Fission-gas releases during post-irradiation annealing of UO2 fuel from WAGR pins of burn-up up to 5500 MWd/te U

Post-irradiation annealing techniques have been applied to UO2 fuel taken from discharged WAGR pins as part of a programme to determine the extent and nature of the release of fission gases at certain fuel temperatures. The UO2 of burn-up up to 5500 MWd/te U was heated to 14000 and 16500C in a helium-hydrogen sweep gas for periods of up to 62 hours. Values for the fission-gas diffusion rate parameter estimated from the steady gas release rates occurring during the 14000C periods ranged from 3 x 10-14 to 2 x 10-11 sec-1. The overall fractional releases for the anneals, to which large contributions were generally made by bursts of gas obtained on heating to the higher temperatures, ranged from 0.13 to 29%. The release of fission gas, which in no case was associated with grain-size changes, frequently occurred in pulses, even under steady temperature conditions. This type of release pattern is thought to be the result of gas bubbles in the grain boundaries coalescing to form open porosity networks. The reasons for the wide variation in overall fractional releases, and possible differences between in-pile and out-of-pile release rates from fuel at the same temperature are discussed. It is concluded that a post-irradiation anneal of several hours duration, particularly if the annealing temperature is much higher than the irradiation temperature, is unlikely to yield a result applicable to fuel at the same temperature in-pile. (author)