Technical Report NTB 12-04
Sorption Data Bases for Argillaceous Rocks and Bentonite for the Provisional Safety Analyses for SGT-E2
In Stage 1 of the Sectoral Plan for Deep Geological Repositories, four rock types have been identified as being suitable host rocks for a radioactive waste repository, namely, Opalinus Clay for a high-level (HLW) and a low- and intermediate-level (L/ILW) repository, and 'Brauner Dogger', Effingen Member and Helvetic Marls for a L/ILW repository. Sorption data bases (SDBs) for all of these host rocks are required for the provisional safety analyses, including all of the bounding porewater and mineralogical composition combinations. In addition, SDBs are needed for the rock formations lying below Opalinus Clay (lower confining units) and for the bentonite backfill in the HLW repository.
In some previous work Bradbury et al. (2010) have described a methodology for developing sorption data bases for argillaceous rocks and compacted bentonite. The main factors influencing the sorption in such systems are the phyllosilicate mineral content, particular the 2:1 clay mineral content (illite/smectite/illite-smectite mixed layers) and the water chemistry which determines the radionuclide species in the aqueous phase. The source sorption data were taken predominantly from measurements on illite (or montmorillonite in the case of bentonite) and converted to the defined conditions in each system considered using a series of so called conversion factors to take into account differences in mineralogy, in pH and in radionuclide speciation. Finally, a Lab → Field conversion factor was applied to adapt sorption data measured in dispersed systems (batch experiments) to intact rock under in-situ conditions. This methodology to develop sorption data bases has been applied to the selected host rocks, lower confining units and compacted bentonite taking into account the mineralogical and porewater composition ranges defined.
Confidence in the validity and correctness of this methodology has been built up through additional studies: (i) sorption values obtained in the manner described above have been compared with those in already existing SDBs for Opalinus Clay and bentonite used in Project Opalinus Clay (Entsorgungsnachweis; see Bradbury & Baeyens 2010), (ii) blind sorption model predictions of isotherms on MX-80 bentonite and Opalinus Clay in realistic groundwater compositions have been compared with measured isotherms (Bradbury & Baeyens 2011), and finally, (iii) blind predictions made using the above methodology have been compared with recent sorption measurements on argillaceous rocks (Baeyens et al. 2014). In all cases the results obtained in the different comparative approaches have been consistent with the predictions made using the methodology described.
In a few cases the mineralogy of the rock type was too poor in clay minerals to apply this approach. SDBs were nevertheless developed but based on a methodology in which calcite was the main sorbing phase. The procedure is fully described in the present report.
Further, a methodology for the derivation of SDBs for a host rock altered by hyperalkaline solutions from a cement based repository is described and the resulting SDBs for Effingen Member and Helvetic Marls are given.