Vollprobabilistische Ermittlung der Fragility-Kurve einer Stahldruckschale bei Wasserstoff-Deflagration

Full-probabilistic investigation of the fragility-curve of a steel shell during hydrogen deflagration

This paper describes the analyses and conclusions reached in regard to the ultimate pressure capacity of the free standing steel containment shell of the Swiss nuclear power plant Beznau during a degraded core event. Such pressure capacities in terms of probabilities of failure are summarized in fragilities curves. The fragility curves for the containment have a strong influence on the results of the Probabilistic Safety Analysis (PSA) Level 2 in terms of Large Early Release Frequency (LERF) and therefore a detailed modeled is required. As used herein the ultimate capacity is defined as the dynamic internal pressure above which the containment would be expected to have excessive leakage significantly beyond the design basis leakage. The analyses were performed using actual material properties based on historical material test protocols. The dynamic structural computation was carried out as a coupled field computation using first a thermodynamic computation to simulate the heating of the steel shell over time and space after the hydrogen deflagration energy release and secondly the structural reaction to pressure caused by the hydrogen deflagration. The structural response was analyzed for simple analytical cases, in a two-dimensional finite element model and in a three-dimensional finite-element model. The two finite-element models consider non-linear material behavior and assume failure if a certain strain and deformation is reached. The results showed a lower probability of failure in comparison to former analyses or an increase in load respectively.