HR: 13:55h
AN: H43I-02 INVITED     [Abstracts]
TI: Determination of In-situ Porosity and Investigation of Diffusion Processes at the Grimsel Test Site, Switzerland.
AU: * Biggin, C
EM: biggin@nagra.ch
AF: Nagra (National Cooperative for the Disposal of Radioactive Wastes), Hardstrasse 73, Wettingen, CH-5430 Switzerland
AU: Ota, K
EM: kunio@tono.jnc.go.jp
AF: Japan Nuclear Cycle Development Institute-Mizunami Underground Reserach Laboratory, 1-64 Yamanouchi, Akeyo, Mizunami, Gifu 509-6 Japan
AU: Siittari-Kauppi, M
EM: marja.siitari-kauppi@helsinki.fi
AF: Laboratory of Radiochemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1) FIN-00014 University of Helsinki, Helsinki, P.O Box 55 Finland
AU: Moeri, A
EM: andreas.moeri@geo-online.com
AF: Geotechnisches Institut AG, Gartenstrasse 13, Bern, CH-3007 Switzerland
AB: In the context of a repository for radioactive waste, 'matrix diffusion' is used to describe the process by which solute, flowing in distinct flow paths, penetrates the surrounding rock matrix. Diffusion into the matrix occurs in a connected system of pores or microfractures. Matrix diffusion provides a mechanism for greatly enlarging the area of rock surface in contact with advecting radionuclides, from that of the flow path surfaces (and infills), to a much larger portion of the bulk rock and increases the global pore volume which can retard radionuclides. In terms of a repository safety assessment, demonstration of a significant depth of diffusion-accessible pore space may result in a significant delay in the calculated release of any escaping radionuclides to the environment and a dramatic reduction in the resulting concentration released into the biosphere. For the last decade, Nagra has investigated in situ matrix diffusion at the Grimsel Test Site (GTS) in the Swiss Alps. The in situ investigations offer two distinct advantages to those performed in the lab, namely: 1. Lab-based determination of porosity and diffusivity can lead to an overestimation of matrix diffusion due to stress relief when the rock is sampled (which would overestimate the retardation in the geosphere) 2. Lab-based analysis usually examines small (cm scale) samples and cannot therefore account for any matrix heterogeneity over the hundreds or thousands of metres a typical flow path The in situ investigations described began with the Connected Porosity project, wherein a specially developed acrylic resin was injected into the rock matrix to fill the pore space and determine the depth of connected porosity. The resin was polymerised in situ and the entire rock mass removed by overcoring. The results indicated that lab-based porosity measurements may be two to three times higher than those obtained in situ. While the depth of accessible matrix from a water-conducting feature assumed in repository performance assessments is generally 1 to 10 cm, the results from the GTS in situ experiment suggested depths of several metres could be more appropriate. More recently, the Pore Space Geometry (PSG) experiment at the GTS has used a C-14 doped acrylic resin, combined with state-of-the-art digital beta autoradiography and fluorescence detection to examine a larger area of rock for determination of porosity and the degree of connected pore space. Analysis is currently ongoing and the key findings will be reported in this paper. Starting at the GTS in 2005, the Long-term Diffusion (LTD) project will investigate such processes over spatial and temporal scales more relevant to a repository than traditional lab-based experiments. In the framework of this experiment, long-term (10 to 50 years) in situ diffusion experiments and resin injection experiments are planned to verify current models for matrix diffusion as a radionuclide retardation process. This paper will discuss the findings of the first two experiments and their significance to repository safety assessments before discussing the strategy for the future in relation to the LTD project.
UR: http://www.grimsel.com
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting