Electron Microscope Observation of the Fission-Gas Bubble Distribution in UO2

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Abstract
The structure of post-irradiation annealed UO2 has been examined by transmission electron microscopy to try to understand the behavior and release of fission gas. At a dose of 2.2 × 1019 fissions/cm3, 5 × 1015 small gas bubbles/cm3 are observed in the material. These precipitate at 1100°C and appear to grow from 25 to 100-Å diam at 1500°C by diffusion of gas atoms from the matrix. Grain-boundary migration sweeps up these bubbles at between 1800 and 2000°C. Re-solution of fission-gas bubbles up to 300-Å diam has been demonstrated on re-irradiation. At a dose of 1.6 × 1020 fissions/cm3, bubbles appear to grow by coalescence and by dislocation sweeping. Precipitates, believed to be solid fission products, are observed. It is concluded that, in a fuel element in which a high-temperature gradient exists, the gas release below 1800°C will be controlled by the migration of bubbles to grain boundaries and by the degree of linking up between the gas-filled voids produced at grain boundaries. At temperatures above 1800°C, large gas-filled voids produced at grain boundaries would be expected to migrate up the temperature gradient by the vapor-transfer mechanism, continuing the process of sweeping up most of the gas started by the initial grown-in porosity.