Formal–statistical methods - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 74

If the effects of ageing of the components significantly affect the
assumptions made in modelling failure sequences or safety features, this
effect can be considered at the level of the structure of event trees (ET)
and/or fault trees (FT) in the PSA model.
In an ageing trend is found in the reliability indicators, it is necessary
to choose adequate models of reliability of equipment and to assess the
impact of reliability parameters on the PSA results.
In Ref. 33, the authors identify two main approaches to understanding
and modelling of the ageing phenomenon:
• ageing, as the concept of reliability associated with the behaviour of
failure rate over time;
• ageing as the physical process of gradual degradation of equipment.
In the first case, reliability is modelled using applied statistical models
with data from operating experience. This approach is mainly used for
simulation of active elements.
In the second case, reliability modelling is based on understanding the
physics and kinetics of the degradation process associated with a specific
ageing mechanism. Such models correspond to the passive elements.
From the viewpoint of the sensitivity of risk assessment to the effects of
ageing of the CCM, special attention should be given initially to failures of
active elements of the safety systems. The active components are explicitly
modelled in FT and ET, and changing the reliability indicators directly
affects the results of PSA and the indicators themselves are calculated on
the basis of statistical data from operating experience.
If sufficient statistical data are available, analysis of the effect of ageing
of the active elements on the safety of a unit may be carried as follows:
• preliminary analysis of statistical data on reliability:
- graphical (visual) analysis of the trend;
- non-parametric methods for testing hypotheses about the presence or
absence of a trend;
- parametric methods for testing hypotheses about the presence / absence
of a trend;
• selection and validation of the reliability model, calculation of
reliability indices;
• Prepare a set of source data for incorporation into models of PSA;
• Assess the risk with ageing taken into account.
Methods for graphical analysis of data on reliability, taking into account
the specificity of the primary data from the operating experience of nuclear
power plant, are presented in Ref. 34-38.
Atwood et al 35 suggests using the cumulative distribution of failures in
time (cumulated failure plot) or interval estimates of failure rates over time.
Klyugel 36 and Prokachchia 37 give examples of using the Nelson-Aalan
and Kaplan-Meier graphical analysis of indicators for the ageing trend.
Two factors that are important from the viewpoint of practical application
of one or another approach are noted in Ref. 38:

￿ ￿ ￿ ￿ ￿