Old plutonium reveals new secrets

by David B. Williams
Thursday, January 5, 2012

Buried treasures come in many forms. Few, however, can help prevent the spread of nuclear weapons to terrorists. But a glass bottle discovered in an old safe buried at the Hanford Site in eastern Washington turned out to contain crucial clues that could help scientists develop new ways to track the spread of nuclear materials.

With more than 750,000 cubic meters of solid waste in 75,000 barrels, Hanford is the largest nuclear waste dump in the Western Hemisphere. In 1989, the federal government began a multi-year cleanup project of the 1,517-square-kilometer site. Fifteen years later, while unearthing material in the southeastern corner of the site, workers found their buried treasure — a safe holding a mysterious bottle containing plutonium.

To determine the origin of the radioactive material, Hanford personnel gave a sample of the plutonium to Jon Schwantes and his colleagues at the nearby Pacific Northwest National Laboratory, one of several labs that conducts nuclear forensics work for the FBI. Schwantes' team made a startling discovery. The bottle contained the second oldest sample of plutonium-239 in the world, at 65 years old. The only older plutonium-239 sample — from the lab of Nobel Prize-winning chemist Glenn Seaborg, co-discoverer of plutonium — is on display at the Smithsonian National Museum of American History in Washington, D.C.

Schwantes subjected the plutonium to a battery of tests, including mass spectrometry, ion chromatography and reactor model simulations, to determine the sample’s history. His fellow researchers also searched written documents, which, along with diagnostic data, revealed that of the thousands of pieces of contaminated waste dug up at Hanford, the safe held the only plutonium produced at a prototype reactor in Oak Ridge, Tenn.

Originally called the Clinton Laboratories, Oak Ridge National Laboratory was one of three Manhattan Project sites for the development of atomic weapons. The prototype reactor, designated as X-10, began producing plutonium-239 from uranium-238 in November 1943. Ten months later, 96 slugs of X-10 fuel, containing an estimated 112 kilograms of irradiated uranium and 400 milligrams of plutonium, were shipped to Hanford. The plutonium was separated from the uranium at Hanford’s T-Plant, which on Dec. 9, 1944, became the world’s first industrial-scale reprocessing plant. Out of this plant also came the plutonium that went into the bombs dropped at Nagasaki, Japan, and the Trinity test site in New Mexico.

For unknown reasons, instead of being used, the X-10 plutonium ended up in a glass bottle in a safe, which was sealed in April 1945 after workers discovered that the safe’s insides had also been contaminated. Buried in 1951, the safe remained undisturbed until a cleanup crew struck and damaged it with an excavator in December 2004. After removing the contents, another team opened the bottle in a fully contained glove box — to prevent dust from the plutonium from entering their lungs, where it can cause cancer — and transferred the slurry into two one-liter polypropylene bottles, which Schwantes received on May 10, 2006.

When Schwantes' team analyzed the plutonium, they discovered that it contained the isotope sodium-22, a byproduct of a secondary nuclear reaction within the bottle. “The sodium has a half life of two and a half years, which given the age of our sample, means it shouldn’t have been there,” Schwantes says. Separating the plutonium into the polypropylene bottles, it turns out, started a chemical reaction — the sodium began to form after the sample was poured into the plastic bottles. Further analysis also indicated the quantity of the original plutonium before the split, the team reported in the journal Analytical Chemistry.

The serendipitous splitting of the original bottle offered new methods and insights into nuclear forensics — the science of identifying the source, point of origin and transport routes of nuclear materials. “If this was an interdicted sample, the FBI would want to know if this was part of a larger batch of material or do we think we interdicted the entire material out there,” Schwantes says. “We could also determine how long ago this material was separated from the larger batch.”

The use of the sodium-22 was “one of the more clever aspects of the paper,” says Ian Hutcheon, a physicist at Lawrence Livermore National Laboratory in Livermore, Calif., and co-author of the primary nuclear forensics reference book called “Nuclear Forensic Analysis.” “People who aren’t paying close attention might easily have overlooked the sodium.” In addition, Hutcheon notes, the work with sodium-22 was the first time this technique had been used and now gives others in the field an additional tool.

Schwantes' work with an unclassified sample shows the power of nuclear forensics methods and acts as a warning to those who might try to deal in illicit materials, Hutcheon says. “The hope is that nuclear forensics acts as a deterrent, and if people come into possession of illicit nuclear material, they are going to know they are going to be caught.”

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