Click on the alphabetic index to display terms beginning with the corresponding letter or search for a term below.

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.

Base load

That part of electricity demand which is continuous, and does not vary over a 24-hour period.


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.

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.


To form fissile nuclei, usually as a result of neutron capture, possibly followed by radioactive decay.


The process of undergoing fission (analogous to burning a fossil fuel) or otherwise becoming denatured in the reactor core.

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 used.


Measure of thermal energy released by nuclear fuel relative to its mass, typically Gigawatt days per tonne of fuel (GWd/t).

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