Experimental study and thermodynamic modelling of the Cu-Fe-Si-U sub-systems

Abstract

The vitrification is a possible treatment for the conditioning of the metallic Long-Lived Intermediate-Level nuclear waste. This process is characterised by the presence of immiscible oxide and metallic liquids that react each other. A thermodynamic model using the Calphad method is then required to predict the composition of the treated waste and the speciation of uranium in this complex system. The quaternary Cu-Fe-Si-U is chosen as the most representative to describe the metallic liquids that appear at high temperature. Key sub-systems of this quaternary are experimentally investigated and thermodynamically modelled. The description of the liquidus in the Si-rich corner of the binary Cu-Si is improved. The Cu-U is assessed with the Calphad method for the first time; it is characterised by a large miscibility gap in the liquid phase. Some new ternary isotherms of the ternary Cu-Fe-Si are then proposed: a large liquid miscibility gap appears also in this system in agreement with the observed behaviour of the metallic liquid during the experimental investigations. The Cu-Si-U system is characterised by three ternary intermetallic compounds: CuSiU, Cu2Si2U and Cu4Si4U3. A first description is proposed, but more data are still required to improve the thermodynamic modelling of this system.