In some countries used fuel is reprocessed to recover its uranium and plutonium, and to reduce the final volume of high-level wastes (HLW).
The plutonium is normally recycled promptly into mixed-oxide (MOX) fuel, by mixing it with depleted uranium.
Where uranium recovered from reprocessing used nuclear fuel (RepU) is to be re-used, it needs to be converted and re-enriched.
This is complicated by the presence of impurities and two new isotopes in particular: U-232 and U-236, formed by or following neutron capture in the reactor, and increase with higher burn-up levels. U-232 is largely a decay product of Pu-236, and increases with storage time in used fuel, peaking at about ten years. Both decay much more rapidly than uranium-235 and uranium-238, and one of the daughter products of 232U emits very strong gamma radiation, which means that shielding is necessary in the plant handling material with more than very small traces of it. One of the daughter products of 232U emits very strong gamma radiation, which means that shielding is necessary in the plant handling material with more than very small traces of it. U-236 is a neutron absorber, which impedes the chain reaction, and means that a higher level of 235U enrichment is required in the product to compensate. Being lighter, both isotopes tend to concentrate in the enriched (rather than depleted) output, so reprocessed uranium which is re-enriched for fuel must be segregated from enriched fresh uranium.
The presence of U-236 in particular means that most reprocessed uranium can be recycled only once - the main exception being in the UK with AGR fuel made from recycled Magnox uranium being reprocessed.
All these considerations mean that only RepU from low-enriched, low-burnup used fuel is normally recycled directly through an enrichment plant.
For instance, some 16,000 tonnes of RepU from Magnox reactors [2] in UK has been used to make about 1650 tonnes of enriched AGR fuel, via two enrichment plants.
Much smaller quantities have been used elsewhere, in France and Japan. Some re-enrichment, eg for Swiss, German and Russian fuel, is actually done by blending RepU with HEU.
A laser process would theoretically be ideal for enriching RepU as it would ignore all but the desired U-235, but this remains to be demonstrated with reprocessed feed. (this can be the RepU that is recovered from the LWR used fuel, it is one of the three components that is recovered in the processing in an ARC, Plutonium for the PRISM reactor and the actinide bundle(for Burial) are the other two things recovered This is what I have been saying for some time, have GLE achieved this?)
After enrichment The enriched UF6 is converted to UO2 and made into fuel pellets—ultimately a sintered ceramic—that are encased in metal tubes to form fuel rods, typically up to four meters long.
A number of fuel rods make up a fuel assembly, which is ready to be loaded into the nuclear reactor.
Depleted Uranium and Deconversion Depleted uranium (DU) is stored long-term as UF6 or preferably, after deconversion, as U3O8, allowing HF to be recycled. To early 2007, about one quarter of the 1.2 million tonnes of DU had been deconverted.
The main deconversion plant is run by Areva NC at Tricastin, France.
This is essentially a dry process, with no liquid effluent, and is the same as that used for the enriched portion, albeit at a scale of 20,000 tonnes per year in the one plant.
The UF6 is first vapourised in autoclaves with steam, then the uranyl fluoride is reacted with hydrogen at 700ºC to yield an HF byproduct for sale and U3O8 powder which is packed into 10-tonne containers for storage. 3UO2F2 + 2H2O + H2 ? U3O8 + 6HF
Ownership title of DU is normally transferred to the enricher as part of the commercial deal.
At present the only deconversion plant is in France, but others are planned. It is sometimes considered as a waste, but usually it is understood as a long-term strategic resource which can be used in a future generation of fast neutron reactors. Any much more efficient enrichment process would also make it into an immediately usable resource as source of more U-235. (This is laser isotope separation=Paducah Tails )
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