COMPASS: A multi-physics coupling platform for safety analysis of lead-cooled fast reactors

• A new multi-physics platform COMPASS is developed for LFR safety analysis. • The coupling method is validated against a PWR pin-cell benchmark with errors below 1%. • UTOP analysis for a LFR shows a 9.5% lower peak power with dynamic coupling. • Dynamic power shift leads to a more prompt and stronger negative feedback. • Fixed-power assumption in traditional safety analysis can be conservative. In the safety analysis of advanced nuclear reactors, the multi-physics coupling of neutronics and thermal-hydraulics is of paramount importance, especially under transient conditions. Conventional system safety analysis methods typically rely on a static, idealized power distribution, which can lead to deviations in the dynamic response of the reactor. To address this issue, this work presents the development and validation of a novel transient safety analysis platform—COMPASS (COupled Multi-Physics Analysis System for Safety). The platform achieves a dynamic coupling between the high-fidelity Monte Carlo code Serpent and the system safety analysis code SAS4A/SASSYS-1 through a semi-implicit Picard iteration scheme, enabling the converged update of neutronics and thermal-hydraulics parameters within each transient time step. The coupling method was validated against the literature data of PWR fuel pin multi-physics modeling. The results show good agreement with the reference solution, with maximum errors of key parameters within 1.00%, thus demonstrating the reliability of COMPASS. Finally, the platform was applied to the analysis of an Unprotected Transient Overpower (UTOP) accident in a small lead-cooled fast reactor (LFR). The results indicate that, compared to the conventional method employing a fixed power distribution, considering the dynamic coupling effect significantly reduces the predicted peak power and fuel temperature. This study not only provides a reliable computational tool for the transient safety analysis of LFR but also highlights the critical importance of the Neutronics-Thermal-Hydraulics Coupling effect in LFR safety analysis.