Radioactivity - glossary
A1 Value
A1 value is the maximum amount of activity for a particular special form radionuclide that is allowed in a Type A package. A1 value for a particular radionuclide is defined as the quantity of that radionuclide resulting in a dose rate of 0.1 Sievert/hr at a distance of 1 meter from the package.
Advanced Gas-Cooled Reactor (AGR)
The second generation of nuclear reactors built in the UK. Uses slightly enriched uranium dioxide clad in stainless steel as fuel, and operates at much higher temperature than do the first generation Magnox reactors.
Advanced Light Water Reactor (AWLR)
A pressurised or boiling water reactor having advanced safety features.
ALARP
As Low as Reasonably Practicable
ALARA
As Low as Reasonably Achievable
Alpha particles
Particles which are emitted from heavy nuclei containing a large number of neutrons and protons, such as uranium isotopes. Alpha particles consist of two neutrons and two protons bound together, which behave as a single particle.
Annual dose
The radioactive dose received in one year.
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.
Authorisation
The granting by a regulatory body or other governmental body of written permission for an operator to perform specified activities.
Background radiation
There is a natural level of ionising radiation in the environment, to which everything is exposed to a greater or lesser extent and for the majority of people it is the major source of radiation exposure. This natural radiation comes from a number of sources, for example cosmic rays from outer space, radioactive elements naturally present in rocks, soils, building materials and in the food and drink that we eat.
Becquerel (Bq)
Unit of radioactivity, one disintegration a second. Since this is such a small unit, large multiples are more commonly used: MBq (a million), GBq (a thousand million), TBq (a million million).
Beta particles
These are high speed electrons that are emitted from the nucleus of an atom.
BPEO
Best Practicable Environmental Option
BPM
Best Practicable Means
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.
Conditioning
Treatment of radioactive waste to put it in a more stable form.
Containment
Methods or physical structures designed to prevent the dispersion of radioactive substances.
Decay (radioactive)
Spontaneous disintegration of a radionuclide accompanied by the emission of ionizing radiation in the form of alpha or beta.
Decommissioning
The process whereby a nuclear facility, at the end of its economic life, is taken permanently out of service and its site made available for other purposes. In the case of a nuclear power station this comprises 3 different states of clearance. Immediately after the final closure, radioactive material such as nuclear fuel and operational waste is removed; secondly, the buildings surrounding the reactor shield are dismantled; and finally the reactor itself is dismantled.
Decontamination
Removal of unwanted radioactive or hazardous contamination by a chemical or mechanical process.
Depleted uranium
Uranium with a lower percentage of uranium-235 than the 0.72% contained in natural uranium. It is produced during uranium isotope separation.
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.
Dose rate
The amount of ionizing (or nuclear) radiation which an individual or material would receive per unit of time. The dose delivered per unit of time, sometimes also called dosage.
Enriched uranium
Uranium in which the content of the isotope uranium-235 has been increased above its natural value of 0.72% by weight.
Enrichment
Physical process of increasing the proportion of uranium-235 to uranium-238.
Exemption orders
Exemption orders are statutory instruments made under the Radioactive Substances Act and specify classes of premises, undertakings or persons, and radioactive material or radioactive apparatus that do not need to be registered or further authorised.
Fission
The nuclear chain reaction which generates heat in a reactor is known as fission. This is because atoms of uranium split up, or undergo fission, and keep the reaction going.
Fission products
Isotopes produced when fissile material is split after colliding with a neutron. E.g. Strontium-90 and Caesium-137.
Gamma radiation
Gamma radiation is a form of electromagnetic radiation emitted from the nucleus and is often emitted with alpha and beta particles. Gamma particles are uncharged, have no mass and travel at the speed of light.
Giga (G)
1 thousand million = 1,000,000,000 = 109
Half-life
The time it takes for HALF of the radioactive element to decay (give off half of its radioactivity). Different radioactive isotopes have different half-lives.
Highly Active Sealed Source (HASS)
HASS is any radioactive sealed source containing more than one-tenth of the A1 value in the IAEA Regulations for the Safe Transport of Radioactive Material.
High Level Waste (HLW)
In which temperature may rise significantly as a result of its radioactivity, so that this factor has to be taken into account in designing storage or disposal facilities.
Ionising radiation
Radiation that produces ionisation in matter. Examples are alpha particles, gamma rays, x-rays and neutrons. When these radiations pass through the tissues of the body, they have sufficient energy to damage DNA.
Intermediate Level Waste (ILW)
Wastes with radioactivity levels exceeding the upper boundaries for low-level wastes, but which do not require heating to be taken into account in the design of storage or disposal facilities.
Isotopes
Nuclides with the same number of protons but different numbers of neutrons.
Nuclides with the same number of protons but different numbers of neutrons. Not a synonym for nuclide.
Low Level Waste (LLW)
Wastes containing radioactive material other than those acceptable for disposal with ordinary refuse, but not exceeding 4 gigabecquerels* per tonne (GBq/te) of alpha or 12 GBq/te of beta/gamma activity.
Magnox
A type of gas-cooled nuclear reactor used for electricity generation at power stations constructed in the 1960s. Takes its name from the magnesium-based alloy in which the natural uranium metal fuel is contained.
Mega (M)
One million
Million = 1,000,000 = 106
Micro (µ)
One millionth = 10-6
Millionth 0.000001 = 10-6
Moderator
Material introduced into a reactor or critical assembly to moderate (slow down) neutrons so that they interact with fissile nuclides.
