Bedrock Characterisation of Four Candidate Repository Sites in Finland as Determined by He-Gas Methods
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ABSTRACT At the end of the year 2000, one of four sites will be chosen as the final repository site in Finland. Therefore accurate and comprehensive statistics of the bedrock characteristics such as porosity [%] and effective diffusion coefficient [m2/s] of these sites are of importance. Altogether 115 rock samples from the four sites were measured by different He-gas methods to achieve this goal. The results obtained indicate that the average bedrock properties at these sites are quite similar. Variations among individual samples and different rock types within one repository site were larger than variations among the averaged values of the four sites. Some indication of increased microfracturing was also found in samples of small size.
INTRODUCTION The programme for disposal of spent nuclear fuel in Finland has now proceeded into a phase in which four locations have been chosen as candidates for repository sites. These sites have been subject to detailed studies over a few years. The disposal concept in Finland is such that the spent nuclear fuel will be enclosed in copper-steel canisters and buried in the bedrock at a depth of about 500 meters. It is therefore important to determine the migration properties of
radioactive elements that may eventually be dissolved in the ground water of these sites. To this end, the migration characteristics of bedrock at the four repository sites, that is their porosity and diffusion properties, were determined. Diffusion and migration properties are usually determined in the liquid phase. In this case measurements on these tight rock samples are time consuming because the relevant processes are slow. We have therefore developed several fast and versatile methods of studying migration phenomena by using inert gases [1-5]. In the developed gas-flow techniques nitrogen is used as the carrier gas and helium as the tracer. In this report we describe two methods: the throughdiffusion and the channel-flow methods. The most important aspect of this report is the sheer amount of data needed for reliable statistics of the migration characteristics of bedrock. For this the fast helium-gas techniques are necessary. Also, the long time scales of the more conventional measurements have lead to such small sample sizes that they may have affected the results. We have found indications of this kind of small-size effect in some of the through-diffusion measurements. The channel-flow measurements were designed for very long drill-core samples in order to get more representative
values in a single measurement. In the actual bedrock, migration always takes place in porous and fractured rock saturated by water. Although measurements are done in the gas phase, the corresponding diffusion coefficients for the water saturated case can be derived. This is based on the fact that the 775 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
diffusion equation is the same in the two cases apart from the actual values of the diffusion coefficients. For helium diffusing in free nitrogen g
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