Formal–statistical methods - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 52

• Identification and evaluation of degradation and failures and failures
of components and systems caused by the effects of ageing;
• Optimisation of operational modes and technologies aimed at reducing
ageing and degradation, increase of reliability;
• Identification of new effects of ageing;
• Evaluation of possible long-term plant operation, including preparation
of applications for renewal of a license;
The existing databases for nuclear power plants are usually insufficient to
assess the reliability and ageing of components of equipment and pipelines,
but they could be used in conjunction with international databases on
failures, for example, those described in NUREG/CR-5750 (Poloski et al.,
1999). This might solve the problem of lack of data for specific components
and small intervals of observation time.
Typically, databases used in the ageing management programs, reliability
and service life, consists of three categories of data:
• Basic data - population, the expected degradation mechanisms,
installation data, qualification data, modification of the project, etc;
• Service data - system and component operating conditions, verifiable
data, failure components, etc;
• Maintenance data - conditions and results of monitoring, service data
(type, date and duration, job description, etc.), cost of repairs, replacements,
etc.
What data are actually needed depends on the capabilities and types of
problems, but mainly on models and approaches used to account for the
effects of ageing in the PSA model.
Studies that were performed in Ref. 20-26 to demonstrate the impact
of ageing effects on the reliability and safety of components and systems
of nuclear power plant included qualitative assessments of periodic tests
and service data, as well as data on failures. In this case, the phenomenon
of ageing could be considered in different ways, depending on the rate of
degradation of the functional components, availability and quality of data
on failures and control conditions.
The general theory of reliability and ageing of components distinguishes
three stages:
• Failures in the early stages of operation;
• A relatively steady stream of failures;
• Increasing the flow of failures after long-term operation.
Identification and evaluation of reliability and ageing in the first (early)
stage are based mainly on operating experience. Quantitative reliability
evaluation is based on the simulation of physical degradation process or
statistical data about the failure. Models of early ageing processes include
the resumption of operation and the non-homogeneous Poisson process.
There are a lot of references describing and discussing in detail ageing
and failures of various types of different components and their mechanisms,
e.g. Refs. 20-26. Below is an example of such approaches to a specific

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