DEVELOPMENT OF NIOBIUM-URANIUM ALLOYS FOR ELEVATED-TEMPERATURE FUEL APPLICATIONS

As a continuation of studies reported in BMI-1400, fabrication characteristics, physical and mechanical properties, and corrosion behavior in NaK, sodium, and water of niobium--uranium binary alloys containing up to 60 wt.% uranium were investigated. Alloys were cast by a skull melting and consumable and nonconsumable arc-melting methods. Fabrication difficulties with alloys containing greater than 25 wt.% uranium were related to coring-type microsegregation during casting. Tensile tests indicated 0.2% offset yield strengths of 16,880, 22,370 and 28,600 psi for niobium2000 deg F. Additional tensile data were obtained for alloys from 1600 to 2400 deg F. Stresses to produce minimum creep rates of 0.001, 0.01, and 0.1%/hr at 1600, 1800, and 2000 deg F were also determined. Both tensile and creep strengths were found to be sensitive to oxygen content. All alloys appeared compatible with NaK at 1600 deg F and with sodium at 1500 deg F. In 600 deg F water, most of the alloys tested exhibited negligible weight changes after 336 days' exposure. Weight changes were greater after 140 days' exposure to 680 deg F water, but corrosion rates were considered satisfactory for a clad fuel. The thermal and electrical conductivities of niobium are lowered by the addition of uranium, while the thermal-expansion characteristics are essentially unaffected. Recrystallization temperatures for 90% cold-reduced niobium-4.38, -14.3, -20, -25.0, and -30 wt.% uranium alloys are 2300, 2300, 2400, 2300, and 2200 deg F, respectively. No appreciable effect of oxygen contents ranging from 100 to 1000 ppm was observed on the composition limits of the gamma immiscibility gap in the niobium-- uranium system. (auth