The US government has released new information about what it claims are illegal nuclear tests conducted by China in 2020.

June 22nd of the same year, Earthquake observatory in remote area A small earthquake was detected in Kazakhstan. Although the scale of the event was only 2.75, the source was China’s main nuclear test site known as Lop Nor, about 450 miles away, according to Christopher Yeo, the State Department’s assistant secretary for arms control and nonproliferation.

“There’s very little chance of anything other than an explosion, a singular explosion,” Yeo said at an event hosted by the Hudson Institute, a conservative think tank in Washington, D.C., on Tuesday. “This is exactly consistent with what you would expect from a nuclear test.”

Independent experts did not immediately agree with that assessment. The ratio of different seismic waves is consistent with an explosion, said Ben Dando, head of seismology and verification. Norsara Norwegian organization that monitors possible nuclear tests. However, the signal was weak and recorded at a single station, he added. Based on these and other limitations, he still believes this could have been a natural event.

“I can’t say there’s really strong conclusive evidence,” Dando told NPR. “At this point, we cannot confirm or deny whether a nuclear test took place.”

in statementthe Comprehensive Nuclear-Test Ban Treaty Organization announced that its monitoring station had received a signal and detected “two very small seismic events 12 seconds apart.” However, it added: “This data alone does not allow us to confidently assess the causes of these events.”

The Chinese embassy did not respond to NPR’s request for comment, but the government has strongly denied the allegations. Chinese Foreign Ministry Spokesperson Lin Jian said, “The United States’ accusations against China’s nuclear explosion test are completely baseless.” press conference last week. “China opposes U.S. policy” [sic] He is concocting an excuse to resume nuclear testing. ”

Test trouble

The world’s major nuclear powers have not tested weapons in decades. The United States conducted its last test in 1992, and China conducted its last official test in 1996. Both countries have signed an agreement. Comprehensive Nuclear Test Ban Treatyprohibiting nuclear tests. However, neither country has formally ratified the Nuclear Test Ban Treaty, and it has not yet entered into force.

Although countries have voluntarily kept their commitments not to conduct nuclear tests, this does not mean that research into nuclear weapons has completely stopped. In the United States, supercomputer simulations are combined with numerous real-world experiments to ensure that the weapons work as designed. Some of those tests are being conducted in tunnels where nuclear tests once took place. NPR gained rare access to these tunnels in 2024 and was shown the test chambers where so-called subcritical tests are conducted. Weapons experts interviewed by NPR at the time said the test would explode a small amount of weapons-grade plutonium, but no nuclear chain reaction would occur within the material.

Tong Zhao, a senior fellow at the Carnegie Endowment for International Peace who studies China’s nuclear weapons program, said China is similarly busy at its Lop Nur test site. He said recent satellite images have shown that the equipment area and personnel housing have been expanded and that at least one new tunnel has been dug.

“China appears to be investing heavily to maintain, if not expand, the test site’s mission,” he said.

big expansion

The U.S. testing program primarily seeks to maintain its current weapons stockpile of about 1,500 deployed weapons, while China aims to expand. Until 2019, China was thought to have around 200 warheads. today, According to the Department of DefenseChina has nearly 600 warheads and aims to have 1,000 warheads by 2030, a number that would bring it closer to parity with the United States and Russia.

But even as China adds hundreds of nuclear warheads to its arsenal, there is relatively little nuclear test data to work with. Before the nuclear testing moratorium began, China had only conducted 45 nuclear tests. According to , about half of those tests were done in the atmosphere and the other half underground. Data from Arms Control Association.

That may be one reason why China is interested in conducting large-scale nuclear tests. Earlier this month, U.S. Under Secretary of State for Arms Control and International Security Thomas DiNanno announced that the United States believes China is skirting the red lines of a moratorium on nuclear tests and plans to go further in the future.

“I can make clear that the U.S. government is aware that China has conducted a nuclear test, including preparations for a test with a designated yield of several hundred tons,” he said in a speech in Geneva, Switzerland.

Speaking at the Hudson Institute on Tuesday, Yeo argued that the 2020 nuclear test was a “yield-producing” nuclear test, meaning it set off a runaway chain reaction of nuclear material. Yeo declined to say how large the test was, but said the Chinese side carried out a “separation” operation to hide the true scale of the test.

Yeo did not elaborate on what China would gain from a possible test, but said: “We know that countries will not take on these risks without expecting significant benefits.”

Dando said that if this was indeed a nuclear test as claimed, the magnitude 2.75 event detected in Kazakhstan would likely correspond to an explosion equivalent to tens of tons of TNT. But by digging a large cavity and placing a nuclear device in its center, it would be possible to hide a much larger explosion, perhaps hundreds of tons, or even a kiloton.

Zhao said China would have many reasons to conduct such an experiment. It may be interested in developing low-yield nuclear weapons for use in a limited war with the United States or testing new designs for hypersonic weapons.

Zhao said he takes the U.S. claim seriously, saying, “This may be supported by secret analysis by U.S. intelligence agencies and appears to be consistent with China’s long-standing highly active nuclear test site at Lop Nur.” (Mr. Yeh declined to answer questions from NPR about whether the U.S. has further information beyond official seismic measurements.)

The revelations about China’s nuclear testing activities come just months after President Trump announced that the United States was considering returning to nuclear testing “on the same basis” as other countries.

Some experts question whether the United States should rush to resume nuclear testing. China has conducted only 45 tests, while the United States has conducted well over 1,000. This means that Chinese nuclear scientists are likely to gain much more knowledge with each new additional experiment than American nuclear scientists.

Mr. Yeo said he expected the United States to be able to enter into negotiations with both China and Russia for a new arms control agreement. However, he added that the Pentagon is currently considering whether to add nuclear weapons to U.S. missiles, bombers and submarines.

“There’s obviously a lot of options on the table,” he said.