paper and even has a limited range in dry air. A neutrino, at the
other extreme, has a low probability of interacting with any matter, even if it
passed through the diameter of the earth.
9.1
Ionization
Radiation can be classified into two general groups, charged and uncharged; therefore, it may be expected that interactions with matter fall into two general types. Charged particles directly ionize the media through which they pass, while uncharged particles and photons can cause ionization only indirectly or by secondary radiation.
A moving charged particle has an electrical field surrounding it,
which interacts with the atomic structure of the medium through which it is
passing. This interaction decelerates the particle and accelerates electrons in
the atoms of the medium. The accelerated electrons may acquire enough energy to
escape from the parent atom. This process, whereby radiation "strips"
off orbital electrons, is called ionization. Uncharged moving particles have no
electrical field, so they can only lose energy and cause ionization by such
means as collisions or scattering. A photon can lose energy by the
photoelectric effect, Compton effect, or pair production.
Because ionizing radiation creates ions in pairs, the intensity of
ionization or the specific ionization is defined as the number of ion-pairs
formed per centimeter of travel in a given material. The amount of ionization
produced by a charged particle per unit path length, which is a measure of its
ionizing power, is roughly proportional to the particle's mass and the square
of its charge as illustrated in the equation below. where:
I = mz2
K.E.
Where:
I is the ionizing power
m is the mass of the particle
z is the number of unit charges it carries
K.E. is its kinetic energy
Since m for an alpha particle is about 7300 times as large as m for a beta article, and z is twice as great, an alpha will produce much more ionization per unit path length than a beta particle of the same energy. This phenomenon occurs because the larger alpha particle moves slower for a given energy and thus acts on a given electron for a longer time.
9.2
Alpha Radiation
Alpha radiation is normally produced from the radioactive decay of heavy nuclides and from certain nuclear reactions. The alpha particle consists of 2 neutrons and 2 protons, so it is essentially the same as the nucleus of a helium atom. Because it has no electrons, the alpha particle has a charge of +2. This positive charge causes the alpha particle to strip electrons from the orbits of atoms in its vicinity. As the alpha particle passes through material, it removes electrons from the orbits of atoms it passes near. Energy is required to remove electrons and the energy of the alpha particle is reduced by each reaction. Eventually the particle will expend its kinetic energy, gain 2 electrons in orbit, and become a helium atom. Because of its strong positive charge and large mass, the alpha particle deposits a large amount of energy in a short distance of travel. This rapid, large deposition of energy limits the penetration of alpha particles. The most energetic alpha particles are stopped by a few centimeters of air or a sheet of paper.
9.3
Beta-Minus Radiation
A beta-minus particle is an electron that has been ejected at a
high velocity from an unstable nucleus. An electron has a small mass and an
electrical charge of -1. Beta particles cause ionization by displacing
electrons from atom orbits. The ionization occurs from collisions with orbiting
electrons. Each collision removes kinetic energy from the beta particle, causing
it to slow down. Eventually the beta particle will be slowed enough to allow it
to be captured as an orbiting electron in an atom. Although more penetrating
than the alpha, the beta is relatively easy to stop and has a low power of
penetration. Even the most energetic beta radiation can be stopped by a few
millimeters of metal.
9.4
Positron Radiation
Positively charged electrons are called positrons. Except for the positive charge, they are identical to beta-minus particles and interact with matter in a similar manner. Positrons are very short-lived, however, and quickly are annihilated by interaction with a negatively charged electron, producing two gammas with a combined energy (calculated below) equal to the rest mass of the positive and negative electrons.
9.5
Neutron Radiation
Neutrons have no electrical charge. They have nearly the same mass
as a proton (a hydrogen atom nucleus). A neutron has hundreds of times more
mass than an electron, but 1/4 the mass of an alpha particle. The source of neutrons
is primarily nuclear reactions, such as fission, but they may also be produced
from the decay of radioactive nuclides. Because of its lack of charge, the
neutron is difficult to stop and has a high penetrating power.
Neutrons are attenuated (reduced in energy and numbers) by three
major interactions, elastic scatter, inelastic scatter, and absorption. In
elastic scatter, a neutron collides with a nucleus and bounces off. This
reaction transmits some of the kinetic energy of the neutron to the nucleus of
the atom, resulting in the neutron being slowed, and the atom receives some
kinetic energy (motion). This process is sometimes referred to as "the
billiard ball effect."
As the mass of the nucleus approaches the mass of the neutron, this reaction becomes more effective in slowing the neutron. Hydrogenous material attenuates neutrons most effectively. In the inelastic scatter reaction, the same neutron/nucleus collision occurs as in elastic scatter. However, in this reaction, the nucleus receives some internal energy as well as kinetic energy. This slows the neutron, but leaves the nucleus in an excited state.
When the nucleus decays to its original energy level, it normally emits a gamma ray. In the absorption reaction, the neutron is actually absorbed into the nucleus of an atom. The neutron is captured, but the atom is left in an excited state. If the nucleus emits one or more gamma rays to reach a stable level, the process is called radiative capture. This reaction occurs at most neutron energy levels, but is more probable at lower energy levels.
9.6
Gamma Radiation
Gamma radiation is electromagnetic radiation. It is commonly referred to as a gamma ray and is very similar to an x-ray. The difference is that gamma rays are emitted from the nucleus of an atom, and x-rays are produced by orbiting electrons. The x-ray is produced when orbiting electrons move to a lower energy orbit or when fast-moving electrons approaching an atom are deflected and decelerated as they react with the atom's electrical field (called Bremsstrahlung). The gamma ray is produced by the decay of excited nuclei and by nuclear reactions. Because the gamma ray has no mass and no charge, it is difficult to stop and has a very high penetrating power. A small fraction of the original gamma stream will pass through several feet of concrete or several meters of water.
There are three methods of attenuating gamma rays:
CHAPTER TWO:INTRODUCTION TO NEUTRON PHYSICS
SECTION
ONE: NEUTRON SOURCES
Neutrons from a variety of sources are always present in a reactor
core. This is true even when the reactor is shut down. Some of these neutrons
are produced by naturally occurring (intrinsic) neutron sources, while others
may be the result of fabricated (installed) neutron sources that are
incorporated into the design of the reactor. The neutrons produced by sources
other than neutron-induced fission are often grouped together and classified as
source neutrons.
In addition to neutron-induced fission, neutrons are produced by
other reactions. The neutrons produced by reactions other than neutron-induced
fission are called source neutrons. Source neutrons are important
because they ensure that the neutron population remains high enough to allow a
visible indication of neutron level on the most sensitive monitoring
instruments while the reactor is shutdown and during the startup sequence. This
verifies instrument operability and allows monitoring of neutron population
changes. Source neutrons can be classified as either intrinsic or installed
neutron sources.
1.1
Intrinsic Neutron Sources
Some neutrons will be produced in the materials present in the
reactor due to a variety of unavoidable reactions that occur because of the
nature of these materials. Intrinsic neutron sources are those
neutron-producing reactions that always occur in reactor materials.
A limited number of neutrons will always be present, even in a reactor core that has never been operated, due to spontaneous fission of some heavy nuclides that are present in the fuel. Uranium-238, uranium-235, and plutonium-239 undergo spontaneous fission to a limited extent. Uranium-238, for example, yields almost 60 neutrons per hour per gram.
1.2
Neutron Production by Spontaneous Fission
Another intrinsic neutron source is a reaction involving natural boron and fuel. In some reactors, natural boron is loaded into the reactor core as a neutron absorber to improve reactor control or increase core life-time. Boron-11 (80.1% of natural boron) undergoes a reaction with the alpha particle emitted by the radioactive decay of heavy nuclides in the fuel to yield a neutron.
The boron-11 must be mixed with, or in very close proximity to, the fuel for this reaction because of the short path length of the alpha particle. For a reactor core with this configuration, this (a,n) reaction is an important source of neutrons for reactor startup. In a reactor that has been operated, another source of neutrons becomes significant. Neutrons may be produced by the interaction of a gamma ray and a deuterium nucleus. This reaction is commonly referred to as a photo-neutron reaction because it is initiated by electromagnetic radiation and results in the production of a neutron.
There is an abundant supply of high energy gammas in a reactor
that has been operated because many of the fission products are gamma emitters.
