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Definitions
Below is a directory of terms used in the field of nuclear power. Most of the terms have been reproduced or adapted from the United States Nuclear Regulatory Commission (US NRC) and the World Nuclear Association (WNA).
Nuclear power is an attractive energy option in the developing world. How has the accident at Fukushima affected plans for the technology, and what do policymakers need to consider when weighing up nuclear against other sources of energy?
B
Boiling water reactor (BWR)
The second most common type of electricity-generating nuclear reactor design after the pressurised water reactor (PWR). In this design, high-pressure water flows upward through the core where it is heated by fission and allowed to boil in the reactor vessel. The resulting steam then drives turbines, which activate generators to produce electrical power.
C
Containment structure
A gas-tight shell or other enclosure around a nuclear reactor to confine fission products that might otherwise be released into the atmosphere in the event of an accident. Such enclosures are usually dome-shaped and made of steel-reinforced concrete.
Core melt accident
An event or sequence of events that results in the melting of part of the fuel in the nuclear reactor core.
D
Decommissioning
The process of safely closing a nuclear power plant after its useful life has ended. This process primarily involves decontaminating the facility to reduce residual radioactivity and then making the property available for use.
Deuterium
A stable isotope of hydrogen with one proton and one neutron in the nucleus. Also known as 'heavy hydrogen'.
E
Enrichment
The physical process of increasing the relative abundance of uranium-235 (which is less abundant than U-238 in naturally occurring uranium) within nuclear fuel. This is required to produce a controlled nuclear reaction.
EPR (European Pressurised Reactor)
A type of pressurised water reactor (PWR) design.
F
Fast neutron reactor
A reactor which does not use a moderator material (see 'moderator') to slow down fast neutrons, thus increasing the likelihood of nuclear fission.
Fission
The splitting of an atom, which releases a considerable amount of energy (usually in the form of heat) that can be used to produce electricity.
Fusion
A reaction in which at least one heavier, more stable atomic nucleus is produced from two lighter, less stable nuclei. Reactions of this type are responsible for an enormous release of energy, such as that given off by stars.
H
Heavy hydrogen
See 'Deuterium'
High-level waste (HLW)
The highly radioactive material arising from a nuclear reaction. 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.
I
Intermediate-level waste (ILW)
Comprises a range of materials produced by nuclear fuel reprocessing and plant decommissioning. It is sufficiently radioactive to require shielding to protect people nearby, and is disposed of in engineered facilities underground.
L
Low-enriched uranium (LEU)
Uranium enriched to contain less than 20 per cent uranium-235. (That used in power reactors usually contains 3.5–5.0 per cent uranium-235.)
Low-level waste (LLW)
Refers to mildly radioactive materials which have become contaminated due to exposure to radiation. It is usually disposed of by incineration.
M
Moderator
A material (such as ordinary water, heavy water, or graphite) used in a reactor to slow down high-velocity neutrons, increasing the likelihood of fission.
P
Power-generating capacity
The maximum amount of electric power that can be produced by a generating unit, usually expressed in megawatts (MW).
Pressurised water reactor (PWR)
A common nuclear power reactor design in which water is heated to a very high temperature by nuclear fission, kept under high pressure (to prevent it from boiling), and converted to steam by a steam generator. The resulting steam is used to drive turbines, which activate generators to produce electricity.
R
Radiation
Energy given off by matter in the form of tiny fast-moving particles (alpha particles, beta particles, and neutrons), or pulsating electromagnetic rays or waves (gamma rays) emitted from the nuclei of unstable radioactive atoms.
T
Thorium
A naturally occurring, slightly radioactive metal which is more abundant than uranium and can be used as a nuclear fuel. Although not fissile itself, Th-232 can absorb slow neutrons to produce uranium-233, which can undergo nuclear fission.
Tritium
A radioactive isotope of hydrogen and a by-product of nuclear fission. Tritium emits a weak form of radiation which does not travel far in air and cannot penetrate the skin, but can contaminate sources of drinking water.
U
Uranium
Uranium is the basic fuel of nuclear energy. It is a radioactive element with two principal natural isotopes: uranium-235 (which comprises 0.7 per cent of natural uranium) and uranium-238 (99.3 per cent of natural uranium).
Uranium enrichment
See 'Enrichment'
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