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.
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
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
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)
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
Burnup: Measure of thermal
energy released by nuclear fuel relative to its
mass, typically Gigawatt days per tonne (GWd/tU).
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
U308 into UF6 preparatory
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
Critical mass: The smallest
mass of fissile material that will support a self-sustaining
chain reaction under specified conditions.
of being able to sustain a nuclear chain reaction.
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.
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.
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).
change in the nucleus of a radioactive isotope as
particles are emitted (usually with gamma rays),
making it a different element.
Element: A chemical substance
that cannot be divided into simple substances by
chemical means; atomic species with same number
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.
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
Fast neutron: neutron
released during fission, travelling at very high
velocity (20,000 km/s) and having high energy (c
Fast neutron reactor:
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,
Fissile (of an isotope):
Capable of capturing a slow (thermal) neutron and
undergoing nuclear fission, e.g. U-235, U-233, Pu-239.
Fissionable (of an isotope):
Capable of undergoing fission: If fissile, by slow neutrons;
otherwise, by fast neutrons.
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.
Fossil fuel: A fuel
based on carbon presumed to be originally from living
matter, eg coal, oil, gas. Burned with oxygen to
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 zircaloy 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
carbon used in very pure form as a moderator, principally
in gas-cooled reactors, but also in Soviet-designed
Gray: The SI unit
of absorbed radiation dose, one joule per kilogram
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
Heavy water: Water containing
an elevated concentration of molecules with deuterium
("heavy hydrogen") atoms.
Heavy water reactor
A reactor which uses heavy water as its moderator,
eg Canadian CANDU.
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.)
In situ leaching (ISL):
The recovery by chemical leaching of minerals from
porous orebodies without physical excavation. Also
known as solution mining.
An atom that is electrically-charged because of
loss or gain of electrons.
(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
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).
Metal fuels: Natural
uranium metal as used in a gas-cooled reactor.
Micro: one millionth
of a unit (eg microsievert is 10-6 Sv).
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
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
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.
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,
Reactor pressure vessel:
The main steel vessel containing the reactor fuel,
moderator and coolant under pressure.
Repository: A permanent
disposal place for radioactive wastes.
treatment of used reactor fuel to separate uranium
and plutonium and possibly transuranic elements
from the small quantity of fission product wastes
products and transuranic elements, leaving a much
reduced quantity of high-level waste (which today
includes the transuranic 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.
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).
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.
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.
of spontaneous radioactive decay.
Tailings: Ground rock
remaining after particular ore minerals (e.g. uranium
oxides) are extracted.
uranium (cf. enriched uranium), with about 0.3%
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).
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.
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
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.
(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 what is left over from
reprocessing used fuel, though some countries regard
used 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.
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.
alloy used as a tube to contain uranium oxide fuel
pellets in a reactor fuel assembly.
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