Nuclear Glossary

Absolute Error: The difference between the approximate and exact value in any calculation.

Accuracy: In measurement indicates that a set of measurements are close to the true answer, though they are not necessarily precise.

Acid: Compounds that contain hydrogen and when dissolved in water (H2O), they increase the concentration of hydrogen ions, H+ (hydronium ions H3O+). Acids are proton donors. Substances with a pH less than 7 are considered to be acidic.

Actinide: an element with atomic number of 89 (actinium) to 102. Usually applied to those above uranium - 93 up (also called transuranics). Actinides are radioactive and typically have long half-lives. They are therefore significant in wastes arising from nuclear fission, eg used fuel. They are fissionable in a fast reactor.

Actinides: The fourteen elements in the bottom row of the inner-transition elements of the periodic table that follow the element Actinium.  Some reference sources include actinium in this series others do not.  For these elements the 5f orbital is the filling orbital. This series is a sub-series of the transition metals.

Activation Product: : A radioactive isotope of an element (eg in the steel of a reactor core) which has been created by neutron bombardment.

Activity: the number of disintegrations per unit time inside a radioactive source. Expressed in becquerels.

ALARA: As Low As Reasonably Achievable, economic and social factors being taken into account. This is the optimisation principle of radiation protection.

Alkali Metal: A metal in the first column of the periodic table (e.g., lithium, sodium, potassium, rubidium, cesium and francium).  With the exception of francium, these metals are all soft and silvery.  They may be readily fused and volatilized with their melting and boiling points becoming lower with increasing atomic mass.  They are the strongest electropositive metals. These elements react vigorously, even violently with water.

Alpha Particle: Consists of the nucleus of an atom of helium (two protons and two neutrons) and is sometimes emitted from the nucleus of some atoms as part of those atoms' radioactive decay. Alpha particles have an energy range of 4-8 MeV. This energy is dissipated in only a few centimeters of air or less than 0.005mm of aluminum, Al. 

Alpha Particle: A positively-charged particle from the nucleus of an atom, emitted during radioactive decay. Alpha particles are helium nuclei, with 2 protons and 2 neutrons.

┼ngstrom (┼): One ten-billionth of a meter (10-10m or 0.0000000001 m) or 1/10th of a nanometer. 

Anion: A negatively charged ion. 

Antimatter (antiparticle): Species of subatomic particles that have the same mass and spin as normal particles, however, they have opposite electrical charges from their normal matter counterparts. In the case of antineutrons they are opposite of neutrons in magnetic moment.   Positrons, which are the counterpart to electrons, have a positive charge and antiprotons have a negative charge.  Photons are their own antimatter counterpart. When a particle of matter collides with a particle of antimatter, both particles are destroyed and their masses are converted to photons of equivalent energy.

Asbestosis: A lung disease (pneumoconiosis) resulting from inhaling fibers of asbestos and marked by interstitial fibrosis of the lung.

Atom: A particle of matter which cannot be broken up by chemical means. Atoms have a nucleus consisting of positively-charged protons and uncharged neutrons of the same mass. The positive charges on the protons are balanced by a number of negatively-charged electrons in motion around the nucleus.

Atom: The smallest possible unit of matter that still maintains an element's identity during chemical reactions.  Atoms contain one or more protons and neutrons (except hydrogen (H), which normally contains no neutrons) in a nucleus around which one or more electrons revolve.

Atomic Mass: The weight of an atom is determined by the number of neutrons and protons that are present in the nucleus. The proton and neutron, which are similar in mass, each weighs approximately 1,836 times greater than a single electron, thus the mass contributed by electrons is insignificant when determining atomic weight or atomic mass. The atomic mass is the sum of the protons and neutrons in the nucleus. Carbon has an atomic mass of twelve. Since there are six protons in carbon (remember, it has an atomic number of six and, therefore, must have six protons), it must have six neutrons:

Atomic Mass = # Protons + # Neutrons
The atomic mass of carbon = 12
The atomic # of carbon = 6 = the # of protons
# neutrons = Atomic Mass - # protons
# neutrons =12 - 6 = 6

The Variability in the Number of Neutrons in Each Element (Nuclides / Isotopes)

While the number of protons and electrons remain constant in the neutral atom, the number of neutrons may vary within different atom species of the same element. As a result, the atomic mass for one atom may be different from another atom of the same element if the number of neutrons varies. Atomic mass must account for all possible species or nuclides (isotopes), of an atom. Carbon 12 with its 6 neutrons is by far the most common isotope of carbon. In reality, there is a carbon 14 which has eight neutrons and an atomic mass of 14. There is also a carbon 11 which has only five neutrons.

The Atomic Mass Average: This is the sum of all nuclide (isotope), masses multiplied by their natural abundance. This weighted average is the relative mass listed in the Periodic Table. The relative percentage of each nuclide (isotope), appears to be similar throughout the world.

Atomic Mass (Atomic Mass Average): The average mass of all nuclides of an element determined by the proportions in which each nuclide of the element are present within the earth and its atmosphere.

