UO2 is the main component of nuclear fuel for most of nuclear power plants. In operation in these plants, it withstands irradiation damage without major changes. Szenes proposed a simple criterion for determining an amorphisation threshold due to swift heavy ions, which works reasonably well for many ionio-covalent compounds. In the case of UO2, this criterion predicts that this threshold is reached for 8.6 keV/nm, which means well under the electronic energy loss of fission products, for example 19.7 keV/nm for 72 MeV iodine. We will address this apparent contradiction. In fact the resistance of a given material to irradiation damage depends on the way it can absorbs the deposited energy by electronic losses before reaching an energy density, generally associated with a threshold temperature that leads to metastable changes. In this paper we will discuss the equilibrium properties of UO2 and how UO2 can absorb energy before reaching this threshold. At thermodynamic equilibrium, the way by which a solid stores energy can be estimated by its heat capacity. In order to better understand UO2 resistance to irradiation, we discuss how the properties of uranium and oxygen sublattice have a drastic impact on the thermodynamic properties of UO2 compared to other fluorite compounds. We will discuss why the unusual behaviour of uranium sublattice is associated with the formation of polarons at high temperature and how this gives UO2 an enhanced irradiation resistance.