Probabilistic seismic safety analysis of multi-component systems

The seismic safety of a nuclear power plant (NPP) is assessed based on the reliable functioning of the entire plant. To control the safety functions, the NPP is divided into several systems based on the functional or structural units. A commonly used method for seismic safety analysis is the Seismic Probabilistic Safety Assessment (SPSA). In SPSA, classical event trees and fault trees are used to perform the system analysis or the accident sequence analysis for the plant. In the fault tree, the structural and functional couplings among the components within a system or subsystem are represented using the logic gates ’AND’ or ’OR’, which are not capable of effectively representing the interaction among the components in a system. An alternative method for system analysis, capable of generating system or subsystem fragility curves, including the interaction among the components called the multidimensional fragility analysis, is proposed in this thesis. The method is based on the multidimensional performance limit state approach. As opposed to the fault tree analysis, where the component fragilities are combined to derive the system fragility, in the multidimensional fragility analysis method, the system fragility curves are generated by combining the probabilistic responses of the components and their limit states. The definition of the system performance limit state allows to include the interaction of the responses of the components. The proposed method can be incorporated into the fault tree analysis software programs. The proposed multidimensional fragility method uses the response surface method (RSM) for the probabilistic analysis as opposed to generally used Monte Carlo Simulation (MCS) to improve computational efficiency. The method is also extended to account for degradation and aging in structure and components, since the structural aging causes brittle failure of the structure or components, compared to that of the predicted failure of the structure or the system. To apply and validate the proposed method, an example system is presented. For this, a reinforced concrete test structure, tested in the international benchmark study, SMART 2013, is selected. To this model, nonstructural components such as pipes and emergency generator are added, forming an example system. For the validation of the multidimensional fragility analysis method, the system fragility curves generated are compared with that generated using fault tree with ’OR’ gate. A comparison of the fragility curves for the chosen example system using different interaction factors shows a decrease in the median capacity with a decrease in the interaction factors values. This implies that the interaction factors have a significant influence on the system fragility curves