Because it is the most used nuclear fuel, uranium dioxide has received attention for more than fifty years. In the last ten years, the increase in calculation capacities made it possible to achieve first principle calculations of its electronic structure and to derive a correct simulation of many of its thermodynamic properties. However simulation of actual irradiated fuel with the same level of accuracy requires modelling the transfer of electronic charges that occurs in UO2 when it is irradiated and chemically doped by fission products. In most studies it is widely admitted that uranium dioxide could incorporate large amounts of 3+ cations produced by fission without any change in its crystalline structure because the 3+ cations take U4+ position and the charge mismatch is compensated by the formation of U5+ or oxygen vacancies. In this contribution we demonstrate that this assumption is not true in the case of the U-Nd-O system. A miscibility gap was evidenced in this system for Nd concentration as low as 6 at % [1]. Thus the existence of two coexisting phases with different contents in oxygen and neodymium for a sample having (U,Nd)O2+/-x composition implies that a different mechanism for charge transfer should be considered for each phase. These mechanisms can be tackled with literature data either using ionic models or thermodynamic modelling. In both models U5+ oxidation state plays a key role. U5+ incorporation in UO2 can also be viewed through the analysis of the U4O9 crystalline structure. Its incorporation requires the formation of clusters of interstitial oxygen atoms. Therefore the coexistence of Nd3+ and U5+ in UO2 crystalline is very likely associated with the existence of complex defect clusters, at least in one phase belonging to U-Nd-O miscibility gap..