ASYST4.1 validation for gas cooled SMR applications using the HTTF experiment

Small Modular Reactors (SMRs) based on High Temperature Gas Cooled Reactors (HTGR) concepts are being considered for remote communities in Canada (power ∼ 5 MWe or less) and elsewhere. One such design is Micro-Modular Reactor (MMR) from USNC (Ultra-Safe Nuclear Corporation). It uses helium as primary coolant and molten salt as intermediate coolant and for thermal energy storage (TES). The molten salt can then be used to generate steam on-demand to match the local communities’ energy requirements. Reliable computational tools are needed for thermal hydraulic analysis of the integrated nuclear and molten salt systems. ASYST4.1 (Advanced SYStem Thermal system) is one such tool which has been recently validated for Solar Salt (40 % NaNO3 + 60 %KNO3) applications and has capability to simulate the helium coolant and reactor core behaviour. To enable the code to model the primary and intermediate loops of MMR there is a need to validate the code for helium coolants and its capabilities to simulate lateral conduction, natural circulation, and radiative heat transfer. The HTTF (High Temperature Test Facility) experiment PG-26, a Depressurized Conduction Cooldown transient, is used for this validation exercise. The results are compared with those of the experiment and those from RELAP5-3D calculations. The reference case and sensitivity analyses for the important parameters are presented. Mass flow rate and temperatures in the primary loop are compared for the steady-state and for the transient PG-26. For the initial assessment ASYST4.1 predictions are found in good agreement with RELAP5-3D, but these results deviate from the experiment. Significant improvement in simulation results is observed when simulations consider the entire trajectory of the warm-up period and of the experiment, indicating that the experiment was not in a steady-state condition at the time of the depressurization test. This is an important factor to be considered in all HTTF cases going forward.