The research conducted was a feasibility study using Lithium Fluoride-Beryllium Fluoride (LiF-BeF2) or FLiBe as a coolant with a natural uranium neutron spallation source applied to an accelerator driven sub-critical molten salt reactor. The study utilized two different software tools, MCNPX 2.6 and FLUENT 12.1. MCNPX was used to determine the neutronics and heat deposited in the spallation target structure while FLUENT was used to determine the feasibility of cooling the target structure with FLiBe. Several target structures were analyzed using a variety of plates and large cylinders of natural uranium with a proton beam incident on a Hastelloy-N window. The supporting structures were created from Hastelloy-N due to their anti-corrosive properties of molten salts such as FLiBe and their resistance to neutron damage. The final design chosen was a "Sandwich" design utilizing a section of thick plates followed by several smaller plates then finally a section of thick plates to stop any protons from irradiating the bottom of the target support structure or the containment vessel of the reactor. Utilizing a proton beam with 0.81 MW of proton beam power at 1.35 mA with proton kinetic energies of 600 MeV, the total heat generated in the spallation target was about 0.9 MW due to fissions in the natural uranium. Additionally, the neutrons produced from the final design of the spallation target were approximately 1.25x1018 neutrons per second which were mainly fast neutrons. The use of a natural uranium target proved to be very promising. However, cooling the target using FLiBe would require further optimization or investigation into alternate coolants. Specifically, the final design developed using FLiBe as a coolant was not practically feasible due to the hydraulic forces resulting from the high flow rates necessary to keep the natural uranium target structures cooled. The primary reason for the lack of a feasible solution was the FLiBe as a coolant; FLiBe is unable to pull enough heat generated in the target out of the target structure. Due to the high energy density of a natural uranium spallation target structure, a more effective method of cooling will be required to avoid high hydraulic forces, such as a liquid metal coolant like lead-bismuth eutectic. / Master of Science

This work 1) draws insights on molten salt structure and properties directly from ab initio molecular dynamics (AIMD) simulations, 2) demonstrates the advantageous symbiosis of computational and experimental collaborations on molten salt research, and 3) simultaneously generates ab initio data sets for fitting an interatomic potential model for classical molecular dynamics (MD) simulations. This work discusses the motivations for AIMD simulations of molten salts, thermophysical properties and structural characteristics of interest, advanced methodologies for AIMD simulations, and several completed AIMD studies on molten salts. Of import are the methodological contributions of this work to AIMD simulations, primarily the radical increase in generalized gradient planewave energy cutoff used to more accurately model the electron distribution across a highly-polarizable molten salt. Cutoffs of up to 2500 Rydbergs are used in this work, but 2000 Rydbergs is found to be sufficient for most AIMD NpT modelling of molten fluorides. The equilibrium liquid density of eutectic FLiNaK as a function of temperature is found to agree with the experimental density reported by Chrenkova et al. to within 0.2%, and the equilibrium liquid density of eutectic FMgNaK is found to agree with experimental measurements reported herein to within 4%. Self-diffusion coefficients in FMgNaK are also considered, with applicability to other halide salts. Molybdenum, Cesium, iodide, nickel, hydrogen, oxide, and uranium complexation are examined. It is found that solvation strength can be qualitatively determine via AIMD simulations, and that poorly solvated solutes will minimize the surface area of interaction with the salt solution. Cesium in particular is shown to be volatile or retainable in FLiBe at 500, 650, and 800 ËšC based on complexation and validated experimentally. It is shown that the chemical potential of an anion varies between melts as influenced by the different cations present in each melt. Hence, attempts to use a common electrochemical reference reaction for different salt mixtures are at best an approximation.