There is a synergy between the possibility of LTO and different plant actions and measures implemented for safety upgrading, power up-rate, improving reliability and plant programmes. Implementation of the safety upgrading programme for ensuring the compliance with national and international 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 modifications, 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 influence 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 fixes 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 mar­ket 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 efficiency of the secondary circuit/turbine and increasing reactor thermal power via imple­mentation 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 sys­tems. The new I&C systems have proper environmental qualification. Aside from their obsolescence, the lack of environmental qualification 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 prac­tically replaceable. To limit local corrosion, the high levels of deposition on tube surfaces should be eliminated to reduce the concentration of the cor­rosion 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 con­densers with stainless steel tubing are leak tight. They in turn allowed the introduction of the high pH water regime in the secondary circuit providing better operational conditions for components of the feed-water system and for the steam generators as well.