A - C,  D - F,  G - I,  J - O,  P - R, S - Z

Actinide: 
an element with atomic number of 89 (actinium) or above. 

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

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

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. 

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. 

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. 

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. 

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 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. 

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). 

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. 

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. 

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

Concentrate: 
See Uranium oxide concentrate (U3O8). 

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. 

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. 

D - F

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. 

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. 

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). 

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. 

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

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. See also SWU 

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

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

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

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. 

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. 

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

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

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

G - I

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

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. 

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

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

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

High-level wastes: 
Extremely radioactive fission products and transuranic elements (usually other than plutonium) in spent nuclear fuel. They may be separated by reprocessing the spent 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.) 

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. 

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. 

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. 

J - O

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

Light water: 
Ordinary water (H20) as distinct from heavy 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). 

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

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

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. 

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. 

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

P - R

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 spent 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 spent reactor fuel to separate uranium and plutonium from the small quantity of fission product waste products and transuranic elements, leaving a much reduced quantity of high-level waste. (cf Waste, HLW).

S - Z

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. 

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. 

Spent fuel: 
Fuel assemblies removed from a reactor after several years use. 

Stable: 
Incapable of spontaneous radioactive decay. 

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. 

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. 

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. 

Waste: 
High-level waste (HLW) is highly radioactive material arising from nuclear fission. It can be recovered from reprocessing spent 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. 

Source: World Nuclear Association