This paper proposes research issues on contribution to safety enhancement for BDBE in structure and material fields.

There are large difference between DBE and BDBE. Objective of DBE is prevention of accident and conservative approach is adopted such as prevention of all assumed failure modes. As for BDBE, objectives are prevention of safety function loss and mitigation of accident consequences, risk approach is expected. Since DEC is a part of BDBE, approach against DEC is different from DBE. DEC requires best estimate.

Probabilistic Risk Assessment consists of best estimate plus uncertainty.

For stress test, identification of the weakest portions and cliff edges become important.

To realize above approach, prediction of realistic failure modes is essential. Furthermore, relative strength evaluation becomes important to predict order of failure location and their mode, even though absolute strength is not clear.

After Fukushima daiichi nuclear accident, there is a tendency to apply DBE design criteria to BDBE, however, conservative criteria for design are inappropriate for best estimate. Therefore, authors proposes failure mode maps to identify realistic failure mode and its application to mitigation to accident sequences such as fracture control.

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