As Units 1, 2, and 3 of the Fukushima Daiichi nuclear power plant (NPP) entered the phase of long-term station blackout following the huge tsunami, the decay heat could not be effectively removed from the reactor vessel and resulted in high in-vessel pressure and temperature. The Tokyo Electric Power Company announced that the safety relief valves of Fukushima Daiichi NPP Unit 1 (1F1) were never manually opened. However, the measured reactor pressure was decreased to ∼1 MPa at 2:43 on March 12, 2011. Such unanticipated depressurization might accelerate core uncovery and on the other hand delay containment failure caused by direct containment heating. In addition, the failure time and the failure path of the boiling water reactor pressure boundary before manual depressurization have a huge impact on the resulting source term. The authors modeled the creep failure of the stainless steel guide tubes of the source range monitor in the core and the main steam line and estimated the possible depressurization mechanism of 1F1 using the SAMPSON (Severe Accident Analysis Code with Mechanistic, Parallelized Simulations Oriented towards Nuclear Field) severe accident analysis code.