Cesium Sorption Rate on Non-crushed Rock Measured by a New Apparatus based on a Micro-channel-reactor Concept

Abstract

Since nuclide migration through rock mediums is an extremely slow process, experimental effort to evaluate the barrier performance of geologic disposal such as the diffusion coefficient (De) and the distribution coefficient (Kd) requires relatively long testing periods and chemically stable conditions. We have developed a fast method to determine both De and Kd by using a non-crushed rock sample. In this method, a fluoroplastic plate with a micro channel (10-200-μm depth) is placed just beneath a rock-sample plate, and a radionuclide solution is injected into the channel at constant rate. A part of radionuclide diffuses into the rock matrix and/or adsorb on the rock surface. The difference between the inlet and outlet radionuclide flux is simply related to the apparent diffusion coefficient (Da) of the rock sample. In this study, we estimated Kd of Cs for granite by using the equilibrium model, finding that Kd decreased with increasing flow rate. This dependence of Kd on flow rate implies the state of sorption equilibrium. The adsorption and desorption curves of 134Cs were thus measured, and the rate constants for both processes were obtained by adopting a first-order rate law. The rate constants of sorption (k+) and desorption (k-) were obtained as a function of flow velocity; constant values of both were observed. Kd was calculated from k+/ k- and then compared with that determined by conventional batch sorption method using a crushed rock sample. The Kd values determined by the present and conventional methods are in good accordance; however, the testing periods for each method are very different, namely, 1 day and 7 days, respectively.