CFD Simulation of Melt Pool Coolability in a Simulated Core Catcher Model

A severe accident involving core melt in a nuclear reactor is a major concern especially after Fukushima. Thus, to mitigate the effects of core melt accidents, an ex-vessel core catcher is being developed for Advanced Indian Nuclear Reactors. The core catcher design envisages using special refractory material. The cooling strategy of the core catcher is one of the key components in the design of the core catcher. Performing a full-scale prototypic experiment is extremely challenging and prohibitory due to the involvement of very high temperature and presence of radioactive materials. Therefore, a computational fluid dynamics (CFD) model capable of simulating the coolability of the melt pool is important to develop. In the present work, a two-dimensional (2D) CFD model was developed to understand the heat transfer phenomenon and solidification of the heat-generating simulant melt pool. The 2D symmetry geometry of the simulated core catcher vessel was used. The CFD model considers appropriate models for melting and solidification to understand crust formation in the melt pool and the k-ε turbulence model to resolve turbulence inside the melt pool. A decay heat of 1 MW/m³ was also considered inside the melt pool. The CFD simulation results were compared with the authors’ experimental results. The experiment involved a scaled-down ex-vessel core catcher model (CCM) employing electrical heaters to simulate decay heat. The experiment was carried out by melting about 25 L of sodium borosilicate glass using a cold crucible induction furnace at about 1200°C and cooling it in the scaled-down CCM. The scaled-down CCM was strategically cooled in three phases, namely, air cooled, indirect side cooling, and complete top flooding. To overcome the complexities of simulation of the initial melt pour condition, the CFD simulation was initialized with the temperatures just after the melt pour was completed in the experiment. Similar to the experimental conditions, the CFD simulations were carried out in three phases by changing the boundary condition. Comparison of the temperatures of the melt pool by the CFD simulations and experiments at different locations gave reasonable agreement. The evolution of crust formation, melt pool temperatures, core catcher inner wall temperatures, and heat flux distribution were investigated in detail using the CFD model.