Thermodynamic Modeling and Sensitivity Analysis of K d Values for Radionuclide Migration in Sedimentary Host Rocks
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7KHUPRG\QDPLF0RGHOLQJDQG6HQVLWLYLW\$QDO\VLVRI.G9DOXHV IRU5DGLRQXFOLGH0LJUDWLRQLQ6HGLPHQWDU\+RVW5RFNV Caterina Talerico1, Michael Ochs1, Shinzo Ueta2, and Noriyuki Sasaki3 1 BMG Engineering Ltd., Ifangstrasse 11, CH-8952 Schlieren-Zürich, Switzerland 2 Mitsubishi Materials Corporation (MMTL), Tokyo, Japan 3 Japan Nuclear Fuel Ltd. (JNFL), Japan $%675$&7 The effects of key geochemical parameters on Kd values for radionuclides in the host rock (pumice, sandstone) of a LLW repository were elucidated through a sensitivity analysis, using a thermodynamic speciation/sorption model for the elements Sr and Ni. The complex mineral assemblage of the rock was approximated by a component-additivity approach. Using published ion exchange and surface complexation parameters, Kd for both Sr and Ni could be well explainned by the same model mineralogy and surface chemistry. Model results suggest that pCO2 can have a significant effect on Kd, and that a correct approximation of groundwater chemistry is a critical component of sorption modeling. ,1752'8&7,21 For the disposal of low level radioactive waste (LLW) in a semi-shallow repository at Rokkasho, Japan, the respective host rocks need to be characterized with regard to their properties for radionuclide (RN) retention. Radionuclide sorption is quantified by distribution coefficients, Kd (L/kg), which are expressed as the ratio of the sorbed concentration (moles/kg) over the dissolved concentration (moles/L). Kd values are strongly influenced by solution chemistry and by the sorbent's surface chemical characteristics. Consequently, Kd values determined under given conditions are usually not valid under different conditions. To select appropriate data for PA, it is therefore important to understand and quantify the influence of different geochemical factors on Kd. To this end, thermodynamic sorption models were used for a sensitivity analysis of the effects of geochemical parameters on the sorption of key radionuclides (Sr, Ni). Specifically, the discussed modeling is aimed i) to explain the relevant mineral and geochemical parameters for sorption, ii) to explain experimental Kd values based on repository site-specific geochemical parameters, iii) to derive Kd as a function of depth in the host rock and iv) to conduct a sensitivity analysis of Kd as a function of pCO2 and groundwater composition (of particular salinity). 6,7(63(&,),&3$5$0(7(56$1''$7$ Available data on site specific geological and chemical conditions [1; 2] include a) Kd measurements performed for Sr and Ni on some rock samples, b) data on groundwater composition as a function of depth, and c) data on the mineral composition of two dominant rock systems, pumice and sandstone. In addition to clay minerals, quartz, and other silicates, both rocks contain a significant fraction of Non-Crystalline (X-ray amorphous) Al-silicate Material (NCM) with unknown properties. Specifically, the average composition of pumice includes 4,5% montmorillonite, 4% chalcedony, 40% plagioclase, 2% pyroxene and 50% NCM
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