Something unusual is happening inside the plutonium facility at Los Alamos National Laboratory in New Mexico. PF-4, as it is known to top government officials, is the heart of America’s nuclear complex, a lab where scientists and engineers study and experiment on highly radioactive materials in tight secrecy. Recently, employees have discovered yellow plastic tents encasing equipment and rendering it inaccessible. At Los Alamos, where even the cleaning crews and firefighters require high-level security clearances, you might think the tents are designed to restrict access to the latest wonder weapon or scientific breakthrough. The truth is more mundane—and more telling. “It’s part of our expansion plans,” Matthew Johnson, a senior lab manager, tells me during a rare tour of the fortified building. “All the old stuff is coming out.”

PF-4 is being transformed from an experimental laboratory that focuses mostly on research into a facility that mass-produces plutonium “pits,” the grapefruit-sized cores inside every nuclear bomb in America’s arsenal. Los Alamos— the lab synonymous with the dark art of nuclear-weapon development—hasn’t produced a certified pit in over a decade and has never had to produce more than 10 in a single year. But in 2018, Congress passed a law mandating that PF-4 produce 30 pits a year by 2026. Around $5 billion has already been spent to overhaul the cramped, aging facilities. The Biden Administration has pumped $4.6 billion into Los Alamos this fiscal year alone—a 130% budget increase over what the lab received just five years ago. Truckloads of new work stations, lathes, and furnaces are set for installation. Coast-to-coast recruiting efforts are underway to increase the lab’s workforce, which is already at a record 17,273.

Not long ago, such ambitions would have been unthinkable. Following the Soviet Union’s collapse in 1991, the U.S. stopped designing, building, and testing new nuclear warheads. Stockpiles were slashed, labs’ budgets cut, and a highly skilled workforce allowed to dwindle. But after a three-decade break from manufacturing nuclear weapons, the U.S. is getting back into the game. Another arms race may be upon us, triggered by China’s growing ambitions and escalating hostilities with Russia, the world’s other nuclear superpower. Since President Biden took office, both nations have wielded their arsenals to threaten adversaries and coerce neighbors. The last remaining nuclear-arms treaty, known as New START, is set to expire in 2026, raising fears about a new era of unchecked expansion. All nine nuclear powers are scrambling to modernize their arsenals and build new weapons.