LIFETIME EXTENSION OF AGEING NUCLEAR POWER PLANTS : ENTERING A NEW ERA OF RISK
Nearly three years on from the Fukushima nuclear disaster, the 25 oldest nuclear reactors in Europe have all passed 35 years of operation. More than two-thirds of US nuclear reactors have received extended licences permitting 60 years of operation, far beyond their original design lifetimes. We are entering a new era of nuclear risk.
At the time of writing (January 2014) the average age of European nuclear reactors has reached 29 years. An increasing number are reaching their design lifetimes of 30 or 40 years. New nuclear reactor construction in the EU is not capable of replacing all the reactors that are approaching the end of their design lifetimes, and the Fukushima disaster acted as a brake on new build programmes. Nevertheless we are seeing an increasing demand for new strategies to avoid a phase-out of nuclear energy, especially in countries that have not developed viable alternatives.
The current strategy of nuclear operators in much of Europe, including Switzerland, Ukraine and Russia, is targeted at a combination of extension of reactor lifetime (also called Long Term Operation) and power uprating. These factors taken together may have an important impact on the safety of the operational reactor fleet in Europe.
The design lifetime is the period of time during which a facility or component is expected to perform according to the technical specifications to which it was produced. Life-limiting processes include an excessive number of reactor trips and load cycle exhaustion. Physical ageing of systems, structures and components is paralleled by technological and conceptual ageing, because existing reactors allow for only limited retroactive implementation of new technologies and safety concepts. Together with ‘soft’ factors such as outmoded organisational structures and the loss of staff know-how and motivation as employees retire, these factors cause the overall safety level of older reactors to become increasingly inadequate by modern standards.
Measures to uprate a reactor’s power output can further compromise safety margins, for instance because increased thermal energy production results in an increased output of steam and cooling water, leading to greater stresses on piping and heat exchange systems, so exacerbating ageing mechanisms. Modifications necessitated by power uprating may additionally introduce new potential sources of failure due to adverse interactions between new and old equipment. Thus, both lifetime extension and power uprating decrease a plant’s originally designed safety margins and increase the risk of failures.