Influence of Platinum Group Metals particles aggregation on a glass melt rheological behavior

Abstract : glasses are generally used as matrices to immobilize nuclear fission products resulting from spent fuel reprocessing. In the high-temperature vitrification process (1200 °C), most elements to be contained react chemically with the vitrification additives to form a homogeneous glass melt. Platinum Group Metal (PGM) particles are not incorporated chemically in the melt and therefore are present as suspended particles a few microns in size. These particles exhibit an intense aggregation tendency and consequently the suspensions may present an anomalously high apparent viscosity. These systems are characterized by shear-thinning and thixotropic behaviors. However, the interplay between the rheological behavior and the aggregation degree is poorly understood. In this work, the aggregation mechanisms of a simulated nuclear glass melt containing 3.0 wt.% (1.02 vol.%) of PGM particles were investigated. The impact of the shear stress and time on the PGM aggregation degree was determined using an imposed-stress rheometer at high temperature followed by an imaging analysis procedure via Scanning Electron Microscopy (SEM). We present three different aggregation scenarios and their impact on suspension rheology. Based on the experimental data acquired, a force balance computation was performed to illustrate these three scenarios.