IVMR modelling with transient effects during molten pool formation and stabilization – Outcomes from models’ comparison performed in the IAEA CRP J46002

In 2020, the International Atomic Energy Agency (IAEA) has started the 4-year Coordinated Research Project (CRP) on In-Vessel Melt Retention (IVMR) with the main objective to harmonize the international understanding of the scientific and technological bases underpinning crucial parts of the safety demonstration of this Severe Accident (SA) management strategy. This strategy consists in maintaining the degraded reactor core (corium) within the vessel by ensuring its cooling thanks to cavity flooding and power extraction through the vessel wall.

In the scope of this CRP, analytical benchmarks were performed focusing on different reactor designs. Among them, a generic 1000MWe PWR design benchmark was set up with a different objective compared to those dedicated to a given scenario and reactor. Its main purpose was to allow detailed comparison of models implemented in capable codes (either integral SA code or dedicated code) based on prescribed and simplified configurations. This paper presents the work done and achievements obtained within this benchmark. Different cases, corresponding to different corium configurations and boundary conditions, were developed with increasing complexity. In total, eight benchmarks are studied covering molten pool formation from solid particles, progressive corium relocation to the lower plenum, progressive molten steel incorporation, vessel wall ablation and possible stratification inversion. Together, they form an efficient and useful tool to better understand IVMR results and code capabilities or limitations. State of codes performance and remaining issues are discussed, focusing mainly on configurations involving progressive molten material arrival in the lower plenum.