A report released by the Electric Power Research Institute (EPRI) on Tuesday examining the underlying technical factors leading to the loss of critical systems at the Fukushima Daiichi reactors concludes parameters set for tsunami flood protection were inadequate and that this led to the eventual loss of all practical cooling paths for the reactors.
The plant lost access to all off-site AC power due to the earthquake and to all on-site AC power due to the submergence of backup power supplies and electrical distribution systems. Also, backup DC power was lost due to the submergence of electrical distribution systems and the depletion of power stored in batteries.
The analysis traced the flooding and submergence to the significant difference between the tsunami height the plant was designed for and the one that impacted the plant. That difference limited the ability of tsunami protection and mitigation measures to effectively address the event. The plant’s primary tsunami protection strategy consisted of locating critical equipment above the design tsunami height. “In establishing a maximum tsunami height that might impact the generating station, the Japanese design method did not factor in multiple geological fault ruptures,” EPRI says.
The report reviews the plant’s design basis for seismic and tsunami events, examines the plant’s response capabilities with respect to seismic and tsunami events, and then compares the plant’s design capabilities with the actual events. The report’s results were based on event information, incident timelines, plant design features, and regulatory and design code requirements.
“The nuclear power industry continues to learn from the Fukushima Daiichi event,” said Neil Wilmshurst, vice president at EPRI. “The knowledge gained from this analysis will inform the identification of safety improvements at nuclear plants worldwide that are both meaningful and appropriate.”