Suppression of third phase in DIDPA/Fe extraction system. Addition of reductants and complexing agents

In a four-group partitioning process, which has been developed, transuranium elements are extracted with diisodecylphosphoric acid (DIDPA) from a high-level radioactive liquid waste (HLW). In a solvent extraction process, a third phase which prevents the separation is formed, when high-dense Fe(III) contained in HLW as a corrosion product is extracted with DIDPA. A variety of reductants and complexing agents were respectively dissolved in 0.038M Fe(NO3)3-0.5M HNO3 solutions. They were extracted with the mixed solvents of 0.5M DIDPA and 0.1M TBP, and the effects of the addition on preventing the third phase formation was examined. Since L-ascorbic acid (0.05M) and sodium thiosulfate (0.05M) reduce Fe(III) to Fe(II), and the distribution ratio of Fe(II) was small, the extraction of Fe was prevented, and the third phase formation could be suppressed. Since ethylenediaminetetraacetic acid (0.05M) and chromotropic acid (0.114M) formed stable hydrophilic complexes with Fe, and 1,10-phenanthroline (0.114M) and 2,2'-bipyridine (0.114M) formed stable hydrophobic complexes with Fe, and the solubilities of the complexes were large, these complexing agents were able to suppress the third phase formation. (author)