Formal–statistical methods - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 81

• maximum year average unavailability (MYAU)) - the maximum value
YAU over the life of the unit.
These unavailability estimates are used for sensitivity analysis of models
by varying the parameters b, T and t .
As mentioned in the previous chapter, the availability of qualitative and
representative data on equipment reliability is a prerequisite for modelling
the effects of ageing in PSA.
In general, there are three types of data sources:
• data from operating experience of nuclear power plants;
• data from operating experience of similar equipment in other industries,
• data from accelerated reliability tests.
Data from the experience of operating nuclear power plants give an idea
about the history and current status of equipment reliability and, under
certain assumptions, allow current trends to be approximated. But this is
only an approximation based on current information.
Experience with operation of similar equipment in other industries can
give an idea about the reliability of equipment over a longer period of
operation, provided the operating time of equipment is longer than that in
nuclear power plant. However, the direct use of data on the reliability of
other industries always requires a careful analysis of the representativeness
of data: comparing the characteristics of equipment, conditions and modes
of operation, maintenance strategies, failure criteria, etc.
The data obtained in accelerated reliability tests can be used to judge the
reliability of equipment for the period simulated in the tests. Nevertheless,
the representativeness of the test results strongly depends on the reliability
of theoretical models of accelerated ageing and completeness of the tests
as regards the ageing mechanisms.
In this section, we consider, as a source of baseline data, only the data
on the reliability of operating experience.

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2.5.1 Data from operating experience of similar

equipment in other industries

First of all, it should be noted that the vast majority of literature data on
equipment reliability did not include the effects of ageing, i.e. reliability
indices are calculated on the assumption that the failure rate or probability
of failure on demand is constant (Poisson flow of failures). Therefore, to
analyse reliability with ageing taken into account, it is necessary to consider
treatment to the primary data to which access is usually limited . Furthermore,
in reality there are not many facilities that have been in operation for a long
period (more than 30 years). In this case, it is necessary to collect and analyse
reliability data for these facilities.
As an example, data on the reliability of equipment of thermal power
plants in North America and Western Europe can be analysed 48 .