Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO2 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 RuO2 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 RuO2-NaNO3 precursor and a sodium borosilicate glass (SiO2-B2O3-Na2O). The key parameter in the formation of acicular or polyhedral RuO2 crystals is the chemistry of the ruthenium compound under oxidized conditions (RuIV, RuV). The precipitation of needle-shaped RuO2 crystals in the melt might be associated with the formation of an intermediate Ru compound (Na3RuVO4) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO2 crystals in the melt followed by an Ostwald ripening mechanism.