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The Effects of Metal Ion Uptake on the Radiolysis of Solvent Extraction Ligands


Used nuclear fuel contains a mixture of highly radiotoxic elements and must be handled and stored with great care due to the potential for environmental damage. As such, special consideration must be given to how long and where the used fuel will be stored. However, used fuel is still quite rich in uranium and plutonium (~96-97% of the initial heavy metal) and therefore can be a viable feedstock for these metals. To minimize the waste volume as well as the storage time scale, various separation techniques to isolate useful material for possible reuse have been proposed [1]. Currently, the most widely practiced method is the Plutonium Uranium Redox Extraction (PUREX) process that was designed to address this issue by utilizing tributyl phosphate (TBP) to selectively extract plutonium and uranium from used fuel by liquid-liquid extraction. More recent efforts have been aimed at the separation of the minor actinides from used nuclear fuel. These minor actinides would still remain in the high-level waste after the PUREX step and would require further extraction steps to be removed. The Actinide-Lanthanide Separation (ALSEP) process, utilizing the solvating extractant N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide (T2EHDGA) as well as the acidic 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) for the co-extraction of lanthanides and actinides from solutions of high nitric acid concentration is currently under development to tackle this problem [2], [3].

Due to the intense radiation of used fuel, solvent extraction processes designed to treat used nuclear fuel experience degradation of the extracting ligands. The products of the degradation can adversely affect the efficiency of the overall extraction process as well as form a third phase or lower the selectivity of the extraction. Prior studies of the radiolysis of TBP without metal contact as a function of various types of radiation have been done [4], [5]. In this study, the degradation is caused by low linear energy transfer (LET) radiolysis and high LET radiolysis (using the 10B (n, α)7 Li reaction) of ALSEP solution with and without nitric acid uptake as well as metal loaded TBP. The results discussed will be of systems of 0.1M TBP/dodecane, 1M TBP/dodecane contacted with nitric acid and varying concentration of uranyl nitrate to observe the effect of increased metal loading. I also establish preliminary degradation constants for the ALSEP ligands HEH[EHP] and T2EHDGA as a function of low and high LET radiation as well as acid uptake.

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