HOW TO EXPLAIN THE EXPERIMENTAL DIFFICULTIES IN THE COFFINITE PREPARATION? HINTS FROM THE STUDY OF URANOTHORITE SOLID SOLUTIONS

The storage of nuclear spent fuel in an underground repository is currently one of the more likely options envisaged to manage the radwaste generated in the back-end of the nuclear fuel cycle. Thus the long-term behaviour of uranium should be assessed in the case of an accidental situation, i.e. break-away of the protection barriers and leaching of the fuel by underground waters. In such conditions, as many potential host rocks exhibit reducing conditions associated to high silicate levels, the concentration of uranium is expected to be controlled by the precipitation of U(IV) silicate, known as USiO₄ coffinite. Nevertheless, very few reliable thermodynamic data concerning coffinite are available in the literature, probably due to some difficulties in obtaining pure and single-phase USiO₄. Indeed, even if Pointeau et al. recently proposed a chemical way of preparation though hydrothermal precipitation, the works based either on dry or wet chemistry methods generally failed to obtain coffinite or led to products insufficiently characterized.

On this basis, this work aimed at the preparation of Th_1-xU_xSiO₄ uranothorite samples to access the thermodynamic data of coffinite through the solid solution approach. The preparation of the samples was performed from a mixture of thorium and uranium in hydrochloric medium and sodium metasilicate. The starting reagents were mixed in the desired stoichiometry then heated in hydrothermal conditions at 250°C for 24 hours.

The resulting powdered samples were then extensively characterized by XRD, SEM, TEM as well as µ-Raman and IR spectroscopies. First, the Rietveld refinement of XRD patterns revealed the presence of two distinct phases in the samples, i.e. uranothorite and uranothorianite. The first one present the classic bypiramidal morphology associated to the zircon-type structure while the second appeared to be nanosized. Also, the amount of dioxide by-product was found to increase significantly with the amount of uranium initially introduced. Finally, the spectroscopic data pointed out the absence of water molecules or hydroxide groups in the compound, thus confirming the chemical formula of the uranothorite series. It also allowed extrapolating the positions of coffinite vibration modes which remained unknown up to now.