In situ corrosion studies on candidate container materials for the underground disposal of high-level radioactive waste in Boom Clay

SCK-CEN has developed in the early 1980's, with the support of NIRAS/ONDRAF and EC, an extensive in situ corrosion program to evaluate the long-term corrosion behavior of various candidate container materials for the disposal of conditioned high-level radioactive waste and spent fuel. The in situ corrosion experiments were performed in the underground research facility, HADES, situated in the Boom Clay formation at a depth of 225 meters below ground level. These experiments place the samples either in 'direct contact' with clay' (type I), in a 'humid clay atmosphere' (type II), or in a 'concrete saturated clay atmosphere' (type III). During the period 1985-1994, twelve in situ corrosion experiments were installed in the underground laboratory. The exploitation of these experiments ended in 1996. All samples were recuperated and analyzed. The purpose of this paper is to summarize and discuss the results from the type I corrosion experiments (samples in direct contact with Boom Clay). Surface analyses tend to indicate that the so-called corrosion-resistant materials, e.g. stainless steels, Ni- and Ti-alloys, remain intact after exposure to Boom Clay between 16 and 170°C, whereas carbon steel presents significant pitting corrosion. Carbon steel seems to be unsuitable for the Belgian repository concept (pits up to 240 urn deep are detected after direct exposure to the argillaceous environment for 2 years at 90°C). The stainless steels look very promising candidate container materials.