TEPCO announced that the safety relief valves of Fukushima Dai-ichi NPP Unit-1 were never manually opened during the accident due to the shortage of DC power. However, the measured reactor pressure was about 1 MPa at 2:43 of March 12. At that moment the reactor vessel bottom head is believed not to have failed according to the severe accident analyses by TEPCO. Such unanticipated depressurization might accelerate the core uncovery and on the other hand delay the containment failure caused by the direct containment heating. The authors modeled the creep failure of the stainless steel guide tubes of source range monitor (SRM) in the core and estimated the possible depressurization mechanism oj the Unit 1 using a severe accident analysis code SAMPSON.
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