The discovery of some intergranular stress corrosion cracks (IGSCC) on some non-isolable pipes of the main primary system of French pressurized water reactors (PWR) in December 2021 led to a large scale inspection programme on all French PWRs, considering the potential impact of those cracks on safety of the reactors.
In the frame of this inspection programme, more than 60 IGSCC cracks above 2mm in depth have been discovered on the welds of the emergency core cooling and the residual heat removal systems. It is important to point out that this mode of degradation was not expected on the welds PWR piping that is made of wrought austenitic stainless steel with a low carbon content.
The understanding of the root causes of IGSCC on these welds is still ongoing. However, the results of the inspections in France show that some auxiliary piping of the main primary system of the reactors are more susceptible than others. According to the current studies, the design of the pipes and the loads resulting from the variation of the water temperature are factors that could affect a pipe susceptibility to IGSCC.
In addition, inspections showed that the welds of auxiliary pipes that had been repaired during the construction of a plant may present the risk of being affected by large IGSCC cracks. The susceptibility of these repaired welds does not seem to be related to the susceptibility of the pipes: it is probably due to the higher level of residual stress and hardening induced by weld repairs. Consequently, a specific inspection programme has been implemented on French PWRs on the welds of the auxiliary pipes of the main primary systems that had been repaired according to the manufacturing reports.
These findings led WENRA to issue the recommendations that are presented in part 1 of this report. Part 2 presents further details on the IGCSC cracks discovered in France, and part 3 a concise background regarding international experience regarding IGSCC on light water reactors.
The recommendations in this paper only deal with IGSCC potentially affecting austenitic stainless steel welds in non-isolable sections of the primary systems of PWRs. Other related issues, such as IGSCC on nickel-base alloys components in PWRs, or transgranular SCC on chloride contaminated materials, etc. may provide relevant feedback, but are not in the scope of these recommendations.