COORDINATION CHEMISTRY OF f-ELEMENTS IN THE TALSPEAK PROCESS

Abstract

A key step in the advancement of the nuclear fuel cycle in the U.S. is transmutation of the long-lived activation product actinides (Am3+ and Cm3+). Transmutation is viable once the minor actinides Am3+ and Cm3+ have been separated from the trivalent lanthanide metals which co-exist in the used fuel matrix. The U.S. is considering the TALSPEAK Process as a preferred method to separate the trivalent lanthanide metals from the minor actinides. TALSPEAK balances the selective complexation of actinide by water-soluble aminopolycarboxylate complexes against lipophilic organophosphorus cation exchanging extractant molecules that (under process conditions) preferentially extract lanthanides. TALSPEAK was developed in the 1960s, and has since been successfully demonstrated at scales up to the pilot plant scale, though the process suffers several practical limitations that are likely to make it more difficult to control at full scale. In recent fundamental studies of TALSPEAK, a simplified thermodynamic model indicated significant deviations between predictions based on a conservative thermodynamic model and experimental results. Most troubling has been the observation of a sharply declining relationship between equilibrium pH and distribution ratios when the thermodynamic model predicts a flat to slightly rising dependence. The opposing trends between predicted and observed pH dependence indicate that the state of understanding of the details of this process are inadequate to support recovery from significant process upset conditions. This dissertation addresses the fundamental chemical processes (in both the aqueous and organic phases) that govern the operation of the TALSPEAK process. The project focuses on the thermodynamics of aqueous phase metal ion coordination, on aqueous phase solute phase transfer processes, on organic phase metal ion coordination, and on characterizing the aggregation state of the organic extractant. The goal is to use the combined results of this study, to explain the conflicting results between the thermodynamic model and the extraction results. Increased understanding of the basic chemistry of TALSPEAK will at a minimum increase the feasibility of using this separation system on an industrial scale. In the ultimate extreme, improved understanding of the specific processes involved in conventional TALSPEAK could enable the development of more effective alternative approaches.