It was recalled that there was still some uncertainty with regard to the mechanism by which fission gas atoms reached grain boundaries during annealing transients. Of the 2 most likely mechanisms, it had been suggested that bubble migration might be too slow to fit the results whereas an alternative model, in which gas atoms could leave bubbles at high temperatures, challenged the older assumption that the solubility of Xe and Kr was effectively zero. In order to avoid the inherent difficulties of experiments involving nuclear fuel, ion implantation techniques were used to form bubbles in samples which were then examined by means of transmission electron microscopy and thermal desorption spectroscopy during annealing at high temperatures. This permitted bubble sub-structure changes to be correlated with gas release. Although the conditions were chosen so as to optimize the detection of re-solution, no clear sign of this mechanism was observed. On the other hand, bubble mobility was observed and could be related to gas release stages and to the evolution of bubble structures. The observations were consistent with a dependence of bubble annealing upon the availability of thermal vacancies. It was possible that a thermal vacancy gradient was important.

J.H.Evans, A.Van Veen, K.T.Westerduin: Journal of Nuclear Materials, 1992, 195[3], 250-9