Role of micropores within minerals in retardation of mass transfer by matrix diffusion and sorption in granitic rock.

ABSTRACT

Understanding the mass transfer characteristics of matrix diffusion and sorption is important in the safety assessment of geological disposal of high-level radioactive waste in crystalline rock (granite) by contributing to radionuclide retardation through mass transfer within the rock body. We present a comparative discussion of the effective diffusion coefficient (De), porosity, and petrological data for rock samples collected from the Toki Granite in central Japan, to evaluate the role of micropores within minerals in retardation by matrix diffusion and sorption in granitic rocks. De was derived from the through-diffusion experiments using uranine, barium, strontium, and chloride ions as tracers. Petrological data consist of the fracture frequency, the extent of hydrothermal alteration in the minerals, the micropore volume in the minerals, and the three-dimensional modal mineralogy (mineral assemblage and ratio) for the target rock samples. The relationship between the De, porosity, and petrological data has the following implications 1) Micropores in minerals related to the alteration act as 'storage pores' that contribute to retardation due to matrix diffusion and sorption; 2) Once the uranine, cations (Ba2+ and Rb+), and anion (Cl-) penetrate the micropores in the minerals through matrix diffusion, the cations are sorbed on the micropore surfaces, whereas the uranine and conservative chloride anion is trapped at the end of the micropore network, resulting in retardation; 3) Regions with a high fracture frequency are associated with not only active advection-dispersion through fractures, but also retardation due to matrix diffusion and sorption; 4) The grain-boundary pores between colorless minerals act as 'transport pores' owing to matrix diffusion, and the retardation within grain-boundary pores is less than that within micropores in minerals.