Research shows nuke weapon pits age gracefully

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LANL: Scaled back manufacturing capability warranted

By The Staff

New research uncovered by scientists at the Lawrence Livermore National Laboratory reveals that plutonium can age gracefully.

A story by Arnie Heller in Science and Technology Review gives a synopsis of the history of the research.
In 1997, the National Nuclear Security Administration launched a comprehensive study at Lawrence Livermore and Los Alamos National Laboratories to examine in detail how plutonium pits age and provide a firmer scientific basis for estimating the service life of these critical weapons components.

The study’s results, announced in late 2006, showed that the slow degradation of plutonium in U.S. nuclear weapons would not affect warhead reliability for decades.

Independent research teams at the two laboratories performed extensive mechanical testing and laboratory-based experiments on aged samples of a plutonium-239 allo y—plutonium mixed with a small amount of gallium to stabilize the material in its delta phase at room temperature.

Alloy samples were taken from 15- to 44-year-old plutonium pits and from plutonium that was artificially aged to 65 years. These tests showed no significant changes in important physical properties such as density and strength. In analyzing the test results, the research teams determined that the minimum lifetime for plutonium pits was at least 85 years — 25 to 40 years longer than previously estimated.

Now, six years later, these same naturally aged samples are 50 years old, and the accelerated alloy samples have reached an equivalent age of 150 years. Both sample lots continue to age gracefully and extremely sensitive tests and high-resolution electron microscope images by Livermore chemists validate the confidence-building conclusions of the earlier study.

“The 2006 report was a work in progress,” said chemist Pat Allen, the deputy program leader for enhanced surveillance and leader of the plutonium aging study. “The 2006 report and recent work continue to show no alarming trends and serve to validate our theories about how plutonium ages. However, we need to keep running tests on naturally aged specimens to see further into the future and make sure there are no surprises.”

Allen said that ongoing monitoring of the accelerated alloys is useful to establish the location of any aging “cliff,” where changes would severely affect performance, even if the cliff did not occur in the near term.

The Los Alamos National Laboratory released the following statement on the findings.
“The article in Science and Technology Review summarizing Livermore’s plutonium aging study, along with an earlier study at Los Alamos, reaffirms the DOE decision not to build a large capacity “Modern Pit Facility” and instead pursue a limited manufacturing capability in existing and planned facilities at Los Alamos. A continued limited manufacturing capability is essential as noted in the article,” the statement said.

“It’s important to note that this study of plutonium aging is only one area of many that could determine pit lifetimes. Extending the observations from plutonium aging as representative of pit lifetimes neglects to take into consideration all of the other factors and could be easily misunderstood.”

Greg Mello of the Los Alamos Study Group says the new research has some interesting implications.
• Pit production for the stockpile is not needed, unless somehow a grossly uneconomical scheme is devised in which the present inventory of roughly 5,000 backup pits, beyond the roughly 5,000 pits now in the nuclear stockpile, is deemed insufficient. 
• Such a scheme indeed has been devised. It is the so-called W78/W88 Life Extension Project, including a large build of new “hedge” warheads.  It may not endure, politically or managerially
• Barring such artificial, created “needs,” no large new plutonium pit manufacturing capability is needed to maintain an extremely large, diverse nuclear stockpile for the foreseeable future — for generations.
• It would be counterproductive to plan or design for such a large facility now because the planning and design bases would change so much prior to any necessity for construction that the effort would need to be updated again and again.
• It cannot be emphasized enough that rising threats to our very survival require our full political and fiscal attention. 

Livermore’s scientists have done the nation a great service by publishing these results, belated though it is.”

Jay Coghlan of Nuclear Watch New Mexico also weighed in.

“Great news coming out of the weapons labs – the science now shows that the plutonium in U.S. nuclear weapons will age gracefully for at least the first 150 years,” Coghlan said. “That is a 50-65 year bump over the previous assessment, which came out in 2006.”