The U.S. government has released fresh intelligence on what it claims was an illicit Chinese nuclear test conducted in 2020.
On June 22 of that year, a remote seismic station in Kazakhstan detected a tiny earthquake. The event registered just 2.75 magnitude, but it originated around 450 miles away at China's main nuclear test site, known as Lop Nur, according to Christopher Yeaw, the assistant secretary for arms control and nonproliferation at the State Department.
"There is very little possibility that it is anything other than an explosion, a singular explosion," Yeaw said at an event hosted Tuesday by the Hudson Institute, a conservative think tank in Washington, D.C. "It is quite consistent with what you would expect from a nuclear explosive test."
Independent experts did not immediately agree with that assessment. The ratios of different seismic waves is consistent with an explosion, said Ben Dando, the head of seismology and verification at NORSAR, a Norwegian organization that watches for possible nuclear tests. But, he added, the signal was weak, and it was recorded at a single station. Based on those limitations and others, he believes that it's still possible that this could have been a natural event.
"I would not say that there's really strong conclusive evidence," Dando told NPR. "We can't really confirm or deny whether a nuclear test took place at this point."
The Chinese Embassy did not immediately respond to NPR's request comment, but the government has forcefully denied the allegations. "The U.S. accusation of Chinese nuclear explosive tests is completely groundless," Chinese Foreign Ministry spokesperson Lin Jian said during a press conference last week. "China opposes the U.S.'s [sic] fabrication of pretexts for its own resumption of nuclear tests."
Testing troubles
The world's major nuclear powers haven't tested their weapons for decades. The U.S. conducted its last test 1992, and China conducted its last official test in 1996. Both nations have signed on to the Comprehensive Nuclear-Test-Ban Treaty, which prohibits nuclear testing. However, neither nation has officially ratified the test ban treaty and it has yet to go into force.
Nations are voluntarily upholding their commitments not to test, but that doesn't mean work on nuclear weapons have stopped altogether. In the U.S., supercomputer simulations are combined with a slew of real-world experiments to ensure weapons still work as designed. Some of those experiments are conducted in tunnels where nuclear testing once occurred. NPR gained rare access to those tunnels in 2024, and was shown the test chamber where so-called sub-critical testing takes place. The tests explode small quantities of weapons-grade plutonium but do not initiate a nuclear chain reaction within the material, according to weapons experts NPR interviewed at the time.
China has likewise been busy at its nuclear test site in Lop Nur, says Tong Zhao, a senior fellow at the Carnegie Endowment for International Peace who studies China's nuclear weapons program. Satellite imagery has revealed an expansion of equipment areas and housing for personnel in recent years, and at least one new tunnel has been dug, he said.
"It looks like China is investing significantly into maintaining, if not expanding, the missions at the testing site," he said.
A big expansion
The U.S. testing program is primarily attempting to maintain its current arsenal of roughly 1,500 deployed weapons, but China is seeking to expand. Until as recently as 2019, China was believed to have around 200 or so warheads. Today, according to the Pentagon, China's arsenal is closer to 600, and the nation has a goal of 1,000 warheads by 2030 – a number that would put it closer to parity with the U.S. and Russia.
But even as China adds hundreds of warheads to its arsenal, it has relatively little nuclear test data to work with. Before the test moratorium began, China had only conducted 45 nuclear tests. About half of those tests were atmospheric and the other half were underground, according to data from the Arms Control Association.
That could be one reason why China would be interested in conducting larger nuclear tests. Earlier this month, U.S. undersecretary of state for arms control and international security, Thomas DiNanno, announced the U.S. believed China was skirting the line with the test moratorium, and was planning to go even further in the future.
"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 in Geneva, Switzerland.
Speaking at the Hudson Institute Tuesday, Yeaw claimed the 2020 test was a "yield-producing" nuclear test, meaning it had triggered a runaway chain reaction in nuclear material. Yeaw declined to say how big the test might have been, but said that the Chinese had conducted "decoupling" operations to hide its true magnitude.
Yeaw did not elaborate on what China was getting out of the possible test, but said "We do know that nations don't take these risks … without an expectation of significant gain."
If it were indeed a nuclear test as claimed, Dando said that a magnitude 2.75 event detected in Kazakhstan would correspond to an explosion equivalent to perhaps tens of tons of TNT. But by digging a large cavity and placing a nuclear device in the center, it would be possible to hide a much larger explosion – perhaps in the hundreds of tons or even a kilotonne.
China would have many reasons to conduct such a test, Zhao said. It might be interested in developing lower-yield nuclear weapons that it could use in a limited war with the U.S., or testing new designs for hypersonic weapons.
Zhao says he takes the U.S. claims seriously: "It may be backed up by some secret U.S. intelligence analysis and it appears consistent with the very active status of China's Lop Nur nuclear testing site over many years," he says. (Yeaw declined to answer a question from NPR on whether the U.S. had additional intelligence beyond the public seismic readings).
The alleged revelations about China's test activity come just months after President Trump announced the U.S. was also considering a return to nuclear testing "on an equal basis" with other countries.
Some experts question whether the U.S. should hastily return to nuclear testing. While China has just 45 tests, the U.S. has conducted well over a thousand. That means Chinese nuclear scientists will likely gain far more knowledge with each new additional test than their U.S. counterparts.
Yeaw said he was hopeful the U.S. could enter negotiations with both China and Russia for a new arms control agreement. But, he added, the Pentagon was currently considering whether to add additional nuclear weapons to America's missiles, bombers and submarines.
"There are obviously a bunch of options on the table," he said.
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