Modelling of chemical evolution in the near-field of the spent nuclear fuel repository: A case study

It has been planned to dispose of spent nuclear fuel in Finland in a repository at a depth of about 500 m in crystalline bedrock. The near-field barriers in the excavated space comprise a compacted bentonite buffer (clay) surrounding a copper-lined iron canister, the canister itself and the spent fuel matrix (UO2). The barriers mutually interact via the groundwater and tend towards a thermodynamic equilibrium with each other and the geochemical environment. The evolution of the near-field chemistry arising from the interactions between the groundwater, bentonite, canister and the spent fuel was calculated for the groundwater and bentonite composition considered in the context of the Finnish nuclear waste disposal concept. The approach employs equilibrium modelling (EQ3/6) with the modelling of three successive closed systems. The pH, Eh and actinide solubilities as well as their solubility-limiting phases were determined in the solution equilibrated with spent fuel.