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The period required for half of the atoms of a particular radioactive isotope to decay and become an isotope of another element.

Heavy water

Water containing an elevated concentration of molecules with deuterium ("heavy hydrogen") atoms.

Heavy water reactor (HWR)

A reactor which uses heavy water as its moderator, eg Canadian CANDU (q.v.) which is a pressurised HWR (PHWR).

High-enriched uranium (HEU)

Uranium enriched to 20% U-235 or more. (That in weapons is about 90% U-235.)

High-level wastes (HLW)

Extremely radioactive fission products and transuranic elements (usually other than plutonium) in used nuclear fuel. They may be separated by reprocessing the used fuel, or the spent fuel containing those isotopes may be regarded as high-level waste.  HLW requires both shielding and cooling.

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