Plutonium-238 dioxide/T-111 compatibility studies

The tantalum-base alloy, T-111, is an ideal radioisotope encapsulant from the aspect of mechanical properties, but unfortunately undergoes severe oxygen embrittlement during long-term, high-temperature exposure to PuO$sub 2$. A study was undertaken in an effort to improve T-111/PuO$sub 2$ compatibility by testing the hypothesis that reduction of fuel stoichiometry to the range PuO$sub 1$.$sub 75$ to PuO$sub 1$.$sub 8$ would suspend the embrittlement process by producing a state of thermodynamic equilibrium within the capsule. Test temperatures ranged from 773 to 1373$sup 0$K, with aging times of 60 days, 240 days, and 2 y. The desired reaction did not proceed rapidly enough at the aging temperatures to stop T-111 embrittlement. Capsules heated above 1573$sup 0$K for 1 hr showed no signs of embrittlement during aging, even after 2 y at temperatures as high as 1173$sup 0$K. Results with test specimens employing pelletized fuel indicated the solid-state diffusion of oxygen from the fuel to the T-111 was the dominating transport process. In nonpretreated capsules oxygen diffusion in T-111 was the rate-controlling process. Pretreatment does result in the desired thermodynamic equilibrium at temperatures up to at least 1173$sup 0$K. (auth)