Interdiffusion and Reaction between Uranium and Iron

Abstract

Metallic uranium alloy fuels cladded in stainless steel are being examined for fast reactors that operate at high temperature. In this work, solid-to-solid diffusion couples were assembled between pure U and Fe, and annealed at 853K, 888K and 923K where U exists as orthorhombic {alpha}, and at 953K and 973K where U exists as tetragonal {beta}. The microstructures and concentration profiles developed during annealing were examined by scanning electron microscopy and electron probe microanalysis, respectively. U{sub 6}Fe and UFe{sub 2} intermetallics developed in all diffusion couples, and U{sub 6}Fe was observed to grow faster than UFe{sub 2}. The interdiffusion fluxes of U and Fe were calculated to determine the integrated interdiffusion coefficients in U{sub 6}Fe and UFe{sub 2}. The extrinsic (K{sub I}) and intrinsic growth constants (K{sub II}) of U{sub 6}Fe and UFe{sub 2} were also calculated according to Wagner's formalism. The difference between K{sub I} and K{sub II} of UFe{sub 2} indicate that its growth was impeded by the fast-growing U{sub 6}Fe phase. However, the thin UFe{sub 2} played only a small role on the growth of U{sub 6}Fe as its K{sub I} and K{sub II} values were determined to be similar. The allotropic transformation of uranium (orthorhombic {alpha} to tetragonal {beta} phase) was observed to influence the growth of U{sub 6}Fe directly, because the growth rate of U{sub 6}Fe changed based on variation of activation energy. The change in chemical potential and crystal structure of U due to the allotropic transformation affected the interdiffusion between U and U{sub 6}Fe. Faster growth of U{sub 6}Fe is also examined with respect to various factors including crystal structure, phase diagram, and diffusion.