Residual life assessment, technical improvements and backfitting of components and systems from the materials standpoint

The safety standard which is attained at the time of initial startup by a nucleaar power plant built in accordance with state-of-the-art design and engineering principles must be assured throughout the plant's entire lifetime. Based on operating experience a plant's original design life should be systematically reevaluated in the light of new findings and developments in safety. The results of an analysis of this type can also be useful for the purposes of preventive maintenance or to prolong a plant's planned or licensed service life. Moreover, they form the basis of decisions regarding technical upgrades and backfits and are of value in optimizing plant reliability and availability. A concept exists for components in light water reactors which provides for prompt identification and remedying of damage due to the deterioration in service of materials and their properties (e.g. fatigue, local corrosion mechanisms, erosion corrosion, neutron irradiation). It may be necessary or appropriate to implement technical upgrades or backfits where application of more up-to-date safety standards (e.g. break preclusion methodology) demands compliance with more stringent requirements than those originally used in the design basis, or where systematic damage cannot be ruled out (unstabilized austenitic steels in BWR plants), or where it is possible to make substantial reductions in the radiation exposure of operating or maintenance personnel (substitution of cobalt-containing materials) during the plant's lifetime. (orig.)