Original Date: 11/03/1996
Revision Date: 01/18/2007
Best Practice : Integrated Refractory Metals Manufacturing
The U.S. space programs have been taking advantage of unique refractory metal design and processing capabilities at the Oak Ridge National Laboratory (ORNL) Metals and Ceramics division. The Materials Processing Group provides key materials support for space system power supplies. This group has successfully invented, manufactured, and delivered hardware for six isotope-powered spacecraft which include Voyager, Galileo, and Ulysses, and worked on hardware for NASA's Cassini mission.
An isotope-powered supply extracts heat from nuclear fuel and converts it to electricity. The design problem given ORNL was to develop a containment system for the plutonium-oxide fuel in a spacecraft power supply that would provide reliable containment under off normal credible accident conditions, such as failure on the launch pad, early launch abort, or spacecraft reentry. The containment system had to be constructed of an alloy that would be compatible with the radioactive fuel and other graphitic parts in the power supply throughout the many years that could be its long-term operational life. The alloy would have to be designed to be ductile at reentry and impact temperatures so as to provide containment on impact in any credible accident scenario.
To meet these unique requirements, ORNL invented and qualified an iridium alloy that could be formed into a thin clad to enclose the fuel. The alloy, composed of iridium, tungsten, thorium, and aluminum, is formed into small cups, half the size of the fuel pellets. Two cups are then welded together over the fuel. Rigorous planning and monitoring were implemented to maintain the quality of the alloy used in the space programs. Inspection and certification represent 40% of the labor cost to produce the iridium blanks before they are formed into the cups. The cup assembly is also subjected to high velocity, high temperature impact tests to demonstrate its containment properties.
ORNL has continued to develop the iridium alloy to reduce the manufacturing costs, improve its performance characteristics and margins, and make the alloy more environmentally acceptable. It is hoped that the radioactive thorium in the alloy can be replaced with cerium in a new process under development.
ORNL's unique capabilities and facilities allow this work to be conducted. Manufacture of this alloy is infrequent and requires high level security, as well as unique and expensive equipment. Oak Ridge estimates that 30% of its work is for private companies. This is the only facility in the country which can extrude up to five-inch diameter ingots at temperatures up to 2,000°C. The facility can consolidate starting materials received in almost any commercial form, purify the materials using state-of-the-art electron-beam melters, and melt the materials using a computer-controlled, consumable arc-melting facility. Facilities also include the pilot-scale facilities necessary to machine, extrude, forge roll, or draw advanced refractory alloys.
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