Irradiation induced volume change in UO2

Two sets of uranium dioxide samples having grain sizes of 2.5 and 5 μm were irradiated with different fission doses in the range of 1.1 X 1014 to 2.9 X 1018 fissions/cm3; the volume change was measured. A steep volume expansion was observed up to a dose of about 5 X 1016 fissions/cm3 which corresponds to the reported dose for maximum elongation of stack length of UO2 fuel elements in the starting period of BWR. At higher doses, an abrupt volume shrinkage was noted. The concentrations of interstitials and vacancies at various doses were evaluated from the changes of the volume and the lattice parameter. Kinetic equations for the densification of fuels based on the concentration of vacancies are proposed. (Auth.)