EXPERIMENTAL AND COMPUTATIONAL STUDY OF STATIC SOLIDIFICATION OF MOLTEN FLUORIDE SALTS FOR REACTOR COOLANT APPLICATION by Louis J. Chapdelaine A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science (Nuclear Engineering and Engineering Physics) at the UNIVERSITY OF WISCONSIN-MADISON 2017 i 1 Abstract A series of experiments were conducted to measure various FLiBe and FLiNaK thermophysical properties, quantify supercooling behavior, and provide solidification modeling validation data. Both fluoride salt purities and compositions were characterized using ICP-MS elemental analysis. DSC analysis was conducted to measure specific heat capacity, latent heat of fusion, and solidification temperature for both FLiBe and FLiNaK using ASTM standard methods. The specific heat capacity measurement was unsuccessful based on the non- physical negative heat flow measurements. The latent heat measurements for FLiBe and FLiNaK were 424.7±6.2 kJ/kg and 412.5±3.3 kJ/kg. The solidification temperature measurements for FLiBe and FLiNaK were 460.8±0.1°C and 465.9±0.1°C respectively. The Density Measurement Experiment was conducted to measure the temperature-dependent density of molten FLiBe and FLiNaK using the Archimedean principle of buoyancy. The density correlation for FLiBe from solidification (~460°C) to 700°C was determined to be 휌 = 2241.6 − 0.42938 ∗ 푇 in kg/m3 with an error of ±0.22%. The density correlation for FLiNaK from solidification (~460°C) to 700°C was determined to be 휌 = 2485.8 − 0.69451 ∗ 푇 in kg/m3 with an error of ±2.7%. The Static Solidification Experiment was designed to characterize FLiBe and FLiNaK supercooling behavior, measure the solidification front propagation, and collect solidification model validation data. It required heating a fluoride sample in a stainless-steel flask to a uniform starting temperature, then transferring it into a cooling control bath. The multi-point thermocouple in the sample then collected spatial temperature data as the sample solidified. Due to significant experimental difficulties like heating element failure and nitrate salt vapor ii pressure in the glovebox, only two experimental trials were conducted. The degree of supercooling was measured as 1.8±0.1°C and 1.9±0.1°C during the two runs. Multiple trials under the same conditions may be necessary to account for the stochastic nature of supercooling. The experimental trials indicated that an isothermal condition for the sample cannot be reached with the current experimental setup. A COMSOL solidification model was developed to conduct pre-predictions of the Static Solidification Experiment and compare to experimental data. The model was developed to include laminar flow, buoyancy, conduction, radiative heat transfer, and phase change physics with appropriate initial and boundary conditions. A mesh refinement study was conducted to determine the optimal mesh size, and a radiative heat transfer sensitivity study was conducted to determine the effect of including radiative heat transfer physics. Phase change and radiative heat transfer benchmarking exercises were performed independently to validate the solidification model accurately implemented the necessary physics. The pre- predictions of the Static Solidification Experiment trials were conducted, which emphasized the inability to reach the expected initial experimental conditions. iii 2 Table of Contents 1 Abstract ........................................................................................................................... i 2 Table of Contents .......................................................................................................... iii 3 Figures.............................................................................................................................v 4 Tables ........................................................................................................................... vii 5 Acronyms and Abbreviations ..................................................................................... viii 6 Acknowledgements ....................................................................................................... ix 7 Copyright Information .................................................................................................. ix 8 Introduction .....................................................................................................................1 9 Background Information .................................................................................................4 10 Literature Review............................................................................................................9 10.1 Fluoride Salt Solidification Studies ............................................................................9 10.1.1 Phase Diagram Studies ........................................................................................9 10.1.2 Fluoride Salt Solidification Related Studies ......................................................11 10.2 Fluoride Salt Thermophysical Properties ..................................................................15 10.2.1 FLiNaK Properties .............................................................................................15 10.2.2 FLiBe Properties ................................................................................................17 10.3 Supercooling Phenomenon ........................................................................................20 10.3.1 General Phenomenon .........................................................................................20 10.3.2 Molten Salt Supercooling ..................................................................................26 11 Pre-Experimental Work ................................................................................................29 11.1 Glovebox Setup .........................................................................................................29 11.2 DAQ and Control System .........................................................................................32 11.3 Fluoride Salt Preparation ...........................................................................................34 11.4 ICP-MS Analysis ......................................................................................................38 12 DSC Analysis ................................................................................................................41 12.1 Methodology .............................................................................................................41 12.1.1 Specific Heat Capacity [45] ...............................................................................42 12.1.2 Latent Heat of Fusion [46] .................................................................................44 12.1.3 Solidification Point [47] .....................................................................................45 12.2 Experimental Equipment ...........................................................................................46 12.3 Results .......................................................................................................................52 13 Density Measurement Experiment ................................................................................57 13.1 Methodology .............................................................................................................57 13.2 Experimental Setup ...................................................................................................59 13.3 Results .......................................................................................................................67 14 Static Solidification Experiment ...................................................................................71 14.1 Experimental Setup ...................................................................................................72 14.2 Results .......................................................................................................................80 15 Solidification Modeling ................................................................................................85 15.1 Modeling Methodology .............................................................................................85 15.1.1 Model Components ............................................................................................85 15.1.2 Sensitivity Studies ..............................................................................................91 iv 15.1.3 Benchmarking Exercises ....................................................................................93 15.2 Results .....................................................................................................................102 16 Conclusions .................................................................................................................107 16.1 Summary of Outcomes ............................................................................................107 16.2 Future Work ............................................................................................................109 17 References ...................................................................................................................112 18 Appendix A: Absorption Coefficient Calculation ......................................................115 19 Appendix B: Glovebox Design and Flow