According to Merriam Webster, a “pit” can be a cavity in the ground, the stone of a drupaceous fruit, or the name of a river in northern California.
Employees at Los Alamos National Laboratory are most familiar with a definition of pit that’s not in the dictionary. To this workforce, a pit is a hollow sphere of plutonium that, when uniformly compressed by explosives inside a warhead or bomb, causes a nuclear explosion.
Los Alamos produced the first plutonium pits in 1945, during the Manhattan Project. These pits were used in the atomic bombs detonated in the Trinity test and above Nagasaki, Japan. Since the end of World War II, Los Alamos has done limited pit production for research purposes and, from 2007 to 2011, to replace the pits in 31 W88 warheads (these warheads are carried on submarine-launched missiles).
From 1952 to 1989, the majority of plutonium pits for U.S. nuclear weapons were manufactured at the Rocky Flats Plant near Denver, Colorado. During that time—the throes of the Cold War— the nuclear weapons stockpile was constantly evolving; new weapons with new pits were designed, manufactured, and tested one after another. At its height in 1967, the stockpile comprised 31,225 weapons, each with a plutonium pit inside.
While pits were produced by the thousands per year in Colorado, Los Alamos was becoming the nation’s Plutonium Center of Excellence for Research and Development (an official title bestowed by the National Nuclear Security Administration [NNSA] in 2009). In other words, the Laboratory was making every effort to learn and understand plutonium’s complexities and the effects of its aging. Because plutonium, a man-made element, had been around only since 1940, a lot was still unknown about its behavior. Los Alamos became the place where scientists went to find out more.
What scientists discovered over the years is that plutonium is unstable and radioactively decays over time. This means that, in an effort to reach a more stable state, plutonium emits alpha particles, neutrons (through spontaneous fission), beta particles, and gamma rays. Plutonium can also absorb neutrons. Eventually, the loss or gain of these particles causes the plutonium to transform into daughter products, such as uranium, neptunium, and americium.
In plutonium pits, these daughters start to build up as impurities. They don’t perform or behave the way plutonium does, and they can even react with the original plutonium. Over decades, as more of the plutonium in the pit is transformed, the total mass of plutonium decreases.
Plutonium decay can also break down molecular bonds in neighboring materials and cause helium bubbles that change the characteristic properties of the plutonium.
But do these changes matter? Do aging plutonium pits pose a risk to the nuclear stockpile?
The most obvious way to learn if aging plutonium pits are less reliable is to test one—to detonate it and study the resulting effects and data. However, the Comprehensive Nuclear-Test-Ban Treaty, signed (but not ratified) by the United States in 1992, prohibits nuclear testing of any kind.
Instead, scientists rely on surveillance (pulling weapons from the stockpile for nonnuclear testing and monitoring) and applied research consisting of nonnuclear experiments, computer simulations, and data from historical nuclear tests.
For example, starting in the early 2000s, Los Alamos’ Franz Freibert led plutonium-aging experiments in collaboration with Lawrence Livermore National Laboratory in which plutonium was aged 60 years in 4 years. These and other experiments offered scientific proof that the material properties of plutonium pits do change over time in ways that could affect the performance of nuclear weapons, even if a pit’s “best before” date is beyond the reach of our current scientific understanding.
In 2019, the independent scientific advisory group Jason released a study that assessed plutonium pit lifetimes. The study, a follow up to the 2006 Jason report that concluded there wasn’t enough proof to support a plutonium aging issue, stated that plutonium aging might in fact eventually impact the reliability of U.S. nuclear weapons. In the unclassified summary, the authors “urge that pit manufacturing be re-established as expeditiously as possible in parallel with the focused program to understand Pu [plutonium] aging, to mitigate against potential risks posed by Pu aging on the stockpile.”
With this sudden paradigm shift, concern about aging pits has become more palpable in recent years. How much longer will pits last?
“We don’t have an immediate concern with aging,” says Los Alamos Director Thom Mason. “Up to this point, the plutonium pits in America’s nuclear weapons have been very robust. But the pits we have today were largely manufactured in the 1980s, and we don’t have the predictive ability to say with certainty that our current, 40-year-old pits will be good until any particular date. It’s sort of glass half full, glass half empty; we can’t prove that they will fail, but we also can’t prove that they will work.”
The best way to deal with this dilemma is to “take it off the table,” Mason explains. “We do that by making new pits, immediately.”
“It’s sort of glass half full, glass half empty; we can’t prove that [pits] will fail, but we also can’t prove that they will work.”
