Multi-state unavailability analysis of safety system redundant components with aging effect under surveillance testing

摘要

Over the recent decades, there has been a growing interest in nuclear power plants to develop maintenance activities in the form of risk-based models. Maintenance planning plays a key role in the probabilistic safety assessment (PSA) applications. The importance of these activities, especially with considering the effects of component degradation, is quite clear in term of compromising between conflicting objectives such as achieving the highest level of availability while minimizing the costs of implementing these activities. Given the limitations of applying current PSA techniques to complex maintenance policies, and also the difficulty in modeling the effects of component maintenance strategies and degradation using fault tree (FT) analysis; in this paper a multi-state continuous-time Markov method is presented to model three test and repair policies (T&R) for components with focus on changing technical specification (TS) such as surveillance test intervals (STIs). First, Markov models are developed for three different test policies taking into account the effects of degradation. Then, these models are applied for analyzing the availability of critical components of emergency core cooling system (ECCS) of the VVER-1000/V446 nuclear power plant (NPP) as a case study. In Policy 1, no additional test on the other components is performed. The test procedure is performed according to the plan test as originally scheduled. In Policy 2, an additional test is performed after the failed component is repaired and in Policy 3, an additional test is carried out promptly on the other redundant components after the first failure detection.