Milli (m)
One-thousandth = 10-3
Thousandth 0.001 = 10-3
MOX
Mixed oxide fuel, a mixture of uranium and plutonium
Millisievert (mSv)
A measure of one thousandth of sievert, the standard unit used to measure radiation.
Natural uranium
Natural uranium is a mixture of uranium-238 (99.27%), uranium-235 (0.72%) and a very low percentage of uranium-234 (0.0056%).
Nuclear power
Power obtained from the operation of a nuclear reactor.
Nuclear power industry
The industry associated with the production of nuclear power. In the UK this includes, the preparation of fuel for nuclear reactors, the operation of reactors, the subsequent reprocessing of the fuel, and the disposal of radioactive wastes.
Nuclear reactor
A device that sustains a controlled nuclear fission chain reaction.
Nuclear safety
The achievement of proper operating conditions, prevention of accidents or mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards.
Orphan source
A source of radioactive material that has been lost from regulatory and management controls.
OSPAR
The Convention for the Protection of the Marine Environment of the North East Atlantic (the OSPAR Convention), to which the UK is a party, agreed a strategy to 'prevent pollution of the maritime area by continuously reducing discharges, emissions and losses of hazardous substances with the ultimate aim of achieving concentrations in the marine environment near background values for naturally occurring substances and close to zero for man-made synthetic substances.’
Partitioning
Is a series of physical and chemical separation processes which separate different radionuclides from a mixture, in order to permit their segregated treatment.
Plutonium
A naturally radioactive, metallic transuranic element, occurring in minute quantities in uranium ores or produced artificially by neutron bombardment of uranium.
PWR (Pressurised Water Reactor)
The most recent type of nuclear reactor to be constructed in the UK which uses pressurised water as both the coolant and the moderator.
Radioactive discharge
Planned and controlled release of (usually gaseous or liquid) radioactive material to the environment.
Radioactivity - general
Radioactivity is a property possessed by radioactive substances of spontaneously emitting energetic particles and energy rays from the disintegration of their atomic nuclei. This process is known as decay and can result in an atom changing to become a different element. A decaying atom may form a number of radioactive isotopes of different elements until it reaches a stable state, which is often referred to as a decay chain.
Radioactive Substances Act
The primary aim of the Radioactive Substances Act is to ensure the control of radiation exposure resulting from radioactive wastes entering the environment. This is achieved through the application of a prior permission regime. Because radioactive wastes are potentially harmful it is important not only that they are disposed of safely by appropriate routes, but also that they are not produced in unnecessary quantities. For this reason the Act controls the use of radioactive materials as well as the wastes themselves. The objective of regulating the use of radioactive substances and the management of radioactive wastes is to protect man and the wider environment.
Radiation
Energy dispersion through matter or space. In atomic physics this term is also extended to fast moving particles (alpha and beta radiation, free neutrons, etc.)
Radioactivity
The spontaneous emission of radiation from the nucleus of an atom. Radionuclides lose particles and energy through the process of radioactive decay.
Radionuclide
Radionuclide. A radioactive species of an atom. For example, tritium and strontium 90 are radionuclides of elements hydrogen and strontium respectively.
Radiotoxicity
Radiotoxicity is a measure of the harm caused by an intake of a radioactive substance.
Radiation dose
The amount of radiation energy that is received by humans can be used to quantify the risk of harm posed to them from the resulting ionisation within human tissue.
Radioactive material
Material designated in national law or by a regulatory body as being subject to regulatory control because of its radioactivity.
Radioactive waste
Is waste that contains or is contaminated with, radionuclides at concentrations or activities greater than clearance levels as established by the regulatory body.
Regulatory body
An authority or a system of authorities designated by the government of a State as having legal authority for conducting the regulatory process, including issuing authorisations, and thereby regulating nuclear, radiation, radioactive waste and transport safety.
Repository
A facility where radioactive waste is emplaced for disposal.
Radon
Radon is a colourless and odourless radioactive gas that occurs naturally from the decay of radium.
Reprocessing
Nuclear reprocessing separates any usable nuclear fuels (e.g., uranium and plutonium) from fission products and other materials in used nuclear reactor fuels.
Shielding
Material used to block or absorb radiation. Often placed between sources of radiation and people or the environment.
Sievert
The international unit of dose equivalent, measuring the effect of radiation dose on persons. Commonly used subunits are the milli-Sievert (one thousandth) and the micro-Sievert (one millionth).
Spent fuel
Spent fuel is irradiated fuel which is removed from a nuclear reactor after final use.
Storage
The holding of spent fuel or of radioactive waste in a facility that provides for its containment, with the intention of retrieval.
Transmutation
Is the destruction of radionuclides by converting them to isotopes of lighter elements which are likely to have shorter half-lives and generally low toxicity.
Transuranic waste
Transuranic waste is material contaminated with radioactive elements heavier than uranium, such as plutonium, neptunium, americium and curium.
Uranium (U)
A naturally occurring radioactive element. It is the basic material for nuclear technology. It is the heaviest naturally occurring (metal) element in the earth's crust, exists as three isotopes in the following percentages by weight: U-238, 99.27%; U-234, 0.72%; and U-235, 0.0056%; by radioactivity: U-238, 48%; U-234, 50%; and U-235, 2%; U-235 is the only fissile isotope (capable of sustaining a nuclear chain reaction), ie it is the uranium "bomb material" and nuclear reactor fuel isotope.
Vitrification
Immobilizing liquid and sludge-type waste by incorporating it in molten glass. The process produces a glass-like solid that captures the radioactive materials.
Waste disposal
Emplacement of waste in an appropriate facility without the intention of retrieval.
Page last modified: 13 September 2007