Designs of space nuclear power systems under consideration specify liquid-metal cooling for the nuclear reactor and heat-rejection loops. NaK--a high-thermal-conductivity eutectic mixture of 78 wt% potassium (K) with the balance sodium (Na)--is one candidate under consideration for use as the heat transfer fluid. As part of the Prometheus Nuclear Power and Propulsion project, the NASA Glenn Research Center initiated an experimental program to examine the compatibility of titanium with NaK. Candidate materials for the ducting were identified, and a literature review indicated that stainless steel and titanium may have very good resistance to NaK. A reasonable understanding of the compatibility for stainless steel has been done in support of land-based nuclear systems, but relatively little compatibility information is available for titanium. Titanium would be the preferred material for space applications because of its lower density. This study developed test techniques and determined the compatibility of NaK with commercial-purity (CP2) titanium.
Typical capsule design and tensile specimen used to evaluate the compatibility of NaK with titanium.
Sealed capsules containing a small tensile specimen, both made of titanium, were filled with about 12 g of NaK, sufficient to completely immerse the titanium tensile specimen. The BASF Company (Evans City, PA), a major supplier of NaK alkali metals in the United States, filled and emptied the capsules and reacted and quenched the NaK. The capsule components were assembled by electron beam welding since this method is performed in a vacuum, which minimizes the possibility of contamination and produces very consistent welds. Evaluations were conducted at 550 and 650 K (277 and 377 °C) for up to 1000 hr. Additional capsules were evacuated and filled with argon, sealed, and then exposed to 650 K for various times to differentiate whether temperature or NaK exposure was responsible should any strength loss occur. Longer exposures, up to 15,000 hr, are in progress.
No significant mechanical property difference was observed in the titanium tensile specimens after the 100- and 1000-hr exposures to NaK at 550 and 650 K.
After the various exposures, the changes in weight of the immersed tensile specimen, the tensile strength, the amount of titanium dissolution into the NaK, and any evidence of chemical attack of the exposed capsule surfaces by optical and scanning electron microscopy were determined. CP2 titanium exposed to NaK at temperatures to either 550 or 650 K for up to 1000 hr showed no significant differences in weight, mechanical properties, or dissolved titanium levels.
Metallography showed surfaces that were largely similar to the as-machined condition, and cross-sectional observations did not show any signs of pitting or crevice corrosion after the 1000-hr exposure at either 550 or 650 K. The 1000-hr results indicated that CP2 is a suitable ducting material candidate for NaK heat-exchanger systems, but longer exposure times, which are in progress, need to be evaluated to confirm this conclusion.
Optical microscopy images of cross sections of capsules exposed to NaK and to argon for 1000 hr at 650 K show no signs of pitting or crevice corrosion.Top: Exposure to NaK. Bottom: Exposure to argon.
ASM International Handbook Committee: Metals Handbook. Ninth ed., Vol. 13, table 20, 1985, p. 687. Foust, O.J., ed.: Sodium-NaK Engineering Handbook. Vol. V, Sodium Purification, Materials, Heaters, Coolers, and Radiators, Gordon and Breach, New York, NY, 1972.
Godard, Hugh P., et al.: The Corrosion of Light Metals. Titanium, R.L. Kane, ed., The Corrosion Monograph Series, John Wiley & Sons, New York, NY, 1967.
Jackson, Carey B., et al.: Liquid-Metals Handbook--Sodium (NaK) Supplement. Third ed., Atomic Energy Commission, Washington, DC, 1955.
Find out more about the research of Glenn’s Materials Division:
http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/
University of Toledo contact:
Dr. Ivan E. Locci, 216-433-5009, Ivan.E.Locci@nasa.gov
Glenn contacts:
Dr. John Gayda, 216-433-3273, John.Gayda-1@nasa.gov
Dereck F. Johnson, 216-433-5038, Dereck.F.Johnson@nasa.gov
Dr. Cheryl L. Bowman, 216-433-8462, Cheryl.L.Bowman@nasa.gov
Authors:
William S. Loewenthal, Dr. Ivan E. Locci, Dr. John Gayda, Dereck F. Johnson, and Dr. Cheryl L. Bowman
Headquarters program office:
Exploration Systems Missions Directorate
Programs/projects:
Project Prometheus
Last updated: December 14, 2007
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