Validation progress and exploratory analyses of three-dimensional simulation code for BWR in-vessel core degradation

Based on the information obtained from the severe accident at the Fukushima Daiichi Nuclear Power Plant Units 1-3 in 2011, Regulatory Standard and Research Department, Secretariat of Nuclear Regulation Authority (S/NRA/R) started development of a detailed three-dimensional simulation code for in-vessel core degradation. The simulation code aims at applying to BWR core degradation assessment with three-dimensional geometry. The present paper describes the progress of validation and the assessment of BWR in-vessel core degradation for future application by parametric surveys. In-vessel core degradation phenomena were validated mainly using the CORA-18 experiment at Karlsruhe Institute of Technology to estimate axial- and lateral-directional motion of molten corium. Additionally, metallic melt relocation around a BWR core support plate and molten corium breakup in a lower head were validated by the XR-2 experiment at Sandia National Laboratory, and the FARO L-19 and the KROTOS K-37 experiments at Joint Research Centre facilities of the European Commission, respectively. The code results match severe accident relevant phenomena qualitatively well. Furthermore, as a validation process, the exploratory analyses using 1/4 Sector-Core Geometry in a hypothetical severe accident condition have been carried out for a typical middle-power-level BWR with the following variable parameters as initial core conditions: a water level, a decay heat level, a radial power shape and an oxidation of fuel cladding. These efforts showed that the code could be used for application to the actual core degradation evaluation with three-dimensional geometry in the near future.