A Detailed Glossary of Nuclear Terms and Phrases


Activation product A radioactive isotope of an element (e.g., in the steel of a reactor core) which has been created by neutron bombardment.
Alpha particle A positively-charged particle from the nucleus of an atom, emitted during radioactive decay.
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.
Atomic Bomb An explosive device whose energy comes from the fission of heavy elements such as uranium or plutonium.
Becquerel (Bq) A unit of radiation equal to one disintegration per second (the SI unit).
Beta Particle A particle emitted from an atom during radioactive decay.
Biological Shield A mass of absorbing material (e.g., 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.
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.
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.
Clastic Derived from clast, a particle of broken down rock. A clastic rock is a sediment composed of fragments of pre-existing clasts. For example, conglomerates, sandstones and shales are clastic rocks.
Control Rods Devices to absorb neutrons so that the chain reaction in a reactor core may be slowed or stopped.
Conversion Chemical process turning U308 into UF6 preparatory to enrichment.
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.
Curie (Ci) A unit of radiation measurement, equal to 3.7x1010 disintegrations per second.
Decay Decrease in activity of a radioactive substance due to the disintegration of an atomic nucleus resulting in the release of alpha or beta particles or gamma radiation.
Decommissioning Removal of a facility (e.g., reactor) from service, also the subsequent actions of safe storage, dismantling and 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 (e.g., from weapons) to make reactor fuel.
Deuterium "Heavy Hydrogen", an isotope having one proton and one neutron in the nucleus. It occurs in nature as 1 atom to 6,500 atoms of normal hydrogen, (Hydrogen atoms contain one proton and no neutrons).
Dose Equivalent The absolute measurement of exposure to a dose of ionising radiation depends upon the type of particle and the body tissue with which it interacts - hence the conversion to dose equivalent, which has units of rem. Rads are converted to rems by multiplying by a factor that depends upon the type of ionising radiation and it's biological effect. For example, with gamma radiation the factor is 1 and a rad is equal to a rem.
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.
Excursion Where the leaching solutions used in the uranium In Situ Leaching (ISL) mining technique escape outside the mining zone (see ISL - Overview or Out of Sight Out of Mind).
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.
Fast Neutron Reactor (FNR) A reactor with little or no moderator and hence utilising fast neutrons and able to utilise fertile material such as depleted uranium.
Fertile (of an isotope) Capable of becoming fissile, by capturing one or more neutrons, possibly followed by radioactive decay. U-238 is an example.
Fissile (of an isotope) Capable of capturing a neutron and undergoing nuclear fission, e.g., U-235, Pu-239.
FissionThe splitting of a heavy nucleus into two, accompanied by the release of a relatively large amount of heat and generally one or more neutrons. It may be spontaneous but usually is due to a nucleus absorbing a neutron.
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.
Fossil Fuel A fuel based on carbon presumed to be originally from living matter, e.g., coal, oil, gas. Burned with oxygen to yield energy.
Fuel Fabrication Making reactor fuel elements, usually from UO2.
Gamma Rays High energy electro-magnetic radiation.
Genetic Mutation Sudden change in the chromosomal DNA of an individual gene. lt may produce inherited changes in descendants. Mutation can be made more frequent by irradiation.
Graphite A form of carbon used in very pure form as a moderator, principally in gas-cooled reactors, but also in Soviet-designed RBMK reactors.
gray (Gy) The SI unit for rads. 1 Gy equals 100 rads.
Greenhouse Gases Thermal 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 (CO2), water vapour (H2O) and methane (CH4)are the most important ones.
Groundwater Water found in the voids or free space of soils and rocks underground.
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 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, e.g., Canadian CANDU.
High-Level Wastes Extremely radioactive fission products and transuranic elements (usually other than plutonium) separated as a result of reprocessing spent nuclear fuel.
Highly (or High)-Enriched Uranium (HEU) Uranium enriched to at least 20% U-235. Uranium in weapons is about 90% U-235.
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.
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 masses, e.g., U-235, U-238.
Joule A unit of energy.
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.
Lixiviant The term used to describe the chemical solutions used in uranium In Situ Leach (ISL) mining.
Low-Enriched Uranium (LEU) Uranium enriched to less than 20% U-235. Uranium in power reactors is about 3.5% 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 (e.g., 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 (e.g., microsievert is one millionth of a Sv).
Mixed Oxide Fuel (MOX) Reactor fuel which consists of both uranium and plutonium oxides, usually with about 5% Pu.
Moderator A material such as light or heavy water or graphite used in a reactor to slow down fast neutrons 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.
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 neutrons can in turn readily cause fission in atoms of some isotopes, e.g., U-235, and fast neutrons can readily cause fission in atoms of others, e.g., 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 U02, 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 with >90% Pu-239, reactor-grade plutonium contains about 30% non-fissile isotopes.
Pressurised Water Reactor (PWR) The most common type of light water reactor (LWR).
Radiation The emission and propagation of energy by means of electromagnetic waves or sub-atomic particles.
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 An element 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).
Radon Daughters Radioactive decay products of radon-222.
rads A unit to measure the absorption of radiation by the body. A rad is equivalent to 100 ergs of energy from ionising radiation absorbed per gram of soft tissue.
rem The unit of dose equivalent.
Reactor see Nuclear Reactor.
Reprocessing Chemical treatment of spent reactor fuel to separate uranium and plutonium from the small quantitiy of fission products (and from each other), leaving a much reduced quantity of high-level waste.
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.

For instance, 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) SI unit indicating the biological damage caused by radiation. One Joule (J) 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. 1 Sv equals 100 rem.
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 (as distinct from Fast Neutron Reactor).
Transuranic Element A very heavy element formed artificially by neutron capture and subsequent beta decay(s). Has a higher atomic number than uranium (92). All are radioactive. Neptunium, plutonium and americium are the best-known.
Uranium 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 raw material of nuclear energy.
Uranium Hexafluoride (UF6) A compound of uranium with fluorine which is a gas above 56oC and is thus a suitable form in which to enrich the uranium.
Uranium Oxide Concentrate (U308) 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 U308. Uranium is exported from Australia in this form.
Vitrification The incorporation of high-level wastes into borosilicate glass, to make up about 14% of the product by mass.
Waste High-level waste (HLW) is highly radioactive material arising from nuclear fission. It is recovered from reprocessing spent fuel, though some countries regard spent fuel itself as HLW and plan to dispose of it in that form. It requires very careful handling, storage and disposal.
Waste Low-level waste is mildly radioactive material usually disposed of by incineration and burial.
Yellowcake Ammonium diuranate, the penultimate uranium compound in U308 production, but the form in which mine product was sold until about 1970.
See also Uranium Oxide.

Last updated - February 8, 1998.
Glossary adapted from the Uranium Information Centre and other sources.
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