Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 239

Operating time

7. 1 7 Determination of the economically optimum time period
between inspections: 1) critical size of discontinuities; 2) allowable
size of operational defects; 3) norms of defects in manufacture.

If the ISI methods or other methods of technical servicing in operation
do not enable the desired level of reliability to be reached, it is economically
reasonable to postpone operation and modernisation of the structure in order
to enhance its reliability (safety).
It can be shown that for the cases described by curves 2 and 3 in
Fig. 7.16, the economically optimum time interval between inspections is
the time during which the size of a discontinuity increases from the size
defined by defect norms in manufacturing to the permissible size of defects
in operation (Fig. 7.17) 106,111 .
The scheme shown in Fig. 7.17 for determining the economically
optimum time between the inspections is only applicable if the inspection
means and methods provide absolute inspection reliability. In fact, the
detectability of defects both at the level of defect norms in manufacture
and at the level of admissible sizes of defects in operation is not 100%. The
actual detection is significantly below 100%, which means that the
definition of the economically optimum time interval between inspections
requires knowledge of the laws of probability of ISI or, more precisely,
the probability characteristics of the detection of defects and inspection
reliability.
Optimisation of inspection sites and inspection volumes throughout
the entire operation and combination of inspection methods also requires
knowledge of the laws of probability of ISI and failure and will be discussed
below.

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7.4.4 The optimum time between the inspections

(probabilistic approach)

Since operational inspection is associated with material costs, the optimum