To do this, the Department of Defense (DOD) and NNSA (which oversees Los Alamos) turned to the only facility in the country where this type of work could be immediately restarted—the Plutonium Facility (PF-4) at Los Alamos. NNSA has tasked the Lab with developing a pit production process and delivering a minimum of 30 pits per year by 2026. PF-4 is the right-size solution for a right-now problem, Mason explains. “Waiting any longer would put us behind the curve in terms of production schedule, which would translate to needing a larger production facility with higher throughput.”
Another facility, the Savannah River Site, in South Carolina, has also been tasked with producing pits. Using the Los Alamos process, Savannah River is planning to deliver 50 pits per year by 2030.
Pits were historically manufactured using new plutonium, but the United States stopped producing fissionable plutonium for nuclear weapons in 1992, when President George H.W. Bush suspended production. So how can new pits be made from old plutonium, especially if aging plutonium is the problem?
On paper, the answer is simple: Los Alamos will salvage usable plutonium from old pits to make new pits.
In reality, this is a long and complex process that begins at the Pantex facility near Amarillo, Texas, where an aging pit is removed from a weapon, packaged, and shipped to Los Alamos. Upon arrival at PF-4, the pit is disassembled.
First, impurities—daughter products—are separated from the plutonium through pyrochemistry, or chemical activity at high temperatures. “This generates purified plutonium metal as a product, while the impurities are separated into a fused salt,” explains David Kimball of the Lab’s Materials Recovery and Recycle group. “These salts and other impure by-products become feed for further purification via aqueous chloride or aqueous nitrate processing. After dissolving the impure salts in acid, the remaining plutonium that was not converted to metal in pyrochemistry operations is recovered and purified into an oxide suitable for storage or conversion back to metal.”
The waste generated from this process is radioactive and requires proper disposal. Both Los Alamos and Savannah River will send this waste to the Waste Isolation Pilot Plant near Carlsbad, New Mexico, for safe, long-term geologic disposal in deep salt beds 2,150 feet underground. “Waste disposal is critical to the ramp-up of pit production,” Mason says, “and it has to be done in real time—keeping up with the production—so that we don’t build a backlog.”
Salvaged plutonium from multiple old pits is necessary to make one new pit. Once enough plutonium has been salvaged, it is sent to be cast into a mold at the PF-4 foundry. The plutonium is inspected at this stage, and samples are sent to the Analytical Chemistry group, where they undergo tests to ensure that the chemical and isotopic properties of the plutonium are within predetermined parameters.
Pieces of cast plutonium are then welded together to form a pit. Pits are carefully examined using a variety of processes and technologies. Computed tomography, for example, is a digital imaging capability similar to a medical CT scan that provides a detailed characterization of a final pit assembly.
Once a pit has passed inspection, it is diamond stamped—literally stamped with a diamond shape—as a visual indicator it has met all design, manufacturing, and quality requirements and that it is ready to be used in the stockpile. Karen Haynes leads the Lab’s Production Agency Quality division, which, upon delegation from NNSA, performs the diamond stamping. “Our division is comprised of quality analysts, engineers, and inspectors,” she says. “We provide the evidence and level of confidence that products meet the exacting quality requirements necessary, such as ensuring there aren’t any defects in a product and that it will function as intended.”
The plutonium’s journey comes full circle as the pit is shipped back to Pantex, where it is placed back into a stockpiled weapon.
Mason describes the whole process as “kind of artisanal,” especially when compared to mass production of pits at Rocky Flats. “Pit production today is more of a craftsman activity,” he says. “It’s very exacting work.”
Bob Webster, deputy Laboratory director for Weapons at Los Alamos, agrees. “We’re talking about making in a year what Rocky Flats could have made in a week,” he says.
Redundancy and relationships
Los Alamos and Savannah River must succeed independently and together to reach the goal of 80 pits per year by 2030. But collaboration is tricky because of the physical distance between the sites—1,500 miles—and because of the need to update facilities at both institutions.
Neither Los Alamos nor Savannah River is currently set up for large-scale pit production.
Los Alamos’ PF-4 was designed for R&D (research and development) and surveillance; the facility at Savannah River was designed for mixed-oxide fuel fabrication (which never happened). Changes—such as new equipment, updated buildings, new employees, and 24-hour operations—are necessary to turn these facilities into functional pit production facilities. Processes and equipment at both sites must be exactly the same so all pits produced are identical. This concept of being able to create a specific product at more than one production facility is called redundancy.
“The level of cooperation and integration across multiple sites that I have witnessed in the first years of this effort has been outstanding,” says Dave Olson, director of the Savannah River Plutonium Production Facility mission. “There is truly a shared vision and commitment to the national plutonium pit production mission.”
In 2018, NNSA completed an engineering assessment and workforce analysis of the sites and found that both locations can meet the needed requirements—and meet them safely. No one wants a repeat of Rocky Flats, which was raided in 1989 after the FBI and other agencies caught wind of environmental crimes. Large-scale pit production came to a sudden halt. The plant was declared a Superfund (hazardous waste) site by the Environmental Protection Agency and was officially shut down in 1992.
