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Fission gas released from UO2 during irradiation was measured by gamma ray spectrometry. Iodine, xenon, and krypton were released by diffusion above 600 C UO2 temperature. About 45 per cent of the 133 isobar and 80 per cent of the 135 isobar emerged in the form of iodine (or a precursor) for the temperature range 700 to 1000 C. The remainder was released as xenon and by diffusion.
Fission gas was emitted in bursts when the UO2 was heated or cooled. The increase in temperature of an already hot fuel specimen resulted in a temporary overshoot of diffusion release but no trapped gases were released. The cooling bursts released stores of trapped gases. The amounts of xenon-133 and xenon-135 released during the cooling burst had a constant relation to the steady state diffusion release before the fuel was cooled. This burst may be due to a phase change of a higher oxide region of the UO2.
The ratio in which radioactive isotopes are emitted from the UO2 is derived for diffusion, recoil, and trapped gas release. The measured ratios confirmed that the cooling bursts were by a different release mechanism than the steady state release.
Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.