Risk reduction by filtered venting in PWR large dry-containments

The potential risk reduction associated with a Filtered-Vented Containment System is evaluated. A low-volume venting strategy has been considered and data referring to the Zion power plant, along with the results of the Zion Probabilistic Safety Study, have been used. An estimate of the reduction factor is first made for a single core melt accident sequence whose containment failure mode is late overpressure. The result, interpreted as a reduction factor applicable to the release category associated with containment late overpressure is then used for the estimation of the overall risk reduction factor. In particular, the case of internal and external risk for the Zion power plant are considered. Because the contribution from seismic events dominates the overall risk, the importance of different assumptions for seismic fragility is also assessed. Finally an uncertainty analysis of the risk reduction factor for a single accident sequence is performed. An estimate is also obtained on the level of confidence with which certain required values of risk reduction can be achieved. (orig.)