FRENCHMAN FLAT, Nev. — In the middle of a dry lakebed northwest of Las Vegas sits a lone section of a bridge, its steel girders bent like spaghetti. Nearby are other oddities — a massive bank vault with no bank for miles; the entrance of an underground parking garage with no lower levels; and domes of rebar and concrete that have been ripped open, leaving their insides exposed to the desert sky.
A half-mile from here, on the morning of May 8, 1953, an Air Force bomber dropped a Mk-6D nuclear bomb from a height of 19,000 feet above the desert floor. It exploded with a yield of 27 kilotons of TNT — creating a shockwave that warped the bridge and blew open the vault. The test, code-named "Encore," was designed to see what, if anything, in the civilian world could survive a nuclear blast (the answer is, apparently, not much).
Nuclear testing seems like a Cold War relic, but there are signs the world's nuclear powers may be readying to test again. Amid growing tensions, Russia, China and the U.S. are all upgrading their nuclear test sites.
"The risk is significant," says Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists. The talk of testing comes at a time when nuclear weapons are resurgent: Russia is designing nuclear weapons to attack satellites and obliterate seaports; China is dramatically expanding its nuclear arsenal; and the U.S. is undergoing a major modernization of its nuclear warheads. After years of declining nuclear stockpiles, the world looks poised to begin increasing the number and types of nuclear weapons being deployed.
Going underground
Amid these growing tensions, the U.S. National Nuclear Security Administration, the civilian agency that maintains America's nuclear stockpile, allowed a small group of journalists to tour a secretive nuclear weapons laboratory beneath the Nevada desert this winter. The effort was part of outgoing NNSA administrator Jill Hruby's effort to display more transparency about what the U.S. is doing with its nuclear stockpile and why.
The work is taking place in a facility known as the Principal Underground Laboratory for Subcritical Experimentation (PULSE). From the surface, it appears to be a small amalgamation of modular buildings next to a large mining hoist. Reporters and officials grab hard hats, flashlights and emergency rebreathers, listen to a safety briefing, and then step into an old mining cage. With a whoosh, it drops into pitch blackness. At the bottom, the gates open to a long corridor that's been carved out of an ancient lakebed. Pipes along the walls carry air, water and power. Workers in hard hats are everywhere.
David Funk, who helps oversee work underground, leads the group in.
"This was designed to be a nuclear test location originally," Funk explains. In the early 1960s, nuclear tests went underground to protect people and the environment from dangerous radioactive fallout. In fact, somewhere in the maze of tunnels we're walking through is a sealed shaft from a full-scale nuclear detonation — the Ledoux test in 1990.
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The Ledoux test was among the last nuclear weapons the U.S. ever detonated. At the end of the Cold War, politicians wanted to show the nuclear arms race had also ended. In 1990, the Soviet Union conducted its last nuclear test. Two years later, the U.S. announced it would no longer test nuclear weapons. One by one, the world's nuclear weapons states followed, with the exception of North Korea, whose last test was in 2017.
The decision to halt testing "was very much an attempt to look around and see what we can do to make it clear that we're not just talking about the end of the Cold War, we're serious about it," Kristensen says.
But these tunnels stayed open. The U.S. still needed to ensure that its nuclear weapons were safe and reliable, so nuclear weapons scientists and engineers embarked on a new program to test the nukes without actually setting them off. Kristensen and most experts agree that the program, known as stockpile stewardship, has been effective in staving off new nuclear testing.
"You didn't need to do nuclear tests to do what you needed to do for the foreseeable future, which is to make sure the nuclear weapons you had worked," he says.
The stockpile stewardship program broadly consists of two arms — supercomputers at the nation's major nuclear weapons labs are used to conduct large-scale digital simulations of nuclear weapons from "button to boom." Highly classified nuclear experiments, like those that take place at PULSE, supply real-world data that ensures that the simulations are accurate.
Critical questions
The experiments are a far cry from full-scale nuclear tests. They are all "subcritical," meaning that they simulate conditions inside a nuclear weapon without triggering a nuclear chain reaction. That runaway chain reaction, in which some atoms split apart and others fuse together, is what gives a nuke its incredible power. Since the testing pause began, the world's nuclear powers have informally used the subcritical threshold to define what constitutes a nuclear test.
After a brief walk down one tunnel, the party arrives at its first stop: a long, empty corridor that's just been dug out for a new experiment.
"This is where the Scorpius machine is going to reside," Funk says. The Scorpius machine sounds like a device from a James Bond movie, but Funk explains it's a giant X-ray machine. "The reason is we need higher-energy X-rays to be able to look through plutonium," Funk says.
