Heat treatment of uranium dioxide - effect on xenon release rate

A long-term study was conducted into the effect of the microstructure of ceramic uranium dioxide on the fission gas release rate in the initial irradiation stages. The measuring instrument and sample preparation for irradiation are briefly described. The results show that even for the same initial powder for UO2 pellet production, the virtual diffusion coefficient of xenon is considerably affected by heat treatment in the post-irradiation annealing range 1498 to 2130 K. Long-term annealing prior to irradiation at 2200 degC results in a decrease in xenon diffusion activation entropy. Intragranular porosity can be observed in the material and the formation of new nonsteady state structure can thus be expected. The migration mechanism remains unaffected. The formation of pores during pre-irradiation treatment is associated with defect migration from grain boundaries to centres, which occurs at a temperature of 2100 degC. (O.K.)