Modelling in-Situ Matrix Diffusion at Palmottu Natural Analogue Study Site in SW Finland
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MODELLING IN-SITU MATRIX DIFFUSION AT PALMOTTU NATURAL ANALOGUE STUDY SITE IN SW FINLAND. KARl RASILAINEN* AND JUHANI SUKSI°" * Technical Research Centre of Finland, Nuclear Engineering Laboratory, P.O. Box 208, SF-02151 Espoo, Finland "*University of Helsinki, Department of Radiochemistry, SF-00170 Helsinki, Finland ABSTRACT Radioactive disequilibria, between U-238, U-234, and Th-230, in crystalline rock adjacent to a fracture, indicates mass transfer of U and Th between water in the fracture and the rock. The matrix diffusion theory was used to interpret the observed profiles of mobilized nuclides around the natural fracture. The interpretation of the profiles was based on the use of uranium series disequilibrium code, URSE, and migration code FTRANS. The model system was characterized using all available site-specific data, and the system evolution was outlined using the geology of the Palmottu site. The simulated concentration profiles, as a function of depth from the fracture surface, indicate that measured profiles can be modelled by matrix diffusion, assuming realistic initial and boundary conditions and diffusion times of 300,000 years. INTRODUCTION The surroundings of a small U-Th deposit at Palmottu have been studied as a migration analogue for radionuclides in spent fuel1 . Rock samples for an in-situ matrix diffusion study were taken from locations clearly separate from the deposit. It is assumed that the activity in the rock above the background level originates from the water flowing in the fractures. The extra activity in the rock around the fracture was studied by autoradiography and phase selective leaching techniques. Uranium-series disequilibrium measurements were used to study the mass transfer of U and Th between water and rock. Since a radioactive decay chain always will develop towards radioactive equilibrium if undisturbed, the observed profiles give a direct indication of the effective penetration depth of matrix diffusion into the rock under in-situ conditions over time periods far longer than can be measured in laboratory or field experiments. This paper describes the first modelling effort to interpret the measured concentration profiles at the Palmottu site. The main emphasis was on the simulation of absolute concentration profiles, with some preliminary analyses of concentration ratio profiles. The basic aim of the simulation exercise was to test the diffusion concept. EXPERIMENTAL Several core sections intersected by natural fractures were subjected to autoradiography and microscopy to establish secondary U accumulations in the rock, that would indicate possible radionuclide mobilization. Core sample 211/R325, obtained from a depth of 50 m, was selected for detailed study. The sample was cut crosswise into slabs, and visually reddish areas that contained strongly altered plagioclase grains were sampled selectively, along the core section in order to follow the diffusion pathway as indicated by autoradiographs. The accumulation of radionuclides around altered plagioclase grains was stu
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