Oxidation and volatilization from tantalum alley during air exposure

Tantalum alloys are one of the refractory metals with renewed consideration for high temperatures in fusion reactor applications. Tantalum alloys perf orm well in protective environments but are oxidized readily in gases conta ining higher oxygen levels. In addition, the radioactive isotope Ta-182 wou ld be produced in tantalum and could be a significant contributor to dose i f mobilized. Other isotopes of importance are produced from tungsten and ha fnium. Mobilization of activated products during an accident with air ingre ss is therefore a safety issue. In this study, we measured the extent of ox idation and mobilization from tantalum alloy T-222 oxidized in flowing air between 500 and 1200 degrees C. This alloy nominally contains 10 wt% tungst en, 2.5 wt% hafnium and 0.01 wt% carbon. We found that the mobilization of Ta and Hf was closely linked to the occurrence of oxide spalling. These ele ments showed no migration from the test chamber. Some W was mobilized by vo latilization as evidenced by transport from the chamber. Tungsten volatiliz ation could occur primarily during initial stages of oxidation before the f ormation of an oxide scale impedes the process. The mobilization of Ta and W are presented in terms of the mass flux (g/m(2)-h) as a function of test temperature. These measurements along with specific designs, activation cal culations, and accident scenarios provide information useful for dose calcu lations of future fusion devices.