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NEW HIGH-PURITY PLUTONIUM SOURCES PRODUCED AT LOS ALAMOS
Plutonium sources used as standards in analytical chemistry must be of extreme purity, 99.99 percent pure, free of contaminants and surface oxidation. Plutonium is extremely prone to oxidation and can burn when exposed to the air. The plutonium sources fabricated at Los Alamos had to be made in an oxygen-free environment, at less than 10 parts per million, and sealed under vacuum in glass ampoules to assure their utility as primary standards. "In the past, analytical standards were made with a 'nibbling' method," said Coriz. "You would start with a thin plutonium plate and nibble little pieces off it. Although it was never proven, wear on the nibbling tool was thought to be a source of contamination with this method. It was for this reason we looked at an extrusion process, with planning that started in 1997." After many delays and technical problems, a press system built into a specially modified glove box is finally in place. The process uses 15,000 pounds of force at 75,000 pounds per square inch to squeeze machined plutonium plugs, one and a half inches long by one half inch in diameter, through a specially designed extrusion die. The plugs, heated to 785 degrees Fahrenheit and roughly the consistency of taffy, come out of the die as a 5/32nd-inch diameter wire, which was then cut into individual sources, each weighing approximately one gram. Each source is about the size of a small hearing aid battery, one quarter inch long and cut to a tolerance of plus or minus one tenth of a gram. "At room temperature plutonium is very hard but also very fragile," said Coriz. "As the soft plutonium wire came out of the extrusion die it hardened very quickly as it cooled. It produced a shape that was easy to cut with a small re-bar cutter even though the material was fairly brittle and, because of the non-reactive materials used in extrusion die, wasn't contaminated." The Laboratory produced 1,200 of the one-gram plutonium sources. The majority of the sources are currently undergoing analytical evaluation at the Department of Energy's New Brunswick Laboratory in Illinois. A few of the sources are also undergoing evaluation at Los Alamos. Once accepted, the sources will be made available to nuclear industry and approved governments worldwide. Some of the sources will be used at Los Alamos in the Laboratory's nuclear weapons surveillance and certification programs. "This was a major challenge," said Coriz. "The project was postponed several times, once during the Cerro Grande forest fire. And there were many difficult technical challenges. But overall, the project was very enjoyable. Even though a lot of things got in our way, we were able to manage and produce an excellent product."
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