Performance-oriented and risk-based regulation for containment testing

In August 1992, the US Nuclear Regulatory Commission (NRC) began a major initiative to develop requirements for containment testing that are less prescriptive and more performance oriented and risk based than current requirements. This action was a result of public comments and several studies that concluded that the economic burden of certain present containment testing requirements was not commensurate with their safety benefits. The rule-making included considering relaxing the allowable containment leakage rate, increasing the interval for the containment integrated leak rate test, and establishing intervals for the containment local leak rate tests on the basis of the performance of containment isolation valves and penetrations. A study has been conducted to provide technical information to establish the preformance criterion for containment tests, i.e. the allowable leakage rate, commensurate with its significance to total public risk. For the study, the results used were from a comprehensive study conducted by the NRC (NUREG-1150, 'Severe accident risks: an assessment for five U.S. nuclear power plants') to examine the sensitivity of containment leakage to public risk. Risk was found to be insensitive to a containment leakage rate up to levels of about 100% volume per day for certain types of containments. 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 integrated containment leakage test from three times to once every 10 years would have an insignificant impact on public risk. Analyses of operational experience data for local leak rate tests show that performance-based testing (valves and penetrations that preform well are tested less frequently) is feasible with a marginal impact on safety. These technical studies have been used to develop efficient (cost-effective) requirements for containment tests.