The transport and settling of platinum-group-metal particles in glass melts
Résumé
Our work is linked with the development of models and numerical simulations of High-Level-Waste (HLW) vitrification processes. Platinum-Group-Metals (PGM) are found in the vitrification process of nuclear waste as fission products. During the melting, the glass generally shows a homogeneous liquid phase seeded with non-soluble heavy platinum-group-metal particles mainly made of palladium and ruthenium dioxide. Previous studies reported spatial discrepancies of the local volume fraction of particles in the melt because of particles settling with time. This migration of particles towards less agitated bottom parts of the crucible impacts the dynamical, electrical and thermal state of the melt because of concentration-dependent electrical conductivity and viscosity of the suspension. A theoretical one-fluid transport model is derived with little hypothesis, with particular attention payed to the hindered settling term. The resulting unsteady transport equation has shown to account for observed volume fraction profiles in seeded glass heat treated at different temperatures and for different time horizons. These experiments have been carried out with a HLW glass simulant and we measured PGM particles local concentration using Laser Induced Breakdown Spectroscopy. We investigate both isothermal and non-isothermal settling and special attention is given to the temperature dependence of Stokes terminal settling velocity and particles' diffusivity due to temperature dependent glass viscosity. The model can easily be coupled with existing 3D thermo-hydraulic numerical codes, thus enhancing the precision of heat flux predictions between the melt and the crucible. The full model will eventually help designing crucibles taking into account PGM transport in the melt.
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