Thermochemical Modeling in Molten Fluoride Salts for Radionuclide Speciation

Abstract

An important aspect of the licensing process for nuclear reactors is providing a reasonable assurance of safety to the general public. This includes modeling potential radionuclide releases from the reactor during normal operations and accident scenarios, which is known as the reactor's source term. A new class of advanced (non-LWR) reactors are being developed which use molten salts as the coolant fluid. Because the molten salt coolant represents a credited barrier for radionuclide transport between the fuel and the environment, a necessary aspect of mechanistic source term (MST) analysis for the KP-FHR is modeling the thermochemistry of molten salts. Provided here is a review of the theory of the thermodynamic principles governing multicomponent phase equilibria, the background of molten salt thermochemistry research, and a summary of the thermochemical data relevant to the KP-FHR coolant salt, Li2BeF4, commonly referred to as "FLiBe". A review of literature is followed by a brief introduction to methods that can be used to model the thermochemical behavior of molten salt mixtures. The methodology outlined is based on the use of a commercial thermodynamic modeling software called FactSage, which is one of only a few available softwares based on the modified quasichemical model (MQM), which is the recommended solution model for molten salts.

Depletion-driven thermochemistry of molten salt reactors: review, method, and analysis

“…Even more recently, the issue of depletion driven thermochemistry or chemistry-integrated fuel evolution was highlighted by the U.S. DOE's Advanced Reactor Technology-Molten Salt Reactor Program (MSRP) (Shahbazi, 2022a). An excellent and thorough review of thermochemical modeling for radionuclide speciation in molten salts was completed and is highlighted here by the authors for the inquisitive reader (Shahbazi et al, 2021). Lastly, radionuclide species transport in the MSRE was conducted albeit without coupling depletion and thermochemistry in the analysis (Shahbazi et al, 2022b).…”

Validation of NEAMS Tools Using MSRE Data

“…The corresponding density values at 650 ºC are only around 1% different from each other, leading to the belief that fission product and actinide evolution in a low power MSR such as the MSRE may not significantly change the density of the fuel salt solution over 10 years. Nonetheless, the impact of this magnitude of cumulative composition differences, and more importantly the effect of adding small amounts of many different elements, must be investigated for all thermophysical properties in addition to thermochemical calculations such as multicomponent phase equilibria [18].…”

Survey of Relevant Data from the MSRP to Guide Development of MSR Chemistry Modeling Benchmarks

“…Aside from accident scenarios, the purity of the helium cover gas during MSRE was assessed for contaminants with mass spectrometric analysis of samples throughout, as seen in Table 2-6. Finally, hydrogen solubility in salts is import as tritium is produced in larger quantities in some of these reactors; this was recently summarized in the context of bubble and radionuclide transport in MSRs and FHRs [22,23].…”

“…As expected, the salt-seeking nuclides display fractions around unity, indicating little to none of these nuclides depart the fuel salt on average. Iodine, which is generally soluble in molten salts [22], does display some volatility compared to other salt seekers (such as the metals which form stable fluorides) as indicated by having fractions near 75-85%. At least part of this may be explained by the substantial portion of iodine radionuclides which are created from precursor decay of tellurium, antimony, and tin isotopes.…”