Environmental Engineering Reference
In-Depth Information
After performing all of the on-site inspections, the fi ndings have to be eval-
uated and any corrective measures identifi ed. The fi nal result of their eval-
uation can result in:
modifi cation of the maintenance procedures
modifi cation of the periods of maintenance
introducing new diagnostic/monitoring measures in order to determine
the necessary additional actions
performing additional evaluation of the situation
modifi cations such as implementing new sealing
replacement of the component for a different type.
The information obtained has to be taken into account while reviewing
and developing the ageing management programmes. Review and revali-
dation of the time-limited ageing analyses can also be considered as part of
the evaluation of the part conditions. Feedback from experience of other
VVER plants and the research results provide some guidance and back-
ground information for the review and evaluation of plant condition.
8.3.2 Ageing of mechanical components
The VVER-440/213 reactor pressure vessel
The design of the VVER-440 RPV is rather specifi c: the relatively small
RPV diameter has to allow its transportation on rails. As a consequence of
its limited diameter, the water gap between the RPV and the core is small,
so the fast neutron fl ux (
E
>0.5 MeV) on the RPV is rather high at 10
15
m
−
2
s
− 1
and the RPV base material should therefore be more resistant to irradia-
tion embrittlement. The RPV is assembled from forged rings without lon-
gitudinal welds. The coolant from the low-pressure emergency core cooling
systems and hydro-accumulators is directly injected into the RPV, and from
the high-pressure system into the cold leg of the loops. The inlet and outlet
nozzles of the loops are separated on different levels. The penetrations for
the instrumentation for core control are on the RPV head.
The ferritic steel reactor pressure vessel is clad internally with austen-
itic stainless steel. The RPVs are made from low alloy steel (15Cr2MVA;
at Loviisa NPP 12Cr2MFA) and the circumferential submerged arc weld-
ing was made using Sv-10CrMoVTi wire. The RPV was covered inter-
nally by a welded clad of two stainless steel layers. The inner layer is a
non-stabilized stainless steel (Sv-07Cr25Ni13, similar to AISI 309) and
that, when in contact with the coolant, is a niobium stabilized stainless steel
(Sv-08Cr19Ni10Mn2Nb; Sv-07Cr19Ni10Nb at Loviisa; both equivalent to
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