Evaluation of availability of nuclear power plant dynamic systems using extended dynamic reliability graph with general gates (DRGGG)

Abstract

To assess the availability of a nuclear power plant's dynamic systems, it is necessary to consider the impact of dynamic interactions, such as components, software, and operating processes. However, there is currently no simple, easy-to-use tool for assessing the availability of these dynamic systems. The existing methods, such as Markov chains, derive an accurate solution but have difficulty in modeling the system. When using conventional fault trees, the reliability of a system with dynamic characteristics cannot be evaluated accurately because the fault trees consider reliability of a specific operating configuration of the system. The dynamic reliability graph with general gates (DRGGG) allows an intuitive modeling similar to the actual system configuration, which can reduce the human errors that can occur during modeling of the target system. However, because the current DRGGG is able to evaluate the dynamic system in terms of only reliability without repair, a new evaluation method that can calculate the availability of the dynamic system with repair is proposed through this study. The proposed method extends the DRGGG by adding the repair condition to the dynamic gates. As a result of comparing the proposed method with Markov chains with regards to a simple verification model, it is confirmed that the quantified value converges to the solution. In addition, the availability of the diverse protection system (DPS), the off-site power supply system, and the representative dynamic system installed in the nuclear power plant, was modeled using the proposed Extended DRGGG. It was verified that the proposed evaluation method can more effectively evaluate the failure and repair characteristics of these dynamic systems than the existing methods.