The effects of dopants on the modification of microstructure of UO

and UO

-4wt%CeO

sintered pellets have been studied in hydrogen and

/CO mixture atmospheres by using

, TiO

and

as sintering additives. The dopant were added as oxide powders and homogenized by attrition milling. The mixed powders were pressed, and then sintered in hydrogen at 1

, or in oxidizing atmosphere using Controlled

/CO mixtures at 125

. Both density and microstructure of UO

are modified by the addition of dopants in reducing atmosphere. The sintered density is increased with

addition up to 0.33wt% and subsequently decreased with higher content of the additive. The effect on the densification and the gain growth are apparent with the addition of 0.24wt%

. With 0.lwt% titania and 0.6wt%

, the sintered density is decreased, but the grain size is increased. In oxidizing atmosphere, the grain sizes for UO

doped with the above additives are smaller than that for pure UO

. The grain size of Ta or Nb-doped UO

is decreased with increasing

/CO ratio, but that of pure UO

or T-doped UO

is increased. A large portion of second phases is observed in UO

doped with 0.lwt% TiO

sintered in hydrogen atmosphere, while, in

/CO atmospheres, the second phases or dopant agglomerates are not observed. For UO

-4wt%CeO

mixed oxide, the effect of additives on the gain growth is not so much as that for the pure UO

. This is attributed to the formation of clusters by dopant cations and Ce ions, so that the additives contribute to a lesser exent to the grain growth for the mixed oxide.