Determination of Matrix Diffusion Properties of Granite
- PDF / 5,714,951 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 64 Downloads / 246 Views
0985-NN11-11
Determination of Matrix Diffusion Properties of Granite Pirkko Holtta1, Marja Siitari-Kauppi1, Nina Huittinen1, and Antti Poteri2 1 Laboratory of Radiochemistry, P.O. Box 55, University of Helsinki, FI-00014, Finland 2 VTT Processes, P.O. Box 1608, VTT, FI-02044, Finland ABSTRACT Rock–core column experiments were introduced to estimate the diffusion and sorption properties of Kuru Grey granite used in block–scale experiments. The objective was to examine the processes causing retention in solute transport through rock fractures, especially matrix diffusion. The objective was also to estimate the importance of retention processes during transport in different scales and flow conditions. Rock–core columns were constructed from cores drilled into the fracture and were placed inside tubes to form flow channels in the 0.5 mm gap between the cores and the tube walls. Tracer experiments were performed using uranine, HTO, 36Cl, 131I, 22Na and 85Sr at flow rates of 1–50 µL·min-1. Rock matrix was characterized using 14C–PMMA method, scanning electron microscopy (SEM), energy dispersive X–ray micro analysis (EDX) and the B.E.T. method. Solute mass flux through a column was modelled by applying the assumption of a linear velocity profile and molecular diffusion. Coupling of the advection and diffusion processes was based on the model of generalised Taylor dispersion in the linear velocity profile. Experiments could be modelled applying a consistent parameterization and transport processes. The results provide evidence that it is possible to investigate matrix diffusion at the laboratory scale. The effects of matrix diffusion were demonstrated on the slightly–sorbing tracer breakthrough curves. Based on scoping calculations matrix diffusion begins to be clearly observable for non–sorbing tracer when the flow rate is 0.1 µL⋅min-1. The experimental results presented here cannot be transferred directly to the spatial and temporal scales that prevail in an underground repository. However, the knowledge and understanding of transport and retention processes gained from this study is transferable to different scales from laboratory to in–situ conditions. INTRODUCTION Crystalline rock is being considered as a host medium for the repository of highly radioactive spent nuclear fuel in Finland and elsewhere. The geosphere would act as the ultimate barrier retarding the migration of radionuclides to the biosphere if radionuclides were to be released through engineered barriers. In crystalline rock water flows through a fracture network and radionuclide transport is thought to proceed along water–carrying fractures. Retardation occurs both in the fractures and within the rock matrix. The experimental column method used in this study was a direct approach for determining the parameters affecting the fracture flow described in radionuclide transport models. Radionuclide transport has been studied in the Finnish program earlier using flow–through fracture and crushed rock columns [1, 2]. Fracture flow and radionuclide transport h
Data Loading...
