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| 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, e.g. used fuel. They are fissionable in a fast reactor. |
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| Activation Energy | Activation energy of a reaction is the amount of energy needed to start the reaction. |
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| Activation Product | A radioactive isotope of an element (e.g. in the steel of a reactor core) which has been created by neutron bombardment. |
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| ALARA | As Low As Reasonably Achievable, economic and social factors being taken into account. This is the optimisation principle of radiation protection. |
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| 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. |
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| 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. |
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| 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. |
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| 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). |
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| Beta Particle | A particle emitted from an atom during radioactive decay. Beta particles may be either electrons (with negative charge) or positrons. |
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| 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. |
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| Boiling Water Reactor | A nuclear reactor in which water is allowed to boil in the core. The resulting steam is used to drive a turbine generating electric power. |
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| Breed | To form fissile nuclei, usually as a result of neutron capture, possibly followed by radioactive decay |
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| 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. |
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| Burnup | Measure of thermal energy released by nuclear fuel relative to its mass, typically Gigawatt days per tonne (GWd/tU). |
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| BWR | Boiling water reactor: 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) |
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| 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. |
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| CANDU | Canadian deuterium uranium reactor, moderated and (usually) cooled by heavy water. |
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| Chain Reaction | A self-sustaining nuclear reaction which takes place during fission. A fissionable substance (i.e., uranium) absorbs a neutron and divides, releasing additional neutrons that are absorbed by other fissionable nuclei, releasing still more neutrons. |
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| Cladding | The metal tubes containing oxide fuel pellets in a reactor core. |
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| 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. |
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| Critical Mass | The smallest mass of fissile material that will support a self-sustaining chain reaction under specified conditions. |
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| 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). |
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| 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. |
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| Delayed Neutrons | Neutrons released by fission products up to several seconds after fission. These enable control of the fission in a nuclear reactor. |
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| 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. |
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| 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). |
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| Disintegration | Natural change in the nucleus of a radioactive isotope as particles are emitted (usually with gamma rays), making it a different element. |
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| 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. |
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| Fast Breeder Reactor | 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. |
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| Fast Neutron | Neutron released during fission, travelling at very high velocity (20,000 km/s) and having high energy (c 2 MeV). |
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| 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). |
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| FERC | Federal Energy Regulatory Agency - The Federal government agency that regulates and oversees energy industries in the economic, environmental, and safety interests of the American public. |
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| Fertile Isotope | Capable of becoming fissile, by capturing neutrons, possibly followed by radioactive decay; eg U-238, Pu-240. |
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| Fissile Isotope | Capable of capturing a slow (thermal) neutron and undergoing nuclear fission, e.g. U-235, U-233, Pu-239. |
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| 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. |
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| 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. |
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| Fissionable | Capable of undergoing fission: If fissile, by slow neutrons; otherwise, by fast neutrons. |
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| Fossil Fuel | A fuel based on carbon presumed to be originally from living matter, eg coal, oil, gas. Burned with oxygen to yield energy. |
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| Fuel Assembly | Structured collection of fuel rods or elements, the unit of fuel in a reactor. |
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| Fuel Fabrication | Making reactor fuel assemblies, usually from sintered UO2 pellets which are inserted into zircalloy tubes, comprising the fuel rods or elements. |
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| Fuel Rod | The uranium fuel is formed into ceramic pellets. The pellets are about the size of your fingertip, but each one produces the same amount of energy as 150 gallons of oil. These energy-rich pellets are stacked end-to-end in 12-foot metal fuel rods. A bundle of fuel rods is called a fuel assembly. |
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| Gamma Rays | High energy electro-magnetic radiation from the atomic nucleus, virtually identical to X-rays. |
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| 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). |
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| Graphite | Crystalline carbon used in very pure form as a moderator, principally in gas-cooled reactors, but also in Soviet-designed RBMK reactors. |
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| 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. |
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| GT-MHR | Gas Turbine Modular Helium Reactor, developed by General Atomics in San Diego, the nuclear energy division of General Dynamics. GT-MHR is based on a primsatic fuel configuration, where tiny three-hundreds of an inch fuel kernals are mixed with graphite and formed into cylindrical fuel rods about two inches long. The fuel rods are then inserted into holes drilled into the hexagonal graphite fuel element blocks..., which also have helium coolant channels, and are then stacked in the reactor core. |
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| Half-life | The period required for half of the atoms of a particular radioactive isotope to decay and become an isotope of another element. |
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| Heavy Water | Water containing an elevated concentration of molecules with deuterium (\"heavy hydrogen\") atoms. |
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| Heavy Water Reactor | A reactor which uses heavy water as its moderator, eg Canadian CANDU (pressurised HWR or PHWR). |
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| HEU | Highly (or High)-enriched uranium: Uranium enriched to at least 20% U-235. (That in weapons is about 90% U-235.) |
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| High-level Waste | 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. |
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| 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. |
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| In Situ Leaching | The recovery by chemical leaching of minerals from porous orebodies without physical excavation. Also known as solution mining. |
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| Ion | An atom that is electrically-charged because of loss or gain of electrons. |
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| Ionising 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. |
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| Irradiate | Subject material to ionising radiation. Irradiated reactor fuel and components have been subject to neutron irradiation and hence become radioactive themselves. |
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| 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. |
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| Light Water | Ordinary water (H20) as distinct from heavy water. |
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| Light Water Reactor | A common nuclear reactor cooled and usually moderated by ordinary water. |
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| Low-enriched Uranium | Uranium enriched to less than 20% U-235. (That in power reactors is usually 3.5 - 5.0% U-235.) |
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| Low-level Waste | Mildly radioactive material usually disposed of by incineration and burial. |
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| Low-level Wastes | Mildly radioactive material usually disposed of by incineration and burial. |
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| Metal Fuels | Natural uranium metal as used in a gas-cooled reactor. |
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| MOX | Mixed oxide fuel: Reactor fuel which consists of both uranium and plutonium oxides, usually about 5% Pu, which is the main fissile component. |
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| 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. |
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| 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. |
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| Nuclear Fission | The splitting apart of atoms. This splitting releases large amounts of energy and one or more neutrons. Nuclear power plants split the nuclei of uranium atoms in a process called fission. |
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| Nuclear Fusion | When the nuclei of atoms are combined or \"fused\" together. The sun combines the nuclei of hydrogen atoms into helium atoms in a process called fusion. Energy from the nuclei of atoms, called \"nuclear energy\" is released from fusion. |
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| 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). |
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| Oxide Fuels | Enriched or natural uranium in the form of the oxide UO2, used in many types of reactor. |
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| PBMR | Pebble Bed Modular Reactor, a reactor which encapsulates the uranium fuel in self-contained tennis-ball sized graphite modules that circle the core. PBMR designs are being developed in South Africa and China.
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| 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. |
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| Pressurized Water Reactor | A reactor in which water, heated by nuclear energy, is kept a high pressure to prevent the water from boiling. Steam is then generated in a secondary coolant loop. |
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| PWR | Pressurised water reactor: 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. |
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| Radioactivity | The spontaneous decay of an unstable atomic nucleus, giving rise to the emission of radiation. |
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| Radiotoxicity | The adverse health effect of a radionuclide due to its radioactivity. |
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| Radium | A radioactive decay product of uranium often found in uranium ore. It has several radioactive isotopes. Radium-226 decays to radon-222. |
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| Radon | Radon (Rn) is a heavy radioactive gas given off by rocks containing radium (or thorium). Rn-222 is the main isotope. |
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