The Charge of an Atom: Since the number of protons (positive charges) always equals the number of electrons (negative charges) in an atom, positive charges equal negative charges and atoms in the elemental state have no charge. Only when an atom takes an electron from another atom does the particle become charged. This charged form of the atom is known as an ion. Positively charged ions are called cations, and negatively charged ions are called anions. For instance, when chlorine accepts an electron from sodium, the sodium ion that is formed will have one more proton than electrons. It will therefore have a positive charge and be called a cation. The chlorine (or chloride) ion will have one more electron than protons. It will take on a negative charge and be called an anion. The compound formed by this transfer of electrons is sodium chloride or table salt, which is nothing like the highly reactive sodium or extremely poisonous chlorine from which it was formed.

Atomic Mass Unit (AMU): A mass unit that is exactly 1/12th the mass of a carbon-12 atom (approximately 1.67E-24g).

Atomic Number: The number of protons always equals the number of electrons in an atom, and that number is equal to the atomic number. For instance, carbon has an atomic number of six and therefore has six protons and six electrons.

Atomic Number: The number of protons in the nucleus of an atom.  This determines an element's structure, properties and location on the periodic table of elements.

Atomic Radius: One-half the distance between two adjacent atoms in crystals of elements.  This varies according to interatomic forces.

Atomic Volume: The atomic mass of an element divided by its density.

Avogadro, Amadeo (1776-1856): An Italian chemist who first stated the principle behind stoichiometry in 1811.

Avogadro's Number: This is the number of atoms in a 12g sample of Carbon-12.  This is equal to 6.0221367E23 atoms. It is named for Italian chemist Amadeo Avogadro (1776-1856) who first stated the principle in 1811.

Azimuthal Quantum Number (angular momentum quantum number): The quantum number that distinguishes orbitals of given n having different shapes. This can be any interger value from 0 to n-1.

Background Radiation: The naturally-occurring ionising radiation which every person is exposed to, arising from the earth's crust (including radon) and from cosmic radiation.

BACT (Best Available Control Technology): Guidelines for municipal waste combustors developed by EPA in 1986 that were intended to add consistency and reduce delay and confusion in the permitting process.

Barns: A measurement of area equal to E -24 cm2.

Base: Compounds that when dissolved in water increase the concentration of hydroxide ions (OH). Bases are proton acceptors. Substances with a pH greater than 7 are considered to basic.

Base Load: That part of electricity demand which is continuous, and does not vary over a 24-hour period. Approximately equivalent to the minimum daily load.

Becquerel: The SI unit of intrinsic radioactivity in a material. One Bq measures one disintegration per second and is thus the activity of a quantity of radioactive material which averages one decay per second. (In practice, GBq or TBq are the common units.)

Beta Particle: A particle emitted from an atom during radioactive decay. Beta particles may be either electrons (with negative charge) or positrons.

Beta Particle: a charged particle emitted from the nucleus of some atoms as part of those atoms' radioactive decay. Positively charged beta particles are positrons and negatively charged beta particles are electrons.  Beta particles can cause burns and are harmful to living tissues, however, protection is afforded by a thin sheet of metal.

Biological Shield: A mass of absorbing material (eg thick concrete walls) placed around a reactor or radioactive material to reduce the radiation (especially neutrons and gamma rays respectively) to a level safe for humans.

Boiling Point: The temperature at which the vapor pressure of a liquid is equal to or slightly greater than the atmospheric pressure of the environment. For water at sea level, its boiling point is 100C (212F).

Boiling Water Reactor (BWR): A common type of light water reactor (LWR), where water is allowed to boil in the core thus generating steam directly in the reactor vessel. (cf PWR)

Breed: To form fissile nuclei, usually as a result of neutron capture, possibly followed by radioactive decay.

Breeder Reactor: see Fast Breeder Reactor and Fast Neutron Reactor.

Burn: cause to fission.

Burnable Poison: A neutron absorber included in the fuel which progressively disappears and compensates for the loss of reactivity as the fuel is consumed. Gadolinium is commonly used.

Burnup: Measure of thermal energy released by nuclear fuel relative to its mass, typically Gigawatt days per tonne (GWd/tU).

CAA (Clean Air Act): Passed by congress to have the air "safe enough to protect the public's health" by 5/31/1975. It was amended in 1990 to require EPA to develop more stringent and specific regulations for air emissions.

Caplan's Syndrome: Pneumoconiosis associated with rheumatoid arthritis.


  1. Chicago Academy of Science
  2. Chemical Abstracts Service (which assigns CAS Numbers to chemicals)

  Calandria: (in a CANDU reactor) a cylindrical reactor vessel which contains the heavy water moderator. It is penetrated from end to end by hundreds of calandria tubes which accommodate the pressure tubes containing the fuel and coolant.

CANDU: Canadian deuterium uranium reactor, moderated and (usually) cooled by heavy water.

Cation: A positively charged ion.

Ceiling: The exposure limit for a substance that should (or in the case of OSHA PEL's "shall") not be exceeded. If instantaneous monitoring is not feasible, then a 15-minute time weighted average is used.

Centigrade (Celsius): The scale for measuring temperature used internationally where the freezing point of water is zero and the boiling point of water at sea level is 100 degrees. To convert from centigrade into Fahrenheit, multiply the centigrade temperature by 1.8 then add 32 to the product (C*1.8+32=F). To convert from Fahrenheit to centigrade, subtract 32 and then divide by 1.8 ((F-32)/1.8).

