Investigation of mineralogy, porosity and pore structure of Olkiluoto bedrock
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Investigation of mineralogy, porosity and pore structure of Olkiluoto bedrock Juuso Sammaljärvi1, Antero Lindberg2, Jussi Ikonen1, Mikko Voutilainen1, Marja SiitariKauppi1, Lasse Koskinen3 1 2 3
Laboratory of Radiochemistry, University of Helsinki, Finland. Geological Survey of Finland, Finland Posiva Oy, Finland
ABSTRACT Spent nuclear fuel from TVO's (Teollisuuden Voima Oy) and Fortum's nuclear power plants will be deposited deep in the crystalline bedrock in Olkiluoto, Western Finland. The bedrock needs to be well characterized to assess the risks inherent to the waste disposal at the site. If radionuclides (RN) are transported, it happens via water conducting fractures. Retardation may occur either by diffusion into stagnant pore water or by immobilization on mineral surfaces of the rock matrix. RN’s retardation from flowing water is linked to parameters defining porosity and microscopic rock pore structure, such as pore size distribution, connectivity, tortuosity and constrictivity, and by the mineralogy and chemical nature of the minerals and charge of the pore surfaces. In this work, centimeter scale rock cores from Olkiluoto were investigated. The work is part of the in situ project REPRO (Experiments to investigate Rock Matrix Retention Properties) where the diffusion and sorption of RN are studied experimentally. Porosity and pore structures were characterized with the PMMA autoradiography method and polarized microscopy, which was used also to ascertain the mineralogy of the samples. The results show that the rock from the REPRO site has low porosity with a mean value of 0.5% and a range of 0.1-1.5%. Rock heterogeneity explains the variation of porosity values. Correlation between the porosity and the mineralogy was found. Areas of high porosity correspond to areas of altered minerals, such as cordierite, biotite and plagioclase, which cover spatially between 10 and 20% of the rock volume INTRODUCTION Understanding the nature of the RN migration through geological formations is essential in any assessment of the confining properties of the barriers [1]. RN migration under natural long-term conditions is a complex process controlled by many parameters. Physical parameters of the rock matrix such as porosity, hydraulic conductivity and diffusivity are used to describe transport properties of materials. RN migration is dependent on the physical parameters and chemical nature and charge of the mineral surfaces of the rock. The interface between aqueous solutions and mineral phases is defined by the relative distribution of solution-filled pores and minerals. Porosity in rocks in the widest sense is understood to mean the entire solution-filled void space between and within the individual mineral grains [2]. Porosity in crystalline rocks can be mainly divided into two types: primary, acquired during magmatic crystallisation, and secondary, acquired since crystallisation through rock alteration. Minerals in the rock are changed chemically with time, first by retrograde metamorphism and then by water–rock inter
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