Before the accident, there was a basic assumption in Japan that the design of NPPs and the safety measures that had been put in place were sufficiently robust to withstand external events of low probability and high consequences.
Because of the basic assumption that NPPs in Japan were safe, there was a tendency for organizations and their staff not to challenge the level of safety. The reinforced basic assumption among the stakeholders about the robustness of the technical design of NPPs resulted in a situation where safety improvements were not introduced promptly.
The accident at the Fukushima Daiichi NPP showed that, in order to better identify plant vulnerabilities, it is necessary to take an integrated approach that takes account of the complex interactions between people, organizations and technology.