Numerical benchmarking of Thermal-Hydraulic System Codes on challenging LFR transient scenarios

The inherent safety features of Lead-cooled Fast Reactors (LFRs) make them an attractive solution for responding to the challenges imposed by the increasing energy demand and decarbonization strategies. Due to the limited operational experience for those concepts, simulations with different codes and benchmarks become crucial in understanding dominant phenomena and supporting the reactor design and analysis phases. In the paper, LFR theoretical cases are presented for investigating different System Thermal-Hydraulic Codes (STHs) responses such as RELAP5/Mod3.3, ASYST-LM and ATHLET. For this purpose, a simplified loop model has been developed implementing several boundary conditions, derived from typical transient scenarios, such as the Unprotected Loss of Flow (ULOF), Unprotected Loss Of Heat Sink (ULOHS), and Unprotected Transient Over-Power (UTOP). Particular attention has been paid to thermal-hydraulic phenomena such as the establishment of natural circulation. After comparisons against analytical solutions, the analyses were performed with STHs codes at first without including neutronic feedback. The main reactivity feedback (Doppler, lead density variations, radial thermal expansion) has been then calculated based on LFR theoretical core configurations and implemented in the models to carry out UTOP transients with the ATHLET code. The obtained results will support the verification and validation efforts of the STH codes applied to LMFRs.