Fission Product Thermodynamics, Solubility, and Extraction in Molten Salts and Lead

Chemical behavior of fission products in systems relevant to two major advanced nuclear reactor systems, namely the lead-cooled fast reactor (LFR) and molten-salt reactor (MSR), has been studied with a heavy emphasis on thermodynamics. The behavior of fission products in molten salts is examined from a theoretical perspective, with an emphasis on the standard state used to define the thermodynamic activity. A standard state extrapolated from infinite dilution behavior, which is fairly common practice in electrochemistry, is found to facilitate faster, easier, and simpler analysis while retaining much of the accuracy of the complex Modified Quasichemical Model. Inductively-coupled plasma mass spectroscopy (ICP-MS) methods for elemental analysis of molten salt samples are optimized and it is found that a method using internal standards can achieve comparable accuracy to the much more time-consuming method of standard addition (approx. 1\% relative error vs. approx. 2\% relative error in the mean of 8 measurements of a single sample). Solubility of metallic fission products in molten lead and their dissolution behavior therein has been studied with the aim of contributing data usable for source term calculations relevant to LFRs in an accident scenario. Expected fission product inventory is generated by a computational model and compared to experimental data. Despite questions about whether the solubility limit was reached, the data indicate that lanthanide fission products and barium are likely soluble at concentrations exceeding the expected fission product inventory over temperature ranges expected in an LFR accident scenario. Transient studies also indicate that lanthanide fission products are likely to dissolve in molten lead on the order of hours. Thermodynamic tools are used to analyze precipitation methods in molten salts and experimental results are compared to theoretical predictions. On the basis of experimental and theoretical results, as well as literature data on the behavior of fission products not studied in this work, recommendations are made and a potential flowsheet is developed for reprocessing spent fuel salt from a magnesium chloride-based molten salt reactor.