Using probabilistic methods for solving the problem of ensuring leak tightness of heat exchanger tubes of nuclear power plant steam generators - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 296

Maximum crack size
(9th inspection)

Crack size, mm

9.23 50, 90 and 99% crack depth.

Table 9.3 Estimated number of newly detected cracks in the 9th
ISI for di0fferent confidence levels

Confidence level Number of detected cracks

50% 45

90% 54

95% 56

9th ISI 1968

Table 9.4 Confidence level for cracks with the size of 50, 90 and 95% of
wall thickness

Crack size, mm

Confidence level, %

50% 90% 95% 99%

50% 3.2 4.9 5.4 6.6

￿ ￿ ￿ ￿ ￿

90% 3.3 5 5.5 7

99% 3.3 5.1 5.6 7.6

9th inspection cycle 6.8 6.8 6.8 6.8

by the ISI because for these cracks PDD and effective PDD have values
close to 1.
Figure 9.23 illustrates the distribution of the probability density of crack
sizes corresponding to 50%, 90% and 95% for the 9th ISI. Distributions
were obtained on 1000 data. The distribution of the maximum size of the
crack can also be obtained but in this case the maximum crack size can be
unusually large. This is due to the probabilistic nature of the crack growth
rate and large errors z in equation [9.3]. Thus, a more accurate value for
the maximum crack size can be obtained if the size distribution of cracks