Physico–statistical models based on the residual defectiveness of structural materials - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 127

flaw inspection highly skilled operators invited from different nuclear power
plants. The DUK66PM flaw detector owned by one of the stations was
used. Inspection was carried out according to the rules valid for irradiating
the fusion zone of the casing with the corrosion-resisting cladding on the
side of the base metal with a direct searching probe with a frequency of
2.5 MHz.
Operators were not given special inspection conditions, they were
asked to implement ispection in accordance with the same requirements as
when working on the reactor vessel at their plants. The length of service
of operator No. 1 was 2 years; operator No. 2 5 years; operator No. 3 2
years; operator No. 4 6 years.
The operators recorded all discontinuities and classified them to
acceptable, with an area of up to 20 mm 2 , and unacceptable, i.e., with a
large area.
As a result, the operator No. 1 detected 3 out of 9 inspected
discontinuities with the area of 12 mm 2 which equalled 33,3% of the 12
discontinuities with the area of 20 mm 2 the operator identified as 3, i.e.
26%. Of the 15 embedded defects with an area of 29 mm 2 6 were detected
and 3 of them were rated as acceptable, i.e. having an area less than 20
mm 2 . The overall detection rate was 40%, but with the correct evaluation
taken into account (3 defects) it was 20%. The total detectablity of the
defects with the area of 63 mm 2 was 50% (5 out of 10 embedded defects),
but in this case two defects were evaluated as acceptable, i.e., having an
area smaller than 20 mm 2 . Therefore, based on correct assessment, the
detectability was 30%. Of the 12 defects with the area of 113 mm 2 8 defects
were found, which accounted for 66.6%, but with such large defects 2 were
rated as acceptable and, consequently, the detectability was 50%. From
the total of 58 defects 25 were detected, i.e. 43.1%, but with the correct
evaluation of the permissibility of defect detection was 34.5%. Similar
results were obtained by all other operators (Table 5.1).
The results can be represented graphically (Fig. 5.7).
The horizontal axis in Fig. 5.7 gives the size of the defect in the form
of its area S , the vertical axis is the value that can be interpreted as the
probability of non-detection of a defect with size S :

￿ ￿ ￿ ￿ ￿

P nod = 1- N det / N emb = 1- R d.d ,

where N emb is the number of defects embedded in a sample of this size; N det
is the number of detected defects of the same size; P d.d is the value which
can be viewed as the probability of defect detection.
These results indicate the probability of non-detection of a defect at
ultrasonic flaw inspection is relatively high and decreases with increasing
size of the inspected defect.
Differences in the curves X 1 - X 4 reveal the influence of the subjective
factor on the inspection results. The curve Σ describes the overall result of