Even after 6 years since the accident, the exact accident progression for each unit and location of core debris has not been clarified. Currently efforts are directed towards robotic inspection with remote cameras, as well as dose and temperature measurements of the environment inside of the Primary Containment Vessels (PCV). In spite of their effort, the observed environmental data do not support the existence of a large radiation heat sources attributable to the molten core at the bottom of the Primary Containments.

Under this situation the author has conducted a forensic engineering study (i.e., different fields of science work together to collect and integrate independent evidences) to clarify the most likely accident scenarios of the Fukushima Daiichi accident. Through this study the author found that the environmental contamination and public exposure could have been substantially mitigated should the following vulnerabilities have been removed before the accident:

(1) The threat of hydrogen generation through “radiation-induced electrolysis”.

(2) Potential threat of “internal hydrogen explosion” in the suppression pools. The atmosphere on top of the suppression pool water (cover gas) should have been nitrogen.

(3) Potential threat of “internal hydrogen explosion” in pipes which had occurred in the case of the Hamaoka Unit 1 accident.

(4) The leak rate of PCV should have been testable at its design pressure. Intrinsic safety factor of the containment flanges against effluent leakage should have been rated as a 3 for functional integrity of the PCV.

(5) Spread of hydrogen gas from vent lines through duct networks connected to SGTS. The hardened vent line should be independent and provided with filters for Dry Well venting. No rupture disks should have been installed in the vent lines.

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