Derivation of self-consistent sorption and diffusion parameters and their uncertainties for compacted MX-80 bentonite

Full text of publication follows: Diffusion-available porosity (ε), effective diffusivities (De) and distribution coefficients (Kd) for the migration of radionuclides in a bentonite buffer are important input parameters to PA calculations. Based on results from [1] for use in a probabilistic safety analysis by SKB, this contribution focuses on issues of self-consistency and on quantification of the uncertainties associated with each parameter. The importance of consistency is twofold. First, the above parameters are conditional in nature. To assure consistency of the conditions assumed for parameter derivation vs. those considered in the safety analysis, all parameters were derived explicitly for the expected in-situ conditions. Based on available systematic datasets recorded as a function of pH and other conditions and/or published thermodynamic sorption models, Kd values were derived for the specific pore water composition (calculated based on [2]) representative for compacted MX-80 in contact with reference groundwater. Where no sorption model was available, extrapolation from experimental conditions to the conditions relevant for compacted bentonite was done with the help of conversion factors taking into account surface and radionuclide speciation as well as sorption site density, using a procedure modified from [3]. De for all radionuclides and ε were evaluated as a function of density. For radionuclides that exist as a mixture of positively, negatively and uncharged species under the relevant conditions, HTO was used to represent average diffusion properties. For Cs and non-sorbing anions, De and ε were derived based on element-specific experimental data and double layer calculations [4]. Second, any set of parameters (ε, De, Kd) selected for a given element needs to be self-consistent. To this end, each selected parameter set was used to calculate apparent diffusivities (Da). These were then compared to independent, experimental Da values. Uncertainties in input parameters can be related to two fundamentally different sources: One source are the uncertainties of the underlying experimental data and models, including uncertainties introduced by extrapolating to reference PA conditions. This was addressed by explicitly deriving uncertainty limits for each parameter and radionuclide. On the other hand, significant parameter uncertainties can also be caused by uncertainties in the conditions expected for PA. The impact of such variability was evaluated by specifically deriving input parameters for the reference PA conditions as well as for several other sets of PA conditions. Because of the highly conditional nature of Kd, this needs to be done by re-calculating bentonite pore water composition for each variation in geochemical boundary conditions. [1] Ochs M. and Talerico C. (2004) SR-Can: Data and uncertainty assessment. SKB TR-04-18. [2] Wanner H., Albinsson Y., Karnland O., Wieland E., Wersin, P., Charlet, L. (1994) Radiochim. Acta 66/67, 157-162. [3] Bradbury M.H., Baeyens B. (2003) Far-field sorption data bases for performance assessment of high-level radioactive waste repository in an undisturbed Opalinus Clay host rock. Nagra Technical Report 02-18. [4] Ochs M., Lothenbach B., Wanner H., Sato H., Yui. M. (2001) J. Contam. Hydrol. 47, 283-296. (authors)