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

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 (De) and the distribution coefficient (Kd) of rock sample. Then, we adopt a fitting procedure to speculate Kd and De values by comparing the observation to the theoretical curve of the two-dimensional diffusion–advection equation.In the present study, we measured De of 3H by using both the micro mock-up method and the conventional through-diffusion method for comparison. The obtained values of De by two different ways for granite sample (Inada area of Japan) were identical: 1.0 × 10−11 and 9.0 × 10−12 m2/s but the testing period was much different: 10 h and 3 days, respectively. We also measured the breakthrough curve of 85Sr and the resulting Kd and De 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.