Preparation for beyond design basis events (BDBE) becomes important as the lessons learned from the Fukushima Daiichi nuclear accident.

The objective of strength evaluation for design basis events (DBE) is a confirmation to prevent structural failure for assumed events.

For BDBE, main objectives are weak point survey, deterministic and probabilistic risk assessment, and planning of countermeasures including potable equipment and accident management.

According to the above objectives, strength evaluation approach have to be different between for DBE and for BDBE.

(1) DBE

Conservative approach to prevent of failure.

Design by analysis concept is basically adopted

Assumption of hypothetical failure modes to prevent actual failure modes

Stress criteria to bond actual strength

Elastic analyses for conservative loading assumption

Design factor to bound uncertainties

(2) BDBE

Best estimation of failure behavior with uncertainties to plan mitigations

Identification of realistic failure modes to identify failure consequences

Criteria by dominant parameters of failure phenomena

Inelastic analyses for realistic loading prediction

Probabilistic evaluation to quantify uncertainties.

Strength evaluation concept has not yet been established for BDBE. It is necessary to discuss from basic philosophy to make sharable concepts.

Adequate criteria is required to meet above concepts. Instead of stress, strain is one of candidate.

New evaluation technics are desired to satisfy above criteria.

This paper indicates the direction of strength evaluation for BDBE with same examples proposals. Its aims is to promote international discussions and to implement new technologies to actual countermeasures against BDBE.

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