Multiphase Modeling of In-Vessel Corium Retention and Corium Concrete Interaction for Nuclear Power Plants

Abstract

Severe accidents in nuclear power plants pose significant safety challenges, with corium behavior and reactor containment integrity being critical concerns. Mitigation strategies, such as in-vessel corium retention (IVCR) and ex-vessel corium retention (EVCR), have been developed to enhance reactor safety by preventing catastrophic containment failure. In a severe accident, core meltdown leads to corium formation, a high-temperature mixture of molten nuclear fuel, structural materials, and control rods. If not adequately cooled, corium may breach the reactor pressure vessel (RPV), leading to Molten Corium Concrete Interaction (MCCI), a process that degrades containment concrete, generates non-condensable gases, and significantly increases the risk of radiological release. Understanding these phenomena is critical for nuclear safety, requiring advanced numerical models to predict corium behavior under various accident conditions.

Among severe accident mitigation strategies, IVCR through reactor vessel external cooling (RVEC) is a widely studied technique to prevent vessel failure by dissipating heat from the RPV lower head. Previous research has typically analyzed IVCR-RVEC as an isolated process, often neglecting the coupled interactions between the internal corium dynamics and external cooling conditions. This study addresses these limitations by developing a conjugate heat transfer (CHT) approach using the enthalpy-porosity method, implemented in OpenFOAM, to simulate corium's melting, natural convection, and solidification processes. The approach was rigorously validated against experimental data, demonstrating its robustness in replicating real-world heat transfer behavior. Further improvements included turbulence modeling using various turbulence models, including the k-ω SST and RNG k-ε models, and sensitivity analyses, which provided more profound insights into IVCR-RVEC performance. The results highlighted the significant influence of turbulence on heat transfer, convective flow patterns, and localized heating effects. However, a key limitation of this model was its inability to account for mass and momentum transfer between different regions, restricting its applicability to more complex accident scenarios.

A multiphase flow heat transfer (MFHT) approach was developed in OpenFOAM to overcome this limitation, and it was numerically calibrated to enhance predictive accuracy for IVCRRVEC simulations. This approach was adapted for IVCR-RVEC and MCCI conditions by incorporating a source term for phase-specific decay heat generation, mass mixing, and multiphase interactions, enabling a more realistic assessment of these severe accident conditions. This study refined key parameters through numerical calibration, improving heat transfer dynamics, corium movement, and phase transition predictions. The MFHT-based simulations provided valuable insights into anisotropic heat and mass transfer processes, which are critical for assessing reactor integrity and containment failure risks.

In the event of RPV failure, corium spills onto the containment concrete, initiating MCCI, where intense heat fluxes cause concrete ablation, generate combustible gases (H2, CO), and increase the risk of containment breach. While previous research has examined MCCI experimentally and numerically, most existing models rely on simplified lumped-parameter methods that do not capture localized thermal and mass transport effects. This study introduces a novel multiphase modeling approach incorporating natural convection, turbulence-driven mixing, and phase change dynamics, offering a more realistic and high-fidelity representation of MCCI phenomena.

The numerical simulations successfully captured critical MCCI phenomena, including anisotropic concrete ablation, crust formation, oxide relocation, and metal penetration into the basemat. The results agreed with experimental data, confirming the model's effectiveness in replicating real-world MCCI behavior. By improving the understanding of corium retention, concrete degradation, and containment integrity, this research substantially advances severe accident modeling and enhances the predictive capabilities of IVCR-RVEC and MCCI simulations.

The validated simulations provide a strong foundation for integrating anisotropic MCCI modeling into broader nuclear safety codes. This study also emphasizes the importance of turbulence effects, material interactions, and decay heat distribution in determining the thermal stability of IVCR-RVEC and MCCI conditions. Implementing the MFHT approach further refines predictive accuracy, making it a valuable tool for severe accident analysis, safety assessments, and emergency response planning.

Future research will further expand this model to analyze multi-layer stratified corium pools, incorporating varying external conditions, coupled chemical reactions, different turbulence models, and advanced computational techniques to improve the accuracy of severe accident mitigation strategies. The findings from this study contribute significantly to nuclear reactor safety, accident management planning, and regulatory decision-making by enhancing predictive capabilities and refining safety margins for next-generation reactor designs.

Date of Award	13 May 2025
Original language	American English
Supervisor	Yacine Addad (Supervisor)