Non-Nuclear Materials Compatibility Testing of Niobium - 1% Zirconium and 316 Stainless Steel for Space Fission Reactor Applications
Mireles, Omar R. (Omar Roberto)
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A new generation of compact and highly efficient power production and propulsion technologies are critically needed in enabling NASAs long-term goals. Nuclear fission power technologies as part of project Prometheus are in development to meet this need. Proposed reactor concepts utilize a combination of refractory metals and stainless steels. One such refractory alloy, Niobium 1% Zirconium (Nb-1Zr), will be used because of its strength at high temperatures, neutron absorption properties, and resistance to corrosion by liquid alkali metals. One potential problem in using Nb-1Zr is that it undergoes rapid high temperature oxidation, even in low oxygen concentrations. Long-term oxidation of the niobium matrix can significantly deteriorate the mechanical properties of the alloy. This thesis reports on experimental studies of the high temperature interaction of 316 stainless steel (316 SS) and Nb-1Zr under prototypic space fission reactor operating conditions. Specifically, how the high temperature oxidation rate of Nb-1Zr changes when in contact with 316 SS at low external oxygen concentrations. The objective of the project is to determine if transport of gaseous contaminants, such as oxygen, will occur when Nb-1Zr is in contact with 316 SS, thereby increasing the oxidation rate and degrading material properties. Experiments were preformed in a realistic non-nuclear environment at the appropriate operating conditions. Thermal Gravimetric Analysis techniques were used to quantify results. Coupons of Nb-1Zr and Nb-1Zr in contact with 316 SS foil are subjected to flowing argon with oxygen concentrations between 4-15ppm and heated to a temperature of 500, 750, and 1000oC for 2 to 10 hours. Experiments were conducted at the Early Flight Fission Test Facility at NASA Marshall Space Flight Center. The experimental results indicate that a complex oxidation process, which depends greatly on temperature and oxygen concentration, occurs at the expected operating conditions. Non-linear regression techniques were applied to experimental data in order to derive correlations for the approximate oxidation rate of Nb-1Zr and Nb-1Zr in contact with 316 SS as a function of time, temperature, and oxygen concentration.