Non destructive examinations and degradations: the use of feedback experience to improve maintenance policy

In France, since 1995, 58 pressurized water reactors have been operating to produce electricity. These reactors were built from 1975 to the beginning of nineties and are ranked in 4 standardized series, 900 MWe CP0 types, 900 MWe CPY types, 1300 MWe types and 1400 MWe types. The plants have undergone evolutions taking into account the feedback experience for manufacturing and construction, net power growth but also maintenance operations. In 1977 the first maintenance programs were based upon the American experience using ASME section XI code. As and when required, they have been modified with the events appeared during the operation. First of them concerned the steam generator tubes with the discover of the first primary water stress corrosion cracking at the end of the seventies then the under-clad cracking phenomenon in the 900 MWe pressure vessel nozzles. At the beginning of years 2000, the new departmental order dedicated to operation required to check and modify, if necessary, the maintenance policy to better take into account the possible damages, fast fracture studies for primary and secondary systems and, last but not least, qualification processes for non destructive examinations. These new requirements have induced a rewriting of the maintenance policies and inspection programs in the way to be more consistent with the need of an improved availability of the plants while keeping a high safety level. Through few examples, we show how the approaches and the practices have been modified in terms of non-destructive examination (NDE) and particularly the impact of the NDE qualification results. In addition, examples of the important choices between in-service inspection and repair policy are also described. (author)