In order to extend the operating life of a reactor, it is necessary to review all aspects of the safety case. Plants are usually designed for a certain life, which is based on knowledge at the time and forecasts extending over a period of 20 years or more. In practice, the actual working life may be different and depend on a number of factors. These might include (Twidale, 1999):

— changes in operating conditions compared with the assumption in the design (these could affect margins);

— findings from maintenance inspections;

— results of test programmes and;

— outcome of safety assessments.

In addition, the operating experience of the plant (and possibly sister plants) and the accumulation of materials and other plant data will also impact the life. If these are favourable, a licensee may seek permission from his regulator for life extension.

As discussed earlier, PSRs have been introduced as a means of reviewing the safety of a plant on a regular basis. The results of PSRs strongly influence decisions for future plant operation and become increasingly important for older reactors or plants where life extension is under consideration. PSRs are conducted, usually at least every 10 years; for some plants they are conducted more frequently, particularly during later life.

The case for plant life extension would have to confirm the plant’s safety for the proposed additional operation. It would include identifying any features that might restrict the plant-operating envelope during this period. A secondary objective may be to assess the plant’s safety standards against current safety standards. This objective is usually realised somewhat partially since it is realised that it is not reasonable to expect older designed plant to wholly meet the safety standards of the day. In this circumstance, consideration would be given to the age of the plant and the intended life extension.

The top priority components for safety justification review are likely to include the reactor pressure vessel, control rod drive mechanisms, internals, supports and the biological shield, the primary circuit including the pressuriser and steam generators, coolant pumps, containment structures, and control and instrumentation. Other more minor components clearly have a safety justification, e. g. valves, smaller pumps, sensors, etc. but these components would have been routinely replaced under regular operation within the original design life.