Systematic analysis and prevention of human errors and common cause failures in relation to maintenance

The human maintenance failures and design deficiencies, remained latent in the system, have an impact on the severity of a disturbance, e.g. by disabling safety-related equipment on demand. Especially common cause failures (CCFs) of safety related systems can affect the core damage risk to a significant extent. The topic was addressed in Finnish studies, where experiences of latent human errors have been searched and analysed in detail from the maintenance history of the Loviisa and Olkiluoto NPPs. The analysis of this experience from the power plant information systems showed that most errors stem from the refuelling and maintenance outage periods. More than half of the multiple errors from the outages remained latent to the power operating periods. The human CCFs involved human, organisational and technical factors. The CCFs were analysed and classified systematically and documented as event reports. The review of the analysed single and multiple errors showed that instrumentation and control and electrical equipment are more prone to human error caused failure events than the other maintenance objects. The review of the analysed multiple errors showed that difficulties with small and normal plant modifications and planning of maintenance and operability were significant sources of CCFs. The most dependent human errors originating from small and normal modifications could be reduced by a more tailored planning and coverage of their start-up testing programs. Improvements could also be achieved by identification in the work planning of those complex or intrusive repair and preventive maintenance works which are prone to errors. Such cases in important equipment require a tailored work planning of installation inspections and functional testing, including a need evaluation of both the component and the system level testing. An earlier and better examination of CCF risks as a part of the failure reporting, and repair and modification processes would also help to identify, investigate and prevent CCFs. The results emphasize the responsibility and requirements of both equipment specialist and functional overview knowledge in performance and planning of maintenance an d operability verification which brings these tasks to a knowledge work. Operability verification of the work objects in plant equipment should be planned and implemented as an integral part of the maintenance process requiring knowledge of both the maintenance and operation branches. Methods for analysis of maintenance history information, examples of presentation of analysis results and conclusions and recommendations for identification and prevention of latent human common cause failures and single errors, are also described in this paper. (author)