One way of increasing significantly the efficiency in designing and qualifying innovative fuels is to enhance the predictive capability of fuel behaviour simulation by developing a more physically based description of nuclear fuels. Basic research approaches combining multiscale modelling and separate effect experiments can bring significant insight into materials properties and key phenomena involved in the evolution of fuels in reactor. We will show the results obtained using state-of-the art electronic structure and empirical calculations on the uranium-plutonium mixed oxide. In particular, the thermal expansion, enthalpy increments and specific heat of (U,Pu)O2 as a function of Pu content will be presented. The defect properties of (U,Pu)O2 and the impact of the disorder on the cationic sublattice will also be discussed.