Radionuclide Retardation in Crystalline Rock Fractures
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RADIONUCLIDE RETARDATION IN CRYSTALLINE ROCK FRACTURES.
P. HOLTI7*, M. HAKANEN*, M. SIITARI-KAUPPI* and A. HAUTOJARVI** * University of Helsinki, Department of Radiochemistry, P.O.Box 55, FIN-00014 University of Helsinki, Finland VTT Energy, Nuclear Energy, P.O.Box 1604, FIN-02044 VT'T, Finland. ABSTRACT
Transport and retardation of slightly sorbing sodium was studied in Syyry area SY-KR7 mica gneiss and in altered porous tonalite. Experiments were performed using dynamic fracture and crushed rock column methods and the static batch method. Flow conditions in 4 the columns were determined using tritiated water and chloride as non-sorbing tracers. ' CPMMA method was used to study the pore structure of matrices and the surface areas were determined by B.E.T. method. Sodium was retarded strongly in altered tonalite owing to homogeneously porous matrix structure and the composition of alteration minerals. An agreement between retardation values in batch and crushed rock column experiments as well as in fracture column experiments was good. 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 released from underground repositories 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 microfissures of the rock. 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.
Radionuclide retardation has been taken into account in transport models by the K4 concept, in which the distribution ratio is applied to radionuclide transport by a retardation factor. The Kd-values used are often measured by static batch method using crushed rocks. The retardation factor can be measured directly by dynamic experiments including crushed and intact rock columns. Relating the parameters measured by static method for a crushed rock to a heterogeneous intact rock may be difficult owing to differences in trace mineral compositions and in mineral availabilities to the aqueous phase. Results from static and dynamic experiments can be compared using hydrologically homogeneous crushed rock columns. The determination of the retardation factor by fracture flow experiments is a direct approach to apply the effects of sorption in radionuclide transport models. Transport behavior of non-sorbing radionuclides in fracture columns has been studied earlier [1]. In this work, retardation of sodium was studied in the same columns. Crushed rock columns and static batch experiments on the same material were also introduced to compare different approaches for measuring the interaction between radionuclides and the geological material. EXPERIMENTAL
Materials Rock chosen for the experiments was from hole SY-KR7 drilled in
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