Copper behaviour in geological nuclear waste disposal

A historical overview of copper behaviour in geological spent nuclear fuel disposal is presented based on the research carried out during the last 40 years. Development of systems for disposal of radioactive waste from nuclear power plants was initiated in Sweden in the mid-seventies. The work resulted in the KBS-3 method, which was approved by the Swedish government in 1984. Since then the KBS-3 method has constituted the reference method in the Swedish programme, and later in the Finnish programme. In KBS-3 method, copper container is designed with an external, self-standing copper shell and an internal structural nodular cast iron insert. This design provides a relatively thick corrosion barrier of 50 mm of copper. Recently, a single shell design envisaging the use of a coating system onto a carbon steel substrate has been developed in Canada consisting of a relatively thin layer of copper (3 mm). This design eliminates the potential of creep failure during the deformation of the external copper shell onto the cast iron insert. However, given its lower copper thickness, this design increases the potential impact of any localised corrosion, and depending on the thickness of the steel structural member, of radiation-induced corrosion due to the higher radiation fields expected outside of the waste containers. Currently it is anticipated that the main degradation mechanism of copper is associated with sulphide present in groundwater, in the buffer, and locally produced by microbiological activity. In compacted bentonite clay, the general corrosion rate is expected to be dominated by the slow transportation of sulphide in the bentonite. In addition to general corrosion, the potential for radiation-induced corrosion, localized corrosion, stress corrosion cracking, and hydrogen-induced cracking are considered. Additionally, creep ductility of copper is examined, since originally it was intended to use pure oxygen-free Cu-OF (without phosphorous) for the canisters. The choice at present is oxygen-free copper with phosphorous alloying (Cu-OFP, 30…100 ppm P).