Shunzo Omori

Fission gas release from UO2-dispersed graphite during irradiation

Abstract In-pile release of 85mKr, 87Kr, 88Kr, 133Xe, 135Xe and 138Xe from natural graphite was measured over a temperature range of 100 to 950°C and that of 133I and 135I was estimated. The effects of fission rate and fission density on the fission gas release were examined. The β-decay process of iodine within and outside the graphite was observed to control the gross release of xenon. Possible mechanisms of an anomalous sink of 88Kr in the plot of log R B versus log λ were discussed. A new model for knock-out release was proposed on the basis of its temperature dependence found. These results were compared with those of post-irradiation experiments and explained by a mechanism whereby fission gas is trapped in defects created in graphite by fission and β-decay energy, and released through annealing of the defects.

In-pile release of fission gas from graphite

Abstract In-pile release of fission krypton, xenon and iodine from graphite was measured and the release mechanism is discussed in detail compared with the data of post-irradiation experiments. No temperature-independent release of fission gases and iodine was observed in the temperature range from 100 to 640 °C. After reactor shutdown with the specimen temperature constant, only xenon was released from the specimen and not krypton and iodine. To explain this phenomenon, a hypothesis is proposed that fission gas releases are induced by β-decay of precursor halogens trapped in defects in the specimen. By the hypothesis, the release behavior of fission gases and iodine can be explained satisfactorily.

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

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 ...