The use of fire modeling in support of the PRA as defined in NUREG/CR 6850 involves a scoping fire modeling phase (Task 8) and a detailed fire modeling phase (Task 11). The primary emphasis of the scoping fire modeling phase is the determination of fire sizes causing limited target damage and the assignment of severity factors to these fires in order to more accurately assess the contribution of high probability, small impact fires. Detailed fire modeling is used to evaluate the effects of larger fires in conjunction with the possibility of fire suppression prior to target damage. A method for scoping fire modeling is described in this paper that allows the use of conservative steady state fire conditions to define zones of influence for fire ignition sources. By evaluating the consequences of these conservative steady state fires, scenarios that warrant analysis using a detailed fire modeling approach can be identified. The scoping fire modeling method presented in this paper correlates the key parameters defining the fire size to the associated zone of influence within which the temperature or heat flux conditions would exceed threshold damage limits. In addition, a series of zone model analyses are performed on bounding room geometries to define the room volume below which hot gas layer effects impact the zone of influence calculations. Using the results of these correlations, site surveys may be conducted to identify fire ignition source zones of influence and the associated target sets for input into an assessment of the consequences of specific ignition source based fire scenarios.
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