Probability Safety Assessments (PSA) by their nature are approximations of the actual risk and consequences of an accident at a nuclear power plant. Today the PSA models are becoming more complex as the computers and PSA software are faster and PSA modeling techniques are improved. Many PSAs now integrate into a single model internal initiators, internal floods, and internal fires. Each of these initiator types has some uncertainties in common and some that are primarily associated with a specific initiator type. This paper discusses some of those uncertainties found in Fire PSAs. Uncertainties in Fire PSAs arise from the broad categories of phenomenological modeling and assumptions, PSA model development and assumptions, data, and human failure events (HFEs). Phenomenological development and assumptions would include the heat release rates, fire durations, zones of influence, damage criteria, fire propagation, and the impact of smoke on equipment. Aside from the model development and assumptions in the traditional internal events PSA, the inclusion or exclusion of mitigating systems into a Fire PSA model generally has greater impact on the results than in the internal events PSA model. Most plants do not have readily available cable information for non-safety related cables. The cost to determine cable routing for non-safety related cable can be very high and, therefore, most Fire PSAs do not include non-safety mitigating systems except where needed. This increases the level of uncertainty and can skew the results. Data uncertainty arises from ignition frequency determinations, fuel available to a fire, the probability of success/failure of fire suppression systems, and the probability of hot shorts and consequential spurious operation. In addition to the HFEs included in an internal events PSA, there are other HFEs specific to fires. These include new human error probabilities (HEPs) for those HFEs that are part of the internal events PSA model due to the fact that there is a fire occurring and added stress and instrumentation failure may result. There are also HFEs associated directly with the fire such as fire detection and suppression. New HFEs will be needed to be modeled for Control Room evacuation. PSAs are being used more and more in the decision making processes of operating nuclear power plants. It is often required that initiators other than the traditional “internal events” be included in these processes. Understanding the uncertainties that are part of the Fire PSA is needed to make an informed decision. This paper addresses each of these in greater detail and provides techniques in understanding the impacts of the uncertainties.

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