US expert on underground nuclear tests held in China for 18 months

By Mike Valerio, Simone McCarthy, CNN

Beijing (CNN) — An American scientist who has studied underground nuclear tests has been detained in China for more than 18 months on spying charges, according to his supporters and a US lawmaker.

Youlin Chen, a seismologist, has been “wrongfully detained” since November 2024, US Senator Edward Markey said in a statement Tuesday.

President Donald Trump raised Chen’s detention and asked for his freedom during a meeting with Chinese leader Xi Jinping in Beijing in May, according to Global Reach, a US non-profit that has been working with Chen’s family on his case.

Chen, who was living in Boston and has a college-age son, is the only American currently held in China designated as wrongfully detained, the non-profit said.

The case adds another point of friction between the US and China as they try to stabilize ties, and ahead of Xi’s expected visit to the US later this fall. Its revelation comes weeks after China confirmed the arrest of another US scholar, Min Zin, who it said is “suspected of spying and endangering Chinese national security.”

Global Reach said there are suspicions that Chen’s detention is linked to China’s recent expansion of its nuclear capabilities, including carrying out an alleged underground nuclear test in 2020. Beijing denies this test.

China and the US have both signed, but not ratified, the Comprehensive Test Ban Treaty (CTBT) – an international agreement prohibiting “any nuclear weapon test explosion or any other nuclear explosion.”

Chen’s work focuses on using seismological data to improve methods to identify and monitor nuclear tests. It has included research into North Korea’s underground nuclear testing.

Chen’s research has been funded by the US State Department and US Air Force Research Laboratory.

In December 2020, he authored a technical report that used regional seismic data recorded across Asia, including data from stations in China, to improve methods for nuclear-test monitoring and yield estimation, according to Global Reach.

A subsequent 2024 study that he authored was also funded by the US Air Force Research Laboratory and the US Department of State and “further reinforces Chen’s expertise in the seismic monitoring and detection of underground nuclear tests,” the group said.

Detained at airport

Chen was detained at the airport after visiting his parents in Beijing, according to Kieran Ramsey, chief investigative officer at Global Reach.

“China is being accused … by the State Department of violating (the CTBT),” Ramsey said. “And at the same time they’re holding the American expert that would be able to identify that exactly.”

“Unfortunately this case is example one of China willing to use hostage diplomacy as part of their great power competition with the US and unfortunately Doctor Chen is the sole example right now officially designated by the US government,” he added.

Earlier this month a pastor who founded one of China’s most prominent underground churches was released from prison and reunited with his family in the United States after being detained in a crackdown in China last year. His release came after Trump raised his case with Xi Jinping during his visit to Beijing in May.

A State Department spokesperson told CNN that the US has raised Chen’s case directly with Chinese officials and called for his immediate release.

When asked by CNN whether the case was raised by Trump during his meeting with Xi, a White House official said that “President Trump has been clear that he wants every American detained abroad to return home.”

China’s Foreign Ministry on Tuesday denied that Chen was “wrongfully detained” and said judicial authorities handle cases in accordance with the law, when asked about the case at a regular press briefing.

Chen was charged with espionage on May 1, 2025, but has not yet stood trial. The family decided to speak out now about his detention because Trump’s request has not been acted on, Global Reach said.

Chen’s wife, Yufang Rong, said she had not spoken to her husband for over 600 days and was concerned for this health.

“Youlin has never held a US government security clearance and to suggest he was involved in espionage is both wrong and inconsistent with the public and collaborative nature of the work that he has done,” she said in a statement provided by Global Reach.

Noting that her husband “works transparently with Chinese colleagues on scientific collaboration,” she added: “He is doing precisely the kind of people-to-people engagement that the Chinese government says it wants.”

US embassy officials have visited Chen several times, but Chinese officials are always present, preventing him from speaking freely, Rong told Reuters in an interview. She retained a Chinese lawyer, but he was allowed to see Chen only after the scientist had been detained for more than 13 months.

Chinese officials have interrogated her husband more than 100 times about his work on the seismographic signatures of North Korean nuclear tests, she said.

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