CERCLA (Comprehensive Environmental Response Compensation and Liability Act, Superfund Act): Is responsible for the "cradle to grave law" that holds the generator of waste responsible for proper waste disposal and provides for cleanup of the most contaminated sites by the government, with the cost of cleanup being charged to the responsible parties. Basically a generator becomes responsible for waste from it's inception until it has been destroyed.

Chain Reaction: A reaction that stimulates its own repetition, in particular where the neutrons originating from nuclear fission cause an ongoing series of fission reactions.

Chemiluminescence: The emission of absorbed energy as light as the result of a chemical reaction. This occurs in thousands of compounds both organic and inorganic. Chemical glow sticks and fireflies are examples of this.

Cladding: The metal tubes containing oxide fuel pellets in a reactor core.

Coefficient of Linear Thermal Expansion: The ratio of change in length per degree centigrade compared to the base length at zero degrees centigrade. The unit of measurement is centimeter per centimeter per degree centigrade (cm/cm/░C). This measurement means that for every centimeter of base length, the length will change X centimeters for every change of one degree centigrade.

Compound: a substance composed of atoms or ions of two or more elements that are chemically combined.  Elements in a compound are present in definite proportions by mass and are bonded with each other in a specific manner.

Concentrate: See Uranium oxide concentrate (U3O8).


  1. Thermal conduction the transfer of heat between two solid materials that are physically touching each other.
  2. Electrical conduction the transfer of electrical current through a solid or liquid.

Control Rods: Devices to absorb neutrons so that the chain reaction in a reactor core may be slowed or stopped by inserting them further, or accelerated by withdrawing them.

Convection: The movement of heat by a moving liquid or gas. Convection results from the differences in the densities of a material at different temperatures. As a liquid or gas raises in temperature, it becomes less dense and thus lighter thereby rising above its cooler and denser counterparts, which in turn sink.

Conversion: Chemical process turning U3O8 into UF6 preparatory to enrichment.

Coolant: The liquid or gas used to transfer heat from the reactor core to the steam generators or directly to the turbines.

Core: The central part of a nuclear reactor containing the fuel elements and any moderator.

Critical Mass: The smallest mass of fissile material that will support a self-sustaining chain reaction under specified conditions.

Criticality: Condition of being able to sustain a nuclear chain reaction.

Cross Section: a measure of the probability of an interaction between a particle and a target nucleus, expressed in barns (1 barn = 10-24 cm2).

Decay: Disintegration of atomic nuclei resulting in the emission of alpha or beta particles (usually with gamma radiation). Also the exponential decrease in radioactivity of a material as nuclear disintegrations take place and more stable nuclei are formed.

Decommissioning: Removal of a facility (eg reactor) from service, also the subsequent actions of safe storage, dismantling and making the site available for unrestricted use.

Decomposition: A reaction where a single compound breaks down into simpler compounds.

Deflagration: The extremely rapid burning of a material. This is much much faster than normal combustion, but slower than detonation.

Delayed neutrons: neutrons released by fission products up to several seconds after fission. These enable control of the fission in a nuclear reactor.

Density: The ratio of mass to unit volume expressed in grams/cm3 for solids and liquids and grams/liter in gases (density=mass/volume).

Depleted Uranium: Uranium having less than the natural 0.7% U-235. As a by-product of enrichment in the fuel cycle it generally has 0.25-0.30% U-235, the rest being U-238. Can be blended with highly-enriched uranium (eg from weapons) to make reactor fuel.

Detonation: The sudden and violent release of mechanical, chemical or nuclear energy from a confined space which creates a shock wave that travels at supersonic speeds. Often used interchangeably with explosion.

Deuterium: "Heavy hydrogen", a stable isotope having one proton and one neutron in the nucleus. It occurs in nature as 1 atom to 6500 atoms of normal hydrogen, (Hydrogen atoms contain one proton and no neutrons).

Diatomic: Elements that are present in the gaseous state as molecules composed of two atoms.   For example: O2, N2, Cl2 and H2 are diatomic.

Disintegration: natural change in the nucleus of a radioactive isotope as particles are emitted (usually with gamma rays), making it a different element.

Dose: The energy absorbed by tissue from ionising radiation. One gray is one joule per kg, but this is adjusted for the effect of different kinds of radiation, and thus the sievert is the unit of dose equivalent used in setting exposure standards.

DOT (USDOT): United States Department of Transportation

Element: A chemical substance that cannot be divided into simple substances by chemical means; atomic species with same number of protons.

Energy Levels: Until now we have focused on the nucleus. Lets turn our attention to the electrons, which surround the nucleus of the atom. Electrons are located in energy levels a term which has replaced the word shells, which was once used to describe the location of electrons. The word shell suggests a fixed position, which is far from reality. We will use energy level to describe the possible location of electrons.

There are seven energy levels. Each has a specific maximum number of electrons that can exist in it. The number of electrons, which an energy level can hold is equal to 2n▓ where n = energy level. The letter n represents the principal quantum number that specifies the energy level of the atom in which an electron is located. The chart below identifies the various energy levels and maximum number of electrons possible. The energy level closest to the nucleus is represented by energy level 1.

