PRA Modelling Under Elevated Ambient Temperature

摘要

PRA results are sensitive to uncertainties in setting threshold for equipment-damage criteria. Equipment-damage criteria under elevated ambient temperature are important in Salem PRA. A number of postulated fire scenarios cause loss of temperature control (HVAC) in fire PRA made the criteria more important. An analysis of equipment operability under elevated ambient temperature was performed. It determined that equipment modelled in PRA could function after the ambient temperature exceeded environmental qualification (EQ). For some areas and scenarios, equipment could operate for the mission time. For the rest, equipment could operate a much longer time window thus providing better opportunity for operator actions. However, due to accelerated thermal aging, equipment failure rates could be significantly worse. Failure rates under the elevated temperature are largely equipment specific. Options to obtain new failure rates were explored and discussed. Engineering approach was selected. Using the engineering approach, new failure rates corresponding to elevated ambient temperature were derived. Fire PRA was re-quantified using new equipment-damage criteria. New CDF and LERF values were significantly smaller. For instance, for 4KV switchgear room, the CDF and LERF values were reduced by a factor of 13 and 16 respectively.