Thixotropic behavior of a glass melt containing PGM particles for nuclear interest
Résumé
Contrary to some other elements such as Iron, Nickel and Chromium, the Platinum Group Metals particles (PGM) are not incorporated chemically in molten glasses. These particles are generally found in nuclear borosilicate glasses resulting from the melting, at 1200°C, of a glass precursor and fission products issued from spent fuel reprocessing. During the melting step, the presence of theses metals as suspended particles of a few microns has an impact on the rheological properties of the material, leading to a Non-Newtonian behavior. Their impact on the process is the object of characterization and modeling of many studies that have established that the melt presents a shear-thinning and thixotropic behavior. At high shear rate, the glass behaves as a suspension of individual particles, whose rheological behavior is controlled by the viscosity of the vitreous matrix. On the other hand, at low shear rate, below a stress threshold, and above a certain volume fraction in PGM particles, the formation of aggregates composed of needle-like Ru/RuO2 chains and Pd-Rh-Te spherical particles, strongly influences the viscosity of the glass. In this work, a deeper analysis of the thixotropic behavior of a simulated nuclear glass melt containing 3.0 wt% (1.02 vol%) of PGM particles will be presented. Steady and transient state measurements were performed over a wide shear rate range using an imposed-stress rheometer at temperatures ranging from 1100 to 1250 °C. Using Houska model, this work shows that it is possible to predict the transient behavior of the sample from their steady-state rheological behavior.