US continues push to restart plutonium pit production

After 37 years producing the radioactive hearts of nuclear weapons, the Rocky Flats facility in Colorado was raided by federal agents in 1989 because of alleged environmental violations. Shortly after, Rocky Flats shut down, and efforts turned toward cleanup—a process that concluded in 2005, cost $7 billion, and transformed the area into a wildlife refuge.

Since the plant stopped production, the US hasn’t produced those plutonium pits at scale. But that will soon change: The Department of Energy’s National Nuclear Security Administration (NNSA) is required by Congress to create the capacity to produce 80 new pits per year by 2030, so engineers can replace the decades-old ones and add pits to new weapons (see Physics Today, February 2020, page 23 ). “It looks like NNSA’s proposed emphasis for the first two decades is not on remanufacturing old pits but on manufacturing new pits for new warheads,” says Frank von Hippel, senior research physicist and emeritus professor of public and international affairs at Princeton University. The agency received $1.37 billion for pit production in fiscal year 2021.

The Departments of Energy and Defense announced in 2018 they planned to split the job between Los Alamos National Laboratory (LANL) in New Mexico, which will make 30 pits per year, and the Savannah River Site in South Carolina, tasked with the remaining 50. Last fall DOE completed its environmental impact analysis of Savannah River, giving it the environmental go-ahead in November. In February, the NNSA began reviewing the facility’s design, safety analysis, cost, and timeline in advance of requesting approval from the deputy secretary. That approval will “launch final design of the repurposed facility and other supporting infrastructure projects,” says Dave Olson, executive vice president of NNSA Capital Projects at Savannah River Nuclear Solutions, which operates the site.

In August the NNSA determined that it did not need to conduct a new sitewide environmental impact analysis for large-scale pit production at LANL’s Plutonium Facility-4 (PF-4) and could instead rely on supplemented versions of analyses done in 2008. That decision was controversial, drawing ire from watchdog organizations, which have also asked the Biden administration to do a more thorough environmental review of the pit-production enterprise.

Controversial, too, is the decision to embark on such a high level of pit production at the present moment. Increased investment in a nuclear workforce and weapons may be seen as provocative to the international community, says Sharon Weiner, an associate professor at American University’s School of International Service. Weiner and other critics also continue to question whether pit production needs to start on this schedule and at this scale, and whether Los Alamos and Savannah River can be properly equipped to produce them safely and effectively.

Refurbishing a pit

“The pit of a nuclear weapon is the thing that contains the material that actually allows it to explode like a bomb,” Weiner says. When a weapon detonates, the plutonium compresses. Squeezed together, the atoms begin fission and sustain a chain reaction, which then triggers fusion and further fission in the secondary.

According to the Bulletin of the Atomic Scientists, the US has about 3800 nuclear warheads, whose cores are between about 30 and 40 years old. Given “the uncertainties regarding plutonium aging and the evolving geopolitical landscape, the United States cannot postpone reestablishing this critical capability,” the NNSA says on its website . The desire to scaffold a new generation of pit producers is also likely part of the production rationale, says Don Hancock, administrator at Southwest Research and Information Center, a regional watchdog group. A whole generation has gone by since the Rocky Flats raid, the thinking goes, and maintaining the infrastructure and skills needed for pit production are also necessary for nuclear security.

Unlike in the past, the US won’t have to acquire plutonium to construct the new pits. It has enough on hand—around 38 tons —largely in the form of pits from decommissioned weapons, each of which contains a few kilograms of the radioactive material. The plutonium is stored at the Pantex weapons assembly/disassembly plant in Texas and at LANL.

The first step in the pit production process will be removing impurities that accrued during the plutonium’s long hibernation. That likely involves filtering out americium, a decay product of plutonium-241, says von Hippel. The purified plutonium will be melt-cast into hemispherical metal shells and machined to precise dimensions. Those shells will be mated, get a quality-assurance once-over, and then go to the NNSA for approval and placement in the arsenal.

Creating the infrastructure to perform that work is no small feat. A 2019 report from the Institute for Defense Analyses, a nonprofit that manages several federally funded research and development centers, called current plans essentially impossible: “No available option can be expected to provide 80 [pits per year] by 2030,” the report concluded. It also noted that “eventual success of the strategy to reconstitute plutonium pit production is far from certain. DOE historical data make clear that difficulties are to be expected in a project of this scale and complexity.”

