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Concept
Reprocessed uranium
Summary
Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material separated during reprocessing.
Commercial LWR spent nuclear fuel contains on average (excluding cladding) only four percent plutonium, minor actinides and fission products by weight. Despite it often containing more fissile material than natural uranium, reuse of reprocessed uranium has not been common because of low prices in the uranium market of recent decades, and because it contains undesirable isotopes of uranium.
Given sufficiently high uranium prices, it is feasible for reprocessed uranium to be re-enriched and reused. It requires a higher enrichment level than natural uranium to compensate for its higher levels of 236U which is lighter than 238U and therefore concentrates in the enriched product. As enrichment concentrates lighter isotopes on the "enriched" side and heavier isotopes on the "depleted" side, 234U will inevitably be enriched slightly stronger than 235U, which is a negligible effect in a once-through fuel cycle due to the low (55 ppm) share of 234U in natural uranium but can become relevant after successive passes through an enrichment-burnup-reprocessing-enrichment cycle, depending on enrichment and burnup characteristics. 234U readily absorbs thermal neutrons and converts to fissile 235U, which needs to be taken into account if it reaches significant proportions of the fuel material. If 235U interacts with a fast neutron there is a chance of a (n,2n) "knockout" reaction. Depending on the characteristics of the reactor and burnup, this can be a larger source of 234U in spent fuel than enrichment.
If fast breeder reactors ever come into widespread commercial use, reprocessed uranium, like depleted uranium, will be usable in their breeding blankets.
There have been some studies involving the use of reprocessed uranium in CANDU reactors.
Official source
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