The effect of irradiation in pressurized hot water loops on bulk UO², UO² dispersed in BeO, and UO² dispersed in Al²O³ are compared. It is shown that when UO² is irradiated to high exposures, the material increases in volume, and increased amounts of fission gases are released. In addition to this effect in BeO and Al²O³, swelling occurs due to fission fragment destruction of the matrix phase. The parameters influencing this behavior are discussed.