Progress Report of Alloy 709 Performance in Liquid Sodium

Abstract

With improved high-temperature properties, Alloy 709 shows great potential for advanced reactor applications. To understand the effects of sodium on A709, small tensile specimens were machined from two commercial heats and subjected to sodium exposure in two forced convection loops operated by Argonne. In parallel, thermal aging experiments were also conducted to isolate any sodium-specific effects. An increasing trend in yield strength and a decrease trend in ultimate tensile strength were observed with increasing exposure time at 600 and 650°C, suggesting microstructural changes at these temperatures. Nonetheless, the thermally aged and sodium exposed samples exhibited similar behaviors, implying no sodium-specific effects at these temperatures. In contrast, 316H SS exposed to sodium at 650°C showed a pronounced sodium effect. The divergent responses of A709 and 316H SS to sodium exposure are noteworthy and necessitate further investigation.A thermodynamic analysis was performed to understand the carburization-decarburization behavior of A709 in sodium environments. The alloy's equilibrium carbon activity was evaluated with M23C6, TiC, and NbC. The carburization-decarburization boundary was calculated with two methods developed previously. Preliminary results suggest that A709 will be carburized in SMT-1 and SMT-2, and decarburized in low-carbon-activity environments such as EBR-II at temperatures exceeding 620-650°C. Nonetheless, A709 should be less likely undergo decarburization compared to 316H SS in identical low-carbon-activity environments.