A Comparative Study on Severe Accident Phenomena Related to Melt Progression in Sodium Fast Reactors and Pressurized Water Reactors

Abstract

The nuclear safety approach has to cover accident sequences involving core degradation in order to develop reliable mitigation strategies for both existing and future reactors. In particular, the long-term stabilization of the degraded core materials and their coolability has to be ensured after a severe accident. This paper focuses on severe accident phenomena in Pressurized Water Reactors (PWR) compared to those potentially occurring in future GenIV-type Sodium Fast Reactors (SFR). Firstly, the two considered reactor concepts are introduced by focusing on safety aspects. The severe accident scenarios leading to core melting are presented and the initiating events are highlighted. The paper focuses on in-vessel severe accident phenomena, including the chronology of core damage, major changes in the core configuration and molten core progression. Regarding the mitigation means, the in-vessel retention phenomena and the core catcher characteristics are reviewed for these different nuclear generation concepts (II, III and IV). A comparison between the PWR and SFR severe accident evolution is provided as well as the relation between governing physical parameters and the adopted mitigation provisions for each reactor concept. Finally, it is highlighted how the robustness of the safety demonstration is established by means of a combined probabilistic and deterministic approach.

The Development of a Multiphysics Coupled Solver for Studying the Effect of Dynamic Heterogeneous Configuration on Particulate Debris Bed Criticality and Cooling Characteristics

“…The decay heat generation rate settled for the Fuel particle in this research was referred from the hypothetical scenario of ULOF in an SFR, which implemented a fuel assembly with an inner duct structure (Fuel Assembly with Inner Duct Structure: FAIDUS). The previous studies suggest that the time from the start of core melting to the onset of FCI is about a few minutes, after which the debris bed is estimated to be piled up on the debris catcher of the lower plenum in around one minute, considering the debris falling rate [1,45]. Therefore, the decay heat generation rate of the debris bed in such a short time can be considered equivalent to the decay heat soon after the normal reactor shutdown (6-7% of operating power) [46].…”