Characterization of Plasma Sprayed Zirconium Coatings on Uranium-Molybdenum Alloy Using Neutron Diffraction

Plasma sprayed zirconium metal coatings onto uranium-molybdenum alloy nuclear reactor fuel foils act as a diffusion barrier between the fuel and the aluminum fuel cladding. This work is in support of the US DOE/NNSA Domestic Reactor Conversion Program to convert reactors using highly enriched uranium to low enriched uranium. Neutron diffraction was performed to investigate the crystallographic phase composition, crystal orientations and lattice parameters of the plasma sprayed Zr and the U-Mo substrate. These results were compared to Zr barrier coatings on U-Mo applied by hot-roll bonding. This unique neutron probing shows that the plasma sprayed Zr coating is crystalline, phase pure (alpha-Zr), and highly oriented. There is little or no modification of the underlying U-Mo following plasma spraying. In particular, there is no detectable allotropic transformation of the starting gamma-U (body-centered cubic) to alpha-U (orthorhombic). The Zr plasma sprayed coating displays a preferred crystalline orientation due to directional solidification. Also, there is a slight (~0.01 angstrom) increase in the plasma sprayed Zr lattice parameter indicating oxygen in the lattice. The unique neutron diffraction capabilities at LANL are ideal for nuclear fuels applications, where conventional x-ray techniques are inadequate.