Optimized initiation criterion for a filtered containment venting in case of a severe accident in a nuclear power plant

•In case of a severe accident, the containment may get challenged by overpressure.•To mitigate this challenge, a containment filtered venting can be initiated.•This work shall help the emergency response staff in the decision finding.•As quantitative measure the minimization of the cesium release risk is chosen. In case of a nuclear accident with core damage, there is a high conditional probability that not only all core cooling systems are lost, but also that all containment cooling systems are lost. Thus, the containment pressurization represents a significant risk for the containment integrity. Nowadays, it is international standard that Generation 2 nuclear power plants are back-fitted with a filtered containment venting system to control the containment pressure passively in case other means have failed. In case of a core damage event, the emergency response team may have to make the decision to either initiate the filtered containment venting to avert an impending containment overpressure failure (accepting a limited fission product release), or to delay the venting to gain additional time for emergency repair works, possibly avoiding the need for the venting (but increasing the probability for an uncontrolled containment failure). In the following, a method is presented to deduce an optimized venting pressure set-point, based on an approach to minimize the cesium release risk. The result of this optimization is that a filtered containment venting shall be initiated after the containment exceeds its design pressure but before the test pressure (usually in the range of 1.2 times the design pressure) is significantly exceeded. A further delay of the venting, accepting even a very small probability for a containment over-pressure failure, significantly increases the overall fission product release risk.