What to know about the U.S. claims of Chinese nuclear tests : NPR

A road-mobile DF-17 medium-range ballistic missile is seen during a military parade in Beijing’s Tiananmen Square on September 3, 2025. China may want to develop new nuclear warheads for its hypersonic weapons.

GREG BAKER/AFP via Getty Images

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GREG BAKER/AFP via Getty Images

Late last week, the United States made an explosive claim: China is planning secret nuclear weapons tests and has already conducted at least one.

China has denied the allegations, but experts fear the claims mark a further erosion of a long-standing global norm against nuclear testing.

The devil is in the details. Here’s what you need to know about the U.S. allegations regarding Chinese testing.

This statement was made publicly and very deliberately

Undersecretary of State for Arms Control and International Security Thomas DiNanno unveiled the main findings of US intelligence services at the Conference on Disarmament in Geneva on Friday.

“I can disclose that the US government is aware that China has conducted nuclear explosive tests, including preparations for tests with a nominal strength of several hundred tons,” he said in a speech to delegates present. He added that China uses a technique known as “decoupling” to hide its activities.

“China conducted such a nuclear test on June 22, 2020,” DiNanno said.

The Chinese Embassy in Washington 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 American attempt to fabricate excuses to justify its own resumption of nuclear tests,” said the statement Monday to AFP.

Most countries have not tested nuclear weapons in years

The last large-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 have experienced the longest period in human history without a nuclear test,” he said. (This is since the United States tested the first atomic bomb in 1945.)

The United States has conducted hundreds of underground tests in Nevada. Each massive explosion created a visible subsidence crater on the surface.

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The United States conducted its last nuclear test in 1992, and China conducted its last official test in 1996. Both countries are signatories to the Comprehensive Nuclear Test Ban Treaty, which prohibits nuclear testing. Although the treaty was never ratified, the major nuclear powers generally respected their obligations not to carry out testing.

An explosion test can mean many different things

Even though the major nuclear powers have not detonated a nuclear bomb in decades, that does not mean that work on nuclear weapons has stopped. In the United States, scientists have embarked on a program to maintain weapons without testing them.

This program includes computer simulations, scientific experiments and underground explosive testing of nuclear weapons components (NPR has been granted rare access to the tunnels where the experiments will take place in 2024).

But these explosions do not trigger a nuclear chain reaction. They are designated as “subcritical” tests and therefore do not violate the US testing moratorium.

China and Russia reportedly have similar programs. Russia has upgraded its nuclear facilities, and in recent years China has expanded its main testing site by digging new tunnels.

The United States has long suspected that China and Russia are conducting experiments that trigger a small nuclear chain reaction, but part of the problem is that “the Comprehensive Nuclear-Test-Ban Treaty doesn’t define what an explosion is,” said Jeffrey Lewis, an arms control expert at Middlebury College.

He added that China and Russia could interpret this to mean they are allowed to carry out small nuclear chain reactions, provided the boom does not become too large.

China’s alleged test took place in 2020 and probably wasn’t too big

Lewis reviewed publicly available seismic data from June 22, 2020, and found no signatures of explosive testing near the Chinese 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 could be that China was covering up the test by resorting to “decoupling.” Decoupling is another way of saying the explosion took place in a large empty cavity, Lewis said. When the shock wave from the explosion hits the walls of the cavity, much of the energy is not transferred to the rock and “the explosion appears much smaller than it really is.”

However, global monitoring systems are extremely effective, so even with decoupling it is extremely unlikely that this would be a large-scale nuclear test.

But the United States is very worried about China’s weapons development.

Regardless of what happened in 2020, the United States also claims that China is planning tests with “designated yields of several hundred tons.” These tests, if they were to take place, would fall in a gray area between a small nuclear chain reaction (sometimes called a hydronuclear test) and a large-scale detonation.

“There are benign explanations and less benign explanations” for why the Chinese might designate such yields, said Panda of the Carnegie Endowment.

A mushroom cloud rises from a test explosion at the Nevada Test Site on June 24, 1957. Major nuclear powers have not conducted tests since the 1990s.

AP/Energy Department

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AP/Energy Department

The mild case is that China is conducting small-scale tests to ensure its weapons will not accidentally explode. Such tests have been carried out in the United States to ensure that weapons are “point safe”, meaning that if an explosive component detonates, the weapon will not detonate its nuclear warhead.

Panda said that if a weapon failed such a safety test, it could “exceed” its intended power, creating an explosion on the order of a few hundred tons. Such a failure would likely lead to safer weapon designs.

The less innocuous theory is that China is attempting to test new nuclear weapon designs. Panda said this could be aimed at developing nuclear warheads for things like hypersonic missiles. Lewis said the Pentagon believes China may try to develop “small” weapons on the order of tens of kilotons (the United States recently deployed such a weapon).

Part of the problem, Panda said, is that China is rapidly expanding its nuclear arsenal. Until 2019, China’s weapons arsenal was around 200, he said. US intelligence estimates suggest that China is now on track to reach 1,000 deployed weapons by 2030.

“China hasn’t told us why these changes are happening. So in the United States the imagination has run wild,” he said.

The Pentagon fears that an expanding Chinese arsenal could be a prelude to invading Taiwan or an attempt to defeat U.S. missile defenses, Panda said.

Norms prohibiting testing are crumbling, and that’s probably a bad thing

China’s nuclear expansion is a key reason why the United States last week let its last arms control treaty with Russia expire and is reconsidering nuclear testing.

President Trump has said he wants to conduct testing “on an equal footing” with other countries, including China.

Lewis said he was very concerned that all this talk about testing could start to snowball.

“There’s not really a stopping point once we all start doing more and more testing,” he said.

He predicts that greater testing will result in more and more nuclear weapons: “It will end up being like the Cold War.”