Environmental Engineering Reference
In-Depth Information
rate of 5 l/h corresponds to a crack length of 2
c
= 8.5 mm. Given that the
critical crack size in the NOE mode is greater than 25 mm, it can be stated
that the recommended margins for the crack size and the sensitivity of the
leak control system are met:
2
c
cr
/2
c
Q
= 5 l/h > 2;
Q
(2
c
cr
) / 5 l/h >> 10.
In the HT mode, the critical crack size is 2
c
cr
= 16.2 mm, which
corresponds to leakage
Q
(2
c
cr
) = 128 l/h.
A leak of 5 l/h corresponds to a crack with the size 2
c
= 5.3 mm.
Thus, the leakage control system at the existing SG of the VVER-1000
with the maximum allowable for operation leakage rate of 5 l/h makes it
possible to detect through-wall cracks long before they reach the critical
size at which complete rupture of the tube takes place. At the same time,
the safety factors satisfy the regulatory guidelines adopted overseas (USA,
Germany) and in Russia in the framework of the'leak before rupture'
concept
97
.
Large leaks through continuous cracks in the NOC mode (up to 128 l/h)
set the conditions for detection and plugging of the tubes on the basis of
the HT results.
At the same time, the size of the leak through a crack of about 6 mm
in the NOC mode and a 4 mm crack in the HT mode is very small (less
than 2 l/h) and may in fact be even smaller because of the influence of the
crack shape and clogging of the crack with corrosion products.
An additional uncertainty in the assessment of operating life, reliability
and safety is caused by the lack of data on the reliability of flaw inspection
by both eddy current method and pneumo-hydraulic (aquarium) inspection.
The detectability of these inspections was assessed by the method
described in Sec. 3.1 and Ref. 139. It is based on mathematical analysis
of test results (Fig. 9.30) and comparison of the inspection results with
monitoring metallographic fractography data (Table 9.11).
The detectability of eddy current testing before chemical cleaning of
tubes even for through-wall defects (
a
= 100%
s
) is less than 50%. This
means that at least half of through-wall defects such as cracks or corrosion
pits are not detected in eddy current testing. After chemical cleaning of
HETs the probability of detecting defects significantly increased (almost
doubled). The estimate of the detectability of eddy current testing in general
correlate with the data obtained in the framework of PISC-III programme
in the laboratory conditions involving about 15 groups of NDT inspectors
with eddy current devices
140
.
If carried out immediately after reactor shutdown, the PHI method
does not offer 100% detection of through-wall defects. 100% detection of
through-wall defects was achieved by PHI conducted only after chemical
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