REACTION OF URANIUM HEXAFLUORIDE GAS WITH ALUMINA AND ZIRCONIA AT ELEVATED-TEMPERATURES

Alumina and zirconia are considered for potential use in uranium hexafluoride (UF6) based nuclear gaseous-core reactors. The corrosion reactions of these oxide ceramics in high-temperature UF6 environment were investigated. Chemical thermodynamic analysis of the reactions was performed by using a computer data base and code package, FACT, to identify stoichiometric products. An alumina reaction tube in a flowing loop test unit was used to expose alumina and calcia-stabilized zirconia samples to UF6 at temperatures from 873 to 1473 K and pressures of about 20.0 to 20.7 kPa. Chemical reaction rates were measured by a discontinuous gravimetric method. X-ray diffraction and EDS analyses and SEM microstructural examination identified corrosion products and the morphology of the exposed surfaces. Results indicate that reactions were not inhibited or slowed down for either oxide. At 1273 K and below, alumina samples formed stable AlF3 scales and showed acceptable compatibility with UF6. The rate of UF6 reaction with alumina and weight loss dramatically increased at 1473 K. Zirconia samples failed to resist degradation and reacted rapidly with UF6 at 1073 K. The maximum service temperatures for alumina and zirconia in UF6 environment do not seem to exceed 1273 and 973 K, respectively.