Modelling of Water Uptake and Salt Leaching in Bituminized Radioactive Waste: Comparison Between Numerical, Analytical a
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0RGHOOLQJRI:DWHU8SWDNHDQG6DOW/HDFKLQJLQ%LWXPLQL]HG5DGLRDFWLYH:DVWH &RPSDULVRQ%HWZHHQ1XPHULFDO$QDO\WLFDODQG([SHULPHQWDO5HVXOWV J. Sercombe, F. Adenot, C. Tiffreau, B. Gwinner, I. Felines CEA Cadarache, DEN/DED/SAMRA, Bât 163, 13108 St-Paul-lez-Durance, France $%675$&7 In this paper, a simplified model which allows the derivation of analytical relations aiming at describing the diffusion-dissolution-swelling processes taking place in bituminized waste exposed to water or salt solutions is presented. The analytical solutions render the comparison with experimental data and tendencies straightforward. The validity of the simplified approach is demonstrated by comparing analytical and numerical solutions with experimental data. ,1752'8&7,21 Bitumen is widely used as a fixation material for low and intermediate level radioactive waste. In general, the bituminized waste contain soluble salts (NaNO3, Na2SO4) and insoluble elements (precipitation sludge particles, radionuclides, …) which originate from the chemical precipitation process used to solidify liquid concentrates. For evaluating the safety of geologic repository, the consequences of water immersion on the transport properties of bituminized materials are of particular interest since the main requirements concern the confinement of radionuclides. As shown in previous investigations, water uptake in bituminized materials containing soluble salts can be important and lead to the development of a solution-filled pore structure in which radionuclides can migrate [1, 2]. The long-term performance assessment of the geologic disposal of bituminized waste materials relies on mathematical models aiming at describing the coupled diffusion-dissolutionswelling processes at the origin of salt and radionuclides leaching. Among them, the COLONBO model has been presented in past communications [3, 4]. It is based on a system of non-linear diffusion equations which is solved numerically. In this paper, it is shown that by the use of adequate assumptions, the system can be simplified and solved to obtain analytical expressions. Analytical expressions are particularly useful for understanding how salt, radionuclides and water interact in leached bituminized waste. Compared to the numerical solution of the complete system, they provide a direct view of the key-parameters and their (theoretical) impact on experimental results. &2167,787,9((48$7,216 In the proposed mathematical description, bituminized waste is assumed to be composed of bitumen and insoluble elements on the one hand, and of sodium nitrate (NaNO3), the dominant soluble salt in most cases, on the other hand. The scale of description is macroscopic such that the bituminized material can be considered as homogeneous. As shown schematically in figure 1,
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