Cesium Sorption on Tonalite and Mica Gneiss
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ABSTRACT Different approaches for measuring the interaction between radionuclides and rock matrix are needed to test the compatibility of transport models and retardation experiments. In this work sorption of cesium (134Cs) was studied on unaltered mica gneiss, and on unaltered, moderately altered and strongly altered tonalite. The crushed rock samples were sieved into seven fractions from 0. 1 mm to 3.15 mm. The mass distribution ratio values for each fraction was determined using the static batch method. Cesium sorption onto different minerals was demonstrated using digital image analysis in order to interpret thin section autoradiographs. The autoradiographic method based on irradiation-induced luminescence properties in feldspars was applied in order to estimate the alteration of tonalites. Cesium sorption on moderately altered and strongly altered tonalite was stronger than on unaltered rocks owing to larger specific surface areas and the composition of alteration minerals. Strongest sorption was found on biotite. Mass distribution ratio, Rdj, values of 0.3- 1.1 3 m 3.kg1 for unaltered rocks and 0.9-3.4 m .kg- for altered rocks were determined. Moderate dependence on surface area was found for mica gneiss after I and 3 days sorption time. For strongly altered tonalite only slight dependence on surface area was found. Rd -values increased as a function of sorption time due to diffusion into the particles. Higher increase in values for larger diameter particles indicated the availability of internal specific surface areas.
INTRODUCTION The Finnish high-level nuclear waste disposal concept is based on a multi-barrier arrangement in crystalline rock, which is the natural barrier isolating radionuclides from the biosphere. Radionuclides possibly released from an underground repository are transported mainly by the groundwater flow in fractures. The migration of dissolved radionuclides is retarded both by interaction with the fracture surfaces and fracture filling materials, and by diffusion into the fissures of the rock. To be able to predict the transport and radionuclide retardation in rock fractures and rock matrices, it is essential to understand the different physical and chemical phenomena involved, as well as the retardation properties of unfractured rock. Owing to complicated interactions of radionuclides and the geologic barrier, several approaches to measuring radionuclide retardation have been developed and discussed [1,2,3]. In transport models, radionuclide retardation has been usually taken into account by the Kd1 concept, in which a retardation factor is used to apply the distribution ratio to radionuclide transport. Determination of the retardation factor by the means of fracture flow experiments is a direct approach to application of the effects of sorption in radionuclide transport models. It may, however, be difficult to relate the parameters measured by the static meihod for crushed rock to a heterogeneous intact rock, owing to differences in trace mineral compositions and in the availabilities
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