Review of Refractory Materials for Alkali Metal Thermal-to-Electric Conversion Cells

Abstract

Refractory alloys are being considered as structural materials in multitube, vapor-anode alkali metal thermalto-electric conversion (AMTEC) cells for future use in radioisotope space electric power systems. In these power systems, the AMTEC cells would operate at a heat source temperature of » 1150 K and a radiator temperature of » 550 K, for a 7-15 year mission lifetime. In addition to high strength, low density, and low brittle-to-ductile transition temperature, suitable materials must be compatible with the sodium working uid and have low thermal expansion and low vapor pressure (<133 nPa). Refractory metals and alloys are reviewed for their potential use as structural materials in vapor-anode AMTEC cells. Results indicate that Nb-1Zr (niobium-1% zirconium) is a suitable choice, particularly for the hot-end structures of the cell (>900 K). C-103 (niobium-10% hafnium-1% titanium-0.5% zirconium) is also suitable, particularly for the cell's colder structure because of its higher strength and lower thermal conductivity.However, the compatibilityof these niobium alloys with sodium at typical operating temperatures and in the presence of minute amounts of oxygen ( >5-10 ppm) for up to 15 years needs further evaluation. Despite the limited availability of rhenium, Mo-Re alloys, with a rhenium content of 14-45%, are also good choices as structural materials in vapor anode AMTEC cells. However, their relatively higher density and thermal conductivity could lower the cell's performance and increase its speci c mass.

A review of refractory metal alloys and mechanically alloyed-oxide dispersion strengthened steels for space nuclear power systems

“…Refractory alloys have high creep strength at high temperatures and excellent compatibility with alkali liquid metals as long as oxygen, nitrogen, carbon, and silicon impurities are kept below appropriate limits, typically in the range of a few to 10Õs ppm [16,17]. For example, Nb-1%Zr suffers embrittlement in as low as 10 ppm oxygen, which detrimentally affects its mechanical properties.…”

“…Molybdenum-rhenium alloys, developed to remedy the lack of ductility of pure molybdenum at low temperatures, and to increase creep strength [19] and the resistance to oxygen embrittlement, are stronger but much heavier than niobium alloys [4,17]. Rhenium increases the re-crystallization temperature of the Mo-Re alloy to 1450-2000 K, depending on the amount of Re (1645 K for Mo-14%Re) [20,21].…”

Uses of Liquid-Metal and Water Heat Pipes in Space Reactor Power Systems

“…Other potential corrosion mechanisms of refractory metals and alloys by alkali metals are the chemical reactions with impurities, particularly nonmetallic, such as oxygen, carbon, nitrogen and silicon (Reid et al, 1991;King and El-Genk, 2001). Corrosion failures could also result Table 2 Operation life tests of alkali-metal heat pipes (Merrigan, 1985;Angelo and Buden, 1985;Houts et al, 1998;Rosenfeld et al, 2004).…”

“…The results of life tests conducted at Los Alamos National Laboratory (Ranken, 1982;Merrigan, 1985;Merrigan and Trujillo, 1992;Reid et al, 1991) demonstrated good compatibility of various alkali metals with refractory metals and alloys ( Table 2), provided that oxygen content is kept very low, less than 1-10 ppm (Reid et al, 1991;King and El-Genk, 2001). Niobium and tantalum are particularly susceptible to oxygen attacks in Na containing < 1-10 ppm O 2 .…”

“…Niobium and tantalum are particularly susceptible to oxygen attacks in Na containing < 1-10 ppm O 2 . Using refractory alloys containing traces of oxygen getters, such as Zr or Hf, helps alleviate this concern (DiStefano, 1989;King and El-Genk, 2001;El-Genk and Tournier, 2005); e.g. the niobium alloys Nb-1%Zr, PWC-11 and C-103 (Table 1).…”