Electron Energy Levels
2n2 Possible # of electrons
1 2(12) 2
2 2(22) 8
3 2(32) 18
4 2(42) 32
5 2(52) 50 (theoretical, not filled)
6 2(62) 72 (theoretical, not filled)
7 2(72) 98 (theoretical, not filled)

Energy Sublevels (Orbitals): Within each principal energy level is one or more energy sublevels (Orbitals) or subshells. The number of sublevels possible for any one principal energy level is equal to the value of the quantum number (n) for that energy level. Note that I have purposely introduced the reader to the term quantum number quite early in this discussion. The idea is not to scare the reader, but rather to begin building up a certain comfort level with a term that may invoke a certain fear of the unreachable to the new chemist. While there are theoretically 7 possible sublevels, only four are actually used for the known elements. The others are not currently needed. Sublevels are numbered with consecutive whole numbers. The first sublevel is 0 followed by numbers 1 through 6. These numbers are the azimuthat quantum numbers, ohm. The value of ohm can never be greater than n-1. Based on the 112 known and verified elements, the following table represents possible sublevels in the atom (Jespersen, 1997).

Energy Sublevels (Orbitals)
Quantum #
Sublevel # Sublevel
1 0 s
2 0,1 s, p
3 0,1,2 s, p, d
4 0,1,2,3 s, p, d, f
5 0,1,2,3 s, p, d, f
6 0,1,2,3 s, p, d
7 0 s
As more elements are identified, the sublevels of 5,6 and 7 will fill.

Enriched uranium: Uranium in which the proportion of U-235 (to U-238) has been increased above the natural 0.7%. Reactor-grade uranium is usually enriched to about 3.5% U-235, weapons-grade uranium is more than 90% U-235.

Enrichment: Physical process of increasing the proportion of U-235 to U-238.

Fallout: The process of the descent to the earth's surface of particles contaminated with radioactive material from a radioactive cloud. The term is also applied in a collective sense to the contaminated particulate matter itself. The early (or local) fallout is defined, somewhat arbitrarily, as those particles which reach the earth within 24 hours after a nuclear explosion. The delayed (or worldwide) fallout consists of the smaller particles which ascend into the upper troposphere and stratosphere, to be carried by winds to all parts of the earth. The delayed fallout is brought to earth, mainly by rain and snow, over extended periods ranging from months to years.

Fast Breeder Reactor (FBR): A fast neutron reactor (qv) configured to produce more fissile material than it consumes, using fertile material such as depleted uranium in a blanket around the core.

Fast Neutron: neutron released during fission, travelling at very high velocity (20,000 km/s) and having high energy (c 2 MeV).

Fast Neutron Reactor: A reactor with no moderator and hence utilising fast neutrons. It normally burns plutonium while producing fissile isotopes in fertile material such as depleted uranium (or thorium).

Fat Man: The implosion bomb dropped on Nagasaki, Japan at 11:02 am on August 9, 1945. The bomb was dropped 2 miles off target. 74,000 people were killed and another 75.000 sustained serious injuries.

Fertile (of an isotope): Capable of becoming fissile, by capturing neutrons, possibly followed by radioactive decay; eg U-238, Pu-240.

Fireball: The luminous sphere of hot gases which form a few millionths of a second after a nuclear explosion. It is the result of the absorption by the surrounding medium of the thermal X-rays emitted by the extremely hot (several tens of million degrees) weapon residues. The exterior of the fireball in air is initially sharply defined by the luminous shock front and later by the limits of the hot gases themselves.
Firestorm:  Stationary mass fire, generally in builtup urban areas, causing strong, inrushing winds from all sides; the winds keep the fires from spreading while adding fresh oxygen to increase their intensity.
First Strike: The launch of a surprise attack to considerably weaken or destroy an adversary's military installations or nuclear forces and thus severely reduce its ability to attack or retaliate.

Fissile (of an isotope): Capable of capturing a slow (thermal) neutron and undergoing nuclear fission, e.g. U-235, U-233, Pu-239.

Fissile Isotope: Isotopes whose nuclei may be split by neutrons moving at various speeds and therefore fission very easily. Uranium-233, Uranium-235, and Plutonium-239 are all fissile isotopes. Fissile isotopes typically undergo fission more easily than other fissionable isotopes.
Fissile Material: Nuclear material, containing a high proportion of fissile isotopes, which is essential for making nuclear explosives. High-enriched uranium (HEU) and weapons-grade plutonium are examples of fissile material.

Fission: The splitting of a heavy nucleus into two, accompanied by the release of a relatively large amount of energy and usually one or more neutrons. It may be spontaneous but usually is due to a nucleus absorbing a neutron and thus becoming unstable.

Fission: The process whereby the nucleus of a particular heavy element splits into (generally) two nuclei of lighter elements, with the release of substantial amounts of energy. The most important fissionable materials are uranium-235 and plutonium-239; fission is caused by the absorption of neutrons.

Fissionable (of an isotope): Capable of undergoing fission: If fissile, by slow neutrons; otherwise, by fast neutrons.

