Environmental Engineering Reference
In-Depth Information
Table 8.1
A comparison of steam generator materials in VVER-440, VVER-1000 and
typical PWR
VVER-440
VVER-1000
PWR
Heat-exchange
tubes
08H18N10T
08H18N10T
Alloy 600, 690 or
800
Tube sheet,
collector
08H18N10T
10GN2MFA, 08H18N10T
and cladding
Low alloy steel
and cladding
SG vessel
22K
10GN2MFA
Low alloy steel
Tube-grid
08H18N10T
08H18N10T
Carbon or
stainless steel
is equivalent to AISI 321. A comparison of the SG materials selected for
VVER-440, VVER-1000 and a typical PWR is shown in Table 8.1.
The oldest VVER-440 type steam generators at Novovoronesh NPP
Units 3 and 4 have been operating for 40 years. The condition of the old-
est VVER-440 steam generators at the Kola and Novovoronesh plants has
allowed a 15 year extension of operation for these plants. According to the
operational history, the feed-water distributor inside the SG shows accel-
erated ageing due to erosion. These elements were replaced at almost all
VVER-440 plants (see the dark coloured new distributor in Fig. 8.1). The
experience regarding the ageing of VVER steam generators is summarized
in TECDOC-1577 from the International Atomic Energy Agency (IAEA,
2007b).
At VVER-440 plants, the lifetime limiting ageing mechanism of the SGs
is outer diameter stress corrosion cracking (ODSCC) of the austenitic stain-
less steel heat-exchanger tubes. The ODSCC indications appear typically
(80%) at the grid structure supporting the tube bundle, where the secondary
circuit corrosion products (with concentrated corrosive agents) are depos-
ited. An eddy current inspection programme is implemented for monitor-
ing the tubes. Samples have been removed from plugged tubes to facilitate
investigations into the phenomenon. The rate of the ODSCC was essentially
slowed down by a series of modifi cations and actions, implemented at dif-
ferent plants and to different extents. The measures implemented are as
follows:
Replacement of the condensers: the new condensers have austenitic
stainless steel tubes.
Removal of copper and copper-bearing alloys from the secondary
circuit.
Replacement of the feed-water distributor (the old one was manufac-
tured from carbon steel).
Cleaning the heat exchanging surface of the SGs.
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