The Effect of Complex Forming Cations on the Thermophysical Properties of Beryllium and Uranium Fluoride Salts for Nuclear Reactor Applications

Molten beryllium and uranium containing fluoride salts, such as NaF-BeF2-UF4-ZrF4 and NaF-BeF2 are examples of fuel solvent and heat transfer salts used in molten salt reactor designs. To observe the behavior of these salts and to ascertain the mechanisms behind the formation of ionic complexes present in their molten state, this work used high temperature rheology and hydrostatic density methods to measure thermophysical properties. Similar to modeling literature, two regions of viscosity were identified: one below 60 molar percentage of complex forming cations, where it is hypothesized that viscosity is driven by the diffusion of small ionic fragments, and one above where it is hypothesized the degree of polymerization of the complexing cation and network formation drives the increase in viscosity.