Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)
144 Simplified Methodology for Dependence among Operator Actions for a Nuclear Plant PRA Model (PSAM-0198)
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An area of continuing development in Human Reliability Analysis (HRA) is the treatment of dependencies among operator actions. The specific items of potential importance revolve around an operating staff error that may occur coincidentally with other errors. These combinations of errors may be such that they cannot be treated as random, independent failures. The difficulty arises in being able to accurately assess dependencies among operator actions, whether they occur concurrently or at different times. To help deal with the complexity of the many variables that can enter into whether one operator action is dependent upon another, a simplified methodology was devised to deal with a large number of unique combinations of operator actions.
Common basic HRA methodology divides operator errors into two components; that is, cognitive errors and errors of execution. This new methodology, however, viewed the cognitive relationship as being the dominant mechanism between two different Human Failure Events (HFEs). The execution relationship was not explicitly considered. The degree of dependency between events was figured based on the functional or system relationship between the two in conjunction with timing requirements and cues. In addition, basic performance shaping factors such as stress were considered to determine whether the extremes of “zero” or “complete” dependence could be assigned. HFEs associated with both similar systems and timing requirements received either a complete or high degree of dependency, whereas those with only one or the other in common received a moderate dependence between the two. HFEs with neither system nor cue/timing similarities were categorized as having either low or zero dependence. The general practice was to consider those actions having similar timing requirements as those that were carried out within 15 minutes of each other. However, some actions may be considered as having dissimilar timing requirements and cues if the second of two HFEs has an available system window that is relatively large in comparison with the first event. After the determination of the degree of dependence of one HFE upon another, the standard equations from NUREG/CR-1278 were then used to determine the factor used to determine the overall joint HFE (JHFE).
The benefit of this simplified methodology is twofold. First, the dependence among HFEs can quickly and efficiently be identified while still allowing for other insights and factors to influence the degree of dependence. The second benefit is that multiple individuals working in parallel on a myriad of combinations for a given PRA model can apply a single methodology that promotes consistent results. Overall, this methodology can be of benefit to the nuclear industry, where a single PRA model that contains 50 or more individual HFEs can result in hundreds of dependent combinations, many which are important to the overall calculation of core damage frequency.