Materials management strategies for VVER reactors - Materials' Ageing and Degradation in Light Water Reactors - page 285

￿ Introducing high pH secondary water chemistry.

￿ Replacement of the high-pressure pre-heaters (with erosion-corrosion

resistant tubes).

All of these measures have been implemented at the Paks NPP, and
have completely changed the conditions and rate of ODSCC in the SGs.
Consequently, a better (i.e. decreasing) plugging trend is experienced, which
can also be expected in the long term. The gaps between the tubes and sup-
port grid are still the critical sites, since any remaining corrosion products
will accumulate there. It is therefore diffi cult to forecast the ODSCC rate in
the gaps and the ageing process has to be closely monitored in the future.
Under the new conditions, sludge may accumulate at the bottom area of
the SG and an effective method for draining it must be found. The reserve
in heat-exchanger surfaces of the SG is relatively large (more than 15%).
Considering past experience and the recent plugging trend of the heat-
exchange tubes, none of the SGs would exceed 10% of plugged tubes by
the end of 50 years operation, due to measures implemented (Katona et al .,
2003; Trunov et al ., 2006b). The number of allowable plugged tubes became
more important at the plants where the primary energy output is increased
for the power up-rate. Therefore, establishing an adequate performance cri-
terion for the steam generators is very important.

Ageing of mechanical components of VVER-1000

In the VVER-1000 models, all primary circuit surfaces are either made
from, or are clad in, stainless steel. The 08X18H10T type stainless steel
(08Cr18Ni10Ti, AISI 321) is used for the core structures, main circulating
pumps and steam generator tubing, whilst the main loop pipework and
steam generator collectors are manufactured from 10GN2MFA type carbon
steel and the cladding is made from 08Cr18Ni10T stainless steel. The pres-
surizer is also made from 10GN2MFA carbon steel, covered by cladding,
with an inner layer of Sv-07Cr25Ni13 (similar to AISI 309) stainless steel
and two layers of Sv-08Cr19Ni10Mn2Nb niobium stabilized stainless steel
(similar to AISI 347). The reactor pressure vessel and head is made from
the low alloy steel 15Cr2MNFA. The cladding of the reactor head has an
inner layer of Sv-07Cr25Ni13 stainless steel and two layers of the niobium
stabilized stainless steel Sv-04Cr20Ni10Mn2Nb (again similar to AISI 347).
The phosphorus and copper contents in the welds of VVER-1000 RPVs are
0.005-0.014% and 0.03-0.08%, respectively.
It has been recognized that the standard surveillance programmes for
VVER-1000/320 reactor pressure vessels have some defi ciencies related to
the design of the surveillance assemblies, for example the non-uniformity
of neutron fi eld within individual specimen sets, large gradient in neutron

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