Computational fluid dynamics model for a C-shaped sodium hydroxide freeze valve

Freeze valves are safety components implemented in molten salt reactor (MSR) designs that utilize a salt’s phase change to isolate two parts of a system from each other. The correct prediction of the behavior of a freeze valve is crucial to the safe operation of MSRs. A freeze valve made from a nickel (Ni-201) tube bent into a C-shape and filled with sodium hydroxide (NaOH) is studied experimentally at the MS1 loop facility at Seaborg Technologies. The molten salt initially fills the freeze valve and is cooled by means of an external air blower and solidifies to close the valve. A computational fluid dynamics code STAR-CCM+ is applied to model the thermodynamic and flow properties of the freeze valve. The model accounts for both conjugate and radiative heat transfer, utilizing a volume-of-fluid method with an enthalpy-porosity-based phase change numerical method. The influence of impurity content on the salt’s melting point is modelled. Experimental temperature measurements are used to determine the model parameters, and the simulation results show a good agreement with the experimental data. Correlation coefficients between the experimental and simulation data are above 0.996, with the average absolute value of their difference being approximately 5 °C. Our sensitivity analysis shows that the performance of the freeze valve is strongly dependent on the latent heat and the flowability threshold of the salt used by the software to model salt dynamics in the mushy zone.