The melting behaviour of uranium/neptunium mixed oxides

•Melting temperatures in the (UO2+NpO2) system have been measured by laser heating and fast pyrometry.•The melting behaviour of this system is satisfactorily described by an ideal solution model.•Raman spectroscopy demonstrates that blending and sintering of UO2 and NpO2 results in the formation of oxygen defects.•Addition of NpO2 to a UO2 matrix does not lead to significant chemical or thermal changes.