1.1. BACKGROUND

The design life of a nuclear power plant (NPP) does not necessarily equate with the physical or technological end-of-life (EOL) in terms of its ability to fulfill safety and electricity production requirements. Operating equipment, generically called systems, structures and components (SSCs) in a NPP is subjected to a variety of chemical, mechanical and physical conditions during operation. Such stressors lead to changes with time in the SSC material properties, which are caused and driven by the effects of corrosion, varying loads, flow conditions, temperature and neutron irradiation, for example.

Even allowing for significant ageing effects in SSCs, it is quite feasible that many NPPs will be able to operate for times in excess of their nominal design lives, provided appropriate and proven ageing management measures are implemented in a timely manner. This aspect has been recognized by operators and regulators alike, as seen in the number of license renewal applications and approvals, respectively, in the USA, and, elsewhere, by extending licensing procedures, primarily based on periodic evaluation of safety, i. e. periodic safety reviews (PSR).

In general, heavy water reactor (HWR) NPP owners would like to keep their NPPs in service as long as they can be operated safely and economically. Their decisions depend upon the business model. They involve the consideration of a number of factors, such as the material condition of the plant, comparison with current safety standards, the socio-political climate and asset management/ business planning considerations.

Historically, most NPP owners (including owners of HWRs) felt that their routine maintenance, surveillance and inspection programmes would be adequate in dealing with the ageing processes that would occur at their plants. However, starting in the early 1990s and following, it has become widely recognized that a more systematic and comprehensive approach generally known as integrated plant life management (PLiM) or life cycle management (LCM) is needed.

Fig. 1 shows the number of HWR reactors by age. 18 reactors have been operated more than 20 years and 5 reactors have been operated more than 30 years.

Fig. 1. Number of HWR reactors by age as of October 2005.