Formal–statistical methods - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 85

Increase as a result of
ageing 0.28

Failure rate
9.1·10 -5

Time to start of
ageing 9.2 years

Years

2.13 Failure rate for a pump of the CCW system - linear model
with a starting point for ageing of 9.2 years.

Table 2.3 Relative increase of the rate of failure per annum , %

Equipment RHR system CCW system

Pumps 30.7% 17%

Electric drive fittings 0.2% 11%

Check valves 1.7% -

Manual valves 21.9% -

Heat exchangers 1.6% 8%

Pressure sensors - 3%

PSA of German plants are analysed in Ref. 50.
Processed information about failures and operating time of equipment
is used in all these cases in one of the formats needed to calculate the
reliability indices:
• operating time to failure and/or censoring of observations,
• interval values of intensities of the flow of failures.
Analysis of the data shows that for a large group of equipment included
in the PSA model analysis of failure rate trends does not make sense due
to lack of data on equipment failures.
For groups of components where available statistics allows for
quantitative analysis of reliability there is no increasing trend of the failure
rate parameter. In cases where the increasing trend is identified, its nature
is close to a linear function with relatively low values of the 'ageing
parameter'.
The applied reliability models do not require any additional information
about the relationship between failure and ageing. On the other hand, there
are difficulties in obtaining other necessary information, such as:
• information on the commissioning date ( T = 0);
• Information on starting and ending time of observation;

￿ ￿ ￿ ￿ ￿