  By first-principles LSDA+U calculations, we revealed that the current
physical picture of defective uranium dioxide suggested solely by neutron
diffraction analysis is unsatisfactory. An understanding based on quantum
theory has been established as a thermodynamical competition among point
defects and cuboctahedral cluster, which naturally interprets the puzzled
origin of the asymmetric O' and O'' interstitials. It also gives a clear and
consistent agreement with most available experimental data. Unfortunately, the
observed high occupation of O'' site cannot be accounted for in this picture
and is still a challenge for theoretical simulations.
