2014:57 Technical Note, Issues in the Corrosion of Copper in a Swedish High Level Nuclear Waste Repository: Phase III. Role of Sulphide Ion in Anodic and Cathodic Processes

The most important findings of this research project show that:

  1. In the case when the bentonite buffer between the copper canister and the granitic rock is not damaged, we can neglect the possibility of general corrosion damage being a threat to canister integrity over a 100,000 year storage period.  However, if the bentonite buffer is damaged for any reason, and does not act as an engineered barrier, it is predicted that the direct access of sulphide species to the canister surface could degrade the canister within a period of up to 48,000 years. An analysis of a highly unlikely event considering high temperature degradation of the canister together with seriously damaged buffer/missing buffer directly after emplacement generated a degradation time of up to 8,500 years for the copper canister. This result highlights the importance of the initial state of all the barriers (the granitic rock / the initial condition of the deposition hole, the bentonite buffer and the copper canister) for the long term stability of the KBS-3 system.
  2. Model predictions show that, in principle, the canister can be protected by the reduction of porosity of the outer layer of the passive film by compression caused by impingement of the growing corrosion product layer against the buffer.  Model calculations show also that the tangential (hoop) stress on the outer layer/bentonite interface can become greater than the tensile strength of the bentonite and cracking of the bentonite may occur.  Such cracking can cause, for example, the appearance of “shortcuts” between the canister and the rock, which might lead to accelerated corrosion. However, all estimates that were performed can be considered as being only “model” in nature, because no experimental data on the elastic properties and initial porosity of bentonite and outer layer of the passive film in this system have been used to verify the outcome of the model.
  3. The most important finding of this analysis is that general corrosion of the copper canister may be self-limiting, because of the compression of the outer layer of the passive film and the near-field buffer, resulting in the porosity of one or both phases going to zero and thereby denying access of water and sulphide ion to the canister surface, which is required for continued corrosion.