The French national radioactive waste management agency (Andra) is investigating disposal of High-Level radioactive Waste (HLW) in a deep geological disposal. The reference concept for HLW disposal cells consists of a multi-barrier system: horizontal tunnels of about 0.7 m in diameter drilled in the Callovo Oxfordian (COx) claystone, cased with carbon steel (C-steel) and containing C-steel overpacks with the HLW packages. To neutralize any acidity due to the oxidation of sulphur-containing minerals present in the claystone, a cement-bentonite mixture is injected between the C-steel casing and the geological medium [1]. As the grout should impose alkaline conditions (pH ~10), passivation of C-steel, or, at least, formation of a magnetite protective layer, is expected after a progressive decrease of the oxygen rate in the environment.
In order to assess the efficiency of the cement-bentonite grout regarding its physical properties, several in-situ experiments were performed at Andra’s Underground Research Laboratory (URL). One of them corresponds to a full-scale disposal tunnel, made by a C-steel tube surrounded by a grout in-filling within the clay host rock. To understand the behavior of carbon steel in this specific grout, samples of the steel-grout interface were taken thanks to a robot capable of coring small samples - about 10 cm length and 2 cm in diameter - from inside a HLW micro-tunnel. To complete the behavior understanding, aerated lab experiment of carbon steel surrounded by the grout was done during 100 days at 25°C following by 90 days at 90°C.
Characterizations of the in-situ samples with different exposure times (6 months and 1 year) and the lab sample were carried out in order to identify the evolution of the interface. The corrosion products were investigated by µ-Raman spectroscopy and by scanning electron microscopy coupled with energy dispersive X-ray analysis.
For a reaction time of 6 months, the C-steel surface exhibits a calamine layer due to elaboration and tube fabrication. In addition, heterogeneous corrosion is observed, with about 10 % of the surface showing large and deep depressions (up to 170 µm) filled with goethite. The corrosion is always heterogeneous after 12 months with a mixture of goethite (majority), lepidocrocite, magnetite and Fe sulphide (mackinawite and greigite). The presence of Fe III compounds points out that the environment is still aerated even after 12 months. However, the observation of magnetite (Fe II) and Fe-sulphide (Fe II or Fe II/Fe III mixture) indicates that the environment could progressively tend towards anoxic conditions. The high temperature seems to accelerate the corrosion behavior since up to 300 µm of corrosion products layer is measured, mainly composed of goethite. Also, the presence of sulfur-containing phases between the transformed medium and the corrosion products layer is observed. The influence of the residual oxygen on the formation mechanism of the corrosion layer and wherefore the Fe-sulphide layer is formed will be discussed.
[1] Michau N, Bourbon X. BOPI. Patent FR3031103(A1), 2014 December 24.
