Reliability trending for Nuclear Power Plant relicensing

The Application for Renewal of Authorisation (ARA) is to be submitted by Operating Nuclear Power Stations to Atomic Energy Regulatory Board (AERB) in India to obtain the consent for operation of plant for the next authorization period. One of the most important elements of this Application for Renewal of Authorisation (ARA) is Reliability Safety Analysis of Safety & Engineered Safety Systems. The purpose of this analysis is to make assessment of the performance and trending of the unavailability of Safety & Engineered Safety Systems. Reliability Safety Analysis demonstrates the adequacy of maintenance programme and testing requirement by comparison with target values given in Safety Analysis Report of Nuclear Power Stations. The analysis is carried out based on the Fault Tree (FT) analysis method. The Fault Trees (FT) were constructed using the immediate cause concept, starting with top event and down to the primary component failure. The traditional method of performing this analysis was using the actual plant specific failure data to report the system reliability over the past review period. However this permitted several systems assessed to have zero unavailability. Now, with the awareness generated through comprehensive Level-1 PSA studies, use of generic failure data, Bayesian updating techniques, impact of Common cause failures on system unavailability, guidelines have been generated to perform this assessment to have a more dependable predictive potential. The Plant Specific Failure Data is exhaustively assessed for different failure modes and Preventive Maintenance (PM) down time. The system Fault Trees are analysed with plant specific failure data aggregated in four categories, enabling Reliability trending and regulatory clearance with higher confidence. Bayesian updating is used for enhancing the dependability of the reliability prediction based on scarce data and zero failure components. The results based on plant specific failure data, obtain- d for Kaiga Generating Station-1&2 from Commercial operation to the license authorization period were compared with target values as given in the Safety Report for the plant. These values were found within the limits. This paper brings out the salient points and insights of the reliability safety analysis as part of ARA.