U-232 and the proliferation-resistance of U-233 in spent fuel

The factors influencing the level of U?232 contamination in U?233 are examined for heavy?water?moderated, light?water?moderated and liquid?metal cooled fast breeder reactors fueled with natural or low?enriched uranium and containing thorium mixed with the uranium or in separate target channels. U?232 decays with a 69?year half?life through 1.9?year half?life Th?228 to T1?208, which emits a 2.6 MeV gamma ray upon decay. We find that pressurized light?water?reactors fueled with LEU?thorium fuel at high burnup (70 MWd/kg) produce U?233 with U?232 contamination levels of about 0.4 percent. At this contamination level, a 5 kg sphere of U?233 would produce a gamma?ray dose rate of 13 and 38 rem/hr at 1 meter one and ten years after chemical purification respectively. The associated plutonium contains 7.5 percent of the undesirable heat?generating 88?year half?life isotope Pu?238. However, just as it is possible to produce weapon?grade plutonium in low?burnup fuel, it is also practical to use heavy?water reactors to produce U?233 containing only a few ppm of U?232 if the thorium is segregated in ?target? channels and discharged a few times more frequently than the natural?uranium ?driver? fuel. The dose rate from a 5?kg solid sphere of U?233 containing 5 ppm U?232 could be reduced by a further factor of 30, to about 2 mrem/hr, with a close?fitting lead sphere weighing about 100 kg. Thus the proliferation resistance of thorium fuel cycles depends very much upon how they are implemented.

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