Abstract
In France, high-level nuclear wastes are confined in glass using a calcination-vitrification process. The waste can also be vitrified by liquid feeding, which imply to add directly in the glass melter the liquid waste. In this study, we focus on nuclear waste vitrification process by direct liquid feeding in order to decipher the dissolution kinetics of liquid waste. We propose an experimental and analytical protocol to study the dissolution in the solid glass frit of the liquid waste dried beforehand. This methodology allows to: i) identify the secondary phases which formed and dissolved during the process; ii) characterize the evolution of waste element concentrations in the glass as a function of time and temperature; iii) define kinetic parameters relative to the process of secondary phases dissolution in the glass. The glass is completely homogeneous at elaboration temperature (1200°C) and three main secondary phases are formed and dissolved as a function of temperature: Ca-REE (Rare Earth Elements)- silicate, Ce- oxide and Zr- oxide. At each temperature step (800 to 1200°C), the evolution of REE (La, Ce, Nd and Pr) and Zr concentrations in the glass shows a fast increase followed by a stabilization in time, which reflects the fast dissolution of the dried waste in the glass, i.e. the fast dissolution of secondary phases. A parameterization methodology of element concentration evolution as a function of time and temperature is proposed here in order to define the kinetic parameters corresponding to the dried waste dissolution (i.e. concentration at equilibrium, characteristic time and activation energy). The results show that the element activation energy might be related to the formation processes of the secondary phases, i.e. whether they constitute intermediate compounds formed in temperature by the interaction between the dried solution and the glass, or whether they are inherited from the dried solution.