DEVELOPMENT OF A GAS METHOD FOR MIGRATION STUDIES IN FRACTURED AND POROUS MEDIA.
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DEVELOPMENT OF A GAS METHOD FOR MIGRATION STUDIES IN FRACTURED AND POROUS MEDIA.
KUSTI VAATAINEN*, JUSSI TIMONEN*, AIMO HAUTOJARVI** *University of Jyviskyl, Department of Physics, P.O.Box 35, SF-40351 Jyvdskyld, Finland **Technical Research Centre of Finland, Nuclear Engineering Laboratory, P.O.Box 208, SF-02151 Espoo, Finland. ABSTRACT
A gas method for fast measurements of diffusion properties of porous materials has been developed. Diffusion coefficients in the gas phase are typically four orders of magnitude larger than those in the liquid phase. For samples whose structures do not change much upon drying it is possible to estimate the diffpision properties of the liquid phase when the properties of the gas phase are known. Advantages of the gas method are quick and easy measurements and therefore they can be used to optimize the liquid-phase measurements which may last months or years. For materials with good correlation between the gas and liquid-phase diffusion, the number of liquid phase measurements can be greatly reduced. Nitrogen was used as the carrier gas and helium as the tracer gas. Helium concentrations were measured by using standard techniques. The equipment was tested for two sample geometries, a column made of a polished granite slab and an acrylic slab with a flow channel in between, and a hollow bore core where diffusion occurs through the sample. Both types of measurement were modelled and apparent diffusion coefficients in the gas phase were determined. INTRODUCTION
The diffusion of solutes from fractures into rock matrix is an important factor in the safety analysis of disposal of radioactive waste. Laboratory measurements are needed to complement field investigations for a reliable deternination of the necessary transport parameters. Measurements of diffusion coefficients in tight rock samples are usually time consuming because the diffusion processes are slow. We have therefore developed a fast and versatile method of studying migration phenomena based on the use of inert gases. It is well known that diffusion coefficients in the gas phase are four orders of magnitude larger than those in the liquid phase. For samples whose structures do not change much upon drying it is possible to estimate the diffusion behavior of the liquid phase when the behavior of the gas phase is known. Advantages of the gas method are quick and easy measurements which can also be used to optimize the liquid-phase measurements that may last months or years. For materials with good correlation between the gas and liquid phase diffusion, the number of liquid-phase measurements can be greatly reduced. Air and nitrogen, e.g., are suitable carrier gases and helium can be used as the tracer gas. Accurate and sensitive determination of helium concentration is possible by using standard techniques like helium leak detection devices. Operating conditions can be arranged such that samples are at ambient pressure and all or part of the gas flow is diverted to a low-vacuum mass-spectrometer for helium detection. This makes
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