Experimental Studies on Retardation Properties of Granite Specimens from Grimsel Test Site, Switzerland

Geological formations of igneous rocks, like granites, have been considered as candidate formations for geological disposal of high level radioactive wastes in many countries. Transport of radionuclides through an igneous rock formation is principally controlled by fluid flow along fracture networks, permeation and diffusion in the directions perpendicular to fracture surfaces, sorption onto geological materials and chain decay. Among these mechanisms, permeation and diffusion in the directions perpendicular to fracture surfaces, and sorption onto geological materials control the retardation properties of a geological formation. In our study, systematic laboratory experiments and analyses aimed to characterize the retardation properties of rock specimens taken from the Grimsel Test Site, Switzerland, have been planned. This paper presents the results obtained from the primary stage of the study, which include chemical and physical properties, and effects of stress and stress history on permeability of test specimens. The results illustrated that: 1) The porosity and wave velocities of Grimsel granite are relatively low compared to average values of igneous rocks indicating that micro-cracks can potentially exist within grain minerals. This inference is further supported by the X-ray scanning visualization. 2) The permeability of Grimsel granite is very sensitive to stress condition. Under the test condition of confining pressures up to about 60 MPa, the intrinsic permeability of Grimsel granite ranged from the order of 1 E-15 to 1 E-19 m2. 3) Permeability is not a simple function of effective confining pressure. It is significantly sensitive to confining pressure at low pressure levels. Besides, the permeability is hysteretic depending on stress history. 4) Similar to the permeability, diffusion coefficient of rock specimens can also be significantly affected by the confining pressure conditions. Laboratory diffusion tests should be designed to incorporate the effects of stress conditions. (authors)