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The formation of crystals by liquid reaction takes place in the many natural and artificial processes, and in particular in reactive crystallization processes. The control of the kinetics of the formation, the size and the morphology of the precipitates is still very challenging. Size control of precipitated powder is also an important issue of nuclear fuel treatment, where plutonium is precipitated as oxalate, before being converted into the oxide used in the manufacture of MOX.
The reference theory for predicting rate of crystal formation, used in process modeling, is the classical theory of nucleation (CNT), which is based on the thermodynamic description of the liquid-vapor equilibrium proposed by Gibbs in 1876. But this model sometimes dramatically fails because it ignores all the disordered intermediate states possibly achieved between the initial solution and the final crystal: clusters, liquid-liquid phase separations, amorphous particles or networks, etc. In particular, such amorphous intermediate states were observed in the precipitation of cerium oxalate, one of the reference simulating systems for plutonium, suggesting a two-stage nucleation process in contradiction with the CNT.
The general objective of this thesis is to characterize the intermediate states of the nucleation of cerium oxalate and their impact on the predictions of classical theory. As a close collaboration between CEA Marcoule and CEA Saclay, this thesis will combine techniques known to be able to tackle this difficult problem: X-ray scattering in laboratory and synchrotron facilities (SAXS/WAXS), microfluidics, thermodynamic and kinetic models.