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Workreport 2017-39

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Name:

Investigation of Rock Matrix Retention Properties – Supporting Laboratory Studies II: Diffusion Coefficient and Permeability

Writer:

Voutilainen, M., Ikonen, J., Sammaljärvi, J., Siitari-Kauppi, M., Lindberg, A., Kuva, J., Timonen, J., Löfgren, M.

Language:

English

Page count:

36

Summary:

Spent fuel from nuclear power plants in Finland will be deposited deep in the crystalline bedrock. To properly estimate the safety of such a repository, the transport properties of the bedrock must be investigated. As a part of such an investigation, a project called rock matrix REtention PROperties (REPRO) has been launched. The project consists of in situ transport measurements and supporting laboratory studies to which this report belongs.

In this study diffusion measurements performed on REPRO rock samples using the water phase through-diffusion measurements with HTO and 36Cl, the gas phase through-diffusion measurements, electrical conductivity measurements and Cl out-diffusion measurements are compiled together to gain deeper knowledge and understanding on the transport properties of the rock and the factors affecting it. This study aims to assist the analysis of the REPRO in situ experiments and to produce data which can be used when comparing the results from the in situ experiments to those from the laboratory experiments. The samples were divided in two groups according to the rock type:veined gneiss (VGN) and pegmatitic granite (PGR) and veined gneiss samples further to three groups according to the in situ experiment they were linked to: Through Diffusion experiment (TDE) that was performed in ONK-PP-324,326 and 327 drill holes, Water Phase Diffusion Experiment (WPDE) that was performed in ONK-PP-323 drill hole and Other VGN samples from ONK-PP-319 drill hole.

The effective diffusion coefficient (De) of HTO as error weighted averages over different sample groups were (1.7 ± 0.2) × 10-13 m2/s (WPDE), (3.9 ± 0.4) × 10-13 m2/s (TDE), (5.7 ± 0.7) × 10-13 m2/s (PGR) and (1.3 ± 0.1) × 10-13 m2/s (Other). The De values of 36Cl were (0.05 ± 0.03) × 10-13 m2/s (WPDE), (3.4 ± 0.5) × 10-13 m2/s (TDE), (5.0 ± 1.0) × 10-13 m2/s (PGR), and (0.07 ± 0.03) × 10-13 m2/s (Other). The effective diffusion coefficients from He-gas through-diffusion experiments presented by correcting to the water phase by a factor of 11 000  were (0.51 ± 0.03) × 10-13 m2/s (WPDE), (1.0 ± 0.1) × 10-13 m2/s (TDE), (5.0 ± 0.3) × 10-13 m2/s (PGR), and (2.0 ± 0.2) × 10-13 m2/s (Other).  The diffusivities were also calculated from the formation factor which was determined from the electrical  conductivity measurements  and they were (3.4 ± 1.1) × 10-13 m2/s (WPDE), (5.9 ± 0.5) × 10-13 m2/s (TDE), (11 ± 1) × 10-13 m2/s (PGR), and (1.6 ± 0.2) × 10-13 m2/s (Other).  The last De values were measured from natural chloride out-diffusion experiment being (0.6 ± 0.3) × 10-13 m2/s (WPDE), (2.9 ± 0.5) × 10-13 m2/s (PGR), (1.0 ± 0.3) × 10-13 m2/s (Other).

The effect of anion exclusion was clearly seen on veined gneiss samples when comparing the results of HTO and 36Cl from the through-diffusion experiment in the water phase. This effect was notable in WPDE samples, where the foliation was found to be perpendicular to the direction of diffusion. All of the TDE samples were however highly foliated and the foliation was found to be parallel in the direction of diffusion, which made the diffusion of the elements faster. Due to this, the anion exclusion effect is not as pronounced in TDE as in WPDE. The diffusivities from the He through-diffusion experiments which were performed in the gas phase were systematically lower than the diffusivities of HTO measured in the water phase in the case of veined gneiss (WPDE, TDE and Other groups). This might be due to the effect of Knudsen diffusion in the veined gneiss samples indicating nano- to micrometer scale pores in the connective porosity of these rocks. Neither anion exclusion nor Knudsen diffusion affected the pegmatitic granite samples in which rock type the connected porosity was formed by intra- and intergranular fissures and fractures in and around of large mineral grains. The electrical conductivity measurements generally overestimated the diffusivities. These diffusivities were determined from the formation factor and yielded the highest values for the PGR due to its more open and direct diffusion routes in the grain boundaries when compared with VGN. The out-diffusion measurements yielded results that were in fair agreement with the other methods.

Keywords:

Crystalline rock, effective diffusion coefficient, permeability, formation factor, porosity, anion exclusion, veined gneiss, pegmatitic granite, REPRO-project

File(s):

WR 2017-39_web (pdf) (3.8 MB)


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