Interdiffusion and Reaction between Uranium and Iron
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.