High-fidelity and high-resolution simulation of two different rod ejection accidents in a NuScale-like small modular reactor with conventional and accident tolerant fuels

This work presents a high-fidelity pin-by-pin simulation approach for a NuScale-like Small Modular Reactor core during a rod ejection accident (REA). We coupled 3D Monte Carlo neutron transport (Serpent), subchannel thermal–hydraulic (SUBCHANFLOW) and fuel performance (TRANSURANUS) codes using the Interface for Code Coupling (ICoCo), which is part of the EU's Salome open source platform. To resolve fuel intra-assembly details, we simulated all the fuel rods and channels, subdividing them into axial slices and transferred calculated data between the codes using scalar fields saved in memory variables. Two different REA scenarios were modelled, and the behaviour of fresh-loaded cores with conventional UO₂ fuel with Zr-4 cladding and accident tolerant fuel (ATF) materials, U₃Si₂ fuel with FeCrAl cladding, were analysed. In both scenarios, the control rod was ejected within 0.1 s, followed by a SCRAM after two seconds. In the first moderate scenario, the control rod ejection occurred at 75% of the nominal power, whereas in the second accident scenario, it occurred at hot zero power (HZP) conditions. In the first scenario, the power increase was around 25%, while in the HZP case it amounted up to 600% and 300% of the nominal power for the core loaded with UO₂ and ATF-fuel and cladding, respectively. Detailed calculations were conducted on a High-Performance Computer (HPC). The results demonstrated the robustness and flexibility of the coupled code system, providing full-core behaviour and rod-level safety parameters and predicting as needed during the safety analysis support of the licensing processes. This paper outlines the system setup, presents rod-level results and underlines the usefulness to assess the performance of SMR-cores loaded with different fuel types under various REA scenarios. In the scenarios considered, we did not observe significant fuel rod deformations, and the core loaded with ATF-fuel and cladding showed a large margin to melting.