Latent failures of safety systems - A generic study performed by the Principal Working Group 1 on Operating Experience and Human Factors. Undetected failures of safety systems - A Generic Study Performed by the Principal Working Group 1 of the Committee on the Safety of Nuclear Installations

Undetected failures of safety systems in nuclear power plants are of a great concern, especially when the failures remain undetected for a long time. Survey of operating experience through the incident reporting system (IRS) and licensee event reports (LERs) revealed that a significant number of latent failures remained undetected during many years of commercial operations sometimes since initial start-up. Also, plants operated under unanalysed conditions, outside their design basis condition, or with long-term unavailabilities associated with high conditional core damage probabilities. Deficiencies affecting safety related systems and their support systems have been discovered in all nuclear activities from initial fabrication, design and installation or commissioning tests, in-service testing, maintenance, inadequate review and testing of safety system modifications. Though a downtrend should normally be observed as a result of the increased integrated operating experience of the nuclear industry, recent events indicate that deficiencies in initial design are still being discovered. Moreover, latent failures have been introduced in the course of inappropriate or ineffective maintenance activities, as well as through incompletely reviewed and tested design changes or modifications. In other cases, deficiencies in the quality organisation as well as failure to implement corrective actions in time have also contributed to maintain these failures during an extended period. This inability to detect problems in a timely manner indicates that existing means to demonstrate the functional capability of equipment to comply with regulatory requirements need to be used more efficiently or upgraded. In the light of the events reviewed in this report which entails a widespread field of failures involving a broad class of systems and a great variety of failures, it appears that some of these failures could have been detected earlier by more complete surveillance tests or prevented by better monitoring and more effective post-maintenance testing. Moreover, as nuclear plants get older, increasing attention should be given to maintenance and modification activities. Therefore efforts should be directed to: 1. Improve existing test program and procedures in order to render them more comprehensive. 2. Assess modifications implementation and subsequent requalification. 3. Establish post-maintenance testing requirements in order to verify the performance of repaired or replaced component and the functional requirements of the whole system. 4. Implement appropriate instrumentation, monitoring or diagnostic techniques, for trending component performances. The report gives an overview of the specific findings, insights, and actions as reported in the individual contributions of the participating countries. It provides some guidance and insights gained from generic studies that could be useful in implementing corrective and preventive actions