AbstractPlenum gas temperature and internal gas pressure buildup were measured during irradiation in four vibrationally compacted UO2-2 wt% PuO2fuel rods. Two each of the four instrumented fuel rods operated to peak burnups of 9900 and 6000 MWd/ MTM, respectively.Experimental results indicate that sorbed gases and moisture released from the fuel react rapidly with the Zircaloy cladding and do not contribute to the internal pressure. The predominant modes of fission gas release from high and lower power fuel rods is different.The rates of pressure buildup in the high and low burnup fuel rods, which correspond to about 34 and 12% fission gas release, respectively, were consistent with fuel temperature-dependent fission gas release fractions predicted from postirradiation gas collection data obtained from similar fuel rods.Measured plenum gas temperatures during irradiation varied directly with coolant outlet temperature.