Tantalum and its alloys have historically been exam­ined for high-temperature nuclear applications, par­ticularly in the various space reactor programs. For reasons similar to those of Nb and its alloys, various alloying combinations of Ta were examined, particu­larly in the late 1950s to 1960s. Much of this effort emphasized the development of solid solution (W and Re additions) and dispersion-strengthened (Hf addi­tion) alloys. While Ta-alloys pay a penalty in higher density over, for example, Nb, and decreases the low temperature density-compensated strength to com­parable values on Nb-base alloys. The higher melting temperature of Ta (3290 k) results in better strength retention above 1000 K and in density-compensated

creep strength.12,41

Early work on substitutional solid solution — strengthened Ta-10W for aerospace applications42 led to limited examination of this alloy for irradiation environments. The improved strengthening by addi­tion of a maximum of 10 wt% allows the retention of suitable nonirradiated ductility and weldability.43,44

However, the use of Ta-10W in space reactor appli­cations where liquid alkali coolants are considered was unacceptable because of the lack of oxide getter — ing elements such as Hf that form stable dispersion — strengthened structures. The T-111 (Ta-8%W-2% Hf) alloy, with its demonstrated compatibility with liquid alkali metals and improved strength over pure Ta while retaining ductility and weldability, has been a lead candidate alloy in space reactor systems since the 1960s.4 Though a considerable effort has been made on the Ta-10W and T-111 alloys, the irradiation properties database is very small. Irra­diated mechanical property behavior follows typical bcc alloys in which radiation hardening effects including limit ductility appear and are expected at temperatures ^0.3 Tm (987 K).3