Contribution of human reliability analysis to the results of probabilistic safety assessment of the nuclear power plant

The human contribution to the undesired events in technological systems is still significant despite automation of systems and processes. In the probabilistic safety assessment, which is a standardized tool for assessment and improvement of nuclear power plant safety, it is possible to consider the human reliability analysis in addition of the risk analysis of component and system failures. The objective of this paper is to show how the human reliability analysis is integrated into the probabilistic safety assessment and to assess effects of human reliability on the nuclear power plant safety compared to the contribution of the selected safety equipment. A method for human reliability analysis was developed. Fig. 1 shows how the human reliability analysis is integrated into the probabilistic safety assessment. Equation 1 provides an expression for quantification of the risk contribution of the selected events. The expression is based on eqs. 2 and 3, which calculate the system unavailability based on failure probabilities of its components and related human errors. A detailed model of a specific nuclear power plant is investigated. Fig. 2 shows the result of parametric deterministic analysis from which a time window for operator action is determined. Results in figure 3 show that the risk contribution of all human errors is comparable to the risk contribution of diesel generators. Abbreviations used in Fig. 3 are explained in Table 1. The results show that the risk contribution of initiator events and pre-initiator events is negligible. The results in Fig. 4 show that only a few human errors notably contribute to the risk. The majority of human errors contributes negligibly or even do not contribute at all. Abbreviations of Fig. 4 are explained in Table 2. Fig. 4 and Table 2 identify the most important human actions to be trained in the plant full scope simulator. Figs. 5 and 6 show partial results of the probabilistic safety assessment focusing only on one initiating event (i. e. loss of offsite power) and to its subsequent scenarios. Figure 5 shows the risk contributions of selected groups of events for all scenarios of the loss of offsite power initiating event. Fig. 6 shows the risk contributions of the same groups for specific scenarios of the loss of offsite power initiating event separately. Results show that the risk contributions of specific events or groups of events vary from one scenario to another and from one initiating event to another. Identification of the most important human actions in general and for specific initiating events is the input for defining training in the full scope plant simulator. It decreases the human error probability for trained human actions. The decrease in the human error probability increases the nuclear power plant safety accordingly.