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Separative Work Unit (SWU)

This is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, ie the extent of increase in the concentration of the U-235 isotope relative to the remainder. The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. Eg, to produce one kilogram of uranium enriched to 3.5% U-235 requires 4.3 SWU if the plant is operated at a tails assay 0.30%, or 4.8 SWU if the tails assay is 0.25% (thereby requiring only 7.0 kg instead of 7.8 kg of natural U feed). About 100-120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. Enrichment costs are related to electrical energy used. The gaseous diffusion process consumes some 2400 kWh per SWU, while gas centrifuge plants require only about 60 kWh/SWU.

Sievert (Sv)

Unit indicating the biological damage caused by radiation dose measured in Gray (q.v.). One Gray of beta or gamma radiation absorbed has 1 Sv of biological effect; 1 Gy of alpha radiation has 20 Sv effect and 1 Gy of neutrons has 10 Sv effect. 


the abrasion and removal of fragments of a target which is bombarded by protons in an accelerator. The fragments may be protons, neutrons or other light particles.

Spent fuel

Used fuel assemblies removed from a reactor after several years use and treated as waste.


Incapable of spontaneous radioactive decay.

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