Fission Bomb: A nuclear bomb based on the concept of releasing energy through the fission (splitting) of heavy elements such as U 235 or Pu 239.
Fission Probability: The probability that a nucleus will split (fission) when an additional neutron is added to the nucleus.
Fission Products: A general term for the complex mixture of substances produced as a result of nuclear fission. A distinction should be made between these and the direct fission products or fission fragments which are formed by the actual splitting of the heavy-element nuclei. Something like 80 different fission fragments result from roughly 40 different modes of fission of a given nuclear species (e.g., uranium-235 or plutonium-239). The fission fragments, being radioactive, immediately begin to decay, forming additional (daughter) products, with the result that the complex mixture of fission products so formed contains over 300 different isotopes of 36 elements.

Fission Products: Daughter nuclei resulting either from the fission of heavy elements such as uranium, or the radioactive decay of those primary daughters. Usually highly radioactive.

Flashblindness: Caused by the reflex glance toward the intense light given off by a nuclear explosion. Its effects can range from permanent blindness to retinal burns to temporary visual loss.
Flash Burn: A burn caused by excessive exposure (of bare skin) to thermal radiation.

Fossil Fuel: A fuel based on carbon presumed to be originally from living matter, eg coal, oil, gas. Burned with oxygen to yield energy.

Fuel Assemblies: Structures that contain about 240 fuel rods of uranium pellets. Fuel for a nuclear power plant is loaded in the reactor core in fuel assemblies.

Fuel Assembly: Structured collection of fuel rods or elements, the unit of fuel in a reactor.

Fuel Cycle: The process of mining and refining uranium for use as reactor fuel. The process usually involves uranium enrichment and/or reprocessing.

Fuel Fabrication: Making reactor fuel assemblies, usually from sintered UO2 pellets which are inserted into zircalloy tubes, comprising the fuel rods or elements.

Fuel Pellets: Cylinders into which nuclear fuel is formed for use in a reactor. A fuel pellet is about the size of your fingertip.
Fuel Rods: 12- to 14-foot-long (3.6- to 4.2-meter-long) rods that hold fuel pellets.
Fusion: Nuclear fusion is a type of nuclear reaction in which two atomic nuclei combine to form a heavier nucleus, releasing energy. For a fusion reaction to take place, the nuclei, which are positively charged, must have enough kinetic energy to overcome their electrostatic force of repulsion. Thermonuclear fusion of deuterium and tritium will produce a helium nucleus and an energetic neutron. This is one basis of the Hydrogen Bomb, which employs a brief, uncontrolled thermonuclear fusion reaction. A great effort is now under way to harness thermonuclear fusion as a source of power.
Fusion Bomb: A nuclear bomb based on the fusing or burning of light elements. Fusion bombs use fission bombs for ignition.

The Gadget: The Gadget was built to test the implosion design. It was placed atop a 100 foot tower and detonated at 5:29:45 am on July 15, 1945. It had a yield of 19 kilotons.
Gamma Radiation: The electromagnetic radiation of high frequency emitted from nuclei when they decay from excited states, similar to but with higher energy than X-rays.

Gamma Rays: High energy electro-magnetic radiation from the atomic nucleus, virtually identical to X-rays.

Gamma Rays: Electromagnetic radiations of high energy photons, originating in atomic nuclei and accompanying many nuclear reactions. They can penetrate deeply into body tissue and many materials. Shielding against gamma radiation requires thick layers of dense materials, such as lead. Gamma rays are potentially lethal to humans.
Gas-Centrifuge Process: A method of isotope separation in which heavy, gaseous atoms or molecules are separated from light ones by centrifugal force.
Gas-Graphite Reactor: A nuclear reactor in which gas is the coolant and graphite is the moderator.
Gaseous Diffusion: A method of isotope separation based on the fact that gas atoms or molecules with different masses will diffuse through a porous barrier at different rates. The method is used to separate uranium-235 from uranium-238. It requires large plants and significant amounts of power.

Genetic Mutation: Sudden change in the chromosomal DNA of an individual gene. It may produce inherited changes in descendants. Mutation in some organisms can be made more frequent by irradiation (though this has never been demonstrated in humans).

Giga: One billion units (eg gigawatt 109 watts or million kW).

Graphite: Crystalline carbon used in very pure form as a moderator, principally in gas-cooled reactors, but also in Soviet-designed RBMK reactors.

Gray: The SI unit of absorbed radiation dose, one joule per kilogram of tissue.

Greenhouse Gases: Radiative gases in the earth's atmosphere which absorb long-wave heat radiation from the earth's surface and re-radiate it, thereby warming the earth. Carbon dioxide and water vapour are the main ones.

Ground Based Interceptor (GBI): The missile intercept of the proposed U.S. National Missile Defense (NMD) system, the GBI will intercept incoming ballistic missile warheads outside the earth's atmosphere (exo-atmospheric) and collide with the incoming ballistic missile, thereby destroying the missile.
Ground Launched Cruise Missile (GLCM)
Ground Zero: The point on the surface of land vertically below or above the center of a burst of a nuclear (or atomic) weapon; frequently abbreviated to GZ. For a burst over or under water the corresponding term is surface zero (SZ). Surface zero is also commonly used for ground surface and underground bursts.
Gun-Type Weapon: A device in which two or more pieces of fissionable material, each less than a critical mass, are brought together very rapidly so as to form a supercritical mass which can explode as the result of a rapidly expanding fission chain reaction.

