Maintenance optimization methods for NPP critical systems, structures and components

Considerable progress has been made in the development and implementation of the plant life management (PLiM) program with formal processes to identify systematically and evaluate the major critical systems, structures and components (CSSCs) in the station, and a plan to ensure that the plant surveillance, operation, and maintenance programs monitor and control component degradation well within the original design specifications essential for the plant life attainment. The paper presents a brief description of most important aspects of the methods used to identify the CSSCs taking into account maintenance activities. The system structures and components (SSCs) that influence decisively the NPP reliability are considered as critical. Also, for the accident conditions, the SSCs, which have a major influence to the system availability/operability, are considered also as critical. PSA techniques serve as a basis for maintenance optimization. In the paper a case of association between operating events and CSSC is presented. The main reference was the PSA study for Cernavoda NPP, Unit 1 CPSE B+. From this study it was selected and presented: - the contribution diagram for the occurrence of one accident (small LOCA) with some severe consequence (e.g. PDS 1 - 2 - early core damage); - dominant accident sequence analyzed; - the initial event that generated the possible dominant sequence; - the most important accident events; - qualitative and quantitative results; - main CSSC identification; - results validation and proposals for system operation performance improving; - documentation of the analysis. For the critical components, if periodical maintenance operations are needed, the maintenance activities must be optimized. In order to ensure these, some sensitivity studies have to be performed. In the system probabilistic models the associated events for the critical components could be the failure events of the components and also the unavailability due to component isolation for maintenance. The AOT (allowed outage time) is optimized in order to accept risk due to maintenance. (authors)