Where was the weakness in application of defense-in-depth concept and why?

Abstract

The accident at Fukushima Daiichi Nuclear Power Station was caused by an unprecedented Magnitude 9.0 earthquake and tsunami. However, the plant was not well prepared to withstand such an unexpected natural hazard. Although defense-in-depth was supposed to be compensating for uncertainties and incompleteness in our knowledge, there were weaknesses in the application of the concept. This paper analyzes where the weakness was and why. Besides technical lessons, the analysis goes to the background of the weakness and concludes with the importance of questioning and critical review of the current practices and provisions, and learning from best practices in order to continuously improve safety. However, it should be considered that this insufficiency in preparedness was not necessarily unique to Japan (its environment and other national factors). Hence, nuclear power countries and those new entrants launching nuclear power programs are expected to learn lessons from this accident, such as the need for continuous re-assessment of design basis natural hazards, understanding of where the cliff edge to core melt exists, how to increase distances to the cliff edge, and, above all, that technical fixes do not solve everything and attitude matters.