All water-cooled reactors have some deuterium present in the coolant in the
reactor core because a small fraction of natural hydrogen is the isotope
deuterium.
The atom percentage of deuterium in the water ranges from close to the naturally occurring value (0.015%) for light water reactors to above 90% deuterium for heavy water reactors. Therefore, the required conditions for production of photo-neutrons exist.
The supply of gamma rays decreases with time after shutdown as the
gamma emitters decay; therefore, the photo-neutron production rate also
decreases. In a few particular reactors, additional Deuterium Oxide, DO2
(heavy water) may be added to the reactor to increase the production of
photo-neutrons following a long shutdown period.
1.3
Installed Neutron Sources
Because intrinsic neutron sources can be relatively weak or dependent upon the recent power history of the reactor, many reactors have artificial sources of neutrons installed. These neutron sources ensure that shutdown neutron levels are high enough to be detected by the nuclear instruments at all times. This provides a true picture of reactor conditions and any change in these conditions.
An installed
neutron source is an assembly placed in or near the reactor for the
sole purpose of producing source neutrons. One strong source of neutrons is the
artificial nuclide californium-252, which emits neutrons at the rate of about 2 x 1012 neutrons per second
per gram as the result of spontaneous fission. Important drawbacks
for some applications may be its high cost and its short half-life (2.65
years).
Many installed neutron sources use the (a,n,) reaction with beryllium. These sources are
composed of a mixture of metallic beryllium (100% beryllium-9) with a small
quantity of an alpha particle emitter, such as a compound of radium, polonium,
or plutonium. The reaction that occurs produces a neutron flux. The beryllium
is intimately (homogeneously) mixed with the alpha emitter and is usually
enclosed in a stainless steel capsule.
Another type of installed neutron source that is widely used is a photo-neutron source that employs the gamma/neutron( g, n) reaction with beryllium. Beryllium is used for photo-neutron sources because its stable isotope beryllium-9 has a weakly attached last neutron with a binding energy of only 1.66 MeV. Thus, a gamma ray with greater energy than 1.66 MeV can cause neutrons to be ejected by the (g, n) reaction.
Density: The ratio of an object’s
mass to its volume.
Departure From Nuclear Boiling Ratio (DNBR): The ratio of the heat flux to cause departure
from nucleate boiling to the actual local heat flux or a fuel rod.
Departure From Nucleate Boiling (DNB): The point at which the
heat transfer from a fuel rod rapidly decreases due to the insulating effect of
a steam blanket that forms on the rod surface when the temperature continues to
increase.
Depleted Uranium:Uranium
having a percentage of uranium-235 smaller than the 0.7 percent found in
natural uranium. It is obtained from spent (used) fuel elements or as byproduct
tails, or residues, from uranium isotope separation.
Derived Air Concentration (DAC): The concentration of radioactive material in
air and the time of exposure to that radionuclide in hours. An NRC licensee may
take 2000 hours to represent one ALI, equivalent to a committed effective dose
equivalent of 5 rems (0.05 sievert).
Derived Air Concentration-Hour (DAC-hour): The product of the concentration of
radioactive material in air (expressed as a fraction or multiple of the derived
air concentration for each radionuclide) and the time of exposure to that
radionuclide, in hours. A licensee may take 2,000 DAC-hours to represent one
ALI, equivalent to a committed effective dose equivalent of 5 rem (0.05 Sv).
Design-basis Accident: A postulated accident
that a nuclear facility must be designed and built to withstand without loss to
the systems, structures, and components necessary to assure public health and
safety.
Design-basis Phenomena: Earthquakes, tornadoes, hurricanes, floods,
etc., that a nuclear facility must be designed and built to withstand without
loss of systems, structures, and components necessary to assure public health
and safety.
Design-basis Threat:
A profile of the type, composition, and capabilities of an adversary. The NRC
and its licensees use the design-basis threat (DBT) as a basis for designing
safeguards systems to protect against acts of radiological sabotage and to
prevent the theft of special nuclear material. The DBT is described in detail
in Title 10, Section 73.1(a), of the Code of Federal Regulations This term is
applied to clearly identify for a licensee the expected capability of its
facility to withstand a threat.
Detector: A material or device that is sensitive to
radiation and can produce a response signal suitable for measurement or
analysis. A radiation detection instrument.
Deterministic (probabilistic): Consistent with the principles of
"determinism," which hold that specific causes completely and
certainly determine effects of all sorts. As applied in nuclear technology, it
generally deals with evaluating the safety of a nuclear power plant in terms of
the consequences of a predetermined bounding subset of accident sequences. The
term "probabilistic" is associated with an evaluation that explicitly
accounts for the likelihood and consequences of possible accident sequences in
an integrated fashion.
Deterministic effect: The health effects of radiation, the severity
of which varies with the dose and for which a threshold is believed to exist.
Radiation-induced cataract formation is an example of a deterministic effect
(also called a non-stochastic effect)
Deuterium: An isotope of hydrogen with one proton and
one neutron in the nucleus.
Deuteron:
The nucleus of deuterium. It contains one
proton and one neutron. See also heavy water.
Differential Pressure (dp or dP): The difference in pressure between two points
of a system, such as between the inlet and outlet of a pump.
Discount Rate: The interest rate used
to assess the value of future cost and revenue streams; an essential factor in
assessing true returns from an investment in energy efficiency, as well as
opportunity costs associated with not making that investment. In this report,
we always use real discount rates that do not include inflation. To obtain the
equivalent nominal discount rate including inflation, simply add the percentage
annual inflation rate to the real discount rate
Distillate Fuel Oil: The lighter fuel oils
distilled off during the refining process. Included are products known as ASTM
grades numbers 1 and 2 heating oils, diesel fuels, and number 4 fuel oil. The
major uses of distillate fuel oils include heating, fuel for on- and
off-highway diesel engines, and railroad diesel fuel.
Doppler Coefficient: Another name used for
the fuel temperature coefficient of reactivity.
Dose: The absorbed dose, given in rads (or in SI units,
grays), that represents the energy absorbed from the radiation in a gram of any
material. Furthermore, the biological dose or dose equivalent, given in rem or
sieverts, is a measure of the biological damage to living tissue from radiation
exposure.
Dose Equivalent: The product of absorbed dose in tissue
multiplied by a quality factor and then sometimes multiplied by other necessary
modifying factors at the location of interest. It is expressed numerically in
rems or sieverts
Dose Rate:
The ionizing radiation dose delivered per unit
time. For example, rem or sieverts per hour.
Dosimeter: A small portable instrument (such as a film
badge or thermoluminescent or pocket dosimeter) for measuring and recording the
total accumulated personal dose of ionizing radiation.
Dosimetry: The theory and application of the principles
and techniques involved in the measurement and recording of ionizing radiation
doses.
Drywell:
The containment structure enclosing a boiling
water reactor vessel and its recirculation system. The drywell provides both a
pressure suppression system and a fission product barrier under accident
conditions.
Earthquake, Operating Basis: An earthquake that
could be expected to affect the reactor plant site, but for which the plant
power production equipment is designed to remain functional without undue risk
to public health and safety.
Effective Dose Equivalent: The sum of the products of the dose
equivalent to the organ or tissue and the weighting factors applicable to each
of the body organs or tissues that are irradiated.
Effective Half-life: The time required for
the amount of a radioactive element deposited in a living organism to be
diminished 50 percent as a result of the combined action of radioactive decay and
biological elimination.
Efficiency, Plant: The percentage of the
total energy content of a power plant's fuel that is converted into
electricity. The remaining energy is lost to the environment as heat.
Elastic Scattering: In this interaction of
radiation with matter. The impinging
particle approaches the target and
Electrical Generator: An electromagnetic
device that converts mechanical (rotational) energy into electrical energy.
Most large electrical generators are driven by steam or water turbine systems.
Electric Dipole Moment: The product of charge and distance of separation for an electric
dipole.
Electric Utility
Restructuring: With
some notable exceptions, the electric power industry historically has been
composed primarily of investor-owned utilities. These utilities have been
predominantly vertically integrated monopolies (combining electricity
generation, transmission, and distribution), whose prices have been regulated
by State and Federal government agencies. Restructuring the industry entails
the introduction of competition into at least the generation phase of
electricity production, with a corresponding decrease in regulatory control.
Restructuring may also modify or eliminate other traditional aspects of
investor-owned utilities, including their exclusive franchise to serve a given
geographical area, assured rates of return, and vertical integration of the
production process.
