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World Nuclear Association - Nuclear fuel terms
Category: Earth and Environment > Nuclear fuel
Date & country: 10/12/2007, UK
Words: 110


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

Activation product
A radioactive isotope of an element (e.g. 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

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

Barn
see Cross section.

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.

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)

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

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

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

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)

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

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
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)

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
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 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

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

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
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)

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

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

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.)

Low-level wastes
Mildly radioactive material usually disposed of by incineration and burial

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

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).

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

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

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

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