POSTIRRADIATION RELEASE OF Xe FROM PYROLYTIC CARBON

摘要

Postirradiation measurements were made at 700 to 1500 C of the release of Xe 133 recoiled into pyrolytic carbon during neutron irradiation in contact with uranium or UO2 powder. The time-dependent release from two types of pyrolytic carbon coatings, laminar and columnar, and from two crystallographic directions in a sample of pyrolytic graphite having a quite perfect crystal structure was studied. Below 1200 C, the release pattern from all specimens was identical: a high rate of loss of Xe33 for several minutes followed by a slower release which was maintained for over 5 hr of heating. The slower process gave an integral fractional release which was linear with the square root of the heating time. At 1200 C and above the initial rapid release persisted for longer times. The release from the laminar coatings was higher than from the other specimens. On the basis of a diffusion model, the apparent diffusion coefficients for the laminar coatings were 10 2 to 103 times higher than those for the columnar coatings with activation energies of 33 and 17 kcal per mole for the laminar and columnar coatings, respectively. There appeared to be a structural dependence to the release from the more perfect specimen.