The findings obtained during the OECD/NEA BSAF activity with the employment of the Sampson code

The Organization for Economic Cooperation and Development, Nuclear Energy Agency (OECD/NEA) launched the Benchmark Study for Analysis of Fukushima Daiichi Nuclear Power Station (BSAF) project after the accident occurrence. The Institute of Applied Energy (IAE) has worked as the technical supporting organization for the operating agent, Japan Atomic Energy Agency (JAEA), of the BSAF project. The IAE has investigated the operative conditions of the plant equipment during the accident in cooperation with TEPCO and gave it to the BSAF member organizations as common understanding. This paper describes some events and phenomena that occurred which have been deemed specific to the Fukushima Daiichi Nuclear Power Plant: (1) direct steam release from the reactor pressure vessels (RPVs) to the drywells, (2) corium release from penetrating pipes at the RPV lower heads onto the pedestal floors, (3) Alternative water injection by the fire pumps, and (4) off-design operation for the reactor core isolation cooling system (RCIC) and the high pressure coolant injection system (HPCI), and calculation results of the accident progressions at Units 1, 2, and 3 at present using the SAMPSON code with new modellings for the above phenomena. Unit 1: 77% of the core materials had already melted down onto the pedestal floor at 15 hours after scram when the alternative water injection started and almost 100% at 100 hour after scram. Unit 2: 25% of the core materials had been relocated in the lower plenum at 105 h after scram when the core was re-flooded by the alternative water injection. Unit 3: Finally 80-100% of the core materials had melted down onto the pedestal floor depending on the HPCI performance and the alternative water mass flow rate into the core by the fire truck.