Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 223

• class of equipment and pipelines 3;
• equipment and pipelines not included in classes 1-3.
However, this classification takes into account primarily the existing
safety barriers so it does not always determine the levels of impact or
correspond to them, although these levels should be clearly differentiated
by using a model based on risk information and subsequent analysis of the
consequences.
The ordinate of the risk matrix is the probability of an event or
probability of failure (POF). The factors in element D 1 important factors
for this axis are:
• estimate of the probability of failure;
• databases;
• models;
• databases and models without inspection taken into account.
As component failures still occur, data on these failures at the two
highest levels of probability (very high and high) may provide the basis for
performance evaluation in this range. Since the probability goes beyond the
accumulated experience, reliability models of structures depend greatly on
the analysis of structural integrity. The lowest level of probability (very low)
is considered as representing very distant events, relating to the category
of consequences with the lack of probability.
Element A (Fig. 7.3) lists the subjects, criteria and methodologies, which
work closely with ISI. It also shows promising possibilities of inspection,
factors affecting the existing methodologies of ISI and its optimisation
criteria derived from the above interaction. Particular attention is given to
the introduction of an approach based on risk information. Considerable
attention is paid to the process of selecting areas of the ISI and the selection
of the initial data needed for this option, and also to the influence of this
choice on the ISI.
On the left side of element A (Figure 7.3) there is a list of the basic
data required for design which cover a large number of criteria and
assumptions. During operation, these criteria, regardless of their values,
may change, leading to the formation of defects, degradation, or damage
that change the state of structural integrity, which in turn are very likely
to lead to changes in the NDSI programme.
Issues related to manufacture are of particular importance for a
re-evaluation completed after upgrades and repairs, as well as in the case
of detection of defects by using a more efficient inspection methodology
during operation.
Once again, it should be noted that during operation the values of these
criteria potentially differ from their values at the time of manufacture. It
is likely that these changes will affect the structural integrity of the
component. Defects formed in the component in the manufacturing process
can grow during operation, and new defects can form in the boundary
conditions that differ from the design and manufacturing conditions.

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