The aging of nuclear power plants

A right assessing of the aging state of the components of a nuclear plant is necessary to extend its operating life. Particularly for 3 components that cannot be replaced: the containment building, the reactor vessel and some electric cables. The impact of irradiation on vessel steels is taken into account through the presence of steel samples in the reactor vessel that receive a higher cumulated radiation dose than the vessel itself. The study of these samples gives the real state of the vessel steel months or years in advance. The aging of the containment building is controlled through sensors embedded in the walls, periodical controls and a pressure test every 10 years. Degradation mechanisms in steels, concretes and in cable sheaths have been studied thoroughly and now reliable simulation are within reach. The example of increased thermal fatigue in mixing zones in pipes where 2 fluids at various temperatures meet, shows the importance of periodical inspections because this phenomena was ignored when the reactors were designed and was discovered for the first time at the Civaux plant in 1998 during inspection. Another example concern the replacement of steam generators (SG). In 1990 EDF launched the replacement of all the SGs of the 34 900-MW-reactors. Today the replacement of the SGs of the 26. unit is being done and it benefits from the last technological improvements: the alloy-600 of the SG tubes is replaced by the alloy-690 that is still based on nickel but has more chromium which enables it to be less sensitive to corrosion and makes maintenance easier. Wear and the fatigue of the components are not the only reasons to limit operating life: a nuclear power plant has to evolve to comply with the most recent safety standards. For instance, life extension implies the implementation of post-Fukushima measures that keep the installation safe in the worst and unlikely accidental situations. (A.C.)