A TEM AND TDS STUDY OF GAS-RELEASE FROM BUBBLES IN KRYPTON-IMPLANTED URANIUM-DIOXIDE

There is still some uncertainty regarding the mechanisms whereby fission gas atoms in UO2 can reach grain boundaries during annealing transients. Of the two most likely mechanisms, it has been suggested that bubble migration may be too slow to fit results while an alternative model using thermal resolution, whereby gas atoms can leave bubbles at high temperatures, challenges the older assumption that the solubility of xenon and krypton in UO2 is effectively zero. This paper describes the results and conclusions of experimental work designed primarily to resolve the competition between the two mechanisms. To avoid the difficulties inherent in experiments with nuclear fuel, the experimental approach used ion implantation techniques to form bubbles in UO2 samples which were then examined with transmission electron microscopy (TEM) and thermal desorption spectroscopy (TDS) during annealing to high temperatures. This combination of techniques allowed bubble substructure changes to be correlated with gas release. In spite of conditions chosen to optimise the detection of thermal resolution, no positive indications of this mechanism were observed. On the other hand, bubble mobility was seen and could be correlated with the gas release stages and the evolution of bubble structures. The observations were consistent with the dependence of bubble annealing on the availability of thermal vacancies. The possible importance of the thermal vacancy gradient is highlighted.