Large-Scale Ex-Vessel Corium Spreading Calculation Using a Particle Method for Estimating Fuel Debris Distribution in Fukushima Daiichi Unit 1: Effect of Cooling Efficiency on Spreading

Abstract

Estimating fuel debris distribution is one of the indispensable approaches for retrieving fuel debris spread within the Primary Containment Vessel (PCV) in Fukushima Daiichi Nuclear Power Plant Unit 1 (1F1). Recent findings from an internal investigation conducted by Tokyo Electric Power Company Holdings (TEPCO) suggest the possibility that molten corium has spread over a vast region of the PCV due to significant Ex-Vessel Corium Spreading. This study utilized a large-scale Ex-Vessel corium spreading simulation, employing a method that couples Moving Particle Hydrodynamics (MPH) with Large Eddy Simulation (LES). Since reliable simulation requires known boundary conditions, some information was referred from Unit 3. Furthermore, sensitivity analyses were performed concerning the dynamic viscosity of corium ranging from 1.0×10⁰ to 1.0×10^-3 (Pa.s), the heat transfer coefficient of vapor surrounding the pedestal, and the magnitude of decay heat, respectively. Calculation results indicated that the magnitude of cooling efficiency, consisting of vapor cooling and heat loss by radiation heat transfer, plays a significant role in determining the termination of spreading due to the formation of a solid crust at the free surface. In contrast, the encapsulated corium below the free surface maintained its liquid phase due to decay heat even after spreading termination. Calculation results were compared to TEPCO findings regarding thickness distribution and thermal neutron flux distribution, providing rational possibilities for fuel debris distribution in 1F1.