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

Fast breeder reactor (FBR)

A fast neutron reactor (q.v.) configured to produce more fissile material than it consumes, using fertile material such as depleted uranium in a blanket around the core.

Fast neutron

neutron released during fission, travelling at very high velocity (20,000 km/s) and having high energy (c 2 MeV).

Fast neutron reactor

A reactor with no moderator and hence utilising fast neutrons. It normally burns plutonium while producing fissile isotopes in fertile material such as depleted uranium (or thorium).

Fertile (of an isotope)

Capable of becoming fissile, by capturing neutrons, possibly followed by radioactive decay; eg U-238, Pu-240.

Fissile (of an isotope)

Capable of capturing a slow (thermal) neutron and undergoing nuclear fission, e.g. U-235, U-233, Pu-239.


The splitting of a heavy nucleus into two, accompanied by the release of a relatively large amount of energy and usually one or more neutrons. It may be spontaneous but usually is due to a nucleus absorbing a neutron and thus becoming unstable.

Fission products

'Daughter' nuclei resulting either from the fission of heavy elements such as uranium, or the radioactive decay of those primary daughters. Usually highly radioactive.

Fissionable (of an isotope)

Capable of undergoing fission: If fissile, by slow neutrons; otherwise, by fast neutrons.

Fossil fuel

A fuel based on carbon presumed to be originally from living matter, eg coal, oil, gas. Burned with oxygen to yield energy.

Fuel assembly

Structured collection of fuel rods or elements, the unit of fuel in a reactor.

Fuel fabrication

Making reactor fuel assemblies, usually from sintered UO2 pellets which are inserted into zircalloy tubes, comprising the fuel rods or elements.

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