The formulation of a radioactive waste packaging glass is the result of a compromise between waste loading, technological feasibility and chemical durability. Maximizing the waste loading rate allows us to reduce the number of vitrified packages produced, and consequently the volume and cost of the underground disposal. On the other hand, increasing this loading rate beyond a certain threshold is likely to lead to the presence of crystals in the vitrification furnaces. However, such an evolution of glass formulations requires, among other things, verification of the impact of the presence of these crystals on the properties of the glass during its production (at 1100-1200°C), in particular its rheology, a key property for the good operation of vitrification furnaces. The aim of the proposed thesis is therefore to measure and then model the effect of crystals on rheology, as a function of time, temperature, and nature and morphology of the crystals, and to take into account the risk of sedimentation. To do this, experimental data will have to be acquired, then modelled using models proposed in the literature, which may need to be adapted. A Master or engineering degree in physico-chemistry or material sciences is needed.