Containment Filtered Venting - The Situation in Switzerland

Abstract

A general decision concerning containment filtered venting has been made some years ago. The implementation of filtered depressurization devices in Swiss nuclear power plants aims at preventing containment failure due to internal pressure build up. Design requirements have been provided based on a specific class of severe accident scenarios. In general the capacity of the system is considered sufficient if it is able to vent the steam production corresponding to a decay heat level of 1 % of the thermal reactor power, but BWRs are allowed to reduce this venting capacity to 0.8 % if relief capacity can be increased due to higher containment pressure. For PWRs the venting capacity can be reduced to 0.5 % if analyses show that this capacity ii sufficient for the relevant accident scenarios. The mitigation capacity for reduction of released radioactive material is specified by a retention factor of 1000 for aerosols to prevent or limit a long term ground contamination and a factor of 100 for elementary iodine to prevent or limit thyroid doses and avoid short term evacuation . The installation of venting devices is expected in the near future. Presently the strategy for initiation of venting in an accident situation is under discussion. It relies primarily on the pressure within the containment but also tries to take into account other influences for example the weather situation.