Year
2016
Abstract
Slight dissension of cladding on unvented plutonium-238 oxide heat sources has been observed, and the origin of the distention investigated. Measurement of engineering strain, radiography of the fuel configuration, and measurement of the pressure and composition of internal gas indicate the condition of the fuel in these heat sources. Gas pressure from helium release by the fuel can be substantial for clads of advanced ages or for clads that have been exposed to temperatures above about 900°C. However, gas is unlikely to accumulate to high pressures, as the weld on the decontamination cover will allow the gas to be released through the vent frit, as designed. Measureable gas in most of these distended clads constitutes less than 1% of the total helium emitted over time. Radiography suggests that the fuel swells until the emitted helium breaches the decontamination cover weld. Then helium is freely released by the fuel, the fuel dimensions become stable, and the clad ceases to expand. In hot environments, gas is evolved promptly from the fuel. The heat sources that have resided in hot environments show cracking of the pellet and very little distension of the fuel pellet volume. In the one example of a clad where the decontamination cover was deliberately removed, swelling of the fuel pellet appears to have been arrested before the pellet occupied the entire volume of the clad.