KANSAS CITY NATIONAL SECURITY CAMPUS, Mo. (AP) — In an ultra-sterile room at a secure factory in Kansas City, U.S. government technicians refurbish the nation’s nuclear warheads. The job is exacting: Each warhead has thousands of springs, gears and copper contacts that must work in conjunction to set off a nuclear explosion.

Eight hundred miles away in New Mexico, workers in a steel-walled vault have an equally delicate task. Wearing radiation monitors, safety goggles and seven layers of gloves, they practice shaping new warhead plutonium cores — by hand.

And at nuclear weapons bases across the country, troops as young as 17 keep 50-year-old warheads working until replacements are ready. A hairline scratch on a warhead's polished black cone could send the bomb off course.

The Associated Press was granted rare access to key parts of the highly classified nuclear supply chain and got to watch technicians and engineers tackle the difficult job of maintaining an aging nuclear arsenal. Those workers are about to get a lot busier. The U.S. will spend more than $750 billion over the next 10 years replacing almost every component of its nuclear defenses, including new stealth bombers, submarines and ground-based intercontinental ballistic missiles in the country’s most ambitious nuclear weapons effort since the Manhattan Project.

It’s been almost eight decades since a nuclear weapon has been fired in war. But military leaders warn that such peace may not last. They say the U.S. has entered an uneasy era of global threats that includes a nuclear weapons buildup by China and Russia’s repeat threats to use a nuclear bomb in Ukraine. They say that America’s aged weapons need to be replaced to ensure they work.

“What we want to do is preserve our way of life without fighting major wars,” said Marvin Adams, director of weapons programs for the Department of Energy. “Nothing in our toolbox really works to deter aggressors unless we have that foundation of the nuclear deterrent.”

By treaty the U.S. maintains 1,550 active nuclear warheads, and the government plans to modernize them all. At the same time, technicians, scientists and military missile crews must ensure the older weapons keep running until the new ones are installed.

Which means those technicians and troops are about to become much busier.

The project is so ambitious that watchdogs warn that the government may not meet its goals. The program has also drawn criticism from non-proliferation advocates and experts who say the current arsenal, though timeworn, is sufficient to meet U.S. needs. Upgrading it will also be expensive, they say.

“They are going to have extreme difficulty meeting these deadlines,” said Daryl Kimball, executive director of the Arms Control Association, a non-partisan group focused on nuclear and conventional weapons control. “And the costs are going to go up.”

He cautioned that the sweeping upgrades could also have the undesired effect of pushing Russia and China to improve and expand their arsenals.

The core of every nuclear warhead is a hollow, globe-shaped plutonium pit made by engineers at the Energy Department’s lab in Los Alamos, New Mexico, birthplace of the atom bomb. Many of the current pits in use come from the 1970s and 80s. That can be problematic, because there’s a lot about plutonium’s aging process that scientists still don’t understand.

The key radioactive atom in the plutonium pit has a half life of 24,000 years, which is the amount of time it would take roughly half of the radioactive atoms present to decay. That would suggest the weapons should be viable for years to come. But the plutonium decay is still enough to cause concern that it could affect how a pit explodes.

President George H.W. Bush signed an order in the 1990s banning underground nuclear tests, and the U.S. has not detonated pits to update data on their degradation since. When the last tests were performed, they provided data on pits that were at most about two decades old. That generation of pits is now pushing past 50.

Bob Webster, deputy director of weapons at Los Alamos, said scientists have relied on computer models to determine how well such old pits might work, but “everything we’re doing is extrapolating,” he said.

That uncertainty has pushed the department to restart pit production. The U.S. no longer produces man-made plutonium. Instead, old plutonium is essentially refurbished into new pits.

This task takes place inside PF-4, a highly classified building at Los Alamos that’s surrounded by layers of armed guards, heavy steel doors and radiation monitors. Inside, workers handle the plutonium inside steel glove boxes, which allow them to clean and process the plutonium without being exposed to deadly radiation.

In the final production steps, a lone employee in the vault takes the almost-completed pit into both of her gloved hands and shapes it into its final form.

“Things have to fit a certain way, and everything is by touch, by feel,” said the Los Alamos employee, who the AP has agreed not to name because she is one of only a handful of people in the U.S., and the only female, who performs this sensitive task.

For about the last 10 years technicians have been practicing on “test” pits that aren’t ready for the stockpile. The U.S. is planning to fully recycle its first weapon-ready pit next year — and quickly increase annual production to as many as 80 new pits.

The painstaking and hazardous work has led a government watchdog to express doubts about whether the U.S. government can meet that goal.

