The U.S. tested nuclear weapons until the early 1990s. Since then, scientists have been using supercomputers and experiments to simulate nuclear test detonations, without detonating any nukes. But there are signs the world's nuclear powers may be readying to test again: Russia, China and the U.S. are all upgrading their nuclear test sites.NPR science correspondent Geoff Brumfiel was among a small group of journalists allowed to tour an underground laboratory where this research happens. Read more of science correspondent Geoff Brumfiel's reporting here. Help shape the future of Short Wave by taking our survey: npr.org/shortwavesurveyListen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: What is the purpose of nuclear testing?
So today on the show, Jeff takes us on a rare trip to tunnels deep under the Nevada desert, where America does science instead of nuclear testing. I'm Regina Barber, and you're listening to Shortwave, the science podcast from NPR.
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OK, so let's start with like a little history on nuclear testing. Like, how did we get to where we are today where we're not doing it anymore?
Chapter 2: When did the U.S. stop nuclear testing?
You know, you think a nuclear test and it's like a mushroom cloud over the desert. But that was actually just a very brief period that those above ground tests were happening. It was mostly in the 1950s and it stopped because of radioactive fallout. The wind was carrying radioactive material much further than expected. And it started showing up on fishing boats.
In milk and baby teeth, places nobody wanted it. So in 1963, the world's nuclear power signed a treaty to move testing deep underground. And it kept going that way all the way until the end of the Cold War.
Which was what, like the late 1980s, early 1990s? That's when the Soviet Union collapsed. Right, right. So why did nuclear testing stop then?
I spoke to Hans Christensen with the Federation of American Scientists. He tracks nuclear weapons all over the world, and he says there were two big reasons.
The first was political. 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. We're serious about it. We're willing to do some things. But the second reason they could stop testing was actually political.
It was this growing power of supercomputers. They could now take these supercomputers and simulate nuclear testing in silicon.
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Chapter 3: What technology replaces nuclear tests today?
You didn't need to do nuclear tests to do what you needed to do for the foreseeable future, which is to make sure that the nuclear weapons you have work. And if there's a problem, a glitch, you can fix it.
So you could still run tests. They were just on computers, not like an actual detonation of a nuke.
Yeah, and that actually takes us to the first stop on this whole nuclear testing road trip.
Okay.
I traveled out to Lawrence Livermore National Laboratory in California, and I got to see their newest computer where they do what they call the button-to-boom calculations.
The button-to-boom.
It's inside a secure vault. The computer is called El Capitan.
We named it after Yosemite. There's a very prominent granite there, and it's in California. Yeah.
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Chapter 4: How do supercomputers simulate nuclear tests?
Terry Quinn oversees high-performance computing at Livermore, and she showed us around. Now, this machine uses advanced processors, similar to those used for generative AI, actually. LCAP can do more than two quintillion calculations per second. Wow. That's two exaflops for those in the supercomputing game. And its only job is to calculate, you know, a nuclear weapon's detonation.
It used to be it might take them a week to do a calculation on how a weapon would detonate.
Now they can get it back by the end of the day or within hours. I mean, that is game-changing, especially in this world where we have to move faster.
And this is what nuclear testing looks like today. Making sure the weapons we do have work, but using supercomputers, right? Like problem solve, like no more real world tests. Not quite. Okay, okay.
As you know, Gina, any good computer model still needs some real world data to make sure it's grounded in reality. Okay. And so, you know, nuclear weapons scientists have continued to collect data. They're doing experiments in the same tunnels underground where they used to set up their nuclear weapons tests.
Okay.
Now, this is not a place many journalists get to go. But I asked and asked and asked very nicely— And eventually, I and a few other journalists made it to Nevada, where they do this testing. We stepped onto an old mining elevator. And the elevator just drops into pitch blackness, nearly a thousand feet underground. Here's what it was like at the bottom.
Already it sounds scary.
