There is no basis set for U built into G03. You will have to provide the U basis set via the input file.  The easiest is probably to try one of the Effective Core Potentials + Valence Basis Set loaded via the SDD keyword. Take a look at the recent literature for guidance regarding what is an acceptable computational protocol for UO2. 

thank u sir,

wil try sdd with different functional combo n wil let u know how d opt went.

thank u .