Computational fluid dynamics for in-vessel retention: Challenges and achievements

During a severe accident in a nuclear reactor, core damage occurs and may lead to the formation of corium, followed by relocation to the vessel lower head. The decay heat released by the corium can threaten the integrity of the vessel, if no effective cooling mechanism is in place. In-Vessel Retention (IVR) is a severe accident mitigation strategy that has been shown to work for low-to-intermediate power reactors. For high power reactors, many uncertainties still exist. In an attempt to remove some of these uncertainties, the European H2020 IVMR project was launched in 2015. The focus of this project is on obtaining additional, necessary, experimental data in order to improve on current modelling strategies. One of the modelling strategies investigated is the potential use of CFD codes in assessing the feasibility of IVR for high power reactors. The main focus of the CFD studies is on two important aspects of IVR: the focusing effect due to the presence of a metallic layer on top of the corium pool and the heat flux distribution on the vessel wall. These aspects are analysed by studying the thermal hydraulics of a thin metal layer and that of a homogeneous pool. In this paper, first the used codes and numerical approaches are presented. The numerical models are subsequently assessed by comparing numerical results with relevant simulant-based experimental data, resulting in general good results. The codes are then used to perform exploratory computations under prototypical conditions. While the behaviours of water and prototypical materials are similar for the oxide pool, significant differences are observed for the metallic layer.