Materials management strategies for VVER reactors - Materials' Ageing and Degradation in Light Water Reactors - page 315

requirements is a precondition for LTO. At the same time, safety is the
most important aspect of public acceptance. The operator commitment
in relation to safety is and will be the decisive point of judgement by the
public.
Most of the safety upgrading measures result in positive technical effects
too. Due to these modifi cations, the safety systems or essential parts thereof
had been practically renewed or reconstructed. Consequently, a large part
of safety systems is un-aged. In some cases, the safety upgrading measures
have a direct infl uence on the lifetime limiting processes. For example, the
new relief valves installed on the pressurizer for the cold over-pressurization
protection eliminate the danger of brittle fracture of the reactor vessel.
Some of the VVER plants implemented an extensive seismic upgrading
programme involving the addition of a large number of new seismic fi xes
and other strengthening measures (see papers in IAEA, 1993). Fixing the
building structures, the anchorage equipment, cabinets, racks and also the
structural support of cable trays can be considered as reconstruction of
these SCs.
The most important economic condition for LTO is preserving the
present cost advantage of nuclear electricity generation within the mar-
ket conditions. By exploiting reserves and advantageous features of the
VVER-440/213 reactors, the electrical output of the plants can be safely
increased up to approximately 500 MWe by improving the effi ciency of the
secondary circuit/turbine and increasing reactor thermal power via imple-
mentation of modernized fuel assemblies. Obviously the power up-rate
should not result in a decrease of the plant safety level and should not cause
stressors of ageing which affect the lifetime extension perspectives and the
plant availability.
The VVER plants replaced the frequently criticized, obsolete I&C sys-
tems. The new I&C systems have proper environmental qualifi cation. Aside
from their obsolescence, the lack of environmental qualifi cation was the
basic issue in the case of the old systems at practically all plants.
One of the major causes of corrosion in the steam generator heat-exchange
tubes local is the high concentration level of corrosion activators (chloride
ions, sulphates, copper oxides, etc.) in the secondary circuit and partially in
the hidden surfaces of the SG secondary side locations. This can be critical
in the case of VVER-440 plants where the steam generators are not prac-
tically replaceable. To limit local corrosion, the high levels of deposition on
tube surfaces should be eliminated to reduce the concentration of the cor-
rosion activators. The most important measure implemented was to replace
the main turbine condenser, for example at Paks NPP (Katona et al ., 2003).
Unlike the old condensers with a copper alloy tube bundle, the new con-
densers with stainless steel tubing are leak tight. They in turn allowed the
introduction of the high pH water regime in the secondary circuit providing

￿ ￿ ￿ ￿ ￿ ￿