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 (U1yNpy)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 C2 mm). 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 (U1y”,Npy”)3O8 phase, this result is in conflict with the assumption of Yamashita et al. (1994).