There's a long corridor that's been carved out of an ancient lake bed. Pipes along the walls carry air, water, and power. Workers in hard hats are everywhere. David Funk, who oversees work underground, leads us in.
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Chapter 5: What does the underground lab look like?
How long have we had tunnels then? Oh, so the tunnels were dug in the 80s.
So this would have been a test shaft, potentially.
Yeah, this was designed to be a nuclear test location originally, and now we do only subcritical experiments in this location.
Subcritical experiments. That means experiments that simulate conditions inside a nuclear weapon without triggering a nuclear chain reaction. That runaway chain reaction is what gives a nuke its incredible power. These tunnels were originally built to contain nuclear explosions. Then in 1992, the US officially stopped all testing.
The Cold War had ended and the thinking was nuclear testing could end too. It would make the world safer. But fast forward to today and nuclear weapons are back on the front pages. Same goes for nuclear testing. China, Russia and America are all upgrading their test sites. And that's why we're here to see what U.S. scientists are up to underground.
Speaking of which, why do they still work down here?
We do this because it is secure, right, so we can control the environment. And we also are concerned about a potential breach of the vessel, and so we want to do it in an environment that's controlled and we don't lose any of the plutonium into the environment. If something goes wrong. If something goes wrong, yeah.
We arrive at our first stop, a long, empty corridor that's just been dug out for a new experiment.
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Chapter 6: What data is still collected by scientists?
So this is where the Scorpius machine is going to reside.
The Scorpius machine. It kind of feels like you're in a James Bond movie. Funk explains it's actually a giant x-ray machine. You guys are familiar with chest x-rays, right? Scorpius is going to work in the same way. It'll create extremely high-powered x-rays.
And the reason is we need higher energy x-rays to be able to look through plutonium.
Plutonium from America's nuclear weapons. Much of it was made decades ago, and it's getting old. The x-rays are used to take a look inside to see how well the plutonium would work if detonated. Scorpius will cost $2 billion to build down here. We walk deeper into the tunnel network. Some of the floors are still rocky. They've just been dug. Next stop, an experiment that simulates a nuclear weapon.
It's called Cygnus, and it's arguably the most secretive scientific project in the U.S. government.
As a reminder, if you're ever going into, no cell phones, any Bluetooth devices, smart watches, smart rings, if you have them, they need to go in the locker over here.
Cygnus is a smaller version of Scorpius. It fires x-rays of plutonium that's inside a spherical steel container about the size of a mini-fridge.
Watch your step, Henry.
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Chapter 7: How do tunnels enhance nuclear research?
Tim Beller is directing the next test, codenamed Knob Hill.
Chapter 8: What are the implications of resuming tests?
So that is the Nob Hill vessel. That's the actual vessel that we will use. That's a three-footer.
In a few months inside this vessel, scientists will blow up a tiny quantity of plutonium using chemical explosives. It's designed to simulate a nuclear detonation. But again, this test will be subcritical. The U.S. government says there will not be a runaway nuclear chain reaction.
Whatever rules they set, I ensure they happen here in Nevada.
OK, so, Jeff, this gives like a real sense of what it was like to visit those tunnels. And like Tim Beller said, is that the U.S. is following these rules. Right. But you mentioned there might be like a return to testing. Is that because scientists need to do like another test they couldn't do with these supercomputers?
No, actually, it isn't. In fact, I heard again and again from nuclear weapons scientists on this trip. There is no technical need to test right now.
Okay. So if it's not scientific, it's not technical, why would America test again?
We'd start testing again for exactly the same reason we stopped testing, and that's politics. Nuclear weapons have a new role in the world. You know, Russia is developing some wild new nuclear weapons designs. China is expanding its nuclear arsenal. And into all of this comes the Trump administration. Now, Project 2025, which is sort of a conservative blueprint for the government. It says the U.S.
should be ready to test another nuclear weapon if it needs to. Trump's former national security advisor has gone further. He's written that the U.S. should test another nuclear weapon.
OK, so if the U.S. did resume underground testing, what would be the consequences of that?
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