Need for improving emergency operating procedures to cope with multiple failure scenarios

The Fukushima Daiichi accident has highlighted the importance of timely accident management actions under extreme external events or multiple failure scenarios. This study evaluates the time adequacy for critical operator actions required at early multiple event/failure scenarios of accident progression, by applying the timeline analysis technique for some selected multiple even/failure scenarios. As a result of the study, it turned out that most of the operator actions under early multiple failure conditions are very time-critical, and for some operator actions, it is very unlikely to initiate the required action within the available time. Therefore, in order for the operators to take timely action for the early multiple failure scenarios, the current EOP structure needs to be revised in a more simple and straightforward way, as well as a clear and direct path to appropriate restoration actions should be provided. (author)