Release of fission products during and after oxidation of trace-irradiated uranium dioxide at 300-900 deg. C

Should defected UO2 fuel pins come into contact with air then oxidation of the fuel may occur, the rate and consequences of which are dependent upon temperature and oxygen partial pressure. At CEGB-BNL an experimental programme is underway investigating the kinetics, and extent, of release of fission products during and after oxidation of trace-irradiated UO2 to U3O8, and reduction of U3O8 to UO2. This paper presents preliminary results and analysis of experiments performed at 300-900 deg. C. Dense sintered UO2 has been oxidised at 300-500 deg. C using a thermo balance with simultaneous counting of released 85Kr. The kinetics of the 85Kr release are shown to correlate with the kinetics of oxidation, and the extent of release has been determined as 3-8% of that in the UO2 converted to U3O8. The release of 106Ru and 137Cs during this oxidation has been estimated by γ-counting of the fuel sample, before and after oxidation, and of glassware in the vicinity of the sample. This indicates slight release of ruthenium and caesium. Greater fission product release is caused by oxidation at higher temperatures or by heating of the oxidation product. U3O8 produced at 400 deg. C has been heated at 800 and 900 deg. C in air for 20 hours. This results in near total release of 85Kr and 106Ru, but still only slight release of 137Cs. The kinetics of the 85Kr release have been analysed and found to follow the Booth diffusion equation at 900 deg. C, but not at 800 deg. C. The fuel burn-up level may also have an effect. Some results of fission product release during reduction of the oxidation product U3O8 are presented, and the influence of chemical effects upon the release of individual fission products is discussed. The future programme is outlined. (author)