Investigations on the corrosion behaviour of structural materials of PWR primary circuits

The today main phenomena relating to the corrosion behaviour of structural materials in PWR primary circuits are mainly connected with stress corrosion cracking. Recently, cracks observed on some vessel head penetrations in US plants increase the interest of studies on Alloy 600 and Alloy 182. The extension of the live time of PWR plants in many countries including France induces to investigate the stress corrosion cracking behaviour of stainless steels materials. We describe the experimental procedures and tests performed for investigating the stress corrosion cracking behaviour of nickel base alloys and stainless steels materials, from constant deformation tests with C-rings or Reverse U-bends, to the crack growth rate monitoring by electrical or acoustic emission techniques. Following the objectives of the investigations, specific tests and results exploitation procedures have to be chosen: constant deformation or constant load tests are used for the evaluation of the sensitivity of materials to SCC, while SCC initiation or propagation may be studied with constant elongation tests. Examples illustrate these today investigations on stress corrosion cracking of Alloys 600, 182 and 690 in PWR primary conditions. Up to now, data acquisition has been the main objective of SCC experiments and semi-empirical correlations have been obtained and are used to predict material behaviour in PWR primary systems. New theoretical developments are needed to increase the confidence in the life time predictions. (author)