A fast and sensitive method for evaluating nuclides migration characteristics in rock medium by using micro-channel reactor concept

摘要

Experimental effort to evaluate the barrier performance of geologic disposal requires relatively long testing periods and chemically stable conditions. We have developed a new technique, the micro mock-up method, to present a fast and sensitive method to measure both nuclide diffusivity and sorption coefficient within a day to overcome such disadvantage of the conventional method. In this method, a Teflon plate having a micro channel (10-200 μm depth, 2, 4 mm width) is placed just beneath the rock sample plate, radionuclide solution is injected into the channel with constant rate. The breakthrough curve is being measured until a steady state. The outlet flux in the steady state however does not meet the inlet flux because of the matrix diffusion into the rock body. This inlet-outlet difference is simply related to the effective diffusion coefficient (D_e) and the distribution coefficient (K_d) of rock sample. Then, we adopt a fitting procedure to speculate K_d and D_e values by comparing the observation to the theoretical curve of the two-dimensional diffusion-advection equation. In the present study, we measured D_e of ~3H by using both the micro mock-up method and the conventional through-diffusion method for comparison. The obtained values of D_e by two different ways for granite sample (Inada area of Japan) were identical: 1.0 x 10~(-11) and 9.0 x 10~(-12) m~2/s but the testing period was much different: 10 h and 3 days, respectively. We also measured the breakthrough curve of ~(85)Sr and the resulting K_d and D_e agreed well to the previous study obtained by the batch sorption experiments with crushed samples. The experimental evidence and the above advantages reveal that the micro mock-up method based on the microreactor concept is powerful and much advantageous when compared to the conventional method.