Consideration of Ageing Effect in Reliability and Safety Analysis of NPP Equipement Functioning

This article presents examples of using Fisher's Chi-square minimisation method for verifying time-dependent reliability model validity, formulating parameters and evaluating uncertainties. The study was performed by State Technical University for Nuclear Power Engineering and EC JRC Institute for Energy, within the context of Ageing PSA Network activities. Sometimes exact times of failure are not known, only an interval of time within which the number of failures are recorded. The ordered by component age sequence of intervals with the corresponding number of failures and the accumulated exposure time during the bin(interval) constitutes the binned data (also known as readout data). For the continuous failure intensity function, it was proposed to apply the following statistical models: 1.Constant failure intensity; 2.Linear failure intensity; 3.Log-linear or exponential failure intensity; 4.Power-low (Weibull) failure intensity mode For models 2-4, the positive shape parameter means a positive trend in time, i.e. the component failure intensity increases with the age of the component. The following tasks were performed: - verification of model validity, - parameters estimation, - characterisation of uncertainties for estimated parameters and whole model, - assessment of possible extrapolation and uncertainties of extrapolation. The developed approach is illustrated with an example of data from operating experience of one of the pump types installed on NPPs. Proposed approach made it possible to identify the component groups with increasing failure intensity and to choose best fitted reliability model. The impact of the choice of model on extrapolation results were analyzed. Detailed results of the study are presented in JRC report EUR23079EN. Keywords: ageing of systems, structures and components, Probabilistic Safety Assessment (PSA), reliability models, ageing trend analysis.JRC.F.5-Safety of present nuclear reactor