Risk assessment of the beneficial impact of a filtered venting containment system in a PWR with large, dry containment

Many of the potential high consequence Class-9 accidents in pressurized water reactors (PWRs) involve containment failure by overpressurization. Filtered Venting Containment Systems (FVCS) have been suggested to prevent this mode of containment failure and to reduce the radioactive releases. This study quantified the beneficial effect of an FVCS on the risk to the public from accidents in a PWR with a large, dry containment (Surry-plant). The accidents considered were those found to be the most likely to cause containment failure by overpressurization. The results for those dominant accidents were then used to infer the risk reduction potential of the FVCS. This study developed a phenomenological event tree to characterize the ex-vessel accident scenarios which might be encountered by the FVCS. The phenomena considered in the tree and their consequences received supportive analyses, including the pressure vessel failure, the melt release from the vessel, the occurrence of ex-vessel events, with and without the operation of the FVCS. The point estimate results obtained in this study show that, for the PWR design considered, the FVCS reduces the risks of early and latent fatalities by one order of magnitude. Only a marginal reduction is attained for the risk of property damage. Uncertainty analysis indicated that values of approx. 1110 of the point estimate results are regarded as more likely. The uncertainty in the other direction is not likely to be greater than a factor of 2.