INTERPRETATION OF TRACER SURFACE DIFFUSION EXPERIMENTS ON UO2 ROLES OF GAS AND SOLID TRANSPORT PROCESSES

The spreading of a tracer from an enriched needle source which contacts the surface of a depleted pellet sink is analyzed rigorously. It is shown that volume diffusion in both the needle and the pellet need to be considered because only by this process is sufficient radioactivity accumulated for measurement after the anneal. Parasitic gas phase processes are of two types-evaporative loss of solid if a flowing gas is used, or molecular diffusion from enriched portions of the surface to depleted zones if the couple is in a closed vessel with a stagnant gas. A complete numerical solution including surface diffusion, solid diffusion, evaporative loss and contact resistance is applied to the UO{sub 2} tracer study of Marlowe and Kazanoff at 1915° C. Based upon UO{sub 2} evaporation experiments, the analysis shows that the evaporative loss effect is not important in these experiments. The UO{sub 2} surface diffusion coefficient deduced from analysis of these data is 0.2{+-} 0.1 cm{sup 2)/s at 1915{degrees}C., which is 10{sup 4} times larger than that predicted by extrapolation of values obtained by mass transfer techniques

Nuclear materials research progress reports for 1979

Research is presented concerning iodide stress corrosion cracking of zircaloy, self-diffusion of oxygen in hypostoichiometric urania, surface chemistry of epitaxial silicon deposition by thermal cracking of silane, kinetics of laser pulse vaporization of UO/sub 2/, gas laser model for laser induced evaporation, solubility of hydrogen in uranium dioxide, thermal gradient migration of metallic inclusions in UO/sub 2/, molecular beam studies of atomic hydrogen reduction of oxides, and thermal gradient brine-inclusion migration in salt. (FS