Half-life: The period required for half of the atoms of a particular radioactive isotope to decay and become an isotope of another element.

Half-Life: The time it takes for one-half of the atoms of an unstable element or nuclide to decay radioactively into another element or nuclide

Halogens: The reactive nonmetals that are in Group 17 of the periodic table. All of these elements are electronegative.

Hazardous Material (HazMat): Any material or substance, which even in normal use, poses a risk to health, safety, property or the environment. Hazardous materials are broken down into nine primary classes by the United Nations, the USDOT, IATA and other regulatory bodies for the purposes of the placarding and identification the hazards of shipments.

HDPE (high density polyethylene): The structure is similar to PVC except that there is no Chlorine, Cl, associated with the molecule.  The tight structure makes it very dense.

Heat: An energy form proportional to and associated with molecular motion.  Conduction, convection or radiation can transfer heat from one mass of matter to another.

Heat of Fusion: The heat required to convert a solid into a liquid with no temperature change.   This is also called the latent heat of fusion or melting.

Heat of Vaporization: The heat required to convert a substance from the liquid to the gaseous state with no temperature change.  This is also called the latent heat of vaporization.

Heavy Water: Water containing an elevated concentration of molecules with deuterium ("heavy hydrogen") atoms.

Heavy Water Reactor (HWR): A reactor which uses heavy water as its moderator, eg Canadian CANDU (pressurised HWR or PHWR).

Heisenberg's Uncertainty Principle: the conclusion that it is impossible to know simultaneously the absolute exact position and the absolute exact speed of a particle such as an electron.  The more precisely the position is known of a particle at a moment in time, the less precisely can its speed be determined. Conversely the more precisely the exact speed of a particle is known, the less precisely its position can be known. Werner P. Heisenberg, a German physicist, developed this principle from his work with quantum mechanics.  He was awarded the Nobel Prize in physics in 1932 for his brilliant work.

As a side note, Star Trek fans will every now and then hear the crew refer to the "Heisenberg compensator" in the transporter pads. This is the writer's nod to this principle and the problems it would create for actually being able to teleport something they way they do in the Star Trek series.

Heterogeneous: A mixture or material consisting of more than one substance. The earth's atmosphere is heterogeneous in that it typically consists of 20.7% oxygen, 78% nitrogen, 0.93% argon and 0.03% CO2 with other gasses making up the remaining percentage. Heterogeneous is Latin for "different kinds".

High-Level Wastes: Extremely radioactive fission products and transuranic elements (usually other than plutonium) in used nuclear fuel. They may be separated by reprocessing the used fuel, or the spent fuel containing them may be regarded as high-level waste.

Highly (or High)-Enriched Uranium (HEU): Uranium enriched to at least 20% U-235. (That in weapons is about 90% U-235.)

Homogeneous: A substance or material that contains only one kind of compound or one element. Homogeneous is Latin for "the same kind". An example of a homogeneous substance would be pure water, which only contains the compound H2O or pure table salt that only contains the compound NaCl.

Hyaluronic Acid: A viscous polysaccharide found in the connective tissue space and the synovial fluid of movable joints and the humors of the eye; it protects tissue.

In Situ Leaching (ISL): The recovery by chemical leaching of minerals from porous orebodies without physical excavation. Also known as solution mining.

Ion: An atom that is electrically-charged because of loss or gain of electrons.

Ionizing Radiation: Radiation (including alpha particles) capable of breaking chemical bonds, thus causing ionisation of the matter through which it passes and damage to living tissue.

Irradiate: Subject material to ionising radiation. Irradiated reactor fuel and components have been subject to neutron irradiation and hence become radioactive themselves.

Isotope: An atomic form of an element having a particular number of neutrons. Different isotopes of an element have the same number of protons but different numbers of neutrons and hence different atomic mass, eg. U-235, U-238. Some isotopes are unstable and decay (qv) to form isotopes of other elements. Light water: Ordinary water (H20) as distinct from heavy water.

Light Water Reactor (LWR): A common nuclear reactor cooled and usually moderated by ordinary water.

Low-Enriched Uranium: Uranium enriched to less than 20% U-235. (That in power reactors is usually 3.5 - 5.0% U-235).

Megawatt (MW): A unit of power, = 106 watts. MWe refers to electric output from a generator, MWt to thermal output from a reactor or heat source (eg the gross heat output of a reactor itself, typically three times the MWe figure).

Mass: a measurement of the quantity of matter contained in a particle or body regardless of its location in the universe and the gravitational force applied to it. Mass is constant, while weight is affected by the distance of the object from the center of the earth (or other such body). Mass is not affected by gravity.

Melting Point (freezing point): The point at which the crystals of a pure substance are in equilibrium with the liquid phase at atmospheric pressure. This point is commonly referred to as melting point when a substance is approaching it from its solid state and is referred to as freezing point when a substance is approaching it from its liquid state.

Mesothelioma: A rare cancer known that affects thin membranes surrounding lungs and other internal organs. Mesothelioma has been associated with asbestos exposure. title="Asbestos: A Manufacturing Health Hazard Dating to Prehistoric Times">More information)

Metal Fuels: Natural uranium metal as used in a gas-cooled reactor.