Electromagnetic Radiation: A traveling wave
motion resulting from changing electric or magnetic fields. Familiar
electromagnetic radiation range from x-rays (and gamma rays) of short
wavelength, through the ultraviolet, visible, and infrared regions, to radar
and radio waves of relatively long wavelength.
Electron: An elementary particle with a negative charge
and a mass 1/1837 that of the proton. Electrons surround the positively charged
nucleus and determine the chemical properties of the atom.
Electron-volt (eV): Energy
unit used as the basis of measurement for atomic (eV), electronic (keV),
nuclear (MeV), and subnuclear processes (GeV or TeV). One electron-volt is
equal to the amount of energy gained by an electron dropping through a
potential difference of one volt, which is 1.6 ´ 10-19 joules.
Electron Capture: A
radioactive decay process in which an orbital electron is captured by and
merges with the nucleus. The mass number is unchanged, but the atomic number is
decreased by one.
Electroweak Interaction: A
theory which unifies the electromagnetic and weak interactions.
Element:
One of the 103 known chemical substances that
cannot be broken down further without changing its chemical properties. Some
examples include hydrogen, nitrogen, gold, lead, and uranium.
Emergency Classifications: Response by an offsite organization is
required to protect local citizens near the site. A request for assistance from
offsite emergency response organizations may be required.
Emergency Core Cooling Systems (ECCS): Reactor system
components (pumps, valves, heat exchangers, tanks, and piping) that are
specifically designed to remove residual heat from the reactor fuel rods should
the normal core cooling system (reactor coolant system) fail.
Emergency
Feedwater: Another
name that may be used for auxiliary feed-water.
Energy: The capacity for doing
work as measured by the capability of doing work potential energy) or the
conversion of this capability to motion (kinetic energy). Energy has several
forms, some of which are easily convertible and can be changed to another form
useful for work. Most of the world's convertible energy comes from fossil fuels
that are burned to produce heat that is then used as a transfer medium to
mechanical or other means in order to accomplish tasks. Electrical energy is
usually measured in kilowatthours, while heat energy is usually measured in
British thermal units.
Energy Saving
Performance Contract: An agreement with a third party in which the overall
performance of installed energy conservation measures is guaranteed by that
party.
Energy Services
Company: A
company which designs, procures, finances, installs, maintains, and guarantees
the performance of energy conservation measures in an owner's facility or
facilities.
ENTOMB: A
method of decommissioning in which radioactive contaminants are encased in a
structurally long-lived material, such as concrete. The entombment structure is
appropriately maintained and continued surveillance is carried out until the
radioactivity decays to a level permitting decommissioning and ultimate
unrestricted release of the property.
Enthalpy: In thermodynamics the
Quantity called enthalpy, denoted by H or h (for the specific
enthalpy) H = U + pV . Where U
is the internal energy, p is the internal pressure and V is
the volume. Enthalpy is a property of a
gas or liquid and it’s units in the British System are Btu/lbm.
Entropy: In thermodynamics the
Quantity called entropy, denoted by S or s (for the specific
entropy) is a measure of the amount of energy in a physical
system not available to do work. As a physical system becomes more disordered,
and its energy becomes more evenly distributed, that energy becomes less able
to do work. The amount of entropy is often thought of as the amount of disorder
in a system.
Environmental
Qualification: A
process for ensuring that equipment will be capable of withstanding the ambient
conditions that could exist when the specific function to be performed by the
equipment is actually called upon to be performed under accident conditions.
Ethanol: A denatured alcohol (C2H5OH)
intended for motor gasoline blending.
Event Notification (EN) System: An internal NRC automated event tracking system used by the NRC Operations Center to track information on incoming notifications of the occurrence of significant material events that have or may affect public health and safety. Significant material events are reported to the NRC Operations Center by NRC licensees, staff of the Agreement States, other Federal agencies, and the public.
Excited State: The state of an atom or nucleus when it possesses more than its normal energy. Typically, the excess energy is released as a gamma ray.
Exclusion
Area: The
area surrounding the reactor where the reactor licensee has the authority to
determine all activities, including exclusion or removal of personnel and
property.
Excursion:
A sudden, very rapid rise in the power level of
a reactor caused by super-criticality. Excursions are usually quickly
suppressed by the negative temperature coefficient, the fuel temperature
coefficient or the void coefficient (depending upon reactor design), or by
rapid insertion of control rods.
Exposure: Being exposed to ionizing radiation or to
radioactive material.
External Radiation: Exposure to ionizing
radiation when the radiation source is located outside the body.
Externalities: Benefits or costs,
generated as a byproduct of an economic activity, that do not accrue to the
parties involved in the activity.
Extremities: The hands, forearms, elbows, feet, knees, leg
below the knees, and ankles. (Permissible radiation exposures in these regions
are generally greater than in the whole body because they contain fewer blood
forming organs and have smaller volumes for energy absorption.)
Fast Fission: Fission of a heavy atom (such as uranium-238)
when it absorbs a high energy (fast) neutron. Most fissionable materials need
thermal (slow) neutrons in order to fission.
Fast Neutron: A neutron with kinetic energy greater than
its surroundings when released during fission.
Feedwater: Water supplied to the reactor pressure vessel
(in a BWR) or the steam generator (in a PWR) that removes heat from the reactor
fuel rods by boiling and becoming steam. The steam becomes the driving force
for the plant turbine generator.
Fermion: A particle having a spin
that is an odd integer multiple of (h-bar)/2.
Fertile Material: A material, which is
not itself fissile (fissionable by thermal neutrons), that can be converted
into a fissile material by irradiation in a reactor. There are two basic
fertile materials: uranium-238 and thorium-232. When these fertile materials
capture neutrons, they are converted into fissile plutonium-239 and
uranium-233, respectively.
Film Badge:
Photographic film used for measurement of
ionizing radiation exposure for personnel monitoring purposes. The film badge
may contain two or three films of differing sensitivities, and it may also
contain a filter that shields part of the film from certain types of radiation.
Fiscal Year:
The 12-month period, from October 1 through
September 30, used by the Federal Government in budget formulation and execution.
The fiscal year is designated by the calendar year in which it ends.
Fissile
material
Although sometimes used as a synonym for
fissionable material, this term has acquired a more restricted meaning. Namely,
any material fissionable by thermal (slow) neutrons. The three primary fissile
materials are uranium-233, uranium-235, and plutonium-239.
Fission (fissioning): The splitting of a
nucleus into at least two other nuclei and the release of a relatively large
amount of energy. Two or three neutrons are usually released during this type
of transformation.
Fission Gases:
Those fission products that exist in the
gaseous state. In nuclear power reactors, this includes primarily the noble
gases, such as krypton and xenon.
Fission Products: The
nuclei (fission fragments) formed by the fission of heavy elements, plus the
nuclide formed by the fission fragments' radioactive decay.
Fissile Nucleus: A
nucleus that may fission after collision with a thermal (slow) neutron or that
fissions spontaneously (by itself).
Fission: The splitting of a heavy
nucleus into two roughly equal parts (which are nuclei of lower-mass elements),
accompanied by the release of a relatively large amount of energy in the form
of kinetic energy of the two parts and in the form of emission of neutrons and
gamma rays.
Fission products: Nuclei
formed by the fission of higher mass elements. They are of medium atomic mass
and almost all are radioactive. Examples: 90Sr, 137Ce.
Fissionable
Material:
Commonly used as a synonym for fissile material, the meaning of this
term has been extended to include material that can be fissioned by fast
neutrons, such as uranium-238.
Fluorescent Lamps: Fluorescent lamps
produce light by passing electricity through a gas, causing it to glow. The gas
produces ultraviolet light; a phosphor coating on the inside of the lamp
absorbs the ultraviolet light and produces visible light. Fluorescent lamps
produce much less heat than incandescent lamps and are more energy efficient.
Linear fluorescent lamps are used in long narrow fixtures designed for such
lamps. Compact fluorescent light bulbs have been designed to replace
incandescent light bulbs in table lamps, floodlights, and other fixtures.
Flux: A
term applied to the amount of some type of particle (neutrons, alpha radiation,
etc.) or energy (photons, heat, etc.) crossing a unit area per unit time. The
unit of flux is the number of particles, energy, etc., per square centimeter
per second.
