EURAD Domain Insight (3.2.3) - Novel containers
The waste container is an essential part of the multibarrier systems designed for deep geological disposal. After emplacement in a deep underground repository, canisters will be exposed to the repository environment and its evolution in time. Assuming the nuclear waste repository remains undisturbed (i.e., there are no earthquakes, human intrusion or related phenomena), the only mechanisms by which radionuclides could reach the biosphere is by the dissolution of the waste in the groundwater followed by migration of radionuclides to the surface, or by transport of volatile radionuclides in the gas phase. Consequently, one of the major factors in selecting a container solution for disposal in an underground repository is its resistance to degradation by groundwater that may eventually penetrate the repository environment. Following disposal, resistance to environmental damage processes, including all relevant corrosion mechanisms, is therefore important (Holdsworth 2013). Currently, steels and copper are the materials proposed for adoption worldwide as nuclear waste disposal canister materials. The accurate prediction of the lifetime of the waste container represents an important input for the safety assessment of the disposal system. This requires a good understanding of the corrosion/leaching behaviour of the canister material over periods of thousands to hundreds of thousands of years. This mechanistic understanding may come from existing information in the literature, from new experimental studies or from numerical models. Advanced material options are of interest due to their potential for optimisation. For example, they could offer longer canister lifetimes, more accurate and robust long-term prediction, advantages related to a reduced impact on the engineered and geological barriers or related to manufacturing.
To satisfy the safety requirements and to have the potential for optimisation of container performance, both the choice of the material and of the production route for the canister are very important. Recent developments (e.g., copper coatings) considered in Canada and investigated in Switzerland (Holdsworth 2018), France and Japan have demonstrated that container optimisation is indeed possible and of value to the radioactive waste disposal community.
A number of materials is currently considered for novel containers, both for the production of single material containers (such as alumina (Al2O3)-based compounds, silicon carbide (SiC), titanium, nickel, copper) or for coated containers (such as titanium oxide (TiO2)-based coatings, chromium nitride (CrN) coatings, copper and Cu/Al2O3 composite coatings) (Baroux 2016, Holdsworth 2013, Holdsworth 2014, Holdsworth 2018). Even though oxygen free phosphorus-doped copper (OFP-Cu) has been widely studied as canister material (making it a traditional container material), quite an effort is currently being made to optimise the long-term performances of bulk copper under repository conditions, opening the way to alternative copper alloys, other than OFP-Cu, as bulk materials for the production of novel canisters.