Today, scientists know more about plutonium handling and hazards (much of it curated by Los Alamos in the 2019 seven-volume second edition of the Plutonium Handbook). “Since the United States last did pit production in any sustained way, a lot of the technology has changed, our understanding of plutonium science has advanced, and the world has changed,” says Dave Eyler, associate Laboratory director for Weapons Production at Los Alamos. “PF-4 was built and operated until relatively recently as a facility for R&D or surveillance activities that are relatively episodic as opposed to a sustained cadence of production. We’re still putting a lot of things in place and learning how to do production while still doing all the R&D and surveillance, too.”
In addition to renovating parts of PF-4 to meet production needs, the Laboratory has also been making improvements to the facility to mitigate all types of potential unexpected events. For example, even though large earthquakes are not common in Northern New Mexico, the columns in PF-4 have been rigorously tested to ensure they’ll withstand a seismic event.
The facility’s fire suppression system has also been upgraded, and empty nuclear material containers have been fire tested and drop tested to help ensure no hazardous material will be released in the unlikely event of an accident.
“Unfortunately, the nuclear deterrent is as relevant as it has ever been. There are certainly people who wish that we didn’t have nuclear weapons. In fact, there are a lot of people who work at Los Alamos who wish we didn’t have nuclear weapons. But we recognize that we do, and as long as we do, the weapons have to be safe and reliable.
Plutonium pits have become almost iconic in the discussion of whether we should have nuclear weapons. What is really an argument against nuclear weapons has become an argument against pit production because, if we don’t have pits, we don’t have nuclear weapons, which is true.
So, the question is: Do you believe that the world as it currently exists would be safer and more stable if the United States unilaterally disavowed nuclear weapons? If so, you wouldn’t make pits. But if you think that the deterrent is important for maintaining stability in an environment in which other states are prone to using coercion if they can get away with it, then we’re going to need to manufacture pits.”
—Thom Mason, Los Alamos National Laboratory director
In short, “PF-4 is probably one of the safest places in New Mexico,” says Matt Johnson, who leads the Lab’s Pit Technologies division. “We want to protect our workers, and we also realize that sustaining public trust and confidence that we can do this safely is priority number one.”
When it comes to actually making pits, all plutonium at Los Alamos is handled inside a glove box—a sealed compartment that is accessed through two holes to which gloves are attached. Technicians insert their hands into the gloves and are able to handle the plutonium with no exposure to the element. Glove boxes are located inside secure rooms, inside a secure building, on a secure road in the middle of a secured Laboratory campus.
“We don’t do the work if we can’t do it safely,” Mason says. “We have to get this work done in order to support the nuclear deterrence mission. So being able to operate safely in a complex environment is a prerequisite.”
Approximately 2,500 people will eventually support the pit mission at Los Alamos. That number includes a handful of people who used to work at Rocky Flats. “But a lot of that expertise is walking out the door as people retire,” Mason says. “We need to transfer that knowledge to younger employees now.”
Finding new employees who meet stringent hiring qualifications is a challenge, but one that the Laboratory is addressing in specific ways. “We have pipeline programs starting or continuing with Northern New Mexico and Santa Fe Community College,” says David Dooley, chief operating officer for Weapons Production at Los Alamos. “And we have plans to provide funding to New Mexico colleges and universities to assist in workforce development.”
All PF-4 employees are part of the Human Reliability Program, which is specific to those who work with nuclear materials. Anything that can cause an employee to be distracted—a stressful life event, for example—is closely monitored so that employees can be removed from contact with nuclear materials for a period of time if that is deemed necessary for safety and security. Employees also undergo yearly physicals, yearly psychological evaluations, random drug testing, and random polygraph tests. “There are a lot of additional demands on this workforce, all of which are designed to keep PF-4 as safe and secure as possible,” Johnson explains.
Los Alamos is currently developing the processes for producing pits for the W87, which is the warhead that tops Minuteman III intercontinental ballistic missiles. The first W87 pit will be delivered in 2023. From there, production will ramp up quickly to 30 pits per year. Down the road, other types of pits for other U.S. nuclear weapons—the B61 gravity bomb, and the W76, W78, and W88 warheads—will be produced at Los Alamos.
These new pits and their corresponding weapons will comprise the future U.S. nuclear deterrent. Maintaining the safety and reliability of the deterrent has always been Los Alamos’ primary mission. “That has been true since 1943 when we made the first pit,” Mason says. “We’ve shown, and will continue to show, that Los Alamos is the right place to lead this effort to support the national security mission.”
Katharine Coggeshall, Virginia Grant, and Whitney Spivey contributed to this article.