PERFORMANCE-ORIENTED AND RISK-BASED REGULATION FOR CONTAINMENT TESTING

In August 1992, the US Nuclear Regulatory Commission (NRC) began a maj or initiative to develop requirements for containment testing that are less prescriptive and more performance oriented and risk based than c urrent requirements. This action was a result of public comments and s everal studies that concluded that the economic burden of certain pres ent containment testing requirements was not commensurate with their s afety benefits. The rule-making included considering relaxing the allo wable containment leakage rate, increasing the interval for the contai nment integrated leak rate test, and establishing intervals for the co ntainment local leak rate tests on the basis of the performance of con tainment isolation valves and penetrations. A study has been conducted to provide technical information to establish the preformance criteri on for containment tests, i.e. the allowable leakage rate, commensurat e with its significance to total public risk. For the study, the resul ts used were from a comprehensive study conducted by the NRC (NUREG-11 50, 'Severe accident risks: an assessment for five U.S. nuclear power plants') to examine the sensitivity of containment leakage to public r isk. Risk was found to be insensitive to a containment leakage rate up to levels of about 100% volume per day for certain types of containme nts. Probabilistic risk assessment methods have also been developed to establish risk-based intervals for containment tests on the basis of experience. Evaluations show that increasing the interval for the inte grated containment leakage test from three times to once every 10 year s would have an insignificant impact on public risk. Analyses of opera tional experience data for local leak rate tests show that performance -based testing (valves and penetrations that preform well are tested l ess frequently) is feasible with a marginal impact on safety. These te chnical studies have been used to develop efficient (cost-effective) r equirements for containment tests.