Environmental Engineering Reference
In-Depth Information
cleaning of HETs and their HT at excess pressure of 24.5 MPa in the
primary circuit coupled with lack of counter pressure in the secondary
circuit.
The low detectability of the results of eddy current testing is also
indicated by direct comparison of eddy current testing data with the results
of metallographic fractographic analysis of HETs cut out on the basis of
the eddy current testing results (Table 9.11). The data in Table 9.11 are not
sufficient for making statistically convincing conclusions, but preliminary
conclusions on the low detectability of quality inspection and a large
number of over-rejected and under-rejected cases can be made.
These data on the actual detectability of eddy current testing and PHI
are crucial for the understanding of the state of the structure, evaluation
of the causes of damage and justification of the forecast of its behaviour.
Soon after unit 2 of the Balakovo NPP was commissioned after
scheduled-preventive maintenance (SPM 96), repeated eddy current testing
on the HET was performed in SPM 98. The results of inspection again
showed a large number of damaged HETs. At meetings of the working
group the participants unanimously expressed the view that the rate of
corrosion cracking in HETs is very high and the steam generators should
be immediately replaced..
However, our analysis of the actual detectability of ECT and of the
residual defects in the tube bundles after the first and second ECTs,
supplemented by estimates of the kinetics of cracks, convinced experts on
the possibility of continued SG operation. Thus, graphs were presented of
the initial defectiveness of a tube bundle prior to ECT in SPM 96, residual
defects after ECT during SPM 96 and plugging of the HET with defects
to more than 70% of tube thickness, and also for the residual defects after
SPM in 1996 during which ECT was repeated and defects were plugged in
more than 75% of tube thickness (Fig. 9.31).
Before the final technical measures in SPM 96 were specified and a
decision was made to continue steam generator operation, the residual
life and safe operation of the tube bundles of the steam generators were
estimated, taking into account the damage detected in SPM 96 and the
detectability of inspection.
The residual life was estimated for the tubes without damage, for
the tubes with damage in the form of cavities, and for the tubes with
cracks. Figure 9.32 shows the kinetics of propagation of a surface crack
with the depth equal to 70% of wall thickness and for a through-wall crack
with the initial length of 11 mm. Thus, in 20 years of operation in the
nominal conditions (including those with the water chemistry regime) the
growth of cracks did not exceed 1%. Analysis of the residual life showed
that when the tubes with defects in more than 70% of wall thickness are
plugged, the steam generator can be operate up to the end of the design
operating life (i.e. 30 years). Curves of the integral probability of the loss
of integrity of tubes are shown in Fig. 9.33.
￿ ￿ ￿ ￿ ￿
 
Search WWH ::




Custom Search