Metals: Elements that form cations. when compounds of it are in solution and oxides of the elements form hydroxides rather than acids in water.  Most metals are conductors of electricity, have crystalline solids with a metallic luster and have a high chemical reactivity.  Many of these elements are hard and have high physical strength.   The metal series includes all elements of the alkali, alkaline-earth, inner-transition (lanthanide and actinide series), transactinides and transition series as well as some elements of the metalloid series (elements: Ge, Sb and Po).

Metalloid: this term is no longer commonly used by chemists, however, it refers to elements that exhibit some properties of metal and nonmetals.  These elements tend to be semiconductors. Silicon is an extremely important example of these elements. Without the semiconductive properties silicon you would not be reading this text right now as most microchips and microprocessors are made with silicon and without these processors computers as we now know them would not exist.

Micro: one millionth of a unit (eg microsievert is 10-6 Sv).

Millibarns (mbarns): A measurement of area equal to 1/1,000 of a barn or E -27 cm2.

Milling: Process by which minerals are extracted from ore, usually at the mine site.

Mixed Oxide Fuel (MOX): Reactor fuel which consists of both uranium and plutonium oxides, usually about 5% Pu, which is the main fissile component.

Mixture: an association of substances, which cannot be represented by a chemical formula. In a heterogeneous mixture the substances are not uniformly dispersed, as is the case in Italian salad dressing. In the purest sense, the term "homogeneous" refers to a chemical composition of only one compound or element, but the term "homogeneous mixture" is loosely used to describe two or more substances that are uniformly dispersed as in a solution of water and sodium chloride (table salt).

Moderator: A material such as light or heavy water or graphite used in a reactor to slow down fast neutrons by collision with lighter nuclei so as to expedite further fission.

Natural Uranium: Uranium with an isotopic composition as found in nature, containing 99.3% U-238, 0.7% U-235 and a trace of U-234. Can be used as fuel in heavy water-moderated reactors.

Neutron: An uncharged elementary particle found in the nucleus of every atom except hydrogen. Solitary mobile neutrons travelling at various speeds originate from fission reactions. Slow (thermal) neutrons can in turn readily cause fission in nuclei of "fissile" isotopes, e.g. U-235, Pu-239, U-233; and fast neutrons can cause fission in nuclei of "fertile" isotopes such as U-238, Pu-239. Sometimes atomic nuclei simply capture neutrons.

Nuclear Reactor: A device in which a nuclear fission chain reaction occurs under controlled conditions so that the heat yield can be harnessed or the neutron beams utilised. All commercial reactors are thermal reactors, using a moderator to slow down the neutrons.

Nuclide: elemental matter made up of atoms with identical nuclei, therefore with the same atomic number and the same mass number (equal to the sum of the number of protons and neutrons).

Oxide Fuels: Enriched or natural uranium in the form of the oxide UO2, used in many types of reactor.

Plutonium: A transuranic element, formed in a nuclear reactor by neutron capture. It has several isotopes, some of which are fissile and some of which undergo spontaneous fission, releasing neutrons. Weapons-grade plutonium is produced in special reactors to give >90% Pu-239, reactor-grade plutonium contains about 30% non-fissile isotopes. About one third of the energy in a light water reactor comes from the fission of Pu-239, and this is the main isotope of value recovered from reprocessing used fuel.

Pressurised Water Reactor (PWR): The most common type of light water reactor (LWR), it uses water at very high pressure in a primary circuit and steam is formed in a secondary circuit.

Radiation: The emission and propagation of energy by means of electromagnetic waves or particles. (cf ionising radiation)

Radioactivity: The spontaneous decay of an unstable atomic nucleus, giving rise to the emission of radiation.

Radionuclide: A radioactive isotope of an element.

Radiotoxicity: The adverse health effect of a radionuclide due to its radioactivity.

Radium: A radioactive decay product of uranium often found in uranium ore. It has several radioactive isotopes. Radium-226 decays to radon-222.

Radon (Rn): A heavy radioactive gas given off by rocks containing radium (or thorium). Rn-222 is the main isotope.

Radon Daughters: Short-lived decay products of radon-222 (Po-218, Pb-214, Bi-214, Po-214).

Reactor Pressure Vessel: The main steel vessel containing the reactor fuel, moderator and coolant under pressure.

Repository: A permanent disposal place for radioactive wastes.

Reprocessing: Chemical treatment of used reactor fuel to separate uranium and plutonium and possibly transuranic elements from the small quantity of fission product wastes, leaving a much reduced quantity of high-level waste (which today includes the transuarnic elements). (cf Waste, HLW).

Scientific Method: a research method that involves: Making an observation Developing a hypothesis Experimentation Testing and retesting (usually by many other scientists) Developing a theory If theory is proven beyond a doubt (e.g.: the law of gravity) a law may result (rarely does the scientific method reach this degree of acceptance as fact)

Second Ionization Potential: To be defined

Separative Work Unit (SWU): This is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, ie the extent of increase in the concentration of the U-235 isotope relative to the remainder. The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms.

