In-pile fission-gas release from fine-grain UO 2

Abstract

According to the defect-trap theory for fission-gas release the rate of escape of gas from UO 2 is controlled by the number of defects in the UO 2 structure. Defects are created by fission and may also be inherent flaws, such as grain boundaries and pores. To test this theory the release rate of gas from single crystal UO 2 was compared to that from fine-grain UO 2. It was found that the single crystal did indeed release much more gas (a factor of 17 at 1000° C) than the fine grain material. Post-irradiation examination showed that the grain boundaries had trapped fission products. The theory further states that the number of radiation produced traps at a given temperature will be roughly proportional to the fission rate, under the conditions of this experiment. At a high fission rate, therefore, there will be more fission-gas but also a higher probability of trapping the gas atoms, and a change in fission rate should not produce much change in the fission-gas release rate. In the fine grain material, a change in fission rate of a factor of 4.5 did not produce a detectable change in the rate of gas release. These results confirm the defect-trap model for fission-gas release, and are directly opposed to any gas release mechanism which is diffusion controlled.