  Swelling of uranium dioxide with ingrowth of defects by irradiation is not
fully understood. Experimental and theoretical groups have attempted to explain
this phenomenon with various complex theories. In this study, experimental
lattice expansion and super saturation of the lattice were well reproduced by
molecular dynamics simulation method. From the resemblance with the
experimental data, here it is manifested that only oxygen frenkel pairs were
created in the fission induced lattice while alpha particle radiation causes
both oxygen and considerable amount of uranium defects. Moreover, in this work,
defects are divided into two sub-groups as obstruction and distortion and it is
shown that obstruction type frenkel pairs merely responsible for the lattice
swelling for both fission and alpha particle radiation. Evidently relative
lattice expansion varies linearly with the obstruction type of survived uranium
defects. Additionally, at high concentrations, some of the obstruction type
uranium frenkel pairs forming double or triple structures with oxygens in their
octahedral cages which increase the slope of the linear dependence.
