EN
In the present paper it is assumed that in the fluence range of 4×10^{19} - 0.5×10^{21} fissions/cm^{3} the main contribution to the fission gas release from an uranium dioxide single crystal is from the bubble traps by knock-out process. An analysis of bubble distribution in the single crystal for different temperatures shows that some experimental peculiarities of the fission gas release during irradiation arise from different conditions of the thin surface layer and the interior of the considered solid. The thin surface layer is assigned by the fission fragment range.