Since 2014 the multi-barriers concept developed for the disposal of high level nuclear wastes, includes a cement bentonite layer. This cement based grout material will be in direct contact with the casing to neutralize the potential acidity resulting from the host rock oxidation induced by the drilling process of the disposal cell. In these pH conditions (around 10.5) and temperature (90°C, maximum expected during in disposal conditions) the metallic materials may be sensitive to stress corrosion cracking (SCC). In this project different environmental conditions (aerated or deaerated, at room temperature or at 90°C) and synthetic solutions are considered to reproduce various periods. A short term (CT) solution has been defined to simulate the cement porewater and a long term (LT) solution for the seepage water going through the cement layer.
The present work is based on SCC tests in order to study the crack initiation sensitivity of carbon steels (P285NH and X65 grades) selected as the reference materials for the metallic components in the multi-barriers concept. Stress specimens are immersed in each solution under aerated or deaerated conditions at 90°C during 8000 hours. Corrosion rate was measured by performing weight loss measurements.
The results showed that after immersion in an aerated solution (CT) simulating the disposal conditions at 90 ° C, the corrosion is heterogeneous, and does not seem to slow down over time. The corrosion rates can reach locally several hundred of microns per year. In direct contact with the cement based grout material, corrosion rates are rather low (between 5 and 12μm/yr) after 8000 hours reflecting a protective nature of the cement. For the specimens immersed in the solution simulating the long-term interstitial water (LT) P285NH steel coupons seem to be less sensitive to corrosion than the X65 ones. In anoxic conditions the corrosion facies are more homogenous.
Concerning the crack initiation investigation, under long-term immersion at the free corrosion potential, several cracks have been observed, but none is related to a stress corrosion cracking phenomenon. The origin of the failure is generally of two natures. On the one hand significant corrosion damage (heterogeneous) of the specimen led to a critical reduction of the section and thus a mechanical rupture of the specimen (stress levels close to the elastic limit). On the other hand the level of stress chosen is likely too high for SCC initiation of the specimens, which failed under 5% of plastic strain.