Formula Quantity: Strategic special nuclear material in any
combination in a quantity of 5000 grams or more computed by the formula, grams
= (grams contained U-235) + 2.5 (grams U-233 + grams plutonium). This class of
material is sometimes referred to as a Category I quantity of material
Fossil Fuel: Any naturally occurring
organic fuel formed in the Earth's crust, such as petroleum, coal, and natural
gas.
Fuel Assembly:
A cluster of fuel rods (or plates). Also called
a fuel element. Many fuel assemblies make up a reactor core.
Fuel Cells: One or more cells
capable of generating an electrical current by converting the chemical energy
of a fuel directly into electrical energy. Fuel cells differ from conventional
electrical cells in that the active materials such as fuel and oxygen are not
contained within the cell but are supplied from outside.
Fuel Cycle:
The series of steps involved in supplying fuel
for nuclear power reactors. It can include mining, milling, isotopic
enrichment, fabrication of fuel elements, use in a reactor, chemical
reprocessing to recover the fissionable material remaining in the spent fuel, re-enrichment
of the fuel material, re-fabrication into new fuel elements, and waste
disposal.
Fuel Reprocessing: The processing of reactor fuel to separate
the unused fissionable material from waste material.
Fuel Rod: A long, slender tube that holds fissionable
material (fuel) for nuclear reactor use. Fuel rods are assembled into bundles
called fuel elements or fuel assemblies, which are loaded individually into the
reactor core.
Fuel Temperature Coefficient of
Reactivity: The
change in reactivity per degree change in the fuel temperature. The physical
property of fuel pellet material (uranium-238) that causes the uranium to
absorb more neutrons away from the fission process as fuel pellet temperature
increases. This acts to stabilize power reactor operations. This coefficient is
also known as the Doppler coefficient.
Full-time
Equivalent: A
measurement equal to one staff person working a full-time work schedule for one
year.
Fusion: A process whereby low mass
nuclei combine to form a more massive nucleus plus one or move massive
particles.
Fusion Reaction: A reaction in which at least one heavier,
more stable nucleus is produced from two lighter, less stable nuclei. Reactions
of this type are responsible for enormous release of energy, as in the energy
of stars, for example.
Gamma Radiation: High-energy, short
wavelength, electromagnetic radiation emitted from the nucleus. Gamma radiation
frequently accompanies alpha and beta emissions and always accompanies fission.
Gamma rays are very penetrating and are best stopped or shielded by dense
materials, such as lead or depleted uranium. Gamma rays are similar to x-rays.
Gamma Ray: A highly
penetrating type of nuclear radiation, similar to x-radiation, except that it
comes from within the nucleus of an atom, and, in general, has a shorter
wavelength.
Gap: The
space inside a reactor fuel rod that exists between the fuel pellet and the
fuel rod cladding.
Gas Centrifuge: A uranium enrichment process that uses a
large number of rotating cylinders in a series. These series of centrifuge
machines, called trains, are interconnected to form cascades. In this process,
UF6 gas is placed in a drum or cylinder and rotated at high speed. This
rotation creates a strong gravitational field so that the heavier gas molecules
(containing U-238) move toward the outside of the cylinder and the lighter gas
molecules (containing U-235) collect closer to the center. The stream that is
slightly enriched in U-235 is withdrawn and fed into the next higher stage,
while the slightly depleted stream is recycled back into the next lower stage.
Significantly more U-235 enrichment can be obtained from a single unit gas
centrifuge than from a single unit gaseous diffusion barrier. No gas centrifuge plants are operating in the
United States, however, Louisiana Energy Services (LES) and the U.S. Enrichment
Corporation (USEC) had plans to submit license applications in 2002 and 2004,
respectively.
Gas-Cooled
Reactor: A
nuclear reactor in which a gas is the coolant.
Gaseous Diffusion Plant: A facility where
uranium hexafluoride gas is filtered. Uranium-235 is separated from
uranium-238, increasing the percentage of uranium-235 from 1 to about 3 percent.
The process requires enormous amounts of electric power.
Gases: A substance possessing perfect molecular
mobility and the property of indefinite expansion, as opposed to a solid or
liquid; any such fluid or mixture of fluids other than air. Normally, these
formless substances completely fill the space, and take the shape of, their
container.
Gas-Turbine Electric
Power Plant: A
plant in which the prime mover is a gas turbine. A gas turbine typically
consists of an axial-flow air compressor and one or more combustion chambers
which liquid or gaseous fuel is burned. The hot gases expand to drive the
generator and then are used to run the compressor.
Gauge Boson: Particle
mediating an interaction. By exchange of the gauge particle, the interaction
between two particles is accomplished.
Geiger Counter: A Geiger-Müller
detector and measuring instrument.
A radiation detection and measuring instrument. It consists of a gas-filled tube containing electrodes, between which there is an electrical voltage, but no current, flowing. When ionizing radiation passes through the tube, a short, intense pulse of current passes from the negative electrode to the positive electrode and is measured or counted. The number of pulses per second measures the intensity of the radiation field. It was named for Hans Geiger and W. Mueller, who invented it in the 1920s. It is sometimes called simply a Geiger counter or a G-M counter and is the most commonly used portable radiation instrument.
Generation (gross): The total amount of
electric energy produced by a generating station as measured at the generator
terminals
Generation (net): The gross amount of electric energy produced
less the electric energy consumed at a generating station for station use.
Global Warming: Global warming is the
increase in global temperatures that the earth has been experiencing this
century. Gases that are thought by many to contribute to global warming through
the greenhouse effect include carbon dioxide, methane, nitrous oxides,
chlorofluorocarbons (CFCs), and halocarbons (the replacements for CFCs). Carbon
dioxide emissions are primarily caused by the use of fossil fuels for energy.
Gluon: A gauge particle mediating the color strong interaction.
Gigawatt: One billion watts.
Gigawatthour: One billion
watt-hours.
Graphite:
A form of carbon, similar to that used in
pencils, used as a moderator in some nuclear reactors.
Gray (Gy): The international
system (SI) unit of absorbed dose. One gray is equal to an absorbed dose of 1
Joule/kilogram (one gray equals 100 rads)
Greenhouse
Gas: Any
gas that absorbs infrared radiation in the atmosphere.
Hadron: A strongly interacting particle.
Half-life: The time in which one half of the atoms of a
particular radioactive substance disintegrate into another nuclear form.
Measured half-lives vary from millionths of a second to billions of years. Also
called physical or radiological half-life.
Half-life, Biological: The time required for
the body to eliminate one half of the material taken in by natural biological
means.
Half-life, Effective: The time required for
a radionuclide contained in a biological system, such as a human or an animal,
to reduce its activity by one-half as a combined result of radioactive decay
and biological elimination.
Half-thickness:
Any given absorber that will reduce the
intensity of an original beam of ionizing radiation to one-half of its initial
value.
Head, Reactor Vessel: The removable top
section of a reactor pressure vessel. It is bolted in place during power operation
and removed during refueling to permit access of fuel handling equipment to the
core.
Health Physics: The science concerned with the recognition,
evaluation, and control of health and environmental hazards that may arise from
the use and application of ionizing radiation.
Heap Leach:
A method of extracting uranium from ore using a
leaching solution. Small ore pieces are placed in a heap on an impervious
material (plastic, clay, asphalt) with perforated pipes under the heap. Acidic
solution is then sprayed over the ore, dissolving the uranium. The solution in
the pipes is collected and transferred to an ion-exchange system for
concentration of the uranium.
Heat Exchanger:
Any device that transfers heat from one fluid
(liquid or gas) to another fluid or to the environment.
Heat
Pump: A
device that extracts available heat from one area (the heat source) and
transfers it to another (the heat sink) to either heat or cool an interior
space. Geothermal heat pumps can operate more efficiently than the standard
air-source heat pumps, because during winter the ground does not get as cold as
the outside air (and during the summer, it does not heat up as much).
Heat Sink: Anything that absorbs heat. It is usually
part of the environment, such as the air, a river, or a lake.
Heatup: The rise in temperature of the reactor fuel
rods resulting from an increase in the rate of fission in the core.
Heavy Water (D2O): Water containing significantly more than the
natural proportions (one in 6,500) of heavy hydrogen (deuterium, D)
atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some
reactors because it slows down neutrons effectively and also has a low
probability of absorption of neutrons.
