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Annette Cary, Hanford News
This article appeared in the January 24, 2009 Hanford News, a publication of the Tri-City (WA) Herald. P&S is grateful to the Herald’s Managing Editor, Rick Larson, for reproduction permission. The original article may be found at http://www. hanfordnews.com/news/2008/story/12725.html – Ed.
Nuclear archaeology has solved the mystery of a jug of plutonium that was found sealed inside a safe dug up as workers cleaned up an early Hanford burial ground. Science showed the plutonium was historic: Researchers at the Department of Energy’s Pacific Northwest National Laboratory in Richland traced its origins to the first batch of weapons-grade materials ever processed at Hanford. It’s also the second oldest known man-made plutonium 239, said Jon Schwantes, a PNNL senior research scientist who led the investigation. The oldest is held in the Smithsonian. The results of the investigation are not just historically significant. Schwantes believes the research also may have applications in the field of nuclear forensics and efforts to keep nations safe from terrorists.
When researchers received the plutonium, they suspected it came from the beginnings of the Atomic Age, after the 586-square-mile Hanford nuclear reservation was created during World War II as the United States raced to make enough plutonium to make an atomic bomb. Hanford’s B Reactor was built as the nation’s first production-scale reactor. It irradiated nuclear fuel that was sent to Hanford’s T Plant, the world’s first industrial-scale reprocessing facility, which chemically extracted the plutonium. During WWII and particularly in the early years of the Cold War, debris from research and operations were disposed of in burial grounds that government officials believed would remain permanently off limits. But to meet modern environmental standards, old burial grounds the size of football fields are being dug up to clean up Hanford.
Among rusty, radioactively contaminated debris in three burial grounds excavated from late 2004 to early 2007 near Hanford’s 300 Area, workers unearthed five forklifts and a flatbed trailer. But the biggest surprise for former contractor Bechtel Hanford may have been the safe in the 618-2 Burial Ground. Bechtel expected to find radioactive material, but not plutonium in the burial grounds, which were used for trash from research work and uranium fuel fabrication. Workers guessed the safe might contain classified documents. But after part of the back separated from the safe as it was lifted out, workers could see six containers. One was a gallon glass jug that appeared to be labeled “Walt’s Group” and still contained liquid. Rather than dispose of the plutonium, Washington Closure Hanford, the next contractor assigned to 300 Area cleanup, turned over the liquid in the jug and a small amount of the solids caked to its walls for research paid for by the Department of Homeland Security.
Schwantes and other PNNL investigators used state-ofthe- art instrumental analyses, reactor model simulations and investigative science techniques that are described in a paper published in the Analytical Chemistry journal this month. “We had some pretty strong clues to start with,” Schwantes said. Writing on the jar included the notation “LaF3” for lanthanum fluoride, a chemical used at T Plant from its startup in 1944 to the mid-’50s. The time frame in which the plutonium originated was further narrowed by studying the ratio of plutonium to uranium in the sample, since plutonium decays over time into uranium. That put the probable date of creation at 1945, give or take 4.5 years. The latest the plutonium could have been made was about 1950. But an analysis of the minor plutonium isotopes was puzzling. Irradiating fuel for weapons production produces not only plutonium 239, the type used in weapons, but also plutonium 238, 240, 241 and 242. Determining the ratio between the amounts of different isotopes created a “fingerprint” of the reactor that produced it. That fingerprint matched a reactor that operated at a power of 3.7 megawatt days per metric ton of uranium. “That was really surprising,” Schwantes said. B Reactor and the two other Hanford reactors operating in the mid-1940s were 200 MWd/MTU reactors, Schwantes said.
There was one other possibility, a research reactor that produced plutonium 239 in the 1940s. The X-10 reactor in Oak Ridge, Tenn., was a prototype for production-scale reactors later built at Hanford. Its power at 3.6 MWd/MTU was close enough to be a match. Historical records also told a story that corroborated what Schwantes concluded. B Reactor and T Plant were built at the same time, but T Plant was finished before B Reactor had irradiated fuel ready to be reprocessed. To test T Plant, plutonium from the X-10 reactor was shipped to Hanford for the plant’s first full-scale test on Dec. 9, 1944, Schwantes said. In the next eight months, Hanford would produce plutonium for the world’s first nuclear explosion in the New Mexico desert on July 16, 1945, and plutonium for the bomb dropped on Nagasaki, Japan, on Aug. 9, 1945, helping end World War II.
Documentation also was found that included information about disposal of a safe in 1951 with contents that matched those found in the safe. The jug belonged not to “Walt’s Group” as workers first read the label, but “Watt’s Group,” a research group led by a scientist named Watt who was in charge of measurements to optimize T Plant operations, Schwantes said. That also explained why the safe with the plutonium from T Plant in central Hanford was unearthed in the 300 Area. The only known sample of plutonium 239 that is older was produced by Glenn Seaborg and his associates in 1940 when they were trying to produce enough plutonium to weigh.
The PNNL study demonstrated the capabilities of nuclear forensics and highlighted the tools that can be used, Schwantes said. It not only demonstrated the ability to trace a sample back to one of several reactors with similar designs, but a red herring in the research may provide information that will help advance nuclear forensics. The plutonium sample included radioactive sodium 22, which has a half life of 2.5 years. With half of its radioactivity decaying every 2.5 years, none should have been detected more than half a century after the sample was produced. Schwantes believes that as the sample was repackaged, the equilibrium between the plutonium and sodium was disturbed. As a result, a reaction between chemicals produced sodium 22 and indicated to researchers when the sample was split. The resulting information could help nuclear forensics experts determine whether a smuggled sample also has been split and more exists.
Nuclear forensics will become more important to protect against the threat of terrorists as nuclear material becomes more available worldwide, Schwantes said. If nuclear forensics can be used to trace nuclear materials to their origin, it provides an incentive for nations to ensure their nuclear materials are well protected and kept off the black market, he said. The national lab in Richland plans more tests on the plutonium sample, and it could be used at other national labs to verify Schwantes’ results. As a well-characterized material, it also can be used in forensics exercises and analytical chemistry
This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.