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The Supercontainer (SC) design is the preferred option for the final disposal of high level nuclear waste and spent fuel in Belgium. The SC consists of a carbon steel overpack surrounded by a very thick concrete buffer, contained within a stainless steel envelope. In this highly alkaline environment of the annulus and under normal conditions (without the ingress of aggressive species), the carbon steel overpack will be protected by a passive oxide film, which is believed to result in very low and almost negligible uniform corrosion rates. This paper discusses the modeling efforts simulating the evolution of various parameters that can potentially influence the corrosion processes (temperature, pH, saturation degree and concentration of aggressive species). The outcome of the Belgian experimental program to study the passive uniform corrosion of carbon steel in concrete is also addressed. Long-term uniform corrosion rates are determined using different independent techniques (hydrogen gas measurements by means of manometric gas cells or pressure transducers, long-term passive current density measurements).

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The leaching behaviour of Pu, Am, Np and Tc from the high-level waste glasses of interest to Belgium has been studied. Corrosion tests were carried out on inactive glasses (the Cogéma R7T7 glass SON68 and the DWK Pamela glasses SM513 and SM527), doped with the radionuclides. Nuclide leaching was studied in contact with a slurry of Boom clay, diluted with claywater, at 90°C and 40°C, for durations of up to 4 years. The retention of Pu and Np in the glass surface layers was considerable for SON68 and SM513, but small for SM527. The retention of these elements decreased at lower temperature. Tc was not retained. The largest part of the leached activity is adsorbed onto the clay in an immobile form. Nuclide concentration in mobile form (smaller than 105 MWU) was found to be independent on duration for Pu and Am. Increasing concentrations were found for Np and Tc. The mobile concentrations were smaller in the presence of corrosion products.