Mechanism of RuO₂Crystallization in Borosilicate Glass: An Originalin SituESEM Approach

Abstract

Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO(2) particles in high-level waste containment glass matrices. These particles are responsible for significant modifications in the physicochemical behavior of the glass in the liquid state, and their formation mechanisms are a subject of investigation. The chemical reactions responsible for the crystallization of RuO(2) particles with acicular or polyhedral shape in simplified radioactive waste containment glass are described. In situ high-temperature environmental scanning electron microscopy (ESEM) is used to follow changes in morphology and composition of the ruthenium compounds formed by reactions at high temperature between a simplified RuO(2)-NaNO(3) precursor and a sodium borosilicate glass (SiO(2)-B(2)O(3)-Na(2)O). The key parameter in the formation of acicular or polyhedral RuO(2) crystals is the chemistry of the ruthenium compound under oxidized conditions (Ru(IV), Ru(V)). The precipitation of needle-shaped RuO(2) crystals in the melt might be associated with the formation of an intermediate Ru compound (Na(3)Ru(V)O(4)) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO(2) crystals in the melt followed by an Ostwald ripening mechanism.

Diffusion-driven and size-dependent phase changes of gallium oxide nanocrystals in a glassy host

“…[10][11][12][13] This is the case of Ga oxide NCs in silicate glass, which is one of the prototypal systems of oxide-in-oxide nanostructured glasses together with SnO 2 -doped silica. 14 Stability and thermal evolution of Ga-oxide nanophases are expected to be largely different from the freestanding and pure compound.…”

Applications of ESEM on Materials Science: Recent Updates and a Look Forward

“…Third, for crystallization/sintering processes occurred at either high or low temperature (relative to room temperature, ≈25 °C), the corresponding morphological evolution and growth mechanism can, of course, be monitored under ESEM by using different temperature‐controlling stages. For example, the evolution of morphology and size distribution of reaction products during crystallization or sintering of glass/ceramic materials can be imaged by ESEM at high temperature . Boucetta et al observed the changes in morphology and composition of the ruthenium compounds formed during an in situ heat treatment in ESEM operated at 30 kV.…”

In Situ ESEM Experiment Applied to the Description of Chemical Processes during Glass Elaboration

“…4. HT-ESEM image sequence illustrating the formation and dissolution of Na2SiO3 intermediary phase (Boucetta et al (2012)) The reaction between NaNO 3 and a simplified sodo-boro-silicate glass yields to the spreading of liquid NaNO 3 at the glass surface and to the formation of intermediary Na 2 SiO 3 and "Na 2 O-B 2 O 3 " phases above 306°C (see Boucetta et al (2012) for more details). The image sequence that illustrates the evolution of the intermediary phases (494-659°C) followed by their local melting in contact with the silicate melt (727-894°C) are reported in figure 4.…”

“…This combination allows studying directly sample morphology modifications occurring during sample heat-treatment. It has been successfully used to study several phenomena such as the corrosion of metals (Jonsson et al, 2011), oxidation of metals (Oquab & Monceau (2001); Schmid et al (2002); Reichmann et al (2008); Mège-Revil et al (2009) ;Quémarda et al (2009); Delehouzé et al (2011)), reactivity at high temperature (Maroni et al (1999); Boucetta et al (2012)), phase changes (Fischer et al (2004); Hung et al (2007); , hydrogen desorption (Beattie et al ( , 2011, redox reactions (Klemensø et al (2006), microstructural modifications (Bestmann et al (2005);Fielden (2005); Yang (2010)), magnetic properties (Reichmann et al (2011)), sintering (Sample et al (1996); Srinivasan (2002); Marzagui & Cutard (2004); Subramaniam (2006); Courtois et al (2011);Joly-Pottuz et al (2011);Goel et al (2012); Podor et al (2012)), thermal decomposition (Gualtieri et al (2008); Claparède et al (2011);Hingant et al (2011);Goodrich & Lattimer (2012)), crystallisation (Gomez et al (2009)) in melts (Imaizumi et al (2003); Hillers et al (2007); Vigouroux et al (2013)) and the self-repairing -self-healing -properties of materials (Wilson & Case (1997); Coillot et al (2010Coillot et al ( , 2011.…”

“…However, applications of this technique in the field of nuclear materials remains limited (Boucetta et al (2012); Clavier et al (2013)) but it potentially offers new opportunities to study and describe complex chemical processes occurring during glass elaboration for the conditioning of radwastes. In this paper, we will report the basics of HT-ESEM and two examples of application in the nuclear field.…”