The In Vessel Melt Retention (IVMR) strategy developed for Light Water Reactors (LWR) is an appealing solution for the mitigation of many Severe Accident (SA) scenarios. Allowing the corium to remain in the Reactor Pressure Vessel (RPV), significantly reduces the stresses on the containment. This approach is typical of innovative reactors like the AP1000 and the Hualong One, both GW-size Pressurized Water Reactors (PWR). In this paper, the IVMR application for integral PWR (iPWR) is investigated. These kinds of reactors are characterized by an electrical power output generally lower than 300 MWe, an integral layout and the use of passive safety systems. During a postulated accidental sequence that leads to fuel melting, the molten corium relocates down to the lower plenum of the RPV. By flooding the reactor cavity where the RPV is immersed, the external face of the vessel is cooled, and corium is maintained inside the lower head. The overall phenomenology occurring during the IVMR in an iPWR is similar to the phenomenology in a high-power LWR, but a few differences can be highlighted due to the different design features. Among the differences, it is interesting to notice that the molten pool is rather shallow and does not occupy a full hemisphere. This has an impact on the heat flux profile along the vessel wall. Another difference is the rather thick oxide crust, for which the modelling approach is adapted with respect to what is usually done for the analysis of high-power reactors. The purpose of this paper is to develop a 0D model for the steady-state analysis of idealized cases in an iPWR. It has been developed in the framework of the SASPAM-SA Horizon Euratom project. It is part of the preliminary activities of Work Package 4, which examines the evaluation of the safety margin for several existing iPWR designs, considering available data and plausible assumptions for the unknown parameters. The aim of this model is to include the specific features of iPWRs and provide first estimates of safety margins. Heat transfer correlations depending on the aspect ratio H/R are used to take into account the shallow geometry of the pool. The absence of a designed channel or baffle around the vessel requires the use of pool boiling correlations for the external cooling. This work will also prepare the next steps of SASPAM-SA, which is the identification of models in SA codes needing some improvements. Integral reactor calculations focused on the IVMR are also planned: they will be compared to the results of the present 0D approach. Finally, we mention that the developed model is not design specific: it may be easily adapted to various designs by changing some geometrical or material parameters. The 0D model was implemented in Python. This tool will be made available for other partners for applications to other designs of interest.

This work has been published in the conference proceedings: Proceedings of the 11th European Review Meeting on Severe Accidents Research (ERMSAR2024).
DOI: https://doi.org/10.5445/IR/1000174165