Abstract
TEPCO announced that the safety relief valves of the Fukushima Daiichi NPP Unit 1 were never manually opened during the accident due to the shortage of DC power. However, the measured reactor pressure was about 1MPa at 2:43 of March 12. At that moment the core damage must already start due to almost no core injection for nearly 11 hours while the reactor vessel bottom head must not fail according to the severe accident analyses so far. Such unanticipated depressurization might accelerate the core “uncovery” and also prevent early containment failure caused by the direct containment heating. The authors modeled the creep failures of the stainless steel guide tubes of the source range monitor (SRM) in the reactor core and the main steam line. Based on such failure models, the possible depressurization mechanism of the Unit 1 and related phenomena were estimated using a severe accident analysis code SAMPSON. The estimated scenario by this study is that the creep failures of the SRM dry tubes failed first and caused the leakage from the core to the containment while the direct cause of the depressurization is the creep failure of the main steam line after the start of core damage.
