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Plutonium aging tests performed at Lawrence Livermore National Laboratory suggest the key nuclear weapons material will last at least 150 years without exhibiting signs of defects that might impair weapon performance, according to a summary of the work published in the December issue of the lab’s Science and Technology Review magazine. The work suggests that the nation will not need a major plutonium manufacturing capability for the foreseeable future, lab chemist Pat Allen said in the article. “In the near term, the nation can save tens of billions of dollars that might be required to build a new production facility,” Allen said.

The article provides the most detailed publicly available look at the latest phase of work that dates to the late 1990s, when Department of Energy nuclear weapons program managers laid out a research effort to try to determine what problems might occur as plutonium ages that could impair nuclear weapon performance. In 2006, former National Nuclear Security Administration chief Linton Brooks told Congress, based on aging studies completed at the time, that pits “have minimum lifetimes of at least 85 years.” That conclusion was based on two analytical paths—plutonium removed from actual aged pits and a number of samples of plutonium put through a process of “accelerated aging.”

Aging Not Leading to ‘Catastrophic’ Effects

The new 150-year lifetime is based on a continuation of those studies along both paths. The accelerated aging is done by spiking weapons-grade plutonium-239 with the plutonium-238 isotope, which undergoes more rapid radioactive decay. That allows scientists to measure the buildup of defects in the material caused by the decay, which creates helium as a byproduct that builds up into microscopic bubbles. One key question has been whether the accumulation of those bubbles would cause changes in key material properties that would impact weapon performance. In addition to the helium inclusions, the decay process also can damage the plutonium’s crystal lattice structure in a way that could potentially affect weapon performance. The accelerated aging plutonium samples tested at Livermore have now reached the equivalent of 150 years in age, along with naturally aged plutonium that is now in excess of 50 years old, and the problems scientists had feared have not materialized in a way that could degrade the plutonium’s performance, according to the report. “Clearly, the findings indicate that lattice damage and helium in-growth are not leading to catastrophic aging effects such as void swelling,” said Livermore chemist Brandon Chung, who has been working a variety of tests of the aged plutonium’s properties for more than 10 years. The results provide confidence in the NNSA’s plans to reuse existing pits in remanufactured nuclear weapons, without the need to make new ones, Livermore officials said.

The results add to the evidence that new plutonium pit manufacturing is not needed, said Greg Mello of the Los Alamos Study Group, whose organization publicized the Livermore Science and Technology Review article. “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,” Mello said.

An Impact on Pit Plans?

The last large-scale plutonium manufacturing in the United States was halted in 1989 when the FBI raided the Rocky Flats plant outside Denver because of violations of environmental laws. Rocky Flats was shut down for good in 1992 as the plant became one of the U.S. nuclear complex’s largest cleanup projects. In the decades since, DOE and NNSA have suggested several paths to construction of a new, large-scale pit production facility, but the agencies finally gave up on that idea with the death of the Modern Pit Facility project in 2007. Instead, the NNSA has established a small-scale pit manufacturing capability at Los Alamos.

Given the lack of any proposals or professed need by the government for large-scale manufacturing, the significance of the new Livermore report is primarily on the required capacity of the smaller-scale pit manufacturing at Los Alamos. The government had said it needs the ability to make 50 to 80 pits per year at Los Alamos. With the February decision to indefinitely defer work on the Chemistry and Metallurgy Research Replacement-Nuclear Facility, federal officials have already said they believe a 30 pit per year capacity would be sufficient by meeting additional needs through pit reuse (NW&M Monitor, Vol. 16 Nos. 8 & 9). But Mello and others suggested the new data mean the Los Alamos capacity requirement could be even lower. —From staff reports

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