Evolution of permeability in a single granite fracture under coupled conditions

Abstract

Waste liquid with high radioactivity generated when reprocessing spent nuclear fuel produced in nuclear power generation is called high level radioactive waste. When geological disposal of high-level radioactive waste is carried out, it is assumed that heat generation from the waste and high overburden earth pressure will affect the permeability characteristics of the surrounding rock mass. Therefore, it is indispensable to evaluate the long-term permeability change of underground rock mass. In this study, a suite of permeability experiments was conducted under various temperature and confining pressure conditions assuming underground environment, and the effects of temperature, pH, confining pressure and fracture surface roughness on permeability characteristics of underground rock mass were evaluated. From the test results, it was confirmed that the higher the pH, the lower the permeability, and that the precipitation phenomenon significantly affects the permeability change, when the permeated water is different. It was also confirmed that when deionized water and high-pH water were used, the permeability more decreased in the high-temperature condition, and when mineral-saturated water and simulated seawater were used, the permeability more decreased in the room-temperature condition. When the confining pressure and fracture surface roughness were different, no clear difference was observed in the permeability change. Moreover, it was confirmed that the elemental concentration of permeated water after permeability was higher in the high temperature condition than in the room temperature condition. This may be due to the promotion of free-face dissolution phenomena by high temperature conditions. From SEM-EDX observations, the precipitation that may be salt crystals was observed under the room temperature condition on the fracture surface after permeability experiments using simulated sea water for permeation water, but it was not confirmed under the high temperature condition. When simulated seawater was used, the precipitation of salt crystals seemed to decrease the permeability in the room temperature condition, because the permeability decreased more than in the high temperature condition. The fracture surface measurements concluded that the roughness of the fracture surface became smoother by carrying out the permeability experiments.