16.6.1 Recycling of uranium from reprocessing

Uranium from processing has generally a 235U nominal content superior to natural uranium (0.72%) so that the use of reprocessed uranium is a gain not only in mass but also as separative work units (SWU), which represent the energy required to increase the concentration of 235U above that of natural uranium. For reprocessed uranium to be used in the fuel assemblies that power a commercial nuclear reactor, it needs to be re-enriched (an LWR will generally need uranium with a 4-5% concentration of 235U). In addition to the 235 and 238 isotopes, reprocessed uranium also contains traces of uranium-236 and uranium-232. These two isotopes cause some difficulties:

• Uranium-236 is neutron-absorbing so that reprocessed uranium needs to be over-enriched in comparison with a fuel prepared with natural uranium.

• Uranium-232 has daughter products (lead-212 and thallium-208) that emit intense py activity. This may pose problems for radiation protection in the enrichment and fuel fabrication plants.

• In addition to these drawbacks, there is a particularly acute commercial difficulty with gaseous enrichment plants where a significant mass of uranium (known as ‘hold-up’) is always retained within the cascade (sequential enriching system). If natural uranium and uranium from reprocessing were to be treated alternately, it would be necessary to empty the cascade between production campaigns, which may be unacceptable economically.

• Co-precipitation of uranium and plutonium can be also used to produce a mixed oxide (MOX) fuel.