Speciation and solubility calculations for waste relevant radionuclides in Boom Clay. First Full Draft

The Boom Clay formation represents the reference host rock for the geological disposal of high-level and/or long-lived radioactive waste in Belgium. The Belgian authority responsible for the long-term management of radioactive waste is ONDRAF/NIRAS. The current focus of the research programme of ONDRAF/NIRAS is the so-called Safety and Feasibility Case 1 (SFC-1), which represents a body of sound arguments and evidences describing, quantifying and substantiating that geological disposal of high-level and/or long-lived radioactive waste is a safe and feasible long-term solution. In order to calculate the release and transport of the waste relevant radionuclides through the near-field of the repository and the host clay layer (far-field), different parameters, such as concentration/solubility limits, retardation factors, diffusion accessible porosity values and pore diffusion coefficients are needed. In this report, the solubility assessment methodology, i.e. results of the speciation calculations and derivation of the RN concentration/solubility limits applicable for "undisturbed" Boom Clay conditions (far-field) will be presented. The solubility and speciation modelling was performed with the geochemical computer code The Geochemist's Workbench (versions 8.08, 8.10 and 8.12). The reference thermodynamic database that was used for the calculations and also developed at SCK•CEN is named MOLDATA thermodynamic database.