Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 218

the mathematical optimisation task can be formulated as follows: minimise
(maximise) the target function with the constraints on the controlled
variables taken into account.
Discussing the optimisation of in-service non-destructive testing (ISI),
various authors have referred to various parameters and various optimisation
criteria.
Thus, V.N. Volchenko 75 investigated the definition of economically
optimum standard forms of defects q , based on the analysis repair costs. In
this case, under the norms of the defects q refer to the allowable percentage
of rejects in a batch of parts of the same type (or welded joints).
In Refs. 105 and 106, the optimumly organised operational of non-
destructive testing is the system providing the required level of safety
specified by the standard documents. The level of safety was estimated by
the probability of failure of structural elements of the reactor, reactor core
melting, and the release of radioactive elements outside the reactor 105 . The
optimisation parameters, as well as the optimisation method are not defined
accurately in Refs. 105 and 106. The advantage of Refs. 105 and 106 is
that they attempted quantitative analysis of ISI as part of safety systems.
An optimisation method of ISI on the basis of the criterion of achieving
maximum reliability of the controlled element of the structure of a nuclear
reactor was proposed in Ref. 107. The optimised parameter is the time
interval between the inspections. This formulation of the optimisation task
is not entirely justified since it is obvious that the higher the frequency
of inspection, the higher reliability can be ensured. Taking this apparent
regularity into account, the time interval between inspections is often
reduced in practice in order to increase the reliability of the given structure.
As discussed in Refs. 75, 105-107, ISI is optimised using probabilistic
representation of strength. In reality, the optimisation can be carried out in
both the deterministic and probabilistic formulations 89 .
Recently, a number of papers have been published on the risk-oriented
organisation ISI which can be called a semi-quantitative approach to
optimisation of ISI. An example of this approach is given in the following
sections 7.2 and 7.3.
A quantitative approach to the optimisation of ISI based on a generalised
methodology for determining the probability of failure is described in
section 7.4 and 7.5 as well as in section 9 in solving the problem of
ensuring the integrity of the heat exchanger tubes of steam generators.

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7.2 Overview of approaches to optimising ISI, based
on information about risks:

Semiquantitative approach

This section has been prepared following the documents ENIQ 108 and IAEA-
TECDOC-1400 109,110 and a document prepared based on them 153 .
The task of ISI based on information about risk is to form a rational