10 Highlights 11  Description of the solubility of the U 1-y Nd y O 2±x system at room temperature. 12  Characterisation of the charge compensation mechanisms and local disorder for the U 1-y Nd y O 2±x 13 system. 14  Analysis of the crystal lattice of the U-Nd-O system assessed complementary by XRD and XAS. 15 Abstract 16 The evolution of the crystal lattice of samples made of UO2 doped with different concentrations of 17 Nd in stoichiometric and hypo-stoichiometric conditions has been systematically studied by X-ray 18 diffraction (XRD) and X-ray absorption spectroscopy (XAS). The substitution of a trivalent cation for 19 the U 4+ initial position is responsible for creating local structural disorder and changes in the 20 oxidation states. In this scenario, the lattice parameter is affected and the concentration of U 5+ and 21 formation of oxygen vacancies as well, since these are the mechanisms necessary to maintain the 22 charge neutrality. The systematic oxidation of U 4+ as predominant charge compensation mechanism 23 over the formation of vacancies can be reduced by performing a thermal treatment under reducing 24 conditions. This paper presents an experimental characterization of the uranium oxidation state