2, UCCS, Villeneuve d'Ascq, , France
3, AREVA, Paris, , France
Spent nuclear fuel contains valuable raw materials, such as plutonium and uranium, which can be separated from other elements in order to be recycled. The industrial process implemented at AREVA-La Hague includes a first stage in which the spent fuel is dissolved in hot nitric acid. During this operation, adverse precipitation involving fission products such as molybdenum and zirconium may occur. Understanding of the precipitation mechanisms and appropriate management of the precipitate require a detailed knowledge of the solid phases formed. This work was aimed at deepening the structural study of the mixed zirconium and plutonium (or cerium) molybdates in order to take into account the possible influence of tetravalent elements on the composition of the precipitates.
The study was first conducted by precipitation in the Mo/ Zr-CeIV surrogate system where cerium can be considered as a chemical analogue of plutonium. The solid composition strongly depends on the tetravalent element initially contained in solution. The results show the predominance of three phases that are ZrMo2O7(OH)2(H2O)2 (1), Ce3Mo6O24(H2O)4 (2) and Ce2Mo3O12(NO3)2(H2O)2.H2O (3). Each of these compounds was proved being a solid solution within which the ZrIV and CeIV atoms can substitute each other. Single–crystals of (2) were obtained by hydrothermal synthesis. The crystal-structure was solved in the no-centrosymmetric space group, Cc, which allows to find a structural arrangement in good agreement with the one previously reported, without disorder. Phase (3) was unknown and not reported in the structural database at the beginning of this study. As no single-crystal could be obtained, the crystal structure was solved ab-initio by combining X-ray powder diffraction and X-ray absorption spectroscopy (beamline MARS, synchroton SOLEIL). The structure of Ce2Mo3O12(NO3)2(H2O)2.H2O derives from Scheelite, ABO4, within which the Mo atoms are in a tetrahedral environment.
The transposition of the study to the Mo/Zr-PuIV system shows that the phases precipitated in the presence of plutonium are isotypes to those formed in the presence of cerium. As such, the mixed zirconium and tetravalent elements molybdates can be seen as a reference system for comparing the behavior of the 4f and 5f elements, even if the precipitation domains differ according to the tetravalent element; the precipitation rate in presence of plutonium appears slower than in presence of cerium.