“The United States has not regularly manufactured plutonium pits since 1989,” the Government Accountability Office noted in a January 2023 report, adding that the Energy Department’s National Nuclear Security Administration has provided “limited assurance that it would be able to produce sufficient numbers of pits.”

Webster has been at Los Alamos since Ronald Reagan was president. He could have retired years ago, but has remained to shepherd the first new plutonium pits through to production. The lab is starting to feel a bit like it did in the 1980s, during the Cold War, he said. Los Alamos scientists are having intense discussions about weapon design — how much each can weigh, its explosive punch, how far it must travel.

“We need our nation to be back making pits,” Webster said. “We just have to be able to do that.”

Completed pits are protected and detonated by an outer warhead layer that is built at the Energy Department’s Kansas City National Security Campus. Inside that three-story windowless factory, workers restore and test those warhead parts, work that a government watchdog said required “a great deal of precision manufacturing to exacting specifications.”

There are thousands of tiny parts inside each warhead, so steady hands are key. That’s why technicians go through a skills assessment that includes disassembling and assembling a mechanical wristwatch.

“Everything is done under a microscope with tweezers,” said Molly Hadfield, a spokeswoman for the Kansas City plant. “And its pass (or) fail. Either the watch works or it doesn’t work.”

This factory would be busy even if an overhaul wasn’t underway. All warheads have regular maintenance requirements. Their plastics age, and metal gears and wiring are weakened by the years and by exposure to radiation.

The factory is also working on warheads for the B-21 Raider, a futuristic stealth bomber, while also supporting the Sentinel, a new intercontinental ballistic missile and on warheads for a new class of submarines.

“There’s a huge modernization effort going on,” said Eric Wollerman, who manages the Kansas City complex for the Department of Energy through its federal contract with Honeywell. “​​If you’re going to update the delivery systems, you would also then update the warheads in the missiles and the bombs that are with them.”

To meet the demand for both maintenance and modernization, the facilities have gone on a hiring spree. The Kansas City plant has 6,700 employees, a 40% jump since 2018, with plans to add several hundred more. The Los Alamos lab has added more than 4,000 employees in that same time frame.

The U.S. nuclear arsenal reveals its age each time troops fix a missile. That can occur as often as twice a week, but only if the equally old tools, or the truck carrying the tools, or the truck needed to transport the missile itself isn’t also broken down, which is often.

That is why Airman 1st Class Jonathan Marrs was dragging a second 225-pound aluminum tow behind him toward a concrete silo in the midst of vast Montana farmland on a recent hot afternoon.

Marrs, 21, and other airmen used a tow and wrenches the size of human femurs to dislodge silo Bravo-9's 110-ton blast door. Underneath its cement and steel cover was a 70,000-pound nuclear missile; the missile's warhead tip needed to be lifted out and trucked to base for work.

Except the blast door wouldn't budge. The first 225-pound tow, or mule, as the troops call it, couldn't generate the power needed to pull back the door.

After attaching a second mule, Marrs and the other airman succeeded in pulling the door free, releasing scores of mice.

The maintainers next unfastened the warhead from the missile and placed it in a specialized truck. It's then escorted by Air Force security forces back to a heavily guarded hangar at Montana's Malmstrom Air Force Base.

Marrs and the other young airmen — known as maintainers — are closely monitored as they handle nuclear weapons, U.S. Air Force officials said.

“If I under-inflate a basketball at the gym, no one will care,” said Chief Master Sgt. Andrew Zahm, the maintenance group senior enlisted leader at F.E. Warren Air Force Base. “If I did something with one of these weapons, the president would know about it in 45 minutes.”

The workload is already a challenge for these troops, and there aren’t many easy ways to relieve it.

While the private-sector managed Los Alamos and Kansas City plants have hired personnel to meet the rising workload, the military has struggled to fill jobs and retain experienced technicians. Instead, the military must do more with fewer maintainers, and for much less money than those troops could make as government contractors.

“Once you start showing a staff sergeant the $80,000" they could make in the private sector, they are going to take it, Zahm said.

Zahm is a rarity. While many have retired or left for private industry, he’s remained to keep serving the military’s nuclear mission. With the U.S. so close to its first new weapon, he’s driven by a desire to see it through. “In 21 years I’ve never seen a new thing,” Zahm said. “I want to see the new stuff.”

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Copp reported from Los Alamos National Laboratory, New Mexico; the Kansas City National Security Campus, Missouri; Malmstrom Air Force Base, Montana and F.E. Warren Air Force Base, Wyoming.

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The Associated Press receives support for nuclear security coverage from the Carnegie Corporation of New York and Outrider Foundation. The AP is solely responsible for all content. Del Wilber is the Washington investigations editor for the AP.