Abstract
Electron probe microanalysis (EPMA), X-ray fluorescence analysis and γ-scanning results are reported. It was found that similar percentages of cesium and xenon were released from the fuel grains above 1200°C. It is proposed, therefore, that the effective diffusion coefficients for xenon previously determined from EPMA measurements on transient-tested UO 2 fuel are also valid for cesium. At the rim of high burn-up fuel where the microstructure has transformed, cesium appears to be retained completely, although a high percentage of fission gas is released. The difference in the release behavior of xenon and cesium at the rim is explained by xenon being a gas, whereas cesium is liquid. Above 1200°C, both cesium and xenon are gaseous and they follow the same release paths. Release occurs by axial migration in the grain boundary porosity in the central region of the fuel. It is concluded that little, if any, cesium uranate forms in the fuel-cladding gap at burn-ups below 8.4 at.%. The solubility of cesium in UO 2 fuel at 1700 to 1950°C is less than 0.06 wt%.
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