Release Process of Fission Xenon Produced by β-Decay of Precursor Iodine in Natural Graphite at 1, 000°C

Abstract

In order to study the release process of fission xenon produced by decay of the precursor iodine in natural graphite powder, post-irradiation experiments were carried out at 1,000°C soon after 20 min irradiation with UO2 powder. In an experiment with interruption (2 hr heating—10 day cooling—10 hr heating), it was found that the decay of 133I to 133Xe caused a fraction of the 133Xe produced to be released rapidly, while the remaining fraction was released gradually. This was followed by a continuous heating experiment, during which the release rates of 133Xe, 135Xe and 135mXe were measured, and in which production and release of xenon occurred simultaneously in a manner similar to conditions prevailing under actual irradiation experiments. The rates of nuclide release were explained by assuming the same release probability for each xenon nuclide as that for 133Xe in the first experiment. The origin of the initial rapid release by decay of iodine to xenon was considered mainly to be the change of chemical interaction between the defects and the atoms trapped therein. The release behavior of xenon after its production was explained by assuming that the activation energies for the release from the defects were lowered by about 10kcal/mol by the decay.