The French national agency for the management of radioactive waste (Andra) is in charge of the deep disposal of high level radioactive waste (HLW). This waste is vitrified and confined in a stainless steel container itself introduced into a container (so called HLW overpack) made of low alloyed steel. These packages will be introduced into disposal cells, which are micro-tunnels drilled in the rock and cased with a micro alloyed steel pipe. Currently the concept includes the injection of a cementitious bentonite grout material between the casing and the rock to neutralize the potential acidity resulting from the excavation of the micro-tunnels and the gradual water resaturation of the rock. During the different stages of the disposal, different risks of corrosion and associated failures have been identified: general corrosion, stress corrosion cracking, H2S assisted corrosion, hydrogen embrittlement and associated cracking. The presented study aims to evaluate the risk of H-assisted cracking in conditions representative of the waste disposal.
Firstly, quantitative data on hydrogen evolution kinetics (polarization curves) were determined in solution simulating the expected disposal environments, particularly after the corrosion products formation. Moreover hydrogen measurements were performed on dedicated samples.
In a second part, the risk of H-assisted cracking was evaluated using slow strain rate tests under cathodic polarization. Different carbon steel grades selected for the disposal concept were studied (X65 and P285NH as references). The hardening effect was studied as well by using pre-deformed tensile specimen (5% strain).
This study is still in progress to provide more results to assess the resistance of steels in various conditions expected in disposal conditions.
