The postirradiation anneal technique was used to determine the release rate of xenon-133 from 95 per cent dense UO₂ pellets between 1000 and 2000 C. The information was required for design of the Experimental Gas-Cooled Reactor (EGCR) fuel elements.

Release rate measurements indicated that below the sintering temperature an initial rapid release of xenon was followed by a slower diffusion-controlled release. The release rate parameter, D′ (where D′ = D/a²), ranges between 10⁻¹⁰ and 10⁻¹⁴ per sec at 1400 C. These data are related to the total surface area of the pellet by the equation: log D′_{1400 c} (sec⁻¹) = −14.0 + 2 log S.A. (sq cm per g) …….. (1) Below approximately 1600 C, the rate of release can be explained in terms of the equivalent sphere model for fission-gas release. Above approximately 1600 C, the rate of release is higher than would be predicted on the basis of diffusion.

The release rate data have been compared with the release rates obtained in-pile. Results showed that the release rate was independent of burnup up to 16,400 Mw days per metric ton of UO₂.