Dissolution behavior and reaction mechanism of strontium oxide in molten salts for used nuclear fuel management

Abstract

Sr-90 is one of the most heat-generating fission products in used nuclear fuel. Because of its chemical stability as an oxide, it is tricky to separate Sr from used nuclear fuel without separating nuclear materials. This paper reports on the thermodynamic behavior of SrO in molten salt systems. This paper studies the respective influences of salt composition and temperature on the dissolution behavior of SrO in molten salts. The results reveal that the solubility of SrO in LiCl and MgCl2 was measured much higher than the solubility in the other molten salts, including NaCl, KCl, LiF, and KF. The difference is caused by the reaction spontaneity for SrO to react with the salt to form Sr4OCl6. The formation of Sr4OCl6 was confirmed using X-ray diffraction, Raman spectroscopy, and extended X-ray absorption fine structure, while any stable strontium oxyfluoride phase was not found in the fluoride systems. The structural stability of oxyhalides calculated from density function theory explains the reason for the different behavior. The results can be utilized to understand the behavior of SrO in pyroprocessing and design a molten salt leaching process to separate high heat-generating fission products without proliferation issues, thereby reducing the environmental footprint during final waste disposal.