Environmental Engineering Reference
In-Depth Information
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The point of intersection of curves 1 and 2 with curve 3 corresponds to
optimum frequency of flaw inspection of the cylindrical part of the reactor
pressure vessel for 30 years of operation. As seen from Fig. 7.18, in the
case of a special reactor shutdown for the inspection of the reactor pressure
vessel the inspection frequency is approximately 0, i.e. in this case, flaw
inspection is economically impractical.
In the case of combining inspection with planned preventive maintenance
(PPM) and refuelling, the optimum frequency of inspection is slightly
higher than 1 which means that the optimum time between the inspection
of the cylindrical part of the reactor is 10 years. The time interval between
inspections adopted in Western countries (US, France and others) is also 10
years. The four-year interval between the inspections of the reactor pressure
vessel, adopted at nuclear power plants in Russia and Germany (once in
four years) is conservative and economically optimum.
7.4.6 ISI optimum frequency of the pressuriser
Analysis was performed for the most stressed sections of the pressuriser
(PRZ) casing - jumpers between the holes in the bottom of the casing. In
these places, refined analysis of stresses revealed elevated levels of total
membrane stresses and a very low safety margin according the criterion
of transition of the jumpers to the plastic state (the actual safety margin is
equal to 1 at a regulatory value of 1.5). In this regard, the actual reliability
of the casing and the conditions for the service inspection was analyzed
in 1976-1977.
The PRZ casing was made from 22K steel. The wall thickness of the
bottom was 145 mm. From the analysis of operating conditions it was
found that the reliability of the PRZ should be evaluated by the criterion
of resistance to: unaccetable plastic deformation; ductile, quasi-brittle and
brittle fracture, as well as by the criterion of resistance to the nucleation
of fatigue cracks.
As a result, computational and experimental studies have identified the
main parameters determining the state of the PRZ and the requirements
to control them during operation (Table 7.3). The characteristics of the
probability of nucleation of fatigue cracks and optimum frequency of their
inspection were obtained as an intermediate result. Figure 7.19a shows the
variation of the probability of nucleation of fatigue cracks in the jumpers
of the dished end of the PRZ, and Fig. 7.19b shows a graphical solution
of the optimisation equation. The optimum frequency of inspection of the
dished ends of the PRZ to detect fatigue cracks is 1 in 30 years. Since
the inspection frequency of developing technological defects is higher, in
practice high frequency of inspection was recommended.
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