Heavy Water Moderated Reactor: A reactor that uses
heavy water as its moderator. Heavy water is an excellent moderator and thus
permits the use of un-enriched uranium as a fuel.
High-enriched Uranium: Uranium enriched to 20
percent or greater in the isotope uranium-235.
High-level Waste: Radioactive materials
at the end of a useful life cycle that should be properly disposed of,
including--
High-level
waste (HLW) is primarily in the form of spent fuel discharged from commercial
nuclear power reactors. It also includes HLW from activities and a small
quantity of reprocessed commercial HLW
High Radiation Area: Any area with dose
rates greater than 100 millirems (1 millisievert) in one hour 30 centimeters
from the source or from any surface through which the ionizing radiation
penetrates. Areas at licensee facilities must be posted as "high radiation
areas" and access into these areas is maintained under strict control.
Homolog (or homologs): Elements
in the same periodic table group that tend to exhibit similar, but not
identical, chemical properties.
Hormesis: Controversial theory which
argues that there is a benefit to health, or decrease in biological damage from
radiation as dose in increased (valid only for very small doses).
Hot: A colloquial term meaning highly radioactive.
Hot Spot: The
region in a radiation/contamination area where the level of
radiation/contamination is significantly greater than in neighboring regions in
the area.
Hubble Constant: Ratio of
outward speed of galaxies to their distances from Earth.
Independent Power Producer: A
wholesale electricity producer (other than a qualifying facility under the
Public Utility Regulatory Policies Act of 1978), that is unaffiliated with
franchised utilities. Unlike traditional utilities, IPPs do not possess
transmission facilities that are essential to their customers and do not sell
power in any retail service territory where they have a franchise.
Individual Plant Examination (IPE): As requested by the
NRC in Generic Letter 88-20, “Individual Plant Examination for Severe Accident
Vulnerabilities” (November 23, 1988), a risk analysis that considers the unique
aspects of a particular nuclear power plant, identifying the specific
vulnerabilities to severe accident of that plant.
Individual Plant Examination for External
Events (IPEEE): While
the “individual plant examination” takes into account events that could
challenge the design from things that could go awry internally (in the sense
that equipment might fail because components do not work as expected), the
“individual plant examination for external events” considers challenges such as
earthquakes, internal fires, and high winds.
Induced Radioactivity: Radioactivity
that is created by bombarding a substance with neutrons in a reactor or with
charged particles produced by particle accelerators.
Infrared Radiation:
Electromagnetic radiation of longer wavelength than visible light.
In Situ Leach:
A process using a leaching solution to extract
uranium from underground ore bodies in place (in other words, in situ). The
leaching agent, which contains an oxidant such as oxygen with sodium carbonate,
is injected through wells into the ore body in a confined aquifer to dissolve
the uranium. This solution is then pumped via other wells to the surface for
processing.
In Vitro:
From the Latin for "in glass,"
isolated from the living organism and artificially maintained, as in a test
tube.
In Vivo: From the Latin for
"in one that is living," occurring within the living.
Integrated Plant Evaluation: An evaluation that
considers the plant as a whole rather than system by system.
Iodine Spiking Factor: The magnitude of a
rapid, short-term increase in the appearance rate of radioiodine in the reactor
coolant system. This increase is generally caused by a reactor transient that
results in a rapid drop in reactor coolant system pressure relative to the fuel
rod internal pressure.
Ion: 1. An atom that has too many or too few
electrons, causing it to have an electrical charge, and therefore, be
chemically active.
2. An
electron that is not associated (in orbit) with a nucleus.
Ion-exchange:
A common method for concentrating uranium from
a solution. The uranium solution is passed through a resin bed where the
uranium-carbonate complex ions are transferred to the resin by exchange with a
negative ion like chloride. After build-up of the uranium complex on the resin,
the uranium is eluted with a salt solution and the uranium is precipitated in
another process.
Ionization:
The process of adding one or more electrons to,
or removing one or more electrons from, atoms or molecules, thereby creating
ions. High temperatures, electrical discharges, or nuclear radiations can cause
ionization.
Ionization Chamber: An instrument that
detects and measures ionizing radiation by measuring the electrical current
that flows when radiation ionizes gas in a chamber, making the gas a conductor
of electricity.
Ionizing Radiation: Any radiation capable
of displacing electrons from atoms or molecules, thereby producing ions. Some
examples are alpha, beta, gamma, x-rays, neutrons, and ultraviolet light. High
doses of ionizing radiation may produce severe skin or tissue damage.
.Irradiate: To expose to some form of radiation.
Isomer: Nuclides with the same
number of neutrons and protons in different states of excitation.
Isomeric Transition: A
relatively long-lived radioactive decay in which a nucleus goes from a higher
to a lower energy state. The mass number and the atomic number are unchanged.
isotope: Isotopes of a given element
have the same atomic number (same number of protons in their nuclei) but
different mass numbers (different number of neutrons in their nuclei). 238U
and 235U are isotopes of uranium.
Any two or more forms
of an element having identical or very closely related chemical properties and
the same atomic number but different atomic weights or mass numbers.
Isotope Separation: The process of
separating isotopes from one another, or changing their relative abundances, as
by gaseous diffusion or electromagnetic separation. Isotope separation is a
step in the isotopic enrichment process.
Isotopic Enrichment: A process by which the
relative abundance of the isotopes of a given element are altered, thus
producing a form of the element that has been enriched in one particular
isotope and depleted in its other isotopic forms.
joule (J): Unit of
energy, equivalent to the work done in lifting a one-newton weight a distance
of one meter.
K-capture: The
capture by an atom’s nucleus of an electron from the innermost electron orbital
(K-shell) surrounding the nucleus.
kelvin (K): Unit of
temperature equal in size to the Celsius degree, but with the zero set by the
absolute zero of temperature, -273.15°C. Ice freezes at 273 K, room temperature
is about 293 K, and water boils at 373 K, at sea level. human body temperature
is 310 K.
keV: One thousand
electron-volts.
Kerosene:
A
petroleum distillate that is used in space heaters, cook stoves, and water
heaters; it is suitable for use as an illuminant when burned in wick lamps
Kilo: A
Greek prefix meaning "thousand" in the nomenclature of the metric
system. This prefix multiplies a unit by 1000.
Kilovolt: The unit of electrical
potential equal to 1000 volts.
Kilowatt
(kW): One
thousand watts of electricity (see Watt).
Kilowatthour
(kWh): One
thousand watthours.
Kinetic energy: The
energy that a body possesses by virtue of its mass and velocity. Also called
the energy of motion.
Lens Dose Equivalent: The external exposure
dose equivalent to the lens of the eye at a tissue depth of 0.3 centimeters
(300 mg/cm2).
Lepton: A particle (such as the electron or neutrino) not subject to
strong interactions.
Lepton Number: Additive quantum number defining leptons; the three lepton
numbers are electron number, muon number, and tau number. These numbers remain
the same in all reactions.
Lifetime: The mean life of a particle
or radioactive nucleus. This is equivalent to the decay time.
Linac: Another name for a linear accelerator.
Linear Accelerator: Particle
accelerator laid out in a straight line.
Lethal Dose (LD): The dose of radiation
expected to cause death to 50 percent of an exposed population within 30 days
(LD 50/30). Typically, the LD 50/30 is in the range from 400 to 450 rem (4 to 5
sieverts) received over a very short period.
Licensed Material: Source material,
special nuclear material, or byproduct material received, possessed, used,
transferred or disposed of under a general or specific license issued by the
NRC.
Licensing Basis: The collection of
documents or technical criteria that provides the basis upon which the NRC
issues a license to possess radioactive materials, conduct operations involving
emission of radiation, use special nuclear materials, or dispose of radioactive
waste.
Light Truck: Two-axle, four-tire
trucks with a gross vehicle weight less than 10,000 pounds.
Light
Water: Ordinary
water as distinguished from heavy water..
Light Water Reactor: A term used to
describe reactors using ordinary water as coolant, including boiling water
reactors (BWRs) and pressurized water reactors (PWRs), the most
common types used in the United States.
Limiting
Condition for Operation: The
section of Technical Specifications that identifies the lowest functional
capability or performance level of equipment required for safe operation of the
facility.
