Los Alamos Experiment Speeds Up Aging Of Nuclear Weapons With "Spiked" Plutonium

Today that element, plutonium, is the main ingredient of weapons in the U.S. nuclear stockpile. Those weapons now are older - and the plutonium inside them has been aging longer - than any earlier stockpile weapons. So researchers at the National Nuclear Security Administration's Los Alamos National Laboratory are trying to hurry along the plutonium aging process to learn how long the metal will last and how that might affect the stockpile.

The nation stopped making new weapons in 1989 and stopped underground nuclear testing in 1992. Researchers at Los Alamos, which designed five of the seven weapon systems in the U.S. stockpile, play a major role in certifying each year that those weapons are safe, secure and reliable.

Certification depends on understanding how the plutonium cores of the weapons, known as pits, will change with age.

"We have to learn how to predict the properties of plutonium as it ages in the weapons, and to do that we need plutonium that's been around as long as plutonium has been on the planet," said Joe Martz, manager of Los Alamos' Enhanced Surveillance Program.

The crucial experiment involves "spiking" samples of nuclear weapons plutonium, the isotope known as Pu-239, with 7.5 percent of the plutonium-238 isotope, which decays about 300 times faster. Plutonium-238, because of its high decay rate, is normally used to provide electrical power for deep-space probes such as the Galileo mission to Jupiter and the Cassini mission to Saturn.

The hamburger-sized spiked samples, cast at Los Alamos on May 13, 2002, should age roughly 16 times faster than the plutonium-239 in U.S. nuclear weapons.

"Every day that passes, the spiked plutonium will be aging more than two weeks, compared to normal weapons plutonium," said Dave Olivas, the metallurgical engineer who is running the experiment with physicist Franz Freibert; both work in Los Alamos' Nuclear Materials Science Group. "When the samples have aged for the equivalent of 60 years, we'll measure all their properties."

This means Olivas and Freibert won't know for certain whether their efforts have paid off for four years, although they plan periodic checks to compare the spiked plutonium to metal inside stockpile weapons.

The researchers began preparing for the experiment in 1998 by building a compact replica of the pit manufacturing line at Rocky Flats, where U.S. nuclear weapon cores were made from the mid-1950s until 1989. They prepared the plutonium-238 metal and combined it with plutonium-239, alloyed the metal with other materials, heat treated it, and shaped it, with every step closely mimicking the way weapons pits were fabricated at Rocky Flats. To ensure its authenticity, the team started with plutonium that was actually made at Rocky Flats just before it closed. They prepared nine identical samples spiked with the Pu-238 and nine of plain Pu-239, and compared their characteristics: fundamental structural, physical, chemical and mechanical properties, such as electrical resistivity and elastic constants, and metallic properties such as density, chemistry and strength.

"At birth our samples are fraternal twins, and we are doing the DNA analysis to make sure," Martz explained.

Now they must wait and see how the metal ages, like watching a time-lapse movie of a plant growing that's played on fast forward.

Aging impacts in stockpile weapons has been subtle so far, but to understand the aging effects after 60 years, scientists can't simply multiply the effects they've seen in 20-year-old plutonium by three, the current age of the oldest weapons in the stockpile. This is because plutonium is the most unpredictable of all the metallic elements, and some aging effects may appear suddenly after years of stable behavior.

The team expects to see some changes in the density of the spiked plutonium and in the growth of helium within its molecular structure, similar to aging effects they've observed in stockpile plutonium.

By analyzing the samples at birth and as they age, the researchers hope to prove they have made plutonium that is nearly identical in nature to metal manufactured into weapons at Rocky Flats. By comparing the samples to the oldest material in the stockpile, they hope to determine whether the accelerated aging process accurately mimics the way weapon pits change as they age.

"Most things age from the outside in, but plutonium is much more unique because it also ages from the inside out," says former Laboratory Director Sig Hecker, a plutonium metallurgist and technical adviser to the experiment.

As plutonium atoms decay, they break down into uranium atoms and helium nuclei, both of which are highly energetic. The helium nuclei eventually combine with other helium nuclei to form helium gas bubbles inside the plutonium metal. The newborn uranium atoms continuously knock plutonium atoms out of place; in fact, about one of every 10 plutonium atoms in a pit is knocked out of position by uranium atoms each year. Most return to their original locale, but some are permanently displaced.

"It's not enough simply to accelerate the initial displacement of the plutonium atoms and the damage due to this radioactive decay, we also have to raise the temperature of our samples to accelerate the subsequent healing of the damage, just as it happens in a real weapon," Olivas explained.

A lot is riding on the experiment. The measurements of density, symmetry and other changes in the spiked plutonium will become sentinels for the plutonium in the stockpile. They will tell the scientists and engineers who continuously track the health of nuclear weapons whether the pits will survive for 60 years and longer.

That predictive capability will be crucial for policy makers who must decide when to replace stockpile pits.

Los Alamos has been working to recapture the capability to make a small number of pits, and is scheduled to produce its first certified stockpile pit in 2007, a year after the experiment should yield results. But the United States has not yet decided when and where to build a facility to make new pits, nor how many that new plant should make each year.

"Intelligent experiments such as this will inform the policy community so that decisions about future, larger-scale manufacturing can accurately assess the environmental and fiscal costs," Martz explained.

"This will provide the only data on 60-year-old plutonium, and that's why this experiment is so crucial to the success of stockpile stewardship," he added.

###Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration of the Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring the safety and reliability of the U.S. nuclear weapons stockpile, developing technical solutions to reduce the threat of weapons of mass destruction and solving problems related to energy, environment, infrastructure, health and national security concerns.

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