The economic case will depend on many factors. The principles for estimating the lifetime extension costs are not different from those used for assessing the costs of plant upgrades, design changes, plant decommissioning or new construction. However, there may be some particular site-dependent factors that need to be taken into consideration.

Plants built during the early days of nuclear power production tended to be diverse; as lessons were learned, designs were improved, and unit capacities were increased. Thus, applications and therefore costs for life extension for early designs are likely to be design specific. This will be less so in the future as later current generation plants reach their design end of life and are considered for life extension.

Even for a given reactor design, there may be significant technical differences in the state of the important components. An obvious example is the state of the reactor pressure vessel. This will largely depend on the integrated neutron fluence experienced during its design life. This will be affected by the power level at which the plant has been operated, the effectiveness of the vessel protection radiation shield, whether the vessel has been annealed, etc. In short, the operating history will be needed to assess the state of the vessel.

Measures may be taken to upgrade certain components during initial planned life. These may be upgrades to improve performance, improve safety, or to improve or mitigate the effects of ageing. For example, steam generator replacement has been performed on a number of plants to improve reliability of operation. Component replacement during normal design life will clearly be beneficial to improving the chances of life extension for the particular plant. Further, higher costs incurred during normal operation could well mean reduced costs for life extension.

Other factors identified in IAEA-TECDOC-1084 (1999) include differences in costs due to differences in regulations between countries, resulting in differences in costs in the safety cases presented by the utilities. Another factor in assessing the comparative economics of lifetime extension costs rests with the proprietary nature of plant data. Utilities are reluctant to release information that might benefit another competitive utility.

Generic cost data for US plants have been published in IAEA-TECDOC-1084 (1999). These show lifetime extension costs relative to the building of new nuclear plant compared

Table 2.10. Lifetime extension versus new building costs ($US per kWe)

Lifetime extension 210- 840 Based on Surry 1 (PWR) and

Monticello (BWR)

New nuclear plants 0 2000 Building costs only

New combined cycle units 700-900 Building costs only

Data from IAEA-TECDOC-1084 (1999).

with combined cycle plants. There are considerable uncertainties on life extension costs but the conclusion is favourable for life extension at least on the basis of building costs of new plant (Table 2.10).

Also in IAEA-TECDOC-1084 (1999), an attempt is made to identify site/plant specific factors that have most influence on the cost and tend to push the cost estimates to either the higher pessimistic or to the lower optimistic figure.

With regard to plant design, steam generator replacement and reactor pressure vessel annealing were key factors for PWRs while replacement of pipes and reactor pressure vessel internals were the key factors for BWRs. For reasons explained earlier, newer plants would be expected to incur lower costs than older plants. However, this is somewhat obscured since the former will tend to have longer design life than the latter.

The schedule for implementing the various measures is important. If the intention is to continue the extended operation immediately beyond the end of design life, it is advantageous to begin lifetime extension measures during earlier scheduled outages. Other costs identified were the costs of replacement power to meet the demand during the intervening period. Such costs are clearly highly power system specific.

Finally, costs to meet the demands of the regulator and also possibly to overcome the concerns of the public also need to be factored into the cost balance.

It is concluded that plant-specific costs be required in order to make realistic cost estimations. To evaluate the competitiveness of life extension options, it will be necessary to compare with other power-producing options, including both nuclear and non-nuclear. However, while not possible to produce a generic economic case for life extension, it is clear that a number of utilities have addressed the issue for their own plants and have come out in favour of the benefits of life extension.