A review of technetium and zirconium extraction into tributyl phosphate in the PUREX process

The fission products of technetium and zirconium have historically been problematic in the reprocessing of spent nuclear fuel by solvent extraction using tributyl phosphate (TBP) containing solvents. One of the reasons for this is that the routing of zirconium and technetium becomes difficult to control due to co-extraction mechanisms with other elements/species in the dissolved spent fuel liquors and through alternative extraction pathways that can occur with the presence of solvent degradation products. Consequently, solvent extraction processes based on the PUREX (Plutonium Uranium Redox EXtraction) process incorporate various strategies to ensure these fission products are not present in the final product streams, increasing plant footprints and operational costs. Next generation spent nuclear fuel reprocessing should minimise the need for such scrubbing operations by applying a complete and thorough understanding of the distribution behaviour of technetium and zirconium to optimise the separations chemistry for higher burn up spent fuels from advanced reactor systems that contain higher inventories of fission products. A substantial body of work exists regarding the distribution behaviour of technetium and zirconium with phosphorus based extractants, but studies have tended to be fragmented using various extraction conditions and approaches. This review collates and reviews these data, supporting the development of predictive process models whilst making recommendations for improved control of technetium and zirconium in an advanced PUREX process. Key findings from this review include the significant increase of technetium distribution ratios by coextraction with zirconium. The distribution ratios of zirconium are also seen to increase with increasing technetium concentrations when zirconium is present with technetium through a synergistic effect. To achieve full decontamination of the U/Pu product stream, significant process modifications are required, which can be achieved by the introduction of scrubbing steps or the saturation of the organic phase with uranium. The use of holdback reagents may improve decontamination factors, but further experimental research is required. High acid scrubs to reject technetium are established options and already used at the industrial scale at La Hague (France).