ANALYSIS OF ACCIDENT PROGRESSION OF FUKUSHIMA DAIICHI NPP WITH SAMPSON CODE - (1) UNIT 1

摘要

Accident progression of the unit-1 of Fukushima Daiichi nuclear power plants was analyzed with SAMPSON code. There were no decay heat removal systems available at the unit-1 except short term intermittent operation of the isolation condenser. The pressure transient in the reactor pressure vessel (RPV) was manually measured after the Tsunami: it was 7.0 MPa at 5 hours and 21 minutes after scram, and decreased to 0.9 MPa at 11 hours and 44 minutes after scram. RPV depressurization must occur at some time in the period of these two measurements. In the initial analysis, depressurization at 9 hours and 44 minutes after scram, when the drywell pressure measurement was 0.6 MPa, was assumed by opening the safety relief valve. The analysis results showed (1) that the RPV bottom failure occurred at 12 hours and 19 minutes after scram, and (2) that about 15.4 ton melt materials fell down into the drywell cavity at 14 hours and 22 minutes after scram. After that, it was clarified through detailed investigation of design specifications that the pressure boundary of the source range monitor (SRM) as one of the in-core monitors was inside the core. Therefore, direct release of reactor steam to the drywell might occur due to the SRM damage and this might be the cause of depressurization. Now, the SRM damage model is being incorporated in SAMPSON and then re-analysis is being performed.