Vitrification process is a technology commonly used for the treatment and the immobilisation of high-level radioactive waste from spent fuels processing. In France, nuclear waste is conditioned in glass matrix to provide a stable and sustainable confinement. The current vitrification facility is a continuous process involving two steps: calcining in a rotary kiln (introduction of high-level wastes in nitric solution) and/then glass melt elaboration in a cold crucible by induction processes (solid feeding). A way to simplify the process could be to introduce the waste solution directly at the glass melt surface (liquid feeding) in order to avoid the calcination step. Simulation studies^1-3 have enabled to propose a 3D model of nuclear waste vitrification in cold crucible by solid feeding, taking into account the fluid mechanics, the induction heating, the thermal and the chemical aspects. However, no modeling of vitrification by liquid feeding has been proposed yet. Such modeling could bring new information on the evolution of the chemical processes and help to predict and optimise the waste vitrification by liquid feeding.

In this context, this study aims to bring new constrain to the 3D modelling on the thermal and chemical reactions occurring during vitrification process by liquid feeding. In situ experiments in temperature have been performed (ESEM and XRD) in order to study the microstructure and to characterise the phases. This first experimental approach has allowed to identify the steps of phase transformation in the liquid in interaction with glass frit (nitrate melting, nitrate decomposition, glass softening, melting and phase dissolution), from the ambient temperature to 1200°C. Ex situ experiments in furnace have also been performed to characterise the evolution of glass composition. Element concentrations have been analysed for different temperature (from 800 to 1200°C) at different dwell time (2, 5, 30, 120 and 480 minutes). The evolution of concentrations allows to determine the liquid dissolution kinetics occuring during vitrification by liquid feeding. The results will pave the way to a global 3D model of liquid radioactive waste vitrification.

1. Paraiso, K. et al. Characterization and modeling of chemical reactions taking place during the vitrification of high level nuclear waste. Journal of Nuclear Materials 569, 153878 (2022).
2. Barba Rossa, G. Modélisation multiphysique de l’élaboration de verre en creuset froid. (Université Grenoble Alpes (ComUE), 2017).
3. Paraiso, K. Modélisation et simulation numérique de l’élaboration du verre dans les procédés de vitrification des déchets nucléaires de haute activité. (Sorbonne Université, 2021).