Eg, to produce one kilogram of uranium enriched to 3.5% U-235 requires 4.3 SWU if the plant is operated at a tails assay 0.30%, or 4.8 SWU if the tails assay is 0.25% (thereby requiring only 7.0 kg instead of 7.8 kg of natural U feed).

About 100-120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. Enrichment costs are related to electrical energy used. The gaseous diffusion process consumes some 2400 kWh per SWU, while gas centrifuge plants require only about 60 kWh/SWU.

Short Term Exposure Limit (STEL): The maximum exposure limit to a substance based on a 15 minute time weighted average.

Sievert (Sv): Unit indicating the biological damage caused by radiation. One Joule of beta or gamma radiation absorbed per kilogram of tissue has 1 Sv of biological effect; 1 J/kg of alpha radiation has 20 Sv effect and 1 J/kg of neutrons has 10 Sv effect.

Solution: a uniformly dispersed mixture of molecules or ions.  The substance being dissolved is the solute, while the substance into which the solute is dissolved is the solvent.

Specific Gravity: the ratio of the density of a substance to the density of a reference substance. Specific Gravity does not have any units. When water is used as the reference material, specific gravity equals density (without units).

Specific Gravity (SG) = (Density of substance g/ml)/(Density of reference substance g/ml)
Notice that g/ml cancels out so that there are no units associated with specific gravity. Specific Gravity then becomes an abstract number.

Specific Heat: the ratio of heat capacity of a substance to the heat capacity of water. Another way of looking at it is the quantity of heat required to raise the temperature of one gram of a substance by one degree centigrade or one degree Kelvin (1 J/gC = 1 J/gK).

Spallation: the abrasion and removal of fragments of a target which is bombarded by protons in an accelerator. The fragments may be protons, neutrons or other light particles.

Spent Fuel: Used fuel assemblies removed from a reactor after several years use and treated as waste.

Stable: Incapable of spontaneous radioactive decay.

Standard Temperature and Pressure (STP): the standard conditions of 273.15 K and 1 atm (0░C at sea level). used as a baseline for calculations involving quantities that vary with temperature and pressure.

Stoichiometry: the quantitative relationship between chemical substances in a reaction.

Tailings: Ground rock remaining after particular ore minerals (e.g. uranium oxides) are extracted.

Tails: Depleted uranium (cf. enriched uranium), with about 0.3% U-235.

Thermal Reactor: A reactor in which the fission chain reaction is sustained primarily by slow neutrons, and hence requiring a moderator (as distinct from Fast Neutron Reactor).

Transmutation: Changing atoms of one element into those of another by neutron bombardment, causing neutron capture and/or fission. In an ordinary reactor neutron capture is the main event, in a fast reactor fission is more common and therefore it is best for dealing with actinides. Fission product transmutation is by neutron capture.

Transuranic Element: A very heavy element formed artificially by neutron capture and possibly subsequent beta decay(s). Has a higher atomic number than uranium (92). All are radioactive. Neptunium, plutonium, americium and curium are the best-known.

Uranium (U): A mildly radioactive element with two isotopes which are fissile (U-235 and U-233) and two which are fertile (U-238 and U-234). Uranium is the basic fuel of nuclear energy.

Uranium Hexafluoride (UF6): A compound of uranium which is a gas above 56oC and is thus a suitable form in which to enrich the uranium.

Uranium Oxide Concentrate (U3O8): The mixture of uranium oxides produced after milling uranium ore from a mine. Sometimes loosely called yellowcake. It is khaki in colour and is usually represented by the empirical formula U3O8. Uranium is sold in this form.

Valence Electron Potential (-eV): Provides a quantitative indication of an elements reactivity and is based on the charge of the valence electrons and the ionic radius.  It is determined by the equation: (-eV) = kn/r. 'k' is a proportionality factor converting ┼ngstroms into centimeters and expressing the force exerted by the valence electrons in electron volts which is equated to 14.399. 'n' is the valence. 'r' is the ionic radius in ┼ngstroms.

Valence Electrons: The electrons involved in the ionization of an element. 

Vapor Pressure: The pressure exerted when a material in its solid or liquid state is in equilibrium with it's gas state. As this is changes as a function of temperature, vapor pressure must be qualified by the relevant temperature.

Vitrification: The incorporation of high-level wastes into borosilicate glass, to make up about 14% of it by mass. It is designed to immobilise radionuclides in an insoluble matrix ready for disposal.

Volatile: Readily passing into the gas state at a relatively low temperature.

Volatility: The tendency of a material to pass into the gas state at a particular temperature from a solid or liquid state.

Volume: The measurement of anything in 3-dimension (cubic magnitude).

High-level waste (HLW)
is highly radioactive material arising from nuclear fission. It can be what is left over from reprocessing used fuel, though some countries regard spent fuel itself as HLW. It requires very careful handling, storage and disposal.
Low-level waste (LLW)is mildly radioactive material usually disposed of by incineration and burial.

Yellowcake: Ammonium diuranate, the penultimate uranium compound in U3O8 production, but the form in which mine product was sold until about 1970. See also Uranium oxide concentrate.

Zircaloy: Zirconium alloy used as a tube to contain uranium oxide fuel pellets in a reactor fuel assembly.