New methods for co-conversion of uranium (IV) and plutonium (IV) in aqueous medium to prepare mixed oxide (U, Pu)O₂ for MOX fuels

At the present time, the uranium-plutonium mixed oxide fuel is synthesized by mechanical mixtures and powder sintering of UO₂ and PuO₂. This work deals with the study of a new way: synthesis of mixed oxides by aqueous way allowing to make an intimate mixture of uranium and plutonium. The (U, Pu) co-conversion consists to precipitate U(IV) and Pu(IV) ions in homologous compounds. The U(IV) and Pu(IV) keeping in nitric acid requires to add a selective actinides (IV) chelating agent, in order to avoid the Pu(IV) reduction by U(IV). The method used is based on the complex formation with CHON (carbon (C), hydrogen (H), oxygen (O) and nitrogen (N)) ligands. These molecules can be potentially incinerated. Two ligands have been studied: the diethylene-tri-amino-penta-acetic acid (DTPA) and the oxalate ions. The thermodynamic and kinetic stability of the U(IV)-DTPA and Pu(IV)-DTPA, has been revealed; the second system has shown the possibility of co-precipitation of the trioxalato actinides (IV) complexes, after having verified the kinetic stability of this solution. The co-precipitates have been obtained at different amounts of Pu and present a complete homogeneity. Their formula is M₂(U_xPu_1-x)(C₂O₄)₃, 6-7 H₂O (with M=alkaline ions, ammonium). The calcination of the formed co-precipitates, has allowed to obtain homogeneous mixed oxides (U, Pu)O₂. This result depends on the analysis of the X rays diffraction diagram of the mixed oxides and of their capacity to be dissolved in nitric acid 6 mol.l^-1. The study of these two systems has been theoretically transposed to others CHON (DOTA) molecules. This way opens a lot of potentialities for the preparation as well as MOX fuels than new fuels or for the preparation of targets for actinides nuclear incineration. (O.M.)