Technical Report NTB 84-32

An assessment of the corrosion resistance of the high-level waste containers proposed by Nagra

The Nagra concept for the final disposal of high-level radioactive waste includes a container ensuring isolation of the waste from the groundwater for a period of time of at least 1'000 years. Two types of containers are proposed by Nagra in current feasibility projects: the first type, for reprocessed vitrified waste is designed as a self-supporting shell; the material is cast steel GS-40. The second type, for spent fuel, is an adaptation of Swedish designs in which the spent fuel elements are cast in lead inside a thick-walled copper container.

The assessment of the corrosion resistance of these containers is based on the conditions expected in a repository in the crystalline bedrock of northern Switzerland. The groundwater is reducing, with salinity up to seawater level and a high sulphate content.

Under these conditions the only significant contributions to the corrosion of copper come from residual oxygen trapped in the bentonite used as backfill material and from sulphate, if it is assumed that the latter can be reduced to sulphide by microbial activity. This is certainly a very conservative assumption, since it implies that sufficient populations of appropriate bacteria can be supported under repository conditions, but it cannot be ruled out on present-day evidence. The use of this together with other conservative assumptions leads to the conclusion that the maximum penetration by corrosion will not exceed 40 mm in 1'000 years.

For iron the same mechanisms must be considered; the stoichiometry, however, is more favourable and the attack is expected to be less uneven than in copper. This results in a maximum depth of attack of 9 mm in 1'000 years. To this must be added a contribution from the direct reduction of water; the relatively few relevant literature data combined with the results of the Nagra corrosion programme indicate that the maximum penetration from this mechanism will not exceed 20 mm in 1'000 years. If the conservatively assessed contributions of the residual oxygen and the sulphate are added to this, a maximum penetration of less than 30 mm in 1'000 years is obtained.

The conclusions are reached using conservative assumptions. The evidence is well documented in the case of copper; it is slightly less complete in the case of iron, but confirming evidence should result from a sustained qualification programme.