Effect of burnup on release of xenon from thoria

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Abstract
Release behaviour of fission Xe-133 from sintered ThO2−1% : 235UO2 pellets and powders has been studied as a function of burnup in the range of 2 × 1014 to 2 × 1018 Fissions/cc by post-irradiation annealing at 1000[ddot]C. It has been observed that below 2 × 1015 fissions/cc the apparent diffusivity and initial burst, F', is more or less dose independent. However, with increase of burnup beyond this level both Dn and F' decrease sharply. The effect is more pronounced for the powder than for the pellets. The decrease in Dn and F' with increasing : ose has been explained on the assumption that at higher dose irradiation a part of gas atoms are immobilized at the radiation induced trapping centres. The fraction of trapping gas atoms and volume damaged per fission event have been evaluated both for powders and pellets. The effect of irradiation dose on the kinetics of initial burst has been : orked out on the assumption that F' is proportional to the fraction of free gas atoms near the surface constituting the burst. A relation between F' and irradiation dose has been derived. The values calculated from this expression agrees well with the experimental values.