In addition, not everyone agrees that revamped pit production is necessary right now, at least not on the basis of aging. A 2007 report from JASON, a group of advisory scientists who analyze defense problems related to science and technology, suggested pits are unlikely to degrade problematically through at least the 2050s. And a 2012 article from Lawrence Livermore National Laboratory has a title that gives away its conclusion: “Plutonium at 150 Years: Going Strong and Aging Gracefully.” The piece was based on studies of artificially and naturally aging plutonium to see how it evolved over time, work first done jointly at Lawrence Livermore and Los Alamos. After that initial research, Lawrence Livermore scientists continued to probe plutonium’s properties. At the time of publication, the naturally aged samples had reached approximately the 50-year mark, while the sped-up samples were the equivalent of 150 years old. “Both sample lots continue to age gracefully,” said the article, “and extremely sensitive tests and high-resolution electron microscope images by Livermore chemists validate the confidence-building conclusions of the earlier study.”

Those making the pits don’t necessarily feel that same sort of confidence. “All of our stewardship of the stockpile, especially in the post-testing era, is through science,” says Bob Webster, LANL deputy director of weapons programs. “And scientifically there are still questions on plutonium aging.” Those relate in part to how self-irradiation and surface reactions affect a weapon’s potency.

But those open questions could have been at least a little more closed by now. The JASON scientists recommended in their 2007 report that DOE continue its extended aging studies. However, von Hippel notes, that didn’t exactly happen. Indeed, in a 2019 JASON study, the scientists noted that “in general, studies on Pu aging and its impacts on the performance of nuclear-weapon primaries have not been sufficiently prioritized over the past decade.”

To help fill that gap, lawmakers recently directed the NNSA to come up with a 10-year research program, with a plan to be submitted by June, noting “concern with the apparent lack of focus on advancing knowledge regarding pit and plutonium aging.”

Evaluating the production facilities

Questions of necessity aside, Weiner and other critics say the new manufacturing sites are physically and financially dubious. At Savannah River, production will take place in what was to become the Mixed Oxide Fuel Fabrication Facility, a spot meant to turn weapons-grade plutonium into reactor fuel. The project was canceled back in 2018 because it was running approximately $13 billion over budget and 32 years behind schedule, according to a Savannah River spokesperson.

Can that semi-finished, 400 000-square-foot, earthquake-resistant infrastructure designed to safeguard against criticality incidents be repurposed into a pit-making factory on time and on budget? “Prove to me that you’re not going to make the same mistake,” Weiner says, referring to both the site itself and NNSA.

“NNSA expects to deliver this project and others according to our commitments,” the NNSA Office of Acquisition and Project Management said in a statement to Physics Today.

The South Carolina lab sees the canceled facility’s pivot differently from Weiner and others with doubts. “The structure of the building is already there, available to be adapted for this use,” according to Olson. To make things safer, the site will create a mock-up facility where new employees can train without radiation risk, and it will install a new “perimeter intrusion detection and assessment system” to make sure only authorized people have access to material and information.

LANL’s PF-4 is currently the only place in the US that can produce pits for the weapons stockpile, having made around 30 since 1996. But in 2013, officials paused portions of its work at PF-4. Around that time, the Defense Nuclear Facilities Safety Board, an independent government advisory organization on health and safety at DOE weapons facilities, identified safety concerns within PF-4. In July of that year, soon after the pause, the board published a report “to aid in the ongoing assessment and development of corrective actions” at Los Alamos. It identified several areas of “noncompliance,” including that “most criticality safety controls are not incorporated into operating procedures,” that “operators typically do not utilize written procedures when performing work,” and that “fissile material labels do not list parameters relevant to criticality safety (e.g., mass).” In addition, the report noted a shortage of criticality safety staff. “Nothing is more important than the safety of the Laboratory’s workforce,” the lab said in a statement.

Recently the lab has been working not just on standards but also on infrastructure—upgrading the glove box stands where workers manipulate plutonium, improving seismic protections (Los Alamos is more tectonically volatile than scientists thought when they first built Atomic City), installing updated power and ventilation systems, and fashioning new anti-fire instrumentation. It has also cut down on the amount of plutonium stored at PF-4 for non-pit projects; these include generating test samples for subcritical experiments at the Nevada National Security Site and producing nuclear fuel for NASA missions.

LANL says that bringing pit production into the modern era is more than “simply restarting former processes that have been inactive.” For instance, the lab will use computed tomography to plumb pits’ depths. Plasma-focused ion-beam scanning electron microscopy, which uses a spotlight of ions to mill samples, will allow scientists to “peel away a sample layer by layer to get to the fundamentals of material behavior,” says the lab.

But Weiner and others still worry about risks. “The thing to remember is, inherently, dealing with plutonium and making pits is dangerous,” she says. “Accidents are always a possibility.”

But barring future changes from the Biden administration—and a deadline delay of up to five years allowed by the 2021 National Defense Authorization Act—pit production plans will continue apace. “This mission is not an option,” says Dave Eyler, LANL associate laboratory director of weapons production. “We’re now in a position where we need to revitalize this capability.”