The propensity for a ~（238）PuO_2 heat source to swell islimited. The moderate strength of the vent cover weldapparently allows excess gas to be released, moderates theextent of swelling, and prevents the iridium encapsulationfrom suffering excessive strain. Where the vent cover hadbeen removed, the pellet had ceased to expand before thevolume of the container was fully occupied.The mechanism of distention is apparently expansion ofthe solid oxide, with subsequent release of external pressureand commencement of slow diffusion of gas out of thepellet. Gas pressures in the distended iridium containers areminimal, even those with ages greater than 20 years.Heat sources exposed to temperatures above about900 °C show less distension of the pellet than thoseremaining at the usual operating 400-500 °C temperature.Pellets in these heat sources show large internal cracks andample free volume between the encapsulation and the pellet.The degree of deformation of the distended iridiumcontainment is not significant enough to cause loss ofstructural integrity of the iridium containment. The plasticstrain of the most deformed iridium containment isestimated to be about 11%, well below the measured strainto failure of 30-35%.
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