Glossary of Terms


An element with atomic number of 89 (actinium) to 103. 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. Minor actinides are americium, curium and neptunium.
Activation Product
A radioactive isotope of an element (e.g. in the steel of a reactor core) which has been created by neutron bombardment.
The number of disintegrations per unit time inside a radioactive source. Expressed in becquerels.
As Low As Reasonably Achievable, economic and social factors being taken into account. This is the optimization principle of radiation protection.
Alpha particle
A positively-charged particle emitted from the nucleus of an atom during radioactive decay. Alpha particles are helium nuclei, with 2 protons and 2 neutrons.
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 almost 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 ionizing radiation which every person is exposed to, arising from the earth's crust (including radon) and from cosmic radiation.
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.
The SI unit of intrinsic radioactivity in a material. One Bq indicates 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 are generally electrons (with negative charge) but may be positrons.
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)




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 spent fuel itself as HLW. It requires very careful handling, storage and disposal.
Intermediate-level Waste (ILW)
Comprises a range of materials from reprocessing and decommissioning. It is sufficiently radioactive to require shielding and is disposed of in engineered facilities underground.
Low-level Waste (LLW)
Is mildly radioactive material usually disposed of by incineration and burial.


Ammonium 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.


Zirconium alloy used as a tube to contain uranium oxide fuel pellets in a fuel rod (part of a reactor fuel assembly).