Environmental Engineering Reference
In-Depth Information
7.4.8 Optimisation of the volume of inspection
in various stages of the life cycle of nuclear
power plant life cycle (factory inspection,
installation and commissioning, operation).
Superinspection
The overwhelming number of defects identified and repaired during the
operation phase formed at time of manufacture or assembly and are of the
technological nature. The reasons for this situation are both objective and,
apparently, subjective factors. The subjective factor can include the desire
of the manufacturers or assembly organisations to deliver the product to
the customer as soon as possible and the absence of material and legal
responsibility for factory rejects (technological nature of the defects)
identified during subsequent operation. The nuclear power plant pays for
almost all repairs of these defects. Input inspection in nuclear power plants
is often carried out by inexperienced NDT inspectors, since the object is
new and the team at the plant has just been formed. The objective factors
include almost complete absence at the present time of NDT methods and
tools allowing with 100% certainty to detect all defects of the technological
nature directly at the manufacturing plant or after installation.
Detection of defects can be significantly increased (2-3 times or more)
even without changing the methods and means of verification. To achieve
this it is necessary to:
• carefully select NDT inspectors on the basis of the psychophysical,
moral and professional characteristics;
• inspect repeatedly the same location inspected by different NDT
inspectors.
Because of these two measures the number of operational defects missed
can be reduced by 5-10 times or more.
Given the above the following tentative scheme can be proposed for the
inspection of responsible elements of vessels and pipelines.
1. Before inspection, NDT inspectors should be selected on the basis
of the results of 'blind inspection' conducted on full-scale test samples of
hidden defects;
2. Following the stage of factory output inspection, representatives of
the nuclear power plant (customer) in the form of two or three independent
teams of NDT inspectors should carry out 100% inspection of all welded
joints. Repairs should then be carried out at the plant using the inspection
results;
3. 100% inspection of all welds should be carried out in the framework
of input inspection. Equipment should then be repaired according to the
inspection results;
4. During the commissioning tests (after hot and cold running of
equipment, including hydraulic tests) all welds should be subjected to
100% inspection.
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