Isothermal diffusion in uranium-plutonium-zirconium alloys
Abstract
Isothermal diffusion couple experiments were performed at 750$\sp\circ$C to investigate diffusion phenomena in body-centered cubic U-Pu-Zr alloys. Because of the difficulties of handling these hazardous and highly oxidizing alloys, special techniques and equipment were developed to assemble, anneal, and analyze isothermal diffusion couples within plutonium glovebox facilities at Argonne National Laboratory. The U-Pu-Zr alloys covered the uranium-rich corner of the ternary phase diagram with plutonium concentrations up to 27 at.% and zirconium concentrations up to 20 at.%. The compositions of the diffusion couple alloys were selected to reveal thermodynamic and kinetic interactions among the components during diffusion. Ternary interdiffusion coefficients were calculated at the common composition between two couples with intersecting diffusion paths. The cross interdiffusion coefficient for zirconium ($\tilde D\sbsp{ZrPu}{U}$) is negative and has a magnitude twice that of the main coefficient ($\tilde D\sbsp{ZrZr}{U}$). In contrast, $\tilde D\sbsp{PuZr}{U}$ is negligible compared with $\tilde D\sbsp{PuPu}{U}$. $\tilde D\sbsp{PuPu}{U}$ is an order of magnitude greater than $\tilde D\sbsp{ZrZr}{U}$. Average effective interdiffusion coefficients for all components in general increase with increasing plutonium concentration and decrease with increasing zirconium concentration. Since the interdiffusion coefficients depend on composition, constant-diffusivity models are limited in their ability to reproduce the concentration profiles. The atomic mobilities $\beta\sb{P\rm u},\ \beta\sb{\rm U},$ and $\beta\sb{\rm Zr}$ were determined from relationships between the chemical potential gradients and the intrinsic fluxes. New equations relating the interdiffusion fluxes to the atomic mobilities and a vacancy wind parameter were also derived. The ratio of $\beta\sb{\rm Pu}{:}\beta\sb{\rm U}{:}\beta\sb{\rm Zr}$ is approximately 4:2:1. The vacancy wind contribution to intrinsic diffusion is appreciable in diffusion couples with large Kirkendall shifts.
Degree
Ph.D.
Advisors
Dayananda, Purdue University.
Subject Area
Materials science|Nuclear physics
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