Multifluid Flow Dynamics of Core Relocation in Pressurized Water Reactors

Nuclear accidents such as those at Fukushima Daiichi, Three Mile Island, and Chernobyl emphasize the urgent need to enhance nuclear safety. Accurately predicting severe accident (SA) phenomena, including critical heat flux (CHF), loss of coolant accidents (LOCA), and core relocation (CR), is vital for mitigating such catastrophic events. Multiphase models with advanced interface capturing techniques are essential for understanding the complex flow dynamics underlying these phenomena. Our previous research focused on the influence of corium composition on flow dynamics during core relocation within a compressible, non-isothermal 2D domain. This study extends this analysis by investigating a broader spectrum of flow conditions, encompassing both compressible and incompressible regimes. Also, this investigation incorporates 3D modeling to provide a more realistic representation of the complex phenomena involved. Employing the compressive advection interface capturing method (CAICM), this study examines corium flow dynamics and the intricate behavior of material interfaces. The aim of this work is to investigate the dynamics of corium flow in post-CHF events using CAICM at different flow conditions. In this research, a novel higher-order flux limiter and adaptive unstructured mesh algorithm were applied to capture the multifluid interface during post-CHF events in pressurised water reactors. To simulate diverse reactor conditions, the investigation considers scenarios involving both two-material and five-material systems within both 2D and 3D domains. Results indicate substantial differences between compressible and incompressible flow regimes. The flow systems with 2-material components, which exhibited symmetric flow patterns and predictable collapse times to the lower plenum with bulk corium viscosity and velocity of 6.5 kg.(m.s)−1 and 0.42 ms−1, respectively. In contrast, 5-material systems displayed complex, asymmetric flow dynamics under the same conditions with bulk corium viscosity and velocity of 3.18 kg.(m.s)−1 and 0.54 ms−1, respectively. Three-dimensional simulations revealed further asymmetry, highlighting the importance of accurate spatial resolution in modeling. These findings highlight the critical role of multiphase models with interface capturing capabilities in analyzing core relocation phenomena. The CAICM approach offers a robust framework for advancing the understanding of SA dynamics and contributes to the development of more effective nuclear safety measures.