Maintenance programmes

Maintenance is the subject of Maintenance Effectiveness Monitoring (MEM), the purpose of which is to control the effectiveness of maintenance on SSCs ensuring they are capable of performing their intended functions. This means ensuring that safety-related SSCs are capable of performing their intended functions; that failures of certain non-safety-related SSCs that could affect safety-related functions will not occur; and failures that could result in scrams or unnecessary actuations of safety-related systems are minimized. The systems within the scope of MEM might be divided into high and low risk-significant categories. The risk significance has been defined quantitatively by PSA or qualitatively by expert judgement. The MEM is an adaptation of 10CFR50.65 for the VVER-440/213 design fea­tures, Hungarian regulatory environment and plant practice.

There are two basic methods applied in the MEM: deterministic method, that is control of maintenance via testing/measuring performance parame­ters of components, and probabilistic method, that is assessing the effective­ness of maintenance via comparison of reliability/availability parameters at the level of component/system or plant. Performance parameters are defined in accordance with safety class and risk significance. The determin­istic method is based on ASME OM Code. For example in case of pumps the performance criteria to be checked are the head, flow-rate and vibration level. Plant level deterministic performance parameters include the capac­ity factor, thermal efficiency of the unit and leakage of the containment (%/day). Risk significance and the probabilistic performance criteria are set on the basis of PSA. High risk significant SSCs are those which are in 90% cut-set, have a high contribution to CDF or high Fussell-Vessely rank. Performance criteria for MEM are based on the reliability or unavailability data of performing safety function. System level performance parameters are, for example, failure rates per demand (failure/start) or run failure rate (failure/time) during operation. Plant level performance parameters are the CDF or some selected contributors to the CDF and other safety fac­tors (unplanned reactor scrams or safety system actuations per year). The MEM is being implemented at Paks; for the implementation of ASME OM Code, the existing in-service and post-maintenance testing programmes of the Paks NPP have to be modified and amended. Probabilistic performance criteria are under development at present. It is expected that the MEM will improve the safety factors and capacity factors for the plant while the main­tenance effort will be optimal. MEM is a prerequisite for license renewal in Hungary, since it provides assurance for the correct functioning of active components.

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