Limiting Safety System Settings: Settings for automatic
protective devices related to those variables having significant safety
functions. Where a limiting safety system setting is specified for a variable
on which a safety limit has been placed, the setting will ensure that automatic
protective action will correct the abnormal situation before a safety limit is
exceeded.
Linear Heat Generation Rate: The heat generation
rate per unit length of fuel rod, commonly expressed in kilowatts per foot
(kw/ft) of fuel rod.
Liquefied Natural Gas: Natural gas (primarily
methane) that has been liquefied by reducing its temperature to -260F
at atmospheric pressure.
Liquefied Petroleum
Gas: Ethane,
ethylene, propane, propylene, normal butane, butylene, and isobutane produced
at refineries or natural gas processing plants.
Loop: In
a pressurized water reactor, the coolant flow path through piping from the
reactor pressure vessel to the steam generator, to the reactor coolant pump,
and back to the reactor pressure vessel. Large PWRs may have as many as four
separate loops.
Loss of Coolant Accident (LOCA): Those postulated
accidents that result in a loss of reactor coolant at a rate in excess of the
capability of the reactor makeup system from breaks in the reactor coolant
pressure boundary, up to and including a break equivalent in size to the
double-ended rupture of the largest pipe of the reactor coolant system.
Low Population Zone (LPZ): An area of low
population density often required around a nuclear installation before it's
built. The number and density of residents is of concern in emergency planning
so that certain protective measures (such as notification and instructions to
residents) can be accomplished in a timely manner
Low-level Waste: A general term for a wide range of wastes
having low levels of radioactivity. Industries; hospitals and medical,
educational, or research institutions; private or government laboratories; and
nuclear fuel cycle facilities (e.g., nuclear power reactors and fuel
fabrication plants) that use radioactive materials generate low-level wastes as
part of their normal operations. These wastes are generated in many physical and
chemical forms and levels of contamination
Low-level radioactive wastes containing source, special nuclear, or byproduct material are acceptable for disposal in a land disposal facility. For the purposes of this definition, low-level waste has the same meaning as in the Low-Level Radioactive Waste Policy Act, that is, radioactive waste not classified as high-level radioactive waste, transuranic waste, spent nuclear fuel, or byproduct material as defined in section 11e.(2) of the Atomic Energy Act (uranium or thorium tailings and waste).
Mass Energy: Energy a
particle has by virtue of its mass (given by m times c2).
Mass Number: The
total number of protons and neutrons in the nucleus: A=Z+N. This
is also the total nucleon number of the nucleus.
Mass-energy Equation: The equation developed
by Albert Einstein, which is usually given as E = mc2,
showing that, when the energy of a body changes by an amount E (no matter what
form the energy takes), the mass (m) of the body will change by an amount equal
to E/c2. The factor c
squared, the speed of light in a vacuum (3 x 108), may be regarded
as the conversion factor relating units of mass and energy. The equation
predicted the possibility of releasing enormous amounts of energy by the
conversion of mass to energy. It is also called the Einstein equation.
Maximum Dependable Capacity (gross): In a nuclear power
reactor, dependable main-unit gross generating capacity, winter or summer,
whichever is smaller. The dependable capacity varies because the unit
efficiency varies during the year due to temperature variations in cooling
water. It is the gross electrical output as measured at the output terminals of
the turbine generator during the most restrictive seasonal conditions (usually
summer).
Maximum dependable capacity (net): In a nuclear power
reactor, gross maximum dependable generating capacity less the normal station
service loads.
Mega: A prefix that
multiplies a basic unit by 1,000,000 (10 to the sixth power).
Megacurie: One million curies.
Megawatt
(MW): One
million watts.
Megawatt
Hour (MWh): One
million watt-hours.
Meson: A particle (such as the
pion) made of quark-antiquark pairs.
MeV: One million electron-volts.
Methanol:
A
light volatile alcohol (CH3OH) used for motor gasoline blending.
Metric Ton:
Approximately 2200 pounds in the English system
of measurements. (Note: In the international system of measurements, 1 metric
ton = 1000 kg.)
Micro: A prefix that divides
a unit into one million parts (0.000001).
Microcurie: One millionth of a
curie. That amount of radioactive material that disintegrates (decays) at the
rate of 37 thousand atoms per second.
Microwaves:
Electromagnetic radiation with wavelength intermediate between radio wave and
infrared radiation.
Milli: A prefix that divides
a basic unit by 1000.
Millirem: One thousandth of a
rem (0.001 rem).
Milliroentgen
(mR): One
thousandth of a roentgen (R). 1mR = 10-3
R = 0.001 R.
Mixed Oxide (MOX) Fuel: A mixture of uranium
oxide and plutonium oxide used to fuel a reactor. Mixed oxide fuel is often
called "MOX." Conventional nuclear fuel is made of pure uranium
oxide.
Moderator:
A material, such as ordinary water, heavy
water, or graphite, that is used in a reactor to slow down high-velocity
neutrons, thus increasing the likelihood of fission.
Moderator Temperature Coefficient of
Reactivity: As
the moderator (water) increases in temperature, it becomes less dense and slows
down fewer neutrons, which results in a negative change of reactivity. This
negative temperature coefficient acts to stabilize atomic power reactor
operations.
Molecule:
A group of atoms held together by chemical
forces. A molecule is the smallest unit of a compound that can exist by itself
and retain all of its chemical properties.
Monitoring of Radiation: Periodic or continuous
determination of the amount of ionizing radiation or radioactive contamination
present in a region, as a safety measure, for the purpose of health or environmental
protection. Monitoring is done for air, surface and ground water, soil and
sediment, equipment surfaces, and personnel (for example, bioassay or alpha
scans).
Multiwire Proportional Counter: Particle detector using changes in the current in wires due
to the passage of ionizing particles nearby.
Muon: A charged lepton about 200
times more massive than an electron.
Muon
Number: Additive quantum number characterizing muons
and muon neutrinos
Nano: A prefix that divides
a basic unit by one billion (10-9).
Nanocurie:
One billionth 10-9
of a curie.
Natural Circulation: The circulation of the
coolant in the reactor coolant system without the use of the reactor coolant
pumps. The circulation is due to the natural convection resulting from the
different densities of relative cold and heated portions of the system.
Natural Gas: A mixture of
hydrocarbons (principally methane) and small quantities of various
non-hydrocarbons existing in the gaseous phase or in solution with crude oil in
underground reservoirs.
Natural Uranium: Uranium as found in
nature. It contains 0.7 percent uranium-235, 99.3 percent uranium-238, and a
trace of uranium-234 by weight. In terms of the amount of radioactivity, it
contains approximately 2.2 percent uranium-235, 48.6 percent uranium-238, and
49.2 percent uranium-234.
Net Summer Capability: The steady hourly
output that generating equipment is expected to supply to system load exclusive
of auxiliary power, as demonstrated by tests at the time of summer peak demand.
Neutrino: An electrically neutral
particle with negligible mass. It is produced in processes such as beta decay
and reactions that involve the weak force.
Neutron: One of the basic particles
that make up a nucleus. A neutron and a proton have about the same mass, but
the neutron has no electrical charge.
Neutron Capture: The reaction that
occurs when a nucleus captures a neutron. The probability that a given material
will capture a neutron is proportional to its neutron capture cross section and
depends on the energy of the neutrons and the nature of the material.
Neutron Chain Reaction: A process in which
some of the neutrons released in one fission event cause other fissions to
occur. There are three types of chain reactions:
Neutron Flux:
A measure of the intensity of neutron radiation
in neutrons/cm2-sec. It is the
number of neutrons passing through 1 square centimeter of a given target in 1
second. Expressed as nv, where n = the number of neutrons per cubic centimeter
and v = their velocity in centimeters per second.
Neutron Generation: The release,
thermalization, and absorption of fission neutrons by a fissile material and
the fission of that material producing a second generation of neutrons. In a
typical nuclear power reactor system, there are about 40,000 generations of
neutrons every second.
Neutron Leakage: Neutrons that escape
from the vicinity of the fissionable material in a reactor core. Neutrons that
leak out of the fuel region are no longer available to cause fission and must
be absorbed by shielding placed around the reactor pressure vessel for that
purpose.
Neutron Number: The total number of neutrons in the nucleus, N.
Neutron Source: Any
material that emits neutrons, such as a mixture of radium and beryllium, that
can be inserted into a reactor to ensure a neutron flux large enough to be
distinguished from background to register on neutron detection equipment.
