Unique Chemical Stability Difference between Uranium Trichloride and Uranium Trifluoride

Abstract

High-temperature chemical stability of molten salts plays a key role in controlling redox reactions between molten salts and structural components in molten-salt applications. The control of redox reactions enables to mitigate the degradation of structural components in molten salt environment. This work investigated the high-temperature chemical stability of molten uranium fluoride and chloride salts, two candidate salts that are being actively considered as fuel salts for molten salt reactors. Highly distinct features about chemical stability of molten uranium chloride salts were identified by comparing the chemical stability of uranium fluoride and chloride salts via phase boundary reaction between U4+/U3+— using first-principles density functional theory (DFT) modeling combing with thermodynamics analysis for both mass transport and charge transfer. The DFT modeling results show that at a representative operation temperature range (800K to 1000K) in a molten chloride salt reactor, a UCl4/UCl3 molarity ratio of 0.01 to 0.02 is expected to be able to prevent the disproportionation reaction of UCl3 into U and UCl4, significantly lower than that for UF4/UF3 molarity ratio (around 2) in fluoride based MSRs. The behavior of the disproportionation of UCl3 versus temperature distinctly differs from that of UF3 salt counterpart. The fundamental mechanism of thermochemical stability of molten salts driven by mass transport and charge transfer unveiled from this study can provide quantitative understanding and guidelines on the salt chemistry control strategy for chloride salt fueled molten salt reactors.

Keywords: Chemical Stability, uranium fluoride and uranium chloride salts, DFT modeling and thermodynamics, molten salt chemistry and thermochemical behavior.