An overcooling transient analysis in a CAREM-like SMR core using Serpent/Subchanflow

This work investigates a 50-s transient scenario in a CAREM-like SMR (Small Modular Reactor) core using a high-detail neutronic/thermal-hydraulic pin/subchannel model with the coupled tool Serpent/Subchanflow. The transient scenario is defined as a core overcooling, which can be described as a colder coolant front entering the reactor core and producing a rise in power due to the positive reactivity. The reactor core is initially at full power (100 MW), and the perturbations that originate the overcooling scenario are initiated by time-dependent boundary conditions of three variables: the coolant mass flow rate, the core inlet temperature, and the core outlet pressure. In the transient analysis with Serpent/Subchanflow, two different solutions with different time step lengths (0.05 s and 0.1 s) are investigated, and similar results are obtained. The reactor power increases by around 16%, the core average fuel temperature increases by 20 °C, and the core average coolant temperature decreases by 2 . 3 °C. The simulation has been performed in the Horeka supercomputer at KIT, using 760 cores, giving a maximum wall-clock simulation time of 19 days for the shorter time step length. As a final step, key parameters are compared at the beginning and end of the transient against two deterministic assembly-based solutions, namely PARCS/Subchanflow and PUMA/Subchanflow. The comparisons show that the three solutions are consistent one to each other since global parameters such as the power and the core average temperatures are in very good agreement. When comparing peak power values, one can also observe the advantage of the high-detail solution since this provides higher values for the fuel temperatures, void fraction, and lower DNB (Departure from Nucleate Boiling) ratio. • Transient coupled simulation with Serpent and Subchanflow codes. • Fifty-second reactor core overcooling simulation. • A high-detail pin-by-pin model is developed for the coupled simulation.