(38b) Overview of Passive Safety Features and Transient Model Validation for the Pebble-Bed Fluoride-Salt Cooled, High-Temperature Nuclear Reactor (PB-FHR)

The Pebble-Bed Fluoride-Salt-Cooled, High-Temperature Reactor (PB-FHR) is an advanced nuclear reactor design using fuel in the form of a randomly packed pebble bed and flibe (Li₂BeF₄) as the primary coolant. One attractive feature of the PB-FHR is the ability for passive cooling during accident scenarios, eliminating the need for external power to recirculate the coolant and remove decay heat from the reactor core. This passive heat removal system relies on natural circulation, which means that coolant flow is driven by the elevation and temperature differences between the reactor core and a heat exchanger acting as a heat sink. At the University of California, Berkeley, we are studying phenomenology that will inform the design and safety analysis of passive heat removal systems. We employ simulant fluids such as Dowtherm A, a heat transfer oil, in place of the fluoride salt coolant. When the relevant non-dimensional numbers are matched between the fluoride salt and the simulant oil, certain thermal hydraulic phenomena can be understood at a reduced geometric and temperature scale, thus allowing for experiments to be easily performed in the laboratory.

We use numerical codes and systems analysis codes to model system transient response, and we design experiments for validation of the system models. FLOWNEX is a systems code that calculates or optimizes flow rates, temperatures, pressures and heat transfer rates in a fluids system. We use it as a tool to model natural circulation in the PB-FHR, as well as experimental natural circulation loops in the laboratory. Comparing computational results between flibe and Dowtherm A and showing that they are equal in a non-dimensional space is a way of proving that Dowtherm A can be used as a simulant fluid for fluoride salts with limited, quantifiable distortions in the results.

The performance of passive decay heat removal systems depends on system-level parameters, but also on component-scale phenomena that we model using the COMSOL Multiphysics finite element analysis software. For instance, we use COMSOL to understand transient natural convection flow of high Prandtl number fluids – such as fluoride salts and Dowtherm A oil – around a hot sphere, and thus predict Nusselt number in a transient regime. Transient natural convection from a single sphere is a simple case, but this can eventually be extended to a packed bed case, which is the fuel configuration in the PB-FHR. We perform scaled heat transfer experiments between the fluoride salt and the Dowtherm A oil to confirm the match in Nusselt number between the two. Experimental data and results from COMSOL are used together to better quantify the difference in Nusselt number between transient and steady state cases. The combination of system-level and component-scale modeling eventually leads to better understanding, designing and optimizing passive safety systems for the PB-FHR.