A Fully Lagrangian Approach of Three-Phase Systems for Debris Bed Formation

Abstract

In a severe accident, if the high-temperature molten core is not adequately cooled, it can erode the concrete in the reactor cavity and be discharged outside the reactor. One of the strategies for cooling molten core is the pre-flooding strategy, which cools the core by filling the reactor cavity with coolant in advance. In the pre-flooding strategy, the high-temperature molten core comes into contact with the coolant in the cavity and is fragmented. The fragmented particles settle and form sediment at the bottom of the cavity. During this process, a complex flow is generated due to natural convection and bubble-induced flow caused by the decay heat from the corium-coolant interaction. This complex flow not only affects the trajectory of particle sedimentation but also influences the debris bed formation. The shape of the debris bed is a critical factor in determining the coolability inside the cavity, including the interface with the coolant and the contact area with concrete. Therefore, in order to design a safer nuclear reactor, it is necessary to analyze the complex flow that affects the shape of the debris bed through multiphase flow analysis. In this study, momentum exchange between fluid and bubbles is achieved through an unresolved coupling between the Discrete Bubble Method (DBM) and the Moving Particle Semi-implicit method (MPS). And momentum exchange between fluid and solid particles is achieved through unresolved coupling using the Discrete Element Method (DEM) and MPS. Each method has been verified by comparing it with various experimental and analytical solutions. The proposed method was validated with experiments to examine the effect of bubble-induced flow on the shape of the debris bed. As a result of comparing with experiment, the trends in sediment shape according to the particle size and bubble flow rate shown in the experiments were well consistent, and it is sufficiently reliable considering the uncertainty that existed in the experiment. Considering the irregularly shaped debris particles that occur in severe accidents, simulations were conducted to investigate the effect of sphericity, particles with the same size but different sphericity were deposited into a tank with a bubble column, and the deposited mass of the particles was observed along the distance from the center. The results indicated that the effect of sphericity on debris bed formation did not show a specific trend, and the impact was very small. The MPS-DBM-DEM method proposed in this study is expected to lead to a more realistic and practical step in three-phase flow analysis that considers the large number of particles and bubbles that occur in actual serious accident phenomena.