Overview and outcomes of the OECD/NEA benchmark study of the accident at the Fukushima Daiichi NPS (BSAF), Phase 2 – Results of severe accident analyses for unit 3

Resilient design in nuclear energy: Critical lessons from a cross-disciplinary analysis of the Fukushima Dai-ichi nuclear accident

“…Although there are previous studies that have suggested that Unit 2 had the major share of the radioactive release, 63 , 64 this study synthesized the in-plant dynamics, wind direction and source term estimation as well as recent simulation studies to confirm the significant contribution of Unit 2, and its PCV venting failure, to the environmental consequences. In fact, a significant portion of the radionuclides from Unit 1 and 3 were considered to be scrubbed in the wet wells, 42 , 43 thereby minimizing the release to the environment ( Figure 2 D). This is despite the fact that Unit 1 was the first to experience core damage and an RPV breach, and that water failed to reach its core, thus leaving it without cooling for at least 11 days—a conclusion drawn from the limited available data and simulation studies.…”

“…41 It's important to highlight that since certain detectors and sensors were compromised during the accident, TEPCO curated these measurements by thoroughly evaluating the available information from different indicators, employing trending analysis, and considering uncertainties. Besides TEPCO’s datasets, we rely on recent simulation studies 42 , 43 , 44 , 45 to verify and timestamp significant physical events like core damage, breach of the RPV, core material relocation, and hydrogen explosion. These simulations draw conclusions by comparing different severe accident computer code outputs—analyzed by various organizations—to establish points of consensus and identify remaining uncertainties and data requirements.…”

“…41 Furthermore, we assign timings to key physical events—such as core damage, breach of the RPV, core material relocation, and hydrogen explosion—by incorporating recent findings from severe accident simulations. 42 , 43 , 44 , 45 Finally we use the latest source term time-series estimates provided by Terada et al. 18 to establish the connection between the onsite events and their offsite consequences.…”

“…10 However, it is estimated that the core was uncovered and had started to melt by that time. 43 With the heat deposited from the RPV, the PCV pressure reached 0.6 MPa by 09:20 ( Figure 2 C), bursting the rupture disk and releasing the excess pressure through the vent lines that had been previously opened. The PCV vent valves closed several times throughout March 13, due to the “loss of air supply”, and were reopened using mobile compressors (and sometimes manually).…”

Simulation-based dynamic probabilistic risk assessment of an internal flooding-initiated accident in nuclear power plant using THALES2 and RAPID

“…28 THALES2 has also been used to evaluate the Fukushima Daiichi NPP accident within the international project called BSAF (Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Station). [29][30][31] To quantify risk using simulation-based dynamic PRA methods, the JAEA is developing a dynamic PRA tool called RAPID (Risk Assessment with Plant Interactive Dynamics) 32,33 to enable explicit modeling of accident scenarios and occurrence probabilities.…”