Document Type:Technical Results
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On March 11, 2011, the Fukushima Daiichi nuclear power plant experienced a seismic event and subsequent tsunami. The accident and the ensuing mitigation and recovery activities occurred over several days, involved a number of incidents, and might provide several opportunities for lessons learned. The objective of this report is to determine the fundamental causative factors for the loss of critical systems at the Fukushima Daiichi reactors that resulted in core damage and subsequent radioactive release. The analytical approach used structured event information to support the identification of causal factors; this information can be used in root cause analysis and corrective action determinations.
The technical analysis traced the cause for the eventual loss of all practical cooling paths for the reactors to the tsunami's flooding of the plant protection. Specifically, the analysis identified the significant difference between the design basis tsunami height and the actual tsunami height, as well as the limitations of beyond-design-basis tsunami protection or mitigation that could address the effects of the actual event. From a causal analysis perspective, these were caused by a methodology that specified that the rupture of combinations of geological fault segments in the vicinity of the plant need not be considered in establishing the design basis tsunami height. The tsunami that occurred was caused by a combined rupture of multiple fault segments.
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