One of the original systems for spent nuclear fuel reprocessing, plutonium uranium reduction extraction (PUREX) is a large-scale solvent extraction process in which the organic phase is composed of tributyl phosphate (TBP) in kerosene and the aqueous phase is comprised of nitric acid. Both uranium (U) and plutonium (Pu) are extracted into the organic phase by nitrate and TBP complexation. Pu is then chemically reduced to its trivalent state and back-extracted into the aqueous phase, thereby separating the two.<br/><br/>This process, however, is complicated by several other elements present in the spent nuclear fuel. Of particular interest for this work is technetium (Tc), particularly Tc-99, a high yield fission product of U-235 with a half-life of 211,000 years. Tc has a wide range of oxidation states from 1- to 7+, which results in complicated redox chemistry that is induced by the radiolysis that occurs from the high radiation fields of spent nuclear fuel. It is also sensitive to the reductants and oxidants used to control the oxidation states of U and Pu to tune their extraction and back-extraction behavior. These factors, as well as the presence of other high yield fission products such as cerium (Ce) and zirconium (Zr), result in Tc being extracted into the organic phase with uranium as either the pertechnetic acid-TBP complex or mixed Tc-metal-TBP complexes. The exact speciation of these complexes has not yet been determined.<br/><br/>This work focuses on understanding the Tc speciation that occurs under PUREX-like solvent extraction conditions. Current investigations focus on cataloging the ways in which Tc, Ce, Zr interact and complex with the TBP-containing organic phase, both separately and together in solution, and what conditions are required for these complexes to form. TBP is known to degrade under radiolysis into dibutyl phosphate, monobutyl phosphate, and phosphoric acid, so current studies also probe the influence of these degradation products on Tc-metal-TBP complexation and on the extractability of high yield fission products into the organic layer.

Investigations of technetium-metal and technetium-metal-TBP binding in PUREX-like solvent extraction conditions

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