SCOTT DETROW, HOST:
Think of a nuclear weapons test and you might visualize a mushroom cloud over the desert.
(SOUNDBITE OF ARCHIVED RECORDING)
UNIDENTIFIED PERSON: Three, two, one.
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DETROW: But for the past 30 years, the U.S. has been running its tests using computers. NPR's Geoff Brumfiel has more on how chips have replaced bombs and why some fear there could soon be a return to real-world explosions.
GEOFF BRUMFIEL, BYLINE: Just outside the Bay Area at a government laboratory in Livermore, California, there's a building that looks like it's a tech company data center. But the computer is locked inside a special classified room.
TERRI QUINN: We're going to go into a vault, so I'm just asking you one more time to make sure you don't have anything transmitting. Check your pockets or else you get to tour security later on.
BRUMFIEL: Terri Quinn oversees high performance computing at Lawrence Livermore National Laboratory. She's leading a select group of journalists inside the vault.
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BRUMFIEL: There are rows and rows of black boxes filled with high-powered computer processors.
QUINN: From here back to the wall is El Capitan and its file system.
BRUMFIEL: El Capitan is the Department of Energy's newest supercomputer.
QUINN: We named it after Yosemite. There's a very prominent granite there, and it's in California.
BRUMFIEL: This supercomputer is among the world's most powerful. Quinn opens one of the black boxes. Inside are racks and racks of microprocessors bristling with fiber optic cables.
QUINN: This is how it's packed. It's very, very dense.
BRUMFIEL: This machine is designed to carry out more than 2 quintillion calculations per second. And it has just one job, to visualize the explosion of a thermonuclear warhead. Rob Neely heads weapon simulation and computing at the lab. He says this new machine can do it all.
ROB NEELY: Button to boom, so everything from the weapon is just sitting there to full yield.
BRUMFIEL: So this is really where the test happens now, inside these machines?
NEELY: Exactly. This is the new test facility.
BRUMFIEL: It wasn't always this way. America began testing nuclear weapons in the atmosphere, then moved underground in the 1960s. It kept testing in tunnels and boreholes all the way until the 1990s, more than 1,000 nuclear tests in total. Hans Kristensen tracks nuclear weapons globally for the Federation of American Scientists. He says after the Cold War ended, politicians wanted to show the nuclear arms race had also ended, and so they decided to stop nuclear testing.
HANS KRISTENSEN: It was very much an attempt to look around and see, what can we do that makes it clear that we're not just talking about the end of the Cold War, and we're serious about it, we're willing to do some things?
BRUMFIEL: And it was possible to end testing thanks to new technologies like lasers and supercomputers. Scientists could rely on small experiments and simulations instead of mushroom clouds. To this day, scientists who work on America's nuclear weapons certify that the stockpile is safe and reliable. And yet, Kristensen says he believes the world is inching towards new nuclear testing.
KRISTENSEN: The risk is significant, and it's significant because of this new dynamic that is not just military but political.
BRUMFIEL: Kristensen says he believes that nuclear testing or the threat of nuclear testing could be used as a show of force by a leader like Russian President Vladimir Putin or Donald Trump.
KRISTENSEN: If the Trump administration needed to do some chest thumping and decided it was necessary to pop a nuke, they could absolutely rupture the situation.
BRUMFIEL: Because the current situation where nobody's testing is entirely voluntary. Tong Zhao is with the Carnegie Endowment for International Peace.
TONG ZHAO: If the United States resumes nuclear testing, I think there's a very high chance that China will follow suit.
BRUMFIEL: But China's tests will be about more than politics. Remember how the U.S. had around 1,000 nuclear tests under its belt? Well, China has only done 45 nuclear tests. Today, it's rapidly growing its nuclear arsenal, and Zhao believes testing could help it learn as it goes.
ZHAO: China could gain much more technologically from resuming nuclear testing than the United States and Russia.
BRUMFIEL: For now, it doesn't look like the Trump administration, Russia or China will test a nuclear weapon soon, but all three nations are updating their nuclear test sites. The world's nuclear ceasefire is increasingly seeming like a tentative truce.
Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.
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