Diffusion in Crystalline Rocks
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DIFFUSION IN CRYSTALLINE ROCKS
K. Skagius and I. Neretnieks Department of Chemical Engineering Royal Institute of Technology S-IO0 44 STOCKHOLM Sweden
SUMMARY the sorption and the rate of diffusion
Laboratory experiments to determine
of cesium and strontium in pieces of granite have been performed. diffusivity,
Dp •*
The effective
p was found to be I - 2 • 10-12 m2 Is for both cesium and
strontium. The diffusion of non-sorbing species in granites and other rock materials have been studied in laboratory scale. tritiated water,
were determined to be 0.7 - 1.3
The non-sorbing species were iodide,
granites the effective diffusivities 10-13 m2 Is for iodide and 1.3 - 1.8 • 10-13
Cr-EDTA and Uranine.
In
m 2Is for tritiated water. Electrical resistivity measurements in salt water saturated rock cores have been performed.
The resistivity is
measured in the saturated core and
in the salt solution with which the core has been saturated. The ratio between these two resistivities has a direct relation to the ratio of the effective diffusivity for a component in in
the rock material and the diffusivity
free water for the same component. The results from the electrical resistivity measurements and the experiments
with diffusion of non-sorbing species are in
fair agreement.
diffusivity for cesium and strontium (sorbing species) are,
The effective however,
more than
ten times higher than expected from the results of diffusion of non-sorbing species and the electrical
resistivity measurements.
This is
interpreted as
an effect of surface diffusion.
INTRODUCTION Questions related to the final disposal of the nuclear wastes are studied intensively in many countries. radioactive waste is
The Swedish concept for a final repository for
to emplace the waste in crystalline rock. Canisters
containing the waste are placed in holes in The nuclides may, however,
stable rock at about 500 m depth.
eventually leak out from the canisters and be
transported with the moving ground water in fissures in the bedrock. estimate the velocity of the moving nuclides in the fissures it
is
To
important
to know the interaction mechanisms between the nuclides and the rock.
The
510 nuclides may sorb on the fissure surfaces and they may also diffuse through the stagnant fluid in the micropores of the rock and sorb on the micropore surfaces as well. This may have a strong impact on the retardation of the radionuclides.
(1)
Three types of experiments in the laboratory scale to determine the sorption and the diffusion of nuclides in crystalline rocks have been performed. o Sorption and diffusion experiments with cesium and strontium in two different granites. o Diffusion experiments with non-sorbing species (iodide,
tritiated water,
Uranine and Cr-EDTA) in different rock materials. o Electrical resistivity measurements in salt water saturated rock cores.
SORPTION OF STRONTIUM AND CESIUM ON GRANITE Laboratory investigation The granites in
these experiments are from Finnsj6n
(quartz-granodiorite)
outside Forsmark on t
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