Development of Niobium Diffusion Barriers for Aluminum-Clad Uranium Alloy Fuel Elements

S>Nickel was used as a diffusion barrier for Al-clad uranium alloy fuel elements, operating at 650 nif- F in organiccooled reactors. Advanced organic-moderated reactor (AOMR) concepts include operating temperatures up to 850 nif- F. Since the interdiffusion of Al --Ni is 150 times faster at 850 nif- F than at 650 nif- F, an alternate diffusion barrier is required. Subsequent studies resuited in the selection of niobium. Several techniques for bonding niobium to the U--10 Mo fuel were investigated. After considerable development effort, electron beam vacuum deposition yielded adherent, nonporous, ductile coatings. Isostatic bonding, at 1050 nif- C and 12,000 psi for 1 hr, also resulted in metallurgically bonded niobium on the major surfaces of U-10 Mo fuel plates. Roll-bonding achieved metallurgically bonded plates on small test specimens, but was never reproduced on full-size plates. Vapor phase deposition, by H/sub 2/ reduction of NbCl/sub 5/, was also partially successful, but not reproducible. Ultrasonic bonding has shown promise in spot welding tests. Plasma-spray coating and electroplating in a fused salt bath were also considered, but not actively evaluated. (auth