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 295

Probable number of cracks formed on the
basis of the calculated data (NIS)

Number of cracks formed during the ninth
operating cycle
Detectability level

Number of pipes with cracks

9.22 Modelling of the probability distribution of the number of
new cracks detected in the 9th ISI.

Table 9.2 Estimated number of newly formed actual cracks in
the 9th operating cycle for different levels of confidence intervals

Detectability level, % Number of new physical cracks

50% 95

90% 109

95% 112

Data for 9th ISI 1988

The projected number of newly formed cracks in the 9th operating
cycle are shown in Table 9.2 with a confidence level of 50%, 90% and
95%. Forecasting is performed using the three-parameter Weibull function,
as described above. The value corresponding to the 50% confidence level,
shows 88 cracks, which is similar to the number of actual cracks, calculated
from the results of the 9th ISI.
It is also necessary to predict the number of newly detected cracks as
well as the number of actual cracks. Figure 9.22 shows the probability
distribution of the number of newly detected cracks in the 9th ISI. The
estimates, corresponding to 50%, 90% and 95% confidence level, are also
shown in Fig. 9.22 and in Table 9.3. Comparison of 68 cracks, obtained
during the 9th ISI, with the simulation results shows that the simulation
gives a slightly smaller number of cracks. The reason for this may be that
the values of PDD are higher than the true values.
The discrepancy between the predicted number of cracks and the ISI data
is associated with shorter cracks. For long cracks the predicted number of
detected cracks is almost always the same as the number of cracks obtained

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