Study on Probabilistic Safety Analysis for Service Water System of Nuclear Power Plant

    Method   In this paper, the fault tree was built to model the full power and shutdown modes of the investigated plant under cold and non-cold weather conditions respectively, and then to assess the reliability of service water system as an accident mitigation measure. The Monte Carlo method was used to analyse the operability of the system as an initiating event. Relationships between equipment failure rate, maintenance cycle and the annual average unavailability were also attained via sensitivity analysis.

    Result   Quantitative results show the failure rates of service water system for different mission time, particularly under the conditions of non-cold weather at full power the failure rates are 1.47E-01 per year and 4.10E-04 per 72 hours (mission time in accident mitigation process), and at shutdown mode the corresponding failure rate is 6.45E-03 per 96 hours. The frequency of the loss of service water is about 2.0E-03 per year and the annual average unavailability time of the system is about 3.4 hours.

    Conclusion   The results suggest that seawater pretreatment system has more significant influence on the unreliability of service water system, of which the operation failure of electric isolation valve at the main pipeline is the dominant contributor due to having no redundant design. In addition, shortening regular maintenance cycle will be a more effective and operable way to reduce the annual average unavailability time compared with reducing the failure rates of components.