Diffusion and Sorption of Radioactive Cesium and Cobalt in Regolith Materials of Central Australia
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Diffusion and Sorption of Radioactive Cesium and Cobalt in Regolith Materials of Central Australia Takashi Itakura1, Chin Jian Leo2, Gordon D. McOrist1 and Timothy E. Payne1 1 Environment Division, Australian Nuclear Science and Technology Organisation PMB1, Menai, NSW, 2234, Australia 2 School of Engineering and Industrial Design, University of Western Sydney Locked Bag 1797 Penrith South DC, NSW, 1797, Australia ABSTRACT Diffusion and sorption are potentially the most important factors governing the transport of radionuclides in clays and under stationary subsurface environmental conditions. Preliminary studies have been performed to measure the transport of 137Cs and 60Co with no advection, using reconstituted samples of three regolith materials collected from a region in South Australia. The samples were saturated with CaSO4 solution to imitate the pore water chemistry of the in situ environment. A double diffusion cell testing apparatus made of polycarbonate resin was used to measure the transport of the selected radionuclides through the samples. A curve fitting procedure employing one-dimensional contaminant equations with a “stop-start” technique was used to estimate the diffusion (D) and sorption coefficients (Kd) from the measured concentration-time profiles. Results from these experiments are compared with those obtained from batch sorption tests. INTRODUCTION A series of studies has been carried out to select a preferred site for construction of a national low-level radioactive waste repository in the central north region of South Australia (CNSA). This region has an arid climate and is characterised as having a low water recharge rate and a deep water table (e.g. Harries et al., 1999). In such an isolated subsurface environment with relatively stationary groundwater conditions, sorption and diffusion are potentially the most important among various factors that govern the transport of radionuclides. The sorption of selected radionuclides on CNSA regolith materials under a range of chemical conditions has hitherto been studied. To further develop the understanding of the radionuclide transport in the region, diffusion tests have been performed for some CNSA regolith materials. This paper presents a preliminary estimate of D and Kd values for 137Cs and 60Cs for these materials. EXPERIMENTAL Three samples were selected from potential repository sites in the region on the basis of drill log and soil properties, with the intention of including the major clay minerals typical of the sites (Table 1). Air-dried soil fractions passing through a 2 mm mesh sieve were used as the samples. Following previous studies for the CNSA region, these fractions were mixed with saturated CaSO4 solution that was used as the main liquid medium (Harries et al., 1999 and 2000). Figure 1 shows the testing apparatus used for the experiment. The apparatus is made of polycarbonate and consists of three parts; source cell (400 ml), sample cylinder of 38 mm JJ5.5.1
diameter and 50 mm long, and a collector cell (430 ml). Analysis of l
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