Cesium Sorption Rate on Non-Crushed Rock Measured by a New Apparatus Based on a Micro-Channel-Reactor Concept
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0985-NN11-08
Cesium Sorption Rate on Non-Crushed Rock Measured by a New Apparatus Based on a Micro-Channel-Reactor Concept Keita Okuyama1, Akira Sasahira1, Kenji Noshita1, and Toshiaki Ohe2 1 Power and Industrial Systems R&D Laboratory, Hitachi Ltd, 7-2-1 Omika, Hitachi-shi, 3191221, Japan 2 Energy Science & Engineering Department, School of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka-shi, 259-1292, Japan
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 (10200-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 adsorbs 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. INTRODUCTION To evaluate the safety of geological disposal of high-level radioactive wastes, the migration of radionuclides leaked from the repository and transported with groundwater through fissures in the bedrock must be predicted. Since the nuclide migration through the rock mediums is an extremely slow process, experimental effort to evaluate diffusion coefficient (De) and the distribution coefficient (Kd) requires relatively long testing periods and chemically stable conditions [1,2]. To shorten the testing time, conventional techniques such as diffusion method and batch sorption methods are normally used for different sample geometries: crushed samples for determining Kd [3] and block specimen for determining De [2]. The size dependence on both De and Kd has been observed; thus, to keep the consistency of data sets, additional correction due to difference in sample geometries is required.
We have thus developed a fast method to determine both De
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