Sensitivity Analysis of Fukushima Daiichi Unit 1 Type Accident with MELCOR

摘要

Investigation of melt behavior in RPV lower plenum and lower head response in Fukushima Daiichi unit 1 type accident was performed with MELCOR 2.1. When not considering penetrations in lower plenum, the lower head failed by creep rupture, resulting in that almost all the debris ejected out of the pressure vessel. Otherwise, the lower head failed by penetration tube rupture and ejection, resulting in that a part of debris still remained in the lower plenum. Most debris discharging into the reactor cavity was in solid phase. The temperature of the remaining debris kept low because of the small fuel fraction, and it didn't remelt before water injection available at 15 hours after the reactor scram by earthquake. After the discharge of debris the vessel wall temperature increased at reduced speed, but it didn't reach the creep failure criterion. Another aspect was that the later RPV depressurization by penetration failure cooled the debris in the lower plenum, which postponed the penetration rupture and debris discharge time.