Fragility curves for steel containments with internal pressure

Containment structures for nuclear power plants have long been designed for static internal pressure. However, since the Three Mile Island accident there has been heightened interest in the behavior of containments under conditions beyond design, particularly the actual, in-place resistance of containments to static internal pressure. Probabilistic methods are being applied to both the loading and resistance quantities so public risk can be accurately assessed. In this paper, a reliability assessment of a steel containment strength under uniform internal pressure is summarized. Typically the containment vessel is a stiffened shell structure. Failure modes for this type of structure are identified as general, inter-ring and panel failure. Simplified methods to predict the resistance of the vessel under internal pressure are presented. The Advanced First Order Second Moment technique is used in conjunction with the simplified methods to construct the cumulative distribution of the containment resistance as a function of pressure. Uncertainty of the basic variables which affects the containment resistance are propagated through the analysis to evaluate the total uncertainty of the system. The resulting distribution considering all possible failure modes is given for a typical Mark III steel containment. Fragility curves for the containment at 95% and 5% certainty are also given