Oxidation of Molybdenum at 800°C and 1000°C in Flowing He with 0.5 to 250ppm O2

Abstract

NorthStar has developed an accelerator-based method to produce Mo-99 without the need for highly enriched uranium. The Mo disk targets will be cooled by Helium flowing at high velocity, with an expected target temperature ranging from 1000°C in the center to 400°C at the edges. Oxidation is a concern since Mo is highly reactive in O2-containing environments at T>~500°C, and the acceptable O2 impurity level in the flowing He gas needs to be determined. ORNL conducted oxidation testing on powder metallurgy and wrought Mo specimens in flowing He at 0.02 to 1m/s with O2 concentrations varying from 0.5 to 250ppm. Results showed that O2 concentration superior to 15ppm would result in unacceptable linear mass losses at 800°C and 1000°C due to the rapid volatilization of the Mo oxide scale formed at the specimen surface. Increasing the temperature from 800°C to 1000°C led to a significant acceleration of the oxidation rates while increasing the flow rate from 0.035m/s to 1m/s had a moderate impact on the Mo oxidation behavior. In flowing He containing less than 5ppm O2, low mass gains or mass losses were observed, consistent with paralinear kinetics typically observed for simultaneous growth and volatilization of the oxide products. Oxide scales few micrometers thick were observed, with some oxide penetrations in the powder metallurgy material due to the presence of open porosities. Oxygen concentrations less than 5ppm seem acceptable from an oxidation degradation point of view, but the impact of oxidation on the Mo material mechanical properties need to be assessed, as oxidation-induced embrittlement of refractory alloys is a known concern.