Hexavalent group actinide separation: exploratory research with copper (III) periodate

Abstract

More significant development of nuclear energy will require scientific advances in the proper management and disposal of used fuel. To reduce the long-term radiotoxicity hazard associated with nuclear waste, the most viable approach is the closed nuclear fuel cycle where actinides, namely U, Np, Pu, and Am, are recovered for transmutation into short-lived radioisotopes. If transmutation of actinides is to be completed, these actinides need be selectively recovered from fission products prior to fast reactor fuel assemblies. Separation of americium from trivalent lanthanides and trivalent actinides is one of the most difficult elemental separation steps due to their similar chemical behavior. One separation method under consideration is oxidization of americium to the hexavalent state, forming a linear dioxo cation (AmO22+). Forcing americium to the hexavalent state provides a path for achieving a group hexavalent actinide separation from lanthanides, trivalent actinides, and remaining fission products. This work evaluates Cu3+ periodate as an oxidant to accomplish a group hexavalent actinide separation with diamyl amylphosphonate (DAAP) and N,N-dialkylamide (monoamide) extractants. Oxidation of Np, Pu, and Am in molar nitric acid and subsequent extraction by DAAP was demonstrated using radiotracer methods and UV-Vis-NIR spectroscopy. The UV-Vis-NIR measurements showed Am3+, AmO2+, and AmO22+ species were present during extraction process while NpO22+ and PuO22+ displayed more stability in solution. Due to AmO22+ reduction, phase contact times were limited to 5 seconds to obtain satisfactory extraction values. The interaction of Cu3+ periodate with other metals encountered in the fuel cycle was also studied, and found to inhibit AmO22+ extraction as well as form precipitates. Straight and branched acyl chain monoamides were synthesized to study their selectivity towards hexavalent actinides. The branched monoamides were shown to be selective for PuO22+ over Pu4+. A UV-Vis analysis of organic phase Pu4+ and PuO22+ showed two species are extracted by straight and branched monoamide ligands. One species was a nitrato charge neutral species extracted by monoamide, and the second were anionic species extracted by a protonated monoamide. Desirable AmO22+ extraction values were achieved using straight chain monoamides pretreated with an oxidant while branched monoamides showed room for improvement.