Neutron, Thermal: A neutron that has (by
collision with other particles) reached an energy state equal to that of its
surroundings, typically on the order of 0.025 eV (electron volts).
Nuclear Binding Energy: The
energy that free nucleons give up in order to be bound inside a nucleus.
Nuclear Reactor: A device
in which a fission chain reaction can be initiated, maintained, and controlled.
Its essential components are fissionable fuel, moderator, shielding, control
rods, and coolant.
Nucleon: A constituent of the nucleus; that is, a proton or a
neutron.
Nucleus: The core of the atom, where
most of its mass and all of its positive charge is concentrated. Except for 1H,
the nucleus consists of a combination of protons and neutrons.
Nuclide: Any species of atom that
exists for a measurable length of time. A nuclide can be distinguished by its
atomic mass, atomic number, and energy state.
Nitrogen Oxides (NOx): A product of combustion
of fossil fuels whose production increases with the temperature of the process.
It can become an air pollutant if concentrations are excessive.
Noble Gas:
A gaseous chemical element that does not
readily enter into chemical combination with other elements. An inert gas.
Examples are helium, argon, krypton, xenon, and radon.
Non-stochastic Effect: The health effects of
radiation, the severity of which vary with the dose and for which a threshold
is believed to exist. Radiation-induced cataract formation is an example of a
non-stochastic effect (also called a deterministic effect)
Non-vital Plant Systems: Systems at a nuclear
facility that may or may not be necessary for the operation of the facility
(i.e., power production) but that would have little or no effect on public
health and safety should they fail. These systems are not safety related.
Non-power Reactor: Reactors used for
research, training, and test purposes, and for the production of radioisotopes
for medical and industrial uses.
Not Applicable
(NA): Specifies
that a particular field is not applicable to the event.
Not Reported (NR): Specifies that
information applicable to the particular field was not included in the event
report.
Nozzle: As
used in power water reactors and boiling water reactors, the interface (inlet
and outlet) between reactor plant components (pressure vessel, coolant pumps,
steam generators, etc.) and their associated piping systems.
NRC Operations Center: Rockville, Maryland,
serves as the focal coordination point for communicating with NRC licensees,
State agencies, and other Federal agencies about operating events in both the
nuclear reactor and nuclear material industry. The Operations Center is staffed
24 hours a day by an NRC Headquarters Operations Officer (HOO), who is trained
to receive, evaluate, and respond to events reported to the Operations Center.
Nuclear Energy:
The energy liberated by a nuclear reaction
(fission or fusion) or by radioactive decay.
Nuclear
Force: A
powerful short-ranged attractive force that holds together the particles inside
an atomic nucleus.
Nuclear Power Plant: An electrical
generating facility using a nuclear reactor as its heat source to provide steam
to a turbine generator.
Nuclear Steam Supply System (NSSS): The reactor and the
reactor coolant pumps (and steam generators for a pressurized water reactor)
and associated piping in a nuclear power plant used to generate the steam
needed to drive the turbine generator unit.
Nuclear Electric Power:
Electricity
generated by an electric power plant whose turbines are driven by steam
generated in a reactor by heat from the fissioning of nuclear fuel.
Nuclear Waste:
A particular type of radioactive waste that is
produced as part of the nuclear fuel cycle (i.e., those activities needed to
produce nuclear fission, or splitting of the atom). These include extraction of
uranium from ore, concentration of uranium, processing into nuclear fuel, and
disposal of byproducts. Radioactive waste is a broader term that includes all
waste that contains radioactivity. Residues from water treatment, contaminated
equipment from oil drilling, and tailings from the processing of metals such as
vanadium and copper also contain radioactivity but are not "nuclear
waste" because they are produced outside of the nuclear fuel cycle. NRC
generally regulates only those wastes produced in the nuclear fuel cycle
(uranium mill tailings, depleted uranium, spent fuel rods, etc.).
Nucleon:
Common name for a constituent particle of the
atomic nucleus. At present, applied to protons and neutrons, but may include
any other particles found to exist in the nucleus.
Nucleus:
The small, central, positively charged region
of an atom. Except for the nucleus of ordinary hydrogen, which has only a
proton, all atomic nuclei contain both protons and neutrons. The number of
protons determines the total positive charge or atomic number. This number is
the same for all the atomic nuclei of a given chemical element. The total
number of neutrons and protons is called the mass number.
Nuclide:
A general term referring to all known isotopes,
both stable (279) and unstable (about 2,700), of the chemical elements.
Occupational Dose: The dose received by
an individual in the course of employment in which the individual's assigned
duties involve exposure to radiation or to radioactive material from licensed
and unlicensed sources of radiation, whether in the possession of the licensee
or other person. Occupational dose does not include dose received from
background radiation, from any medical administration the individual has
received, from exposure to individuals administered radioactive materials and
released in accordance with NRC regulations, from voluntary participation in
medical research programs, or as a member of the general public.
Operable:
A system, subsystem, train, component, or
device is operable or has operability when it is capable of performing its
specified functions and when all necessary attendant instrumentation, controls,
electrical power, cooling or seal water, lubrication, or other auxiliary
equipment that are required for the system, subsystem, train, component, or
device to perform its functions are also capable of performing their related
support functions.
Operational mode: In a nuclear power
reactor, an operational mode corresponds to any one inclusive combination of
core reactivity condition, power level, and average reactor coolant
temperature.
Orphan
Source: See
unwanted radioactive material
Oxygenates:
Any
substance which, when added to motor gasoline, increases the amount of oxygen
in that motor gasoline blend.
Ozone: Three-atom oxygen
compound (03) found in two layers of the Earth’s
atmosphere. One layer of beneficial ozone occurs at 7 to 18 miles above the
surface and shields the Earth from ultraviolet light.
Several
holes in this protective layer have been documented by scientists. Ozone also
concentrates at the surface as a result of reactions between byproducts of
fossil fuel combustion and sunlight, having harmful health effects.
Parent: A radionuclide that decays to another nuclide.
A radionuclide that upon radioactive decay or disintegration yields a specific nuclide (the daughter).
Particulates:
Visible
air pollutants consisting of particles appearing in smoke or mist.
Parts Per Million (ppm): Parts (molecules) of a
substance contained in a million parts of another substance (e.g., water).
Uranium:
A radioactive element with the atomic number 92
and, as found in natural ores, an atomic weight of approximately 238. The two
principal natural isotopes are uranium-235, U235. (0.7
percent of natural uranium), which is fissile, and uranium-238, U238
(99.3 percent of natural uranium), which is fissionable by fast neutrons and is
fertile. Natural uranium also includes a minute amount of uranium-234, U234.
Uranium Fuel
Fabrication Facility: A
facility that:
1.
Manufactures reactor fuel containing uranium
for any of the following:
A.
preparation of fuel materials;
B.
formation of fuel materials into desired
shapes;
C.
application of protective cladding;
D.
recovery of scrap material;
E.
storage associated with such operations;
2. Conducts
research and development activities.
Uranium Hexafluoride Production Facility: A facility that
receives natural uranium in the form of ore concentrate, processes the
concentrate, and converts it into uranium hexafluoride (UF6).
Van de Graaff Accelerator: Device
using a high voltage terminal to accelerate charged particles.
Vapor: The gaseous form of
substances that are normally in liquid or solid form.
Very High Radiation Area: An area accessible to
individuals, in which radiation levels exceed 500 rad (5 gray) in one hour at 1
meter from the source or from any surface that the radiation penetrates (see 10
CFR 20.1003).
Viability Assessment: A Department of Energy
decision making process to judge the prospects for geologic disposal of
high-level radioactive wastes at Yucca Mountain based on;
Void: In
a nuclear power reactor, an area of lower density in a moderating system (such
as steam bubbles in water) that allows more neutron leakage than does the more
dense material around it.
Void Coefficient of Reactivity: A rate of change in
the reactivity of a water reactor system resulting from a formation of steam bubbles
as the power level and temperature increase.
Waste, Radioactive: Radioactive materials
at the end of a useful life cycle or in a product that is no longer useful and
should be properly disposed of.
Watt: An
electrical unit of power. 1 watt = 1 Joule/second. It is equal to the power in
a circuit in which a current of one ampere flows across a potential difference
of one volt.
Watt-hour:
An electrical energy unit of measure equal to 1
watt of power supplied to, or taken from, an electrical circuit steadily for 1
hour.
