IASCC susceptibility under BWR conditions of welded 304 and 347 stainless steels

Core components of light water reactors (LWR), made of austenitic stainless steels (SS) and nickel alloys, subjected to stress and exposed to relatively high fast neutron flux may suffer a cracking process termed as Irradiation Assisted Stress Corrosion Cracking (IASCC). This degradation phenomenon is a time dependent process in which neutron and gamma radiation are directly implicated in the initiation and propagation of cracking [1]. Although this type of cracking was first recognized in boiling water reactors (BWR), later service failures attributed to IASCC were also observed in pressurized water reactors (PWR) components [2]. Cracking of welded reactor pressure vessel (RPV) internal components, such as core shrouds, has increased in BWR during recent years. Most of these cracking incidents were associated with the heat-affected zone (HAZ) of the welded components. As cracking was located in the HAZ, some core shroud failures have been attributed to classical Intergranular Stress Corrosion Cracking (IGSCC) of thermally sensitized stainless steels, due to the significant grain boundary carbide precipitation occurring in the HAZ during the welding process. However, the intergranular cracking of stabilized and L-grade materials, where carbide precipitation is minimized, cannot be sufficiently explained by the thermal chromium depletion mechanism [3]. Although the maximum end-of-life dose for BWR core shrouds is about 3 × 1020 n/cm2 [4], below the threshold fluence (5 × 1020 n/cm2) for IASCC in BWR of annealed materials, the influence of neutron irradiation in the HAZ of welds is still an open question. As a consequence of the welding process, residual stresses, microstructural and mechanical changes are induced in the welded stainless steels. In addition, neutron radiation can lead to critical modifications in material characteristics and in the surrounding water environments, which can modify the stress corrosion cracking resistance of the components. While the IASCC of base materials is being widely studied, the specific conditions of weldments are rarely addressed.