Three-Dimensional Modeling of Debris Mixing and Sedimentation in Severe Accidents Using the Moving Particle Semi-Implicit Method Coupled with Rigid Body Dynamics

Particle mixing and sedimentation, related to corium debris bed formation and coolability in severe accidents, is investigated using a new computational fluid dynamics tool: the Analysis of Debris Dynamics and Agglomeration (ADDA) code. ADDA was developed based on an enhanced numerical method combining the moving particle semi-implicit algorithm with a rigid body dynamic model. The analysis successively simulates the entire process of debris bed formation, including corium jet breakup, mixing, and sedimentation. The methodology allows identification of key characteristics in the formation of the corium debris bed. Two-dimensional (2-D) and three-dimensional (3-D) simulations were utilized to model the detailed flow structures and mixing phenomena, along with the final sedimentation process, and were compared to the Q21 QUEOS test performed at Forschungszentrum Karlsruhe, Germany. For the analysis of debris bed formation, it is recommended that full 3-D simulations be utilized to provide enhanced accuracy related to corium debris field prediction. The 2-D simulations were found to be insufficient because of the debris field dependence on particle agglomeration and mixing, prior to debris settling.