Late last week, the U.S. leveled an explosive claim: China is planning secret nuclear weapons tests and has already conducted at least one.
China denied the allegations, but experts are worried that the claims mark a further unraveling of a long-standing global norm against nuclear testing.
The devil is in the details. Here is what to know about the U.S. claims of Chinese testing.
The claim was made publicly and very deliberately
The undersecretary of state for arms control and international security, Thomas DiNanno, disclosed the major U.S. intelligence finding at the Conference on Disarmament in Geneva on Friday.
"I can reveal that the U.S. government is aware that China has conducted nuclear explosive tests, including preparing for tests with designated yields in the hundreds of tons," he said in a speech to the attending delegates. He went on to say that China is using a technique known as "decoupling" to hide its activities.
"China conducted one such yield-producing nuclear test on June 22 of 2020," DiNanno said.
The Chinese Embassy in Washington, D.C., did not respond to NPR's request for comment, but in a statement to AFP, China's Foreign Ministry called the claims "outright lies."
"China firmly opposes the US attempt to fabricate excuses for its own restarting of nuclear tests," said the statement to AFP on Monday.
Most nations haven't tested nuclear weapons in years
The last full-scale nuclear test took place in North Korea in 2017, said Ankit Panda, a nuclear weapons expert at the Carnegie Endowment for International Peace.
"As of this month, we are in the longest period in human history without a nuclear test," he said. (That's since the U.S. tested the first atomic bomb in 1945.)
The U.S. conducted its last nuclear test in 1992, and China conducted its last official test in 1996. Both nations are signatories to the Comprehensive Nuclear-Test-Ban Treaty, which prohibits nuclear testing. Although the treaty has never been ratified, major nuclear powers have generally upheld their obligations not to test.
An explosive test can mean many different things
While the major nuclear powers have not detonated a nuclear bomb in decades, that doesn't mean work on nuclear weapons stopped. In the United States, scientists embarked on a program to maintain the weapons without testing them.
That program includes computer simulations, science experiments and underground explosive tests on components of nuclear weapons (NPR was granted rare access to the tunnels where the experiments take place in 2024).
But these explosions do not trigger a nuclear chain reaction. They are designated "subcritical" tests and thus do not violate the American testing moratorium.
China and Russia are believed to have similar programs. Russia has been seen modernizing its nuclear facilities, and in recent years, China has expanded its main test site by digging new tunnels.
The U.S. has long suspected that China and Russia are conducting experiments that do trigger a small nuclear chain reaction, but part of the problem is that "the Comprehensive Nuclear-Test-Ban Treaty does not define what an explosion is," said Jeffrey Lewis, an arms control expert at Middlebury College.
He said that China and Russia may interpret it to mean they are permitted small nuclear chain reactions, so long as the boom doesn't get too big.
China's alleged test was in 2020 and probably wasn't too large
Lewis has reviewed publicly available seismic data from June 22, 2020, and found no signature of an explosive test near China's test site. The Comprehensive Nuclear-Test-Ban Treaty Organization also said in a statement that it had not seen seismic evidence of a test.
When asked for more information about the alleged Chinese test, the State Department told NPR, "More information will be released soon."
One reason for the lack of seismic evidence might be that China was hiding the test through "decoupling." Decoupling is another way of saying it conducted the explosion in a large, empty cavity, Lewis said. When the shock wave from the blast reaches the walls of the cavity, much of the energy doesn't transfer to the rock and "the explosion looks much, much smaller than it really is."
Global monitoring systems are extremely good, however, so even with decoupling, it's extremely unlikely this was a full-scale nuclear test.
But the U.S. is very worried about China's weapons development
Regardless of what happened in 2020, the U.S. is also alleging China is planning tests with "designated yields in the hundreds of tons." Those tests, should they happen, would be in a gray area between a small nuclear chain reaction (sometimes called a hydronuclear test) and a full-scale detonation.
"There are benign explanations and there are less benign explanations" for why the Chinese might designate such yields, the Carnegie Endowment's Panda said.
The benign case is that China is doing small-scale testing to ensure its weapons won't explode accidentally. Such tests have been done in the U.S. to ensure that weapons are "one-point safe," meaning that if one explosive component goes off, the weapon won't detonate its nuclear charge.
Panda said if a weapon failed such a safety test, it could "overrun" the predicted yield, creating an explosion in the range of a few hundred tons. Such a failure would presumably lead to safer weapon designs.
The less benign theory is that China is attempting to test designs of new nuclear weapons. Panda said it might be for the development of warheads for things like hypersonic missiles. Lewis said the Pentagon believes China may be trying to develop "small" weapons with yields in the tens of kilotons (the U.S. recently deployed such a weapon).
Part of the problem, Panda said, is that China is in the process of rapidly expanding its nuclear arsenal. Until as late as 2019, China's arsenal of weapons was in the low 200s, he said. U.S. intelligence estimates suggest that China is now on track to reach 1,000 deployed weapons by 2030.
"China has not told us why these changes are happening. And so in the United States, imaginations have run wild," he said.
The Pentagon worries that an expanding Chinese arsenal could be a prelude to invading Taiwan or an attempt to beat America's missile defenses, Panda said.
The norms against testing are unraveling, and that's probably bad
China's nuclear expansion is a big reason that the U.S. allowed its last arms control treaty with Russia to end last week and is reconsidering nuclear testing.
President Trump has said he wants to test "on an equal basis" with other countries, including China.
Lewis said he's very worried all this testing talk could start to snowball.
"There's no real stopping point once we all start doing bigger and bigger tests," he said.
He predicts bigger tests will lead to new nuclear weapons, and more of them: "It will end up, in the end, looking just like the Cold War."
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