Plutonium uranium recovery by extraction (PUREX) and mixed oxide (MOX)

The history of recycling of used nuclear fuels traces its lineage back to the 1940s and the Manhattan Project (Rhodes, 1986). Having begun with co­precipitation technologies, the multiple advantages of solvent extraction — based processing significantly improved the efficiency and throughput of recycling. Introduction of solvent extraction methods also dramatically reduced the amount of HLW produced during recycling operations. During the first several decades of this enterprise, the principal driver for reprocess­ing was plutonium production to support the weapons programs of the nuclear nations. Dating from the mid-1950s, the system of choice for pluto­nium isolation and recovery from dissolved used fuel has been the PUREX (plutonium uranium recovery by extraction) process. When introduced, the PUREX process represented a significant leap forward in efficiency over the solvent extraction processes that it replaced (REDOX and BUTEX; Nash et al, 2006).

In PUREX, contact of a 3-6 m HNO3 solution containing all but the most volatile products of fission with an immiscible kerosene solution of tributyl phosphate (TBP) allowed selective extraction of UO2(NO3)2 and Pu(NO3)4 from a mixture of fission products and mixed actinides. With careful control of the conditions of extraction, all fission products, the transplutonium actinides and neptunium remain in the aqueous raffinate for ultimate dis­posal as high level wastes. Operation of the solvent extraction system with multiple stages and a counter-current flow of aqueous and organic phases results in a high purity Pu/U nitrate organic solution almost completely devoid of fission products or minor actinides (Np, Am, Cm).

From this solution the plutonium is readily (and selectively) removed with the application of a suitable reducing agent (early applications employed Fe(II) sulfamate, more recently U(IV) or HN2OH/N2H4 have been employed as reducing agents). The resulting pure Pu product is well suited to further purification to the metallic state or the creation of MOX ceramic fuel in combination with fresh UO2. Most of the uranium is removed from the TBP phase by reducing the acidity; the remainder is scrubbed with complexants to allow the extractant to be recycled back to the head end of the process. Because the isotopic distribution of the uranium is altered during its time in the reactor, this material is not at present recycled back to the MOX fuel, but rather stored for future use. The PUREX process has seen more than 50 years of process improvements and remains today the standard for Pu/U recycle. Various adaptations of this extraction system include options for extraction of Np(VI or IV) and the preparation of mixed U/Np/Pu mixtures to increase the proliferation resistance of the fissile material.

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