Abstract
Mass transports in a rock formation has been studied since it can work as natural barriers against the migration of radionuclides in geological disposal of high level radioactive waste. The rock formation can be categorized into crystalline rock as a fractured medium and sedimentary rock as a porous medium. However, the Wakkanai formation (siliceous mudstones) in Horonobe underground research laboratory (URL) in northern Hokkaido, Japan, has unique features having both porous and fractured medium. Therefore, matrix diffusion toward the crack-free porous media should be considered in addition to the advection-dispersion through fractures in the porous medium. In this study, in-situ dipole tracer migration tests were conducted at the G.L. -250 m gallery of the Horonobe URL. Laboratory experiments were also conducted to determine the apparent diffusivity (D_a) and sorption coefficient (K_d) of cesium and water (HTO) in the rock taken at the gallery to evaluate the performance of Wakkanai formation as a natural barrier. In the in-situ dipole tracer migration tests, a non-sorbing tracer (Uranine) and a sorbing tracer (cesium) were injected to a section in one borehole, and collected at a section in the other borehole to obtain the data on the mass transport between two parallel boreholes where a single fracture crosses. The D_a of cesium and water were determined from the laboratory tests by a non-steady, one-dimensional diffusion method. The K_d of cesium were also determined by a batch method. The breakthrough curves of non-sorbing tracer (Uranine) obtained in the in-situ dipole tracer migration tests were well described by a dual-channel model in which one-dimensional advection dispersion was taken into account. This suggests that the tracers migrate through at least two different pathways in the fracture. The breakthrough curves also indicate that the peak concentration of the sorbing tracer (cesium) was much smaller than that of the non-sorbing tracer (Uranine), suggesting that the Wakkanai Formation has a high sorptive and low diffusive properties for cesium. This specific property of the rock was confirmed at the laboratory experiments. The D_a value obtained for cesium was about 2.9×10^<-12> m^2/s, which is significantly smaller than that of water (3.4×10^<-10> m^2/s), and the K_d value of cesium was determined to be 488 ml/g. These new findings can be useful for understanding the mass transport in the fractured sedimentary rocks having unique features.
