Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 237

Norm rejection size of discontinuity

7. 1 5 Justification of the economic optimum norm of defects in
operation, identified using fracture mechanics methods: 1) defect
norms for manufacture; 2) losses due to repair; 3) optimum normas
of operational defects; 4) losses due to failures; 3) critical size of
discontinuities.

ISI it is necessary to create a mathematical model of inspection in relation
to reliability and economic characteristics of the object operation depending
on inspection. An example of solution of this optimisation problem is given
Sec. 7.4.4. However, often the best solution may be obvious and can be
justified without constructing exact quantitative inspection models. Examples
of this approach are given in Sec. 7.4.8.

7.4.2 Cost-optimum norms of defects in service
(deterministic approach)

￿ ￿ ￿ ￿ ￿

The optimum norms of operational defects are the norms based on fracture
mechanics. Indeed, the left branch of the curve in Fig. 7.15 reflects the
loss of the organisation as a result of repair of non-hazardous defects. The
left branch has the flowing form since with increasing size the number of
discontinuities decreases and, consequently, maintenance costs also decrease.
The right branch of the curve (see Fig. 7.15) is ascending in nature,
since which norms of the defects approach the critical size of discontinuities
(decreasing safety margins) the number of accidents caused by defects
should increase and, consequently, losses from accidents should grow.
The minimum value of the curve of losses in Fig. 7.15 correspond to
the norms of defects identified as permissible defects in operation (subject
to properly safety margins).
As noted above, the Russian nuclear power plants use the norms of
defects for manufacture. In connection with this the volume of repairs at
nuclear power plants to ensure safe operation exceeds the required level