The Sorption of Uranium and Technetium on Bentonite, Tuff and Granodiorite
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THE SORPTION OF URANIUM AND TECHNETIUM ON BENTONITE, TUFF AND GRANODIORITE G M N BASTON, J A BERRY, M BROWNSWORD, M M COWPER, T G HEATH AND C J TWEED AEA Technology, 220 Harwell, Oxfordshire, UK ABSTRACT A combined experimental and modelling study of the sorption of uranium and technetium on geological materials has been carried out as part of the PNC programme to increase confidence in the performance assessment for a high-level radioactive waste (I-LW) repository in Japan. Batch sorption experiments have been performed in order to study the sorption of uranium and technetium onto bentonite, tuff and granodiorite from both equilibrated seawater and de-ionised water under strongly-reducing and nonreducing conditions. A preliminary study of the sorption of uranium on mineral surfaces in granodiorite has also been undertaken using a nuclear microprobe. Mathematical modelling using the geochemical speciation program HARPHRQ in conjunction with the HATCHES database has been carried out in order to interpret the results of the sorption experiments. INTRODUCTION The research programme described here has investigated major chemical parameters which may govern the sorption behaviour of uranium and technetium, and is part of the PNC programme to increase confidence in the performance assessment for a proposed Japanese high-level radioactive waste (HLW) repository. Measurements have been made of the sorption of the two radioelements onto bentonite (Kunigel V1)1, which may be used as a backfill material in the repository, and onto two rocks, tuff and granodiorite. Conditions were chosen to simulate, as closely as possible, those that might pertain to a Japanese HLW repository. These included both stronglyreducing and non-reducing conditions, and both saline and non-saline groundwaters. A preliminary surface analysis study using a nuclear microprobe was also undertaken to determine the minerals that are important in uranium sorption on granodiorite. Mathematical modelling using the geochemical speciation program HARPHRQ2 and the HATCHES3 database has been used to interpret the experimental results. The aims of the modelling were to predict the chemical speciation of uranium and technetium in the bentonite-, tuff- and granodiorite-equilibrated groundwaters under both stronglyreducing and non-reducing conditions, and to use this information, together with data on sorption properties of the relevant minerals, to derive simple plausible mechanisms to explain the observed sorption behaviour. A full description of the contribution from every sorbing phase in these complex natural materials was beyond the scope of the study. EXPERIMENTAL Batch sorption experiments were carried out at room temperature (21*C ± 3*C) in polypropylene centrifuge tubes in a nitrogen-atmosphere glove-box with an oxygen level of less than 1 ppm. All solutions were de-oxygenated prior to use. Samples of the crushed rocks (
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