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Understanding the behaviour of heavy metals involved in the nuclear fuel cycle is of paramount importance to the reprocessing and storage of spent nuclear fuel. These studies have attempted to obtain a greater understanding of the fundamental chemistry of these systems, by investigating extraction performance and speciation in current (PUREX) and proposed (GANEX) extraction processes. Various complexes have been shown to exist in the post-extracted organic fraction of the systems analysed. For Zr(IV), U(VI) and Np(VI) separated from aqueous nitric and hydrochloric using TBP, the complexes [Zr(NO3/Cl)4(TBP)4], [UO2(NO3/Cl)2(TBP)2] and [NpO2(NO3/Cl)2(TBP)2] formed, respectively. For Zr(IV) separated from aqueous mixtures of HNO3 and HCl at equal concentration, a preference was shown to [Zr(Cl)4(TBP)4] over the analogous nitrate complex. For U(VI) separated from aqueous mixtures of HNO3 and HCl, a preference was shown to [UO2(Cl)2(TBP)2], even at high aqueous nitrate concentrations. NMR data for Pu(IV) separated from aqueous HNO3, HCl and mixtures of both, using TBP were presented, where possible complexation was observed. It is thought that [Pu(NO3)4(TBP)4] or [PuCl4(TBP)4] species existed within the organic fraction for Pu(IV) separated from aqueous HNO3 and HCl, respectively. These systems showed high distribution ratios where an increase was observed with increasing aqueous acid concentration overall. Distribution ratio data were presented for the lanthanide series separated from aqueous nitric acid, using the proposed GANEX solvent system(s). The lanthanides analysed showed an increase in distribution ratio with increasing aqueous nitric acid concentration and with increasing TODGA concentration in the organic fraction. Heavier lanthanides were observed to give higher distribution ratios overall. The best distribution ratios were observed for lanthanides separated using 0.2 M TODGA with 1-octanol (5 % by volume) over the nitric acid concentration range analysed. For lanthanides separated using 0.5 M DMDOHEMA, an optimum distribution ratio was observed at around 6 M aqueous nitric acid concentration. The distribution ratio data for lanthanides separated from a range of DMDOHEMA concentrations, were observed to increase with increasing organic DMDOHEMA concentration. The distribution ratios observed for isotopes of Np, Am, Eu and Pu separated using 0.2 M TODGA, increased with increasing aqueous nitric acid concentration. The same trend was observed for the aforementioned isotopes separated using 0.5 M DMDOHEMA. However, pertechnetate separated using 0.2 M TODGA from aqueous nitric acid, showed a decrease in the distribution ratios observed over the acid concentration range analysed. This was contrary to pertechnetate separated from aqueous nitric acid using 0.5 M DMDOHEMA, where a small increase in distribution ratio was observed over the concentration range analysed. For Np(VI) separated from some proposed GANEX solvents, the 0.2 M TODGA/0.5 DMDOHEMA combination gave the best distribution of neptunium into the organic fraction. For Np(VI) separated using 0.5 M DMDOHEMA, the complex [Np(DMDOHEMA)2(NO3)4] was observed. Additional attempts to analyse Np(VI) behaviour under GANEX style conditions via EXAFS, were not successful due to immediate reduction of the Np(VI) on the beam line.

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Spent nuclear fuel (SNF) resultant from the generation of nuclear power is a chemically and radiologically diverse system which is advantageous to chemically process prior to geologic disposal. Hydrometallurgy is the primary technology for chemical processing for light water reactor spent fuels, where spent fuel is dissolved in an acid for liquid based separations. The primary means for recovery of desired metals from the SNF solution is liquid-liquid extraction which is based on distribution (partitioning) of the metal ions between two immiscible phases based on thermodynamic favorability. One of the means of increasing this favorability is by designing extractant molecules to be either "harder" or "softer" bases, which will more preferentially extract harder or softer metal ions respectively. This technique is used in designing extractant molecules for targeted extraction as actinides are slightly softer than lanthanides, and precious metals produced in significant quantities from the fission process are especially soft metals. The work performed in this thesis involved the synthesis of a novel soft electron donor organic extractant molecule for testing of targeted soft metal extraction. The molecule synthesized was bis-dibutanethiolthiophosphinato-methane, or S6, a bidentate neutral extractant molecule with significant thiolysis for a softer electron environment. The synthesis technique was refined and the molecule composition and structure was confirmed by ¹H NMR, ³¹P NMR, and elemental analysis. Two metal groups, f-elements (actinides and lanthanides) and soft transition metals were tested for their extractability from nitric acid solutions into an S6 solution in n-dodecane. Aqueous solutions of nitric acid and n-dodecane as an organic diluent are typical liquid-liquid extraction conditions in spent nuclear fuel reprocessing. As extraction experiments were performed with radiotracers, for the soft metal extraction experiment, a mixture of the selected metals was neutron-activated in the OSU TRIGA reactor, as was europium to create a lanthanide radiotracer. Actinides and lanthanides were not seen to effectively extract into the organic or form a precipitate at all, making their partitioning with this extractant seemingly ineffective. Through gamma spectroscopy of an irradiated metal solution post-extraction, it is seen that only silver and palladium preferentially complex in the mixed metal samples into an insoluble organic ligand, dropping out of solution. This effect was more pronounced at higher acid concentrations, but silver was seen to slightly extract to the organic phase at all acid concentrations as well. This testing has shown that the S6 extractant can be used to recover silver and palladium from a mixed metal aqueous solution, such as one resultant from advanced spent nuclear fuel reprocessing operations. This result shows promise for future development of sulfur based organophosphate ligands for targeted extraction of precious metals from solutions. / Graduation date: 2013

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