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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. 

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 optimisation 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.

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