Phase relationships in the U - Np - O system

U-Np-O phase equilibria was experimentally studied by X-ray diffraction in reducing and oxidizing conditions. Both (U1-yNpy)O2 stoichiometric and hyper-stoichiometric compounds were characterized after a heating treatment and resulting lattice parameters were compared to that of previous studies. Synthesized stoichiometric compounds are monophasic and their lattice parameters as a function of Np content are in nice agreement with the Vegard's law between UO2 and NpO2 end-member. Resulting compounds after an oxidizing heat treatment of the stoichiometric samples confirm previous reported U-Np-O relationship. For low Np content, a biphasic hyper-stoichiometric compound is observed: a (U1-y'Npy')O2+x phase (space group fm-3m) and a (U1-y'',Npy'')3O8 phase (space group C2mm). For higher Np content (with starting material y = 0.7), only one monophasic hyper-stoichiometric phase (U1-y'Npy')O2+x appears. Lattice parameters of the (U1-y'',Npy'')3O8 phase differ from the ones of pure U3O8, indicating some Np solubility in this compound. Although Np might remain tetravalent in the (U1-y'',Npy'')3O8 phase, this result is in conflict with the assumption of Yamashita et al. (1994). (authors)