DEVELOPMENT OF LARGE SCALE HIGHLY VISCOUS FLUID-STRUCTURE INTERACTION CALCULATION METHOD FOR CLARIFYING SEDIMENTATION BEHAVIOR OF FUEL DEBRIS IN UNIT 3

抄録

The 3D reconstructed image results of the inner pedestal region of Unit 3 at the Fukushima Daiichi Nuclear Power Plant underscore the critical need to understand the interaction between highly viscous corium and large-scale structures. This interaction is essential for accurately estimating the distribution of fuel debris within Unit 3. In this study, Fluid-Structure Interaction (FSI) solver is developed to calculate these interactions. Moving Particle Hydrodynamics (MPH) with an implicit pressure and viscosity solver is employed to model both the highly viscous fluid and structural components. The governing equations are modified to ensure that both fluid and structure maintain angular momentum, which is vital for accurately representing rigid body orientation. The structural material is modeled as a rigid body and calculated using Passively Moving Solid (PMS) methods. A penalty method, incorporating springs, dashpots, and friction sliders, is applied to simulate solid-solid interactions, similar to the Discrete Element Method (DEM). Phase changes between solid and fluid are modeled by adjusting the dynamic viscosity value. For verification and validation (V&V), several benchmark studies, including rigid body dam failure scenarios, were conducted. The results demonstrate the method's capability to accurately reproduce rigid body motion, both in terms of translation and orientation. Light rigid bodies were splashed by water, while heavier ones sank due to the density ratio. In melting scenarios, the method conserved angular momentum, successfully reproducing the rotational motion of complex rigid body geometries. Overall, the robustness and efficiency of this calculation method suggest its potential for simulating large-scale corium-structure interactions.