Environmental Engineering Reference
In-Depth Information
5. During the first 20 thousand hours of operation 100% inspection of
all welds should be conducted;
6. The next inspection should be carried out after a period defined
on the basis of quantitative analysis of the inspection results, equipment
reliability, rate of damage of the metal in service, as well as taking into
account other factors which define the state of the structure. When using this
inspection schedule the number of inspections of the important elements of
the reactors can be reduced to 1-2 for the entire period of operation (and
in some cases to 0) without prejudice to the reliability of the inspected
structural element.
A similar inspection scheme can be applied to inspect the field welds
and can be adjusted depending on the type of structure and operating
conditions. The advantages of this inspection control scheme are obvious.
Firstly, the total number of inspections does not exceed the number
of inspections for the entire lifetime according to the current normative
documents (5-7 inspections). In this case, the cost of inspections will
be lower as inspection is carried out on a clean (not contaminated with
radioactive elements and corrosion products) construction. For this reason,
the efficiency of inspection must also be higher. In addition, the quality of
repairs conducted at the manufacturing plant and(or) on clean equipment,
will be higher and the cost lower.
Secondly, the detected defects can be removed at the expense of the
manufacturer or installation company as the technological nature of such
defects is obvious.
Thirdly, the volume of repairs during operation will be sharply reduced
(5 times or more).
Fourthly, the losses associated with the outages of the nuclear power
units for will be lower.
Fifthly, the reliability of pressure vessels and pressure piping of nuclear
reactors will improved because of more efficient inspection and repair
conducted at the factory or on clean elements of structure for nuclear
power plants.
NPP operating experience provides an example proving the validity
of the above proposals. This case was analyzed in one VNIIAES reports
(Scientific Research Institute of Atomic Power Stations) drawnup under the
guidance of one of the authors of this topic (A.F. Getman).
In the mid 80-ies, the newly commissioned units of nuclear power plants
with RBMK reactors showed an increased number of defects in welds of
the DN800 pipeline of the repeated forced circulation circuit. In this regard,
experts at TsNIITMASh (Central Scientific Research Institute of Heavy
Engineering) proposed to conduct the so-called superinspection. The results
of factory inspection are shown in Fig. 7.21. More extensive inspection was
also conducted before operation of the nuclear power plant. The subsequent
results of in-service inspection of welds on DN800 pipelines of unit 3 of
the NPP which had been subjected to superinspection prior to operation,
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