Reprocessed uranium
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Reprocessed uranium (RepU) is the uranium recovered from nuclear fuel reprocessing. This uranium actually makes up the bulk of the material separated during reprocessing. Spent fuel contains, on average, only four percent plutonium, minor actinides and fission products by weight.
The present economic situation of the uranium market limits the interest in uranium recycling, however the technology for making reprocessed uranium fuel is well established and there is no technical reason limiting its adoption. In fact the scarcity of natural uranium, which was expected to drive demand for reprocessed uranium, never materialized, and the economic outlook for commercial reprocessing plants dimmed during the late 1970s. At the same time, concerns that plutonium from civilian spent fuel could be used for nuclear weapons helped end most support for commercial reprocessing in North America. Reprocessed uranium will not become competitive unless it were to attract a price premium compared with conversion of natural uranium.
However opening permanent disposal facilities has proven more expensive, technically more difficult, and politically more problematic than expected, generating renewed interest in the practicability of burning reprocessed uranium in current reactors. In some countries spent fuel is reprocessed to recover its uranium and plutonium, and to reduce the final volume of high-level wastes. The plutonium is normally recycled promptly into mixed oxide fuel (MOX), by mixing it with depleted uranium.
Only certain countries such as France, Japan the United Kingdom and Russia have so far decided to reprocess this in order to separate fissile uranium and plutonium for recycling in reactors. Reprocessing plant and MOX fuel fabrication capacities together with the historically low uranium price serve to limit the current significance of spent fuel as a resource, but potentially within 10 years, reprocessed uranium and MOX fuel could substitute for 7,000 tonnes of uranium per annum.
Where uranium recovered from reprocessing spent nuclear fuel 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: uranium-232 and uranium-236, which are formed by neutron capture in the reactor. Both decay much more rapidly than uranium-235 and uranium-238, and one of the daughter products of U-232 emits very strong gamma radiation, which means that shielding is necessary in the plant. U-236 is a neutron absorber which impedes the chain reaction, and means that a higher level of U-235 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.
There have been some studies involving the use of reprocessed uranium in CANDU reactors. CANDU is designed to use natural uranium as fuel; the U-235 content remaining in spent PWR/BWR fuel is typically greater than that found in natural uranium, allowing the re-enrichment step to be skipped. Fuel cycle tests also have included the DUPIC fuel cycle, or direct use of spent PWR fuel in CANDU, where used fuel from a pressurized water reactor is packaged into a CANDU fuel bundle with only physical reprocessing (cut into pieces) but no chemical reprocessing.
