OBSERVATIONS ON THE RELEASE OF CESIUM FROM UO2 FUEL

Electron probe microanalysis (EPMA), X-ray fluorescence analysis and g amma-scanning results are reported. It was found that similar percenta ges of cesium and xenon were released from the fuel grains above 1200 degrees C. It is proposed, therefore, that the effective diffusion coe fficients for xenon previously determined from EPMA measurements on tr ansient-tested UO2 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 i s released. The difference in the release behavior of xenon and cesium at the rim is explained by xenon being a gas, whereas cesium is liqui d. Above 1200 degrees C, both cesium and xenon are gaseous and they fo llow the same release paths. Release occurs by axial migration in the grain boundary porosity in the central region of the fuel. It is concl uded that little, if any, cesium uranate forms in the fuel-cladding ga p at burn-ups below 8.4 at.%. The solubility of cesium in UO, fuel at 1700 to 1950 degrees C is less than 0.06 wt%.