Environmental Engineering Reference
In-Depth Information
For simplicity, these equations will be written as:
P
= exp [
−g
(
aa
−
);
[5.26]
a
a
D
P
= exp [
−g
( )].
cc
−
c
c
D
Constants γ
a
and γ
c
characterise the rate of decrease of the probability of
existence of discontinuities with increase of their size; these constants will
be referred to as the probabilistic coefficient of residual defectiveness. The
values of
a
d
and
c
d
characterise the threshold size of discontinuities; below
this size the discontinuities exist with the highest probability and these
values can be referred to as the threshold values of the reliably existing
discontinuities (defects).
Quantitative estimates of the probabilistic part of residual defectiveness
(Fig. 5.40) were made in 1982-1983. All values of
a
d
and c
d
, shown in these
figures, lie in the range of unacceptable sizes of discontinuities, if evaluation
is carried out in accordance with PK1514 developed for evaluating the quality
in the manufacturing stage and valid for the service stage
Later, operational inspection of the above structural elements revealed
discontinuities classified as defects according to PK1514 and lying in the
range (
a, c
) > (
a
d
, c
d
). For example, in the 1991 the reactor vessel of block
2 of the Kola Nuclear Power Plant was inspected in service with automated
ultrasound installation Reaktortest made in Czechoslovakia. The detected
12 defects, unacceptable according to PK1514, were found in the reliable
parts of residual defectiveness in the casing of the VVER-440 reactor. This
can be verified this by comparing the geometric characteristics of the
detected defects with the values of
a
d
values in Fig. 5.41. The identified
defects were of technological nature and were missed in because of the
insufficient of reliability of the plant and input inspections. As will be
shown in section 5.3.2, these defects do not influence the design service
life and were kept in service without repair according to the analysis of
strength and residual life.
Cases like the one described above for the reactor vessel were also
detcted for the main pipelines for the VVER-1000 reactors.. They testify
to the correctness of the above-described methods of quantitative analysis
of residual defectiveness and the reliability of non-destructive testing.
The quantitative characteristics of the probability of residual
defectiveness in the cladding and base metal of the reactor vessels and
piping of the VVER-1000 reactors are shown in Table 5.10.
In conclusion, it should be noted that these examples of estimation of
a
d
and
c
d
comparison of the estimates with the permissible and critical
dimensions of discontinuities show that the probabilistic part of residual
defectiveness largely determines the strength, longevity, reliability and
remaining service life of the investigated structural elements.
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