Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 226

the signal amplitude and phase, and in radiography a catalog of reference
defects.
This means that such indications are based on assumptions of models
of defects which are used - through abstraction, generalisation and
simplification - to describe the true state of the defects. Taking into account
the determining intrinsic factors of defects, such as their orientation,
structure, depth, and material (e.g. stainless steel) or geometry, it becomes
clear that there may be discrepancies between the results obtained when
using the indication and the actual defect state. In the case of low efficiency
of non-destructive testing (e.g., based on identifying only reflectors of
simple geometric shape for ultrasound inspection), these discrepancies can
be significant. They may even mean the complete absence of any meaningful
testing results.
Compared with element B 1 (Fig. 7.1), representing the commonly used
scheme for evaluating the effectiveness of NDT, the scheme presented
in element B (Fig. 7.4) goes beyond simple ratios of the depth of the
defect with detectability, recommending to consider the real importance
of defects (depending on the types of defect) and distribute them to the
major categories.
The scheme shown in Fig. 7.4, should read as follows:
The left side of the scheme to the part indicated by black vertical lines
are the results of non-destructive testing, characterised by the degree of
importance which may also be known as the indication. Different levels
of indication are characterised by typical criteria relating to NDT, as the
noise level of sensitivity, the level of fixation, etc. Vertical rectangles in the
central part of the scheme (black, gray and white) correspond to different
categories of defects, which are designated by the same tones and are
placed in the right side of the scheme. The diagram shows that the change
of indication values for the category 'allowable defects' is located within
the 'level of fixation' and the 'rejection level' extending behind at both
the lower and the upper boundary. The latter means over-rejection. If all
allowable defects should be recorded, under-rejection should also be taken
into account in the inspection method.
To determine whether a particular method and system of non-destructive
testing are effective or not possible, it is necessary to take into account
the degree of correlation between the indication obtained with ISI and the
actual defect state. The ideal inspection system assumes the existence of
direct correlation.
As stated above, the given correlation demonstrates acceptable but not
a perfect inspection system, since as regards the allowable defects there
would be some under-rejection as the range of indications for these defects
falls below 'the level of fixation' indication. For the allowable defects the
system also permits some over-rejection in the range of indications reaching
the level of rejection.
The most objective information on the effectiveness of inspection can

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