UC Berkeley

Molten actinide containing halide salts are used to fuel molten salt nuclear reactors. Before these reactors can be deployed, it is necessary to characterize the thermophysical properties of these fuel salts for fuel qualification, licensing, and technological applications on the front end (supply chain), reactor operations (maintenance), and back end (end of life) of their fuel cycles. To connect experimental property measurements to simulation and thermodynamic predictions, it is also necessary to understand the mechanisms behind this thermophysical property behavior. In this dissertation, internal structure is used to understand macroscopic properties to better understand how occurrences in a fuel system may affect reactor behavior.Activation energy of viscosity in halide molten salts is shown to scale with increasing concentration of strong complex-forming cation species. This amount by which this parameter scales is dependent on the degree of polymerization and coordination number, which in turn depend on oxidation state of each species, of each complex-forming cation and resulting size of the oligomeric species present. This behavior is demonstrated in beryllium and uranium containing fluoride and chloride fuel and heat transfer salts. Coordination number and network formation are used to predict the viscosity behavior of a molten chloride fuel salt system at an equilibrium state in the reactor with fission and transmutation products present.Ideal thermal expansivity, calculated through molar volume additivity, is shown to be better fitted by a linear density assumption rather than a linear molar volume assumption. For fuel salt systems containing uranium fluoride or chloride, the redox potential of the salt, and thus the U3+/U4+ ratio can affect the measured density. This sensitivity is demonstrated through ideal density trend calculation of NaCl-UCl3 and NaCl-UCl4 melts.Similar to observations of the Molten Salt Reactor Experiment fuel salt, FLiBe is measured to have a ~3% volume shrinkage upon freezing. Imaging of solid FLiBe also revealed strong preferred orientation in its internal structure of its unit cell axes. This results in anisotropy and directionally varied strain during thermal expansion.The molten salt characterization in this work reports new data regarding the properties of fuel and heat transfer salts for advanced molten salt reactor fuel qualification and operation. The thermophysical property uncertainty propagation calculated as well as the sensitivity analysis of fuel salt properties provides information for nuclear reactor safety systems and parameters such as freeze valves and tolerance ranges. The fundamental science exploration of the structural mechanisms of molten salt thermophysical properties deepens the understanding of molten salt behavior and provides validation data for simulated predictions.