Thermodynamic and thermophysical investigation on the Cu-Fe-Si system
Résumé
The ternary Cu-Fe-Si plays a crucial role in the phase equilibria at high temperatures during the vitrification of metallic waste contaminated by uranium and plutonium oxides, as result of the fabrication of MOx (Mixed Oxide) nuclear fuel. In such process, the metallic contaminated devices (mainly made of stainless steel, copper and aluminum) are melted and merged together with an oxide bath, leading to the coexistence of two immiscible liquids. A third liquid may appear at high temperature, as result of the demixing between Cu and Fe. Moreover, Redox reactions between the oxide and the metallic melts affect each other their compositions, resulting in the total oxidation of Al and in the reduction of a small amount of Si in the metallic state. Because of these phenomena, the phase equilibria during the whole vitrification process are even more difficult to predict.
A thermodynamic model of the ternary Cu-Fe-Si through the Calphad (CALculation of PHAse Diagram) method is a good way to obtain a better knowledge on the behavior of the metallic phases of this complex system. Differential Thermal Analysis and Energy Dispersion X-ray spectroscopy have been used in order to establish the transition temperatures and the phase equilibria compositions of selected samples. These thermodynamic data are then implemented in new assessments of the binary Cu-Si and of the ternary Cu-Fe-Si, capable of predicting the liquidus/solidus temperatures, as well as the extent of the miscibility gap in the liquid phase. In order to provide useful information for the development of such process on industrial scale, measurements of surface tension of liquid Cu-Si alloys are also provided, using an aerodynamic levitation setup coupled with a laser heating technique.