Plutonium makes up the core of America's nuclear weapons. Much of it was made decades ago, and it's getting old. "Right now the oldest plutonium samples are about 80 years old," says Ivan Otero, a nuclear weapons scientist at Lawrence Livermore National Laboratory. As the plutonium ages, it decays radioactively, releasing helium atoms. Those atoms can form bubbles and damage the plutonium metal's structure. "We need to know whether the helium bubbles or the damage to the lattice have a significant impact on the material response," he says.
The $2 billion Scorpius machine will see how very small quantities of plutonium react when detonated explosively. The X-rays are used to take a series of images of the plutonium as a shockwave generated by conventional explosives passes through it. In addition to addressing the aging question, the tests will help with new upgrades and modifications to existing nuclear weapons.
Not far away, Funk is clearing another tunnel for a second machine, called Zeus, that will bombard the plutonium with subatomic particles known as neutrons. The neutrons will show how plutonium behaves in a higher radiation environment such as the core of a bomb.
The final stop on the tour is an experiment that's getting ready to test plutonium later this spring. It's called Cygnus, and it's arguably the most secretive scientific project in the U.S. government.
Tim Beller is directing the next test, code-named "Nob Hill."
Beller gestures to a small metal sphere about the size of a mini-fridge that's surrounded with scientific equipment: "That is the actual vessel that we will use," he says. A few months from now, inside this vessel, scientists will blow up a small quantity of plutonium using chemical explosives. The explosion will be tiny, an unimaginably small fraction of a nuclear weapon's true power, and the U.S. government says there will not be a runaway nuclear reaction, even a little one.
"Whatever rules they set I ensure they happen here in Nevada," he says.
New nuclear rule
Globally, there are signs the rules might soon be changing. A treaty to permanently ban nuclear testing is stalled. China has been building up its nuclear arsenal and expanding its test site at Lop Nur. "There were new buildings constructed in recent years," says Tong Zhao, a senior fellow with the Carnegie Endowment for International Peace. In the southeastern part of the test area, drill rigs have also suddenly appeared. "One possibility is China developing vertical shafts that can be used for future nuclear explosive testing," he says.
Russia has also been making upgrades to its nuclear test site at Novaya Zemlya. Last fall, Russia's Deputy Foreign Minister Sergey Ryabkov told the state-run Tass news agency that the test site was "fully prepared for a resumption of nuclear tests."
Russia's stockpile is aging just like America's, but its scientific program is less vigorous, says William Courtney, an adjunct senior fellow at the Rand Corp. who helped forge several arms control treaties at the end of the Cold War.
"I'm pretty sure there are a number of Russian nuclear scientists who would like to resume nuclear testing, and Putin may want to as part of his nuclear scare tactics over Ukraine," Courtney says.
The increased activity has led some in the U.S. to call for a return to testing. In the July issue of Foreign Affairs, Trump's former national security adviser, Robert O'Brien, wrote that to keep up with China and Russia, "Washington must test new nuclear weapons for reliability and safety in the real world for the first time since 1992 — not just by using computer models."
Project 2025, a conservative agenda for the U.S. government published by the Heritage Foundation, stops short of calling for a return to testing, but it does say the government should be able "to conduct nuclear tests in response to adversary nuclear developments if necessary."
The idea "is not just to start testing for giggles," says Robert Peters, the Heritage Foundation's research fellow for nuclear deterrence. Peters worries that the current lack of test-readiness might unduly constrain the president in a crisis. "If you're engaged in high-stakes poker with nuclear weapons, I don't want to box the president out," he says.
But arms control experts warn that the U.S. would be taking on enormous risks if it played a role in returning the world to nuclear testing. Before nuclear testing ended in the 1990s, China conducted just 45 tests. The U.S. by contrast, conducted more than 1,000 nuclear tests.
"The U.S. has a technical advantage locked in by this moratorium on nuclear testing because it undertook so may more tests during the Cold War period," says Jamie Kwong, a fellow at the Nuclear Policy Program at the Carnegie Endowment for International Peace. Kwong and others predict that if the nuclear test moratorium breaks down, China — as well as nuclear newcomers like India, Pakistan and North Korea — would likely rush to conduct more nuclear tests to advance their weapons programs.
Many of the scientists working in the tunnels beneath Nevada feel there is no immediate technical reason to conduct another underground test.
Although a full-scale nuclear detonation would be "complementary" to the battery of experiments being planned, "our assessment is that there are no system questions that would be answered by a test, that would be worth the expense and the effort and the time," says Don Haynes, a nuclear weapons scientist from Los Alamos National Laboratory, which tested the first bomb in 1945.
Still, he adds, the decision whether or not to test is above his pay grade. Ultimately, it will be up to the politicians and the generals to choose whether the U.S. should detonate another nuclear weapon in these tunnels.
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