Weighting factor (WT): Multipliers of the
equivalent dose to an organ or tissue used for radiation protection purposes to
account for different sensitivities of different organs and tissues to the
induction of stochastic effects of radiation
Well-logging:
All operations involving the lowering and
raising of measuring devices or tools that contain licensed material or are
used to detect licensed materials in wells for the purpose of obtaining
information about the well or adjacent formations that may be used in oil, gas,
mineral, groundwater, or geological exploration (see 10 CFR 39.2).
Wheeling Service: The movement of
electricity from one system to another over transmission facilities of
intervening systems. Wheeling service contracts can be established between two
or more systems.
Watt (Electric): The electrical unit of
power. The rate of energy transfer equivalent to one ampere of electric current
flowing under a pressure of one volt at unity power factor.
Watthour (Wh): The electrical energy
unit of measure equal to 1 watt of power supplied to, or taken from, an
electric circuit steadily for one hour.
Weak Interaction: The
interaction responsible for weak decays of particles, mediated by the exchange
of W± and Z0 gauge bosons.
Whole-body Counter: A device used to
identify and measure the radioactive material in the body of human beings and
animals. It uses heavy shielding to keep out naturally existing background
radiation and ultrasensitive radiation detectors and electronic counting
equipment.
Whole-body Exposure: Whole body exposure
includes at least the external exposure, head, trunk, arms above the elbow, or
legs above the knee. Where a radioisotope is uniformly distributed throughout
the body tissues, rather than being concentrated in certain parts, the
irradiation can be considered as whole-body exposure (see also 10 CFR 20.1003).
Wipe Sample:
A sample made for the purpose of determining
the presence of removable radioactive contamination on a surface. It is done by
wiping, with slight pressure, a piece of soft filter paper over a
representative type of surface area. It is also known as a "swipe" or
"smear" sample.
Wind Energy: The kinetic energy of
wind converted into mechanical energy by wind turbines (i.e., blades rotating
from a hub) that drive generators to produce electricity.
X-radiation:
Electromagnetic radiation usually produced in transitions of the inner
electrons of atoms. The wavelength is between ultraviolet and gamma rays.
X-ray: Electromagnetic radiation
with wavelengths between ultraviolet and gamma rays.
Radiation from cosmic sources; naturally occurring radioactive materials, including radon (except as a decay product of source or special nuclear material) and global fallout as it exists in the environment from the testing of nuclear explosive devices. It does not include radiation from source, byproduct, or special nuclear materials regulated by the Nuclear Regulatory Commission. The typically quoted average individual exposure from background radiation is 360 millirems per year.
Penetrating
electromagnetic radiation (photon) having a wavelength that is much shorter
than that of visible light. These rays are usually produced by excitation of
the electron field around certain nuclei. In nuclear reactions, it is customary
to refer to photons originating in the nucleus as x-rays.
Yellowcake:
Yellowcake is the product of the uranium
extraction (milling) process; early production methods resulted in a bright
yellow compound, hence the name yellowcake.
The material is a mixture of uranium oxides that can vary in proportion and in
color from yellow to orange to dark green (blackish) depending at which
temperature the material was dried (level of hydration and impurities). Higher
drying temperatures produce a darker, less soluble material. Yellowcake is
commonly referred to as U3O8 and is assayed as pounds U3O8 equivalent. This
fine powder is packaged in drums and sent to a conversion plant that produces
uranium hexafluoride (UF6) as the next step in the manufacture of nuclear fuel.
APPENDIX C: FUNDAMENTAL CONSTANTS:
Quantity Symbol
or Definition Value
________________________________________________________________
Atomic
mass unit amu 1.66054 x 10-24grams
Avogadro’s
number NA 0.6022137 x 1024
(g-mole)-1
Boltzmann’s
Constant k 1.38066 x
10-23 J/0 K
Compton’s
Wavelength lC 2.42631 x 10-10
cm.
Of
the Electron
Electron
rest mass mo 9.10939 x 10-31
Kg.
5.485799
x 10-4 amu
Elementary
Charge e 1.602192 x
10-19 coul.
Neutron
rest Mass Mn 1.674929 x 10-27
Kg.
1.008665
amu
Plank’s
constant h 6.616075 x
10-34 J-sec.
Proton
rest Mass Mp 1.67262 x 10-27
Kg.
1.007276
amu.
Velocity
of Light c 2.997925 x
108 m/sec.
APPENDIX D: TABLE OF ELEMENTS
|
Element name |
Element symbol |
Atomic number |
|
H |
1 |
|
|
Helium |
He |
2 |
|
Lithium |
Li |
3 |
|
Beryllium |
Be |
4 |
|
Boron |
B |
5 |
|
Carbon |
C |
6 |
|
Nitrogen |
N |
7 |
|
Oxygen |
O |
8 |
|
Fluorine |
F |
9 |
|
Ne |
10 |
|
|
Na |
11 |
|
|
Mg |
12 |
|
|
Al |
13 |
|
|
Si |
14 |
|
|
P |
15 |
|
|
S |
16 |
|
|
Cl |
17 |
|
|
Ar |
18 |
|
|
K |
19 |
|
|
Ca |
20 |
|
|
Sc |
21 |
|
|
Ti |
22 |
|
|
V |
23 |
|
|
Cr |
24 |
|
|
Mn |
25 |
|
|
Fe |
26 |
|
|
Co |
27 |
|
|
Ni |
28 |
|
|
Cu |
29 |
|
|
Zn |
30 |
|
|
Ga |
31 |
|
|
Ge |
32 |
|
|
As |
33 |
|
|
Se |
34 |
|
|
Br |
35 |
|
|
Kr |
36 |
|
|
Rb |
37 |
|
|
Sr |
38 |
|
|
Y |
39 |
|
|
Zr |
40 |
|
|
Nb |
41 |
|
|
Mo |
42 |
|
|
Tc |
43 |
|
|
Ru |
44 |
|
|
Rh |
45 |
|
|
Pd |
46 |
|
|
Ag |
47 |
|
|
Cd |
48 |
|
|
In |
49 |
|
|
Sn |
50 |
|
|
Sb |
51 |
|
|
Te |
52 |
|
|
I |
53 |
|
|
Xe |
54 |
|
|
Cs |
55 |
|
|
Ba |
56 |
|
|
La |
57 |
|
|
Ce |
58 |
|
|
Pr |
59 |
|
|
Nd |
60 |
|
|
Pm |
61 |
|
|
Sm |
62 |
|
|
Eu |
63 |
|
|
Gd |
64 |
|
|
Tb |
65 |
|
|
Dy |
66 |
|
|
Ho |
67 |
|
|
Er |
68 |
|
|
Tm |
69 |
|
|
Yb |
70 |
|
|
Lu |
71 |
|
|
Hf |
72 |
|
|
Ta |
73 |
|
|
W |
74 |
|
|
Re |
75 |
|
|
Os |
76 |
|
|
Ir |
77 |
|
|
Pt |
78 |
|
|
Au |
79 |
|
|
Hg |
80 |
|
|
Tl |
81 |
|
|
Pb |
82 |
|
|
Bi |
83 |
|
|
Po |
84 |
|
|
At |
85 |
|
|
Rn |
86 |
|
|
Fr |
87 |
|
|
Ra |
88 |
|
|
Ac |
89 |
|
|
Th |
90 |
|
|
Pa |
91 |
|
|
U |
92 |
|
|
Np |
93 |
|
|
Pu |
94 |
|
|
Am |
95 |
|
|
Cm |
96 |
|
|
Bk |
97 |
|
|
Cf |
98 |
|
|
Es |
99 |
|
|
Fm |
100 |
|
|
Md |
101 |
|
|
No |
102 |
|
|
Lr |
103 |
|
|
Rf |
104 |
|
|
Db |
105 |
|
|
Sg |
106 |
|
|
Bh |
107 |
|
|
Hs |
108 |
|
|
Mt |
109 |
|
|
Ds |
110 |
|
|
Rg |
111 |
|
|
Cp |
112 |
|
|
Uut |
113 |
|
|
Uuq |
114 |
|
|
Uup |
115 |
|
|
Uuh |
116 |
|
|
Uus |
117 |
|
|
Uuo |
118 |
APPENDIX E: BIBLIOGRAPHY;