AUTHOR=Mohammadhasani F. , Pirouzmand A. 

TITLE=Multi-state Risk-Based Maintenance Analysis of Redundant Safety Systems Using the Markov Model and Fault Tree Method

JOURNAL=Frontiers in Energy Research

VOLUME=Volume 9 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2021.685634

DOI=10.3389/fenrg.2021.685634

ISSN=2296-598X

ABSTRACT=The risk-based maintenance strategy has received special attention in the safe operation of nuclear power plants (NPPs). Simultaneous quantification of the positive and negative effects of maintenance activities (MAs) and components degradation effect makes it possible to evaluate the risk criterion for NPP safety systems, accurately. However, it is difficult to integrate the effects of maintenance and components degradation into the standard reliability approaches (SRAs). A straightforward approach for considering components degradation and different maintenance policies, is to make use of Markov maintenance models (MMMs). In this paper, the effectiveness of MAs (include changes in the surveillance test intervals and alteration in the different maintenance policies) on the components unavailability with considering aging effects is quantified using MMMs and then by coupling these models and FT method, the risk measure is upgraded from the component level to the system level. The proposed models are applied to evaluate the unavailability of two safety systems of VVER-1000/V446 NPP as case studies. The results show that Markov method due to its multi-state nature is effective in the conservative evaluation of risk measures so that the unavailability computed by the coupling process is higher than the original unavailability (calculated by system FT using NPP PSA data) for all maintenance policies. In addition, this study illustrates that the developed MMMs could be applied to the large-scale whole plant level and provide a proper transition from the classical PSA methods to new techniques. This could integrate the effects of maintenance strategies and components degradation and consequently make available a practical and a more accurate tool for technical specifications (TSs) determination of a real nuclear power plant from the risk point of view.