Post-irradiation examination of high burnup Mg doped UO2 in comparison with undoped UO2, Mg–Nb doped UO2 and Ti doped UO2

The pellets of UO2, magnesium doped UO2 (Mg–UO2), magnesium and niobium doped UO2 (Mg–Nb–UO2) and titanium doped UO2 (Ti–UO2) were irradiated to burnups ranging from 19 to 94 GWd/tU at temperatures 550–930°C. The solubility of magnesium in UO2 was low around 2 mol%. The addition of magnesium and titanium caused to form large grain sized pellet on sintering. The swelling of pellets during irradiation was unchanged by magnesium addition below 60 GWd/tU in agreement with the literature rate for UO2. The thermal conductivity of unirradiated Mg–UO2 was higher than that of undoped UO2, which seemed to also hold for irradiated specimens. Pellet fracturing occurred by irradiation mainly by thermal stress. The undoped and metal doped UO2 pellets in the 84–94 GWd/tU range at the irradiation temperatures of 560–640°C showed large bubbles and sub-divided grains of sub-micron size and the rim structure formation all over the surface. The xenon release from the pellets during irradiation increased with increasing burnup. In the fuels of close burnups, the xenon release increased rapidly with increasing temperature above about 600°C. At high burnups, the effect of metal addition seemed to recede unclear perhaps due to the formation of heavily damaged fuel matrix.