Sorption Aspects for in Situ Matrix Diffusion Modelling at Palmottu Natural Analogue Site, Sw Finland
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SORPTION ASPECTS FOR IN SITU MATRIX DIFFUSION MODELLING AT PALMOTTU NATURAL ANALOGUE SITE, SW FINLAND. MARKUS OLIN* KARI RASILAINEN*, JUHANI SUKSI**, MARTTI HAKANEN**,Helsinki, Department of * VT" Energy, P.O. Box 1604, FIN-02044 VTT, ** University of Radiochemistry, P.O. Box 5, FIN-00014 UNIVERSITY OF HELSINKI, *** VTI Chemical Technology, P.O. Box 1404, FIN-02044 VTT
ABSTRACT Concentration profiles in rock matrix around water-carrying fissures were measured at Palmottu U deposit. The profiles were interpreted by the classical matrix diffusion concept. Site-specific sorption studies were performed for U using standard batch experiments and surface complexation modelling; the response of sorption isotherms was also tested. Sitespecific matrix properties as well as initial and boundary conditions were used in simulations. Our results indicate that matrix diffusion alone cannot explain the observed enrichment of U and its daughters in the rock matrix.
INTRODUCTION The Palmottu U-Th deposit in south-western Finland is being studied to improve the scientific basis for assessing the long-term performance of a nuclear waste repository in fractured crystalline bedrock. The deposit contains about 1 000 tonnes U in an irregular and discontinuous geometry from the ground surface to a depth of at least 300 m. This study concentrates on transport mechanisms in fractured rock. Measured concentration distributions of natural decay chain nuclides around water-carrying fissures were interpreted by the classical matrix diffusion model in an attempt to validate the concept. The general constudy, and the exercise using site-specific physical data have been reported cept feasibility 12 earlier " . In this work we study critically the earlier sorption assumptions by means of sitespecific batch experiments and surface complexation modelling. When validating a model by comparing measured and simulated concentration profiles, experimental quality assurance is essential. The measured profile acts as the yardstick, and it should represent natural conditions free of experimental artefacts. A realistic site-specific simulation needs consistent and representative input (initial and boundary conditions, matrix properties, and time). Site-specific data, however, are difficult to acquire, due to the heterogeneity of natural rock. The small length scales in diffusion problems set further resolution requirements on matrix characterisation.
MEASURED CONCENTRATION PROFILES Drill core sections, intersected by natural fractures, containing elevated radioactivity were sampled outside the deposit. In such samples we can assume that the surplus of radioactivity in the matrix is of secondary origin, and may thus originate from the groundwater that has been flowing in the fracture. Drill core sample 21 l/R325 taken 25 m from the mineralization at a depth of 50 m was chosen for detailed interpretation. The drill core was cut into slabs parallel with the fracture surface; the distance in Table I refers to the centre of the slab. Autoradiographical and micr
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