"Nuclear Archaeologists" Find World War II Plutonium

Brian Handwerk
for National Geographic News
March 5, 2009
A plutonium sample recently found at a U.S. waste dump is leftover from a batch used by the Manhattan Project, which produced the world's first nuclear bomb test, a team of chemists has announced.

A nuclear waste cleanup team unearthed the 0.01 ounces (400 milligrams) sample in a waste pit at the Hanford Nuclear Reservation in Washington, where it was sealed in a glass jar and enclosed in a safe.

The discovery highlights new techniques in the emerging field of nuclear archaeology that could become key factors in nuclear deterrence.

Although the mysterious material was unearthed in 2004, its origins were unknown until the researchers used state-of-the art methods to identify its age and history.

The clues led the team to the X-10 reactor at Oak Ridge, Tennessee, which was once part of the Manhattan Project. The spent sample was sent to Hansford for reprocessing—or removing bomb-grade plutonium from nuclear waste products.

"This kind of nuclear archaeology is unclassified, so it gives the public a rare glimpse at what can be done to potentially identify the origin of these types of materials," said nuclear chemist and team member Jon Schwantes, of the Pacific Northwest National Laboratory in Richland, Washington.

(Related: "Archaeologists Explore Cold War Nuclear Test Site".)

Finding "Fingerprints"

The work, described in the journal Analytical Chemistry, highlights experts' growing ability to pinpoint the origins of dangerous nuclear materials.

Because plutonium is rare in nature, weapons-grade reactors must process uranium into the right version, or isotope, of plutonium. To find a plutonium sample's age, scientists chart the element's rate of decay back into uranium.

The levels of different plutonium isotopes then provide a unique fingerprint that can reveal the reactor of origin.

"Each reactor that's producing this material has a slightly different makeup to it," Schwantes explained. "It may be burning a different type of fuel or [running at] higher or lower burnup levels," resulting in different isotope ratios in the finished product.

More clues come from levels of reactive agents, such as iron, added during processing and reprocessing.

For the Hansford sample, the team could tell that the plutonium was reprocessed using the bismuth-phosphate method, the only industrial-scale technique used in the U.S. in the 1940s.

Nuclear Forensics

Harold Smith, of the Goldman School of Public Policy at the University of California, Berkeley, noted that "nuclear forensics" efforts actually began in 1949.

That year, the U.S. used airplanes near the Chinese coast to monitor airborne debris and determined that the Soviet Union had detonated its first nuclear weapon.

Today, if plutonium is captured in transit or terrorists detonate a device, tracking techniques like those Schwantes used could help determine the material's origins.

But that requires comparing the stolen material, or the debris from an explosion, to data banks held by makers of nuclear material, Smith said. Such information is not universally available.

"You can have hundreds of fingerprints at a crime scene, but it won't do you much good unless you have a data bank of prints to compare it to. That data bank is going to be a pressing issue in the decade ahead."

Access to information on reactors and nuclear production is a sensitive national security issue. But Smith said that collaboration among nations is improving, and agencies such as the International Atomic Energy Agency have the beginnings of data banks.

Such a system could give pause to those who would acquire nuclear materials illegally.

"If Russia and the U.S. were willing to say they will be able to hold accountable anyone who supplies fissile material to terrorists, it could have a significant deterrent effect," Smith said.

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