Modelling and Experimental Studies of Sorption in the Near Field of a Cementitious Repository
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MODELLING AND EXPERIMENTAL STUDIES OF SORPTION IN THE NEAR FIELD OF A CEMENTITIOUS REPOSITORY
P.L.Brown , A.Haworth, R.McCrohon , S.M.Sharland and C.J.Tweed, Theoretical Physics Division, Harwell Laboratory, Oxfordshire, UK, #Chemistry Division, Harwell. Permanent address: Environmental Science Division, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia. ABSTRACT. A joint experimental and modelling programme is reported, which aims to improve our understanding of sorption processes of radionuclides onto repository materials. Diffusion/sorption experiments of sorption onto cement are described, although results are limited at this stage. The modelling studies use the coupled chemical equilibria and transport code CHEQMATE to simulate some of these experiments. The chemical part of the model is based on a simple mass-action model of sorption. More detailed comparisons will continue when the experiments are terminated, and the samples are sectioned. 1. INTRODUCTION. The process of sorption of nuclides on to solids is an important feature in the construction of a safety case for a nuclear waste repository. In the near field, sorption on repository materials such as the backfill will limit the aqueous concentration of certain nuclides which, in turn, will significantly reduce nuclide levels in the groundwater leaving the repository. Current plans for a low- or intermediate-level waste repository in the UK include a cementitious backfill. At the start of the cement lifetime, it will provide a high pH environment and act as a homogeneous sorbent material. However, as the repository evolves there may be changes in the chemistry and structure of the near-field (i.e. the engineered barriers and geology immediately surrounding the repository), which will alter the sorptive capacity.
In this paper, a joint experimental and modelling approach is described, which is aimed at improving our understanding of sorption processes. There are several experimental techniques utilised for measuring sorption of radionuclides onto repository and geological materials. The most common is 'batch sorption' in which a volume of solution containing a radionuclide is added to a known mass of solid. The final concentration of the sorbing nuclide is measured and the ratio of the amount sorbed to the amount in solution is calculated. This ratio is known as the distribution Rd, or when the system is at equilibrium, Kd. A linear sorption isotherm corresponds to an Rd that is independent of solution concentration. An important factor in batch sorption experiments with geological materials appears to be the solid to liquid ratio in the experiment since Rd can vary significantly with different experimental conditions. However, for sorption onto cement, experimental Rd values show little or no dependence on such factors [1]. For some nuclides though, the Rd values do vary with solution concentration. A second experimental technique is 'through-diffusion' or 'diffusion/sorption' where nuclides diffuse t
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