Atomic Diffusion in the Uranium-50wt% Zirconium Nuclear Fuel System

Atomic diffusion phenomena were examined in a metal-alloy nuclear fuel system composed of ?-phase U-50wt%Zr fuel in contact with either Zr-10wt%Gd or Zr-10wt%Er. Each alloy was fabricated from elemental feed material via melt-casting, and diffusion samples of nominal 1.5 mm thickness were prepared from the resulting alloy slugs. The samples were assembled into diffusion couples and annealed for periods of 14, 28, and 56 days at temperatures of 550?C, 600?C, and 650?C. Thus, the U-50Zr/Zr-10Er system and the U-50Zr/Zr-10Gd system were each annealed for three different time periods at each of three different temperatures, for an initial total of 18 diffusion interfaces that were to be studied. In practice, data was collected from only 12 of the 18 interfaces. At 650?C, the U-50wt%Zr alloy exists in the ?-phase region, which enabled the comparison of diffusion behavior between the ? phase and ? phase.

Diffusion samples were examined by collecting composition profiles across the diffusion interface for each element via electron probe microanalysis. From the resulting experimental data diffusion coefficients were evaluated. Diffusion coefficients were found to be on the order of 10^-19 m2/s in the ?-phase systems, and 10^-17 m^2/s in the ?-phase systems.

It was observed that atomic mobility of all diffusing species was generally greater in the U-50Zr/Zr-10Gd system than in the U-50Zr/Zr-10Er system; furthermore, it was found that diffusion rates were considerably higher above the phase transformation temperature into the ? phase, as indeed would be expected in the more open structure of the body-centered cubic ? phase, as compared to the hexagonal ?-phase U-Zr. However, values for diffusion coefficients measured in this study were considerably smaller than those found in past studies of ?-phase U-Zr, which are on the order of 10^-17 m^2/s. It is likely that diffusion was inhibited by the formation of stable metal oxides resulting from oxygen contamination; it is also possible that diffusion was suppressed by the presence of the erbium and gadolinium.