Diffusion Measurements on Crystalline Rock Matrix
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DIFFUSION MEASUREMENTS ON CRYSTALLINE ROCK MATRIX
KARl HARTIKAINEN', A. HAUTOJARVI**, H. PIETARILA* and J. TIMONEN* "University of Jyviskyld, Department of Physics, P.O.Box 35, FIN-40351 Jyvdskyl~i, Finland; "..VTT/Energy, P.O.Box 1604, FIN-02044 V7I', Finland. ABSTRACT A new gas flow technique is introduced such that experiments on very long samples are possible. This new technique together with increased accuracy of the measurements, allows the observation of power law tails in the break-through curves. Dispersion in these experiments can be controlled in great detail, and therefore the power law tails can be used to determine very accurately the parameters relevant in matrix diffusion. Results for rock and metal samples are shown, and they are fitted with model calculations which include both dispersion and matrix diffusion. The introduced technique, which is designed for ordinary drill cores, is suitable for scanning a large number of samples in a very short time. INTRODUCTION The Finnish programme for disposal of spent nuclear fuel has now proceeded into a phase where a few places have been chosen as candidates for possible repository sites. These sites will be subjected to a more detailed study over the next few years. The repository concept in Finland is that of copper-steel canister to be buried in bedrock not crossed by fracture zones at the depth of about 500 m. It is therefore of importance now to determine the migration properties of radioactive elements dissolved in ground water in the bedrock of the chosen sites. To this end one needs to have the characteristics of the bedrock including
e.g. porosity and diffusion coefficients. From usual experiments on small samples one cannot reliably determine the macroscopic characteristics of rock because of its local variations in a scale bigger than the sample size. Measurements on large or long samples are needed for this purpose. We have recently introduced 2 a new gas flow technique which allows quick and easy measurements on rock samples for determination of their porosity and effective diffusion coefficient. It was possible with this technique 2' 3 to gain information also of matrix diffusion, which is supposed to have a significant effect on retardation in flow in bedrock fractures. In the gas flow technique, nitrogen is used as the carrier gas and helium as the tracer. Diffusion of helium in nitrogen is about four orders of magnitude faster than diffusion of species in water, and this makes such experiments possible which are either extremely difficult or practically impossible to realize in liquid phase 2". The results of the gas flow experiments can be extrapolated to liquid phase by simple scaling (with some provisos of course). Preliminary comparison of rock properties measured in gas phase and in liquid phase show a good agreement '. In liquid phase there exist, however, phenomena that cannot be studied in gas phase but knowing the basic physical and geometrical rock properties helps to study such phenomena in liquid phase. In order to reliably ob
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