Abstract
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.
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