Fire Risk Analysis

Abstract: Recently, the fire protection programs at nuclear power plants have been transitioned to a risk-informed approach utilizing Fire Probabilistic Risk Assessment (Fire PRA). One of the main limitations of the current methodology is that it is not capable of adequately accounting for the dynamic behavior and effects of fire due to its reliance on the classical PRA methodology (i.e., Event Trees and Fault Trees). As a solution for this limitation, in this paper we propose an integrated framework for Fire PRA. This method falls midway between a classical and a fully dynamic PRA with respect to the utilization of simulation techniques. In the integrated framework, some of the fire-related Fault Trees are replaced with a Fire Simulation Module (FSM), which is linked to a plant-specific PRA model. The FSM is composed of simulation-based physical models for fire initiation, progression, and post-fire failure. Moreover, FSM includes the uncertainty propagation in the physical models and input parameters. These features will reduce the unnecessary conservatism in the current Fire PRA methodology by modeling the underlying physical phenomena and by considering the dynamic interactions among them.

Fire has been recognized as a major contributor to Nuclear Power Plant (NPP) risks. The current practice for Fire PRA builds on earlier work, advancing the state of modeling in many areas (initiator statistics, expert-opinion and experimentally- based assessments of cable damage, etc.), while leaving in place other techniques (physical fire propagation modeling to and through cable trays).

  1. Change quantification techniques of the Fire risk analysis of the NPPs from FT/ET to a simulation- based approach in a “Fire Simulation” module. The plan is to extract the majority of fire-related fault trees from PRA and, instead, cover them in a simulation-based context. Current Fire PRA methods have used time-based physical models for some aspects of the phenomena. Our purpose is to integrate the existing time-dependent fire physical models (to the extent practical and accurate) into a simulation module so that their dynamic interactions can be more adequately covered.
  2. Propagate uncertainty in the Fire simulation module. The Fire Simulation module integrates the physical phenomena and propagates uncertainty in physical models and parameters from fire initiation to potential core damage precursors. Uncertainty propagation can be accomplished by assuming that the input parameters are random variables with distributions derived from historical data, experimental data, expert elicitation, physics, or a combination of these sources. Finally, these values would be propagated through the Fire Simulation module to yield as output, an estimator of a key performance measure, such as the probability of a subsystem failure, which is then passed to the PRA model.
  3. Link the Fire Simulation module to PRA in order to incorporate the calculated probabilities from the Fire Simulation module to FT/ET of PRA.