The kinetics of fission gas venting from saturated grain boundaries in irradiated UO2

The kinetics of fission gas release from post-irradiation annealed UO2 has been analysed using the stand-alone version of the ENIGMA fission gas release model. This model calculates the arrival rate of fission gas to grain boundaries under circumstances where there is a competition between migration to grain boundaries and absorption by and emission from a coarsening population of intragranular bubbles. The fraction of the fission gas vented from the boundaries is critically dependent on the temperature ramp rate and for low ramp rates there is insignificant gas release until the grain boundary concentration reaches a critical value. The subsequent evolution of gas from the saturated boundaries is consistent with a grain boundary model based on a quasi-crystalline treatment of the lenticular grain face bubbles. At higher ramp rates the entire inventory of the grain boundaries is vented and there appears to be a critical temperature dependent threshold between normal incubation and total venting. (author)