Reliability of diesel generators in the Finnish and Swedish nuclear power plants

Diesel generators are used as emergency AC-power sources in nuclear power plants and they produce electric power for other emergency systems during accidents in which offsite power is lost. The reliability of diesel generators is thus of major concern for overall safety of nuclear power plants. In this study we consider the reliability of diesel generators in the Swedish and Finnish nuclear power plants on the basis of collected operational experience. We classify the occurred failures according to their functional criticality, type and cause. The failures caused by human errors in maintenance and testing are analysed in detail. We analyse also the reliability of the diesel generator subsystems. Further, we study the effect of surveillance test and the type of test on the reliability. Finally we construct an unavailability model for single diesel generator unit and discuss the findings of the study giving some practical recommendations