Thorium Diffusion in Pyrolytic Carbon and Porous Graphite

Abstract

Rates of migration of heavy metals in carbons and graphites are of practical importance, though accurate determination of diffusion behavior in such systems presents formidable problems both in experiment and in theory. Diffusion coefficients . . h' 2 3 d ph'te 4 for uraOlum 10 grap Ites t an a pyro-gra 1 have been reported; we now report coefficients for diffusion of thorium recently obtained from specimens prepared by ion bombardment.The pyrolytic carbon used was prepared by the thermal decomposition of methane. Porosity determinations with helium indicated a poreless structure. This finding was consistent with measured crystalloo 0 graphic parameters (a = 2.46 A, c = 6.86 A) and density (p = 2.2 g/cm 3 ). X-ray determinations using a rocking-curve techniqueS with flat specimens revealed preferred-orientation indices 6 of 80-90 based on (QQ2) reflections. The average distance between coherently diffracti~ domain boundaries (in the c direction) was 150 A, determined by line broadening of the (QQ2) reflections.The effects of lattice strains were not accounted for in the index and broadening re suIts; additional characterization studies 7 are in progress. Nevertheless, it is clear that the pyrolytic carbon used possessed a high degree of orientation, a high density, and a columnal structure arising from the presence of conical grains perpendicular to the c direction of the material.The porous graphite was prepared by impregnating a fine-grained' graphite with an organic compound that was subsequently carbonized. The density of CONTENT ANALYSIS

Development of Fueled Graphite Containing Pyrolytic-Carbon-Coated Carbide Particles for Nonpurged, Gas-Cooled Reactor Systems

“…16, 17, and 18 are summarized in Fig. 19 in terms of a log D vs (T)"1 plot as well as some previously reported (11) results for a porous graphite. At this stage of the investigation, major attention has been focused on the magnitudes of the coefficients rather than activation energies.…”

Development of Fueled Graphite Containing Pyrolytic-Carbon-Coated Carbide Particles for Non-purged, Gas-Cooled Reactor Systems

“…16, 17, and 18 are summarized in Fig. 19 in terms of a log D vs (T)"1 plot as well as some previously reported (11) results for a porous graphite.…”

“…Details pertinent to materials and experimental procedures used appear elsewhere (11). We briefly note, regarding procedures, that surfaces of carbon specimens were bombarded with actinide ions, heated in vacuum, then assayed for diffused material either during or after precision polishing operations.…”