Speciation and solubility calculations for waste relevant radionuclides in Boom Clay: Topical Report - Status 2014

The Belgian radioactive waste management organisation, ONDRAF/NIRAS, is responsible for the long-term management of radioactive waste in Belgium. At present, the long-term solution proposed by ONDRAF/NIRAS is a deep disposal facility for low-level and intermediate-level long-lived radioactive waste (LILW-LL) (Category B waste), vitrified high-level radioactive waste (HLW) and spent nuclear fuel (SNF, or SF-spent fuel), together comprising Category C waste. The primary aim of the ONDRAF/NIRAS programme for Category B and C wastes is to establish the feasibility of a deep disposal facility, without any presumption about a precise repository location. This report is part of the ONDRAF/NIRAS RD&D Plan in support of the establishment of a Safety and Feasibility Case (ONDRAF/NIRAS, 2013). In order to calculate the release and transport of the waste relevant radionuclides (RNs) 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 (2010_MOLDATA_nov_b.dat; MOLDATA TDB, version 1).