Modeling and verification of three-dimensional simulation for BWR in-vessel core degradation

A Multifunction Model has been developed for describing BWR in-vessel core degradation from initial cladding temperature rise to eventual molten materials accumulation in the lower part of reactor vessel as a fundamental safety research by Regulatory Standard and Research Department, Secretariat of Nuclear Regulation Authority (S/NRA/R). The Multifunction Model aims to reduce the uncertainties of severe accident assessment related to physical models on core damage progression, melt formation and material relocation processes in the reactor vessel. The Multifunction Model consists of a thermal-hydraulic module, a fuel pin behavior module and a neutronic module with three-dimensional geometry. For reducing the uncertainties, the thermal-hydraulic module incorporated multi-phase, multi-component and multi-velocity field with multiple chemical and eutectic reactions. Four fuel failure models were incorporated into the fuel pin behavior module in order to simulate the failures of cladding rupture, brittle fracture, cladding melting and pellet dissolution. In order to decrease the calculation time, the data table of macroscopic cross-section was prepared for neutronic diffusion calculation of three fuel conditions in advance. The verification results revealed that the Multifunction Model demonstrated a reasonable capability to simulate the BWR in-vessel core degradations. The validation will be carried out for existing severe accident experiments.