RESULTS AND INSIGHTS FROM LEVEL-1 PROBABILISTIC SAFETY ASSESSMENTS FOR NUCLEAR-POWER-PLANTS IN FRANCE, GERMANY, JAPAN, SWEDEN, SWITZERLAND AND THE UNITED-STATES

The level-1 results of 14 PSAs from 6 countries were evaluated, leading to the following conclusions: benefits-in terms of reduced CDF-from the increased redundancy, improved spatial separation and enhanced automation of the systems, that exist in newer plants, are only seen for PWRs, In the older BWRs, the absence of such features is compensated by a high degree of intermeshing and greater flexibility for using alternate RPV-injection and heat removal. Two old plants that have been equipped with completely independent, bunkered emergency systems, compare favourably with modern plants; preventive AM is highly effective for transients in PWRs, and for accident situations involving the loss of heat removal in BWRs; for the newer plants, PWR as well as BWR, the contribution to the total CDF from common cause failures is consistently higher, and the contribution from operator errors consistently lower, than for the older plants; the CDF for BWRs is, in general, lower than for the PWRs. The largest impacts on the results and on their uncertainties are consistently observed for human errors, common cause failures, and initiator frequencies for LOCAs, in particular SLOCAs, in PWRs. The largest impacts on the results and on their uncertainties are consistently observed for human errors, common cause failures, and initiator frequencies for LOCAs, in particular SLOCAs, in PWRs.