Washington releases seismic data from a 2020 explosion; Beijing denies all allegations
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No country has acknowledged conducting a nuclear test since 1998. But on February 6, 2026, the United States accused China of secretly breaking that taboo—claiming Beijing detonated a nuclear device at its Lop Nur test site in June 2020 and masked the explosion using underground caverns that muffle seismic waves.
The allegation arrives at a precarious moment: the New START treaty between the US and Russia expired days earlier, China is rapidly expanding its nuclear arsenal, and the Trump administration has signaled openness to resuming American nuclear tests. If substantiated, a Chinese violation would undermine the fragile international norm against nuclear testing. If fabricated, it could provide diplomatic cover for the US to resume its own tests. The stakes extend far beyond Washington and Beijing.
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International Day Against Nuclear Tests 2015
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Operates the International Monitoring System to detect nuclear explosions worldwide, though the treaty itself has not entered into force.
Washington-based policy research organization analyzing defense and international security issues.
Assistant Secretary Yeaw presents detailed seismic data at the Hudson Institute, citing a Kazakhstan monitoring station's detection of a magnitude 2.76 event inconsistent with earthquakes or mining.
Under Secretary DiNanno tells the UN Conference on Disarmament in Geneva that China conducted a 'yield-producing' nuclear test on June 22, 2020. China immediately denies the allegation.
The New START treaty between the US and Russia expires, leaving no binding limits on either country's strategic nuclear arsenal for the first time in over 50 years.
Thomas DiNanno sworn in as Under Secretary of State for Arms Control and International Security, leading US nuclear diplomacy.
US and Chinese officials meet in Washington for the first bilateral arms control discussions since the Obama administration.
Researchers identify a new tunnel under construction at China's Lop Nur nuclear test site, along with drill-rig sites used for vertical shafts.
A Kazakhstan seismic station records a magnitude 2.76 event approximately 450 miles from China's Lop Nur test site. A second event follows 12 seconds later.
The Comprehensive Nuclear-Test-Ban Treaty opens for signature. Both the US and China sign but neither ratifies, preventing the treaty from entering into force.
China conducts its 45th and final acknowledged nuclear test at Lop Nur before declaring a testing moratorium.
The United States conducts its final nuclear test, Operation Julin's 'Divider,' before entering a moratorium on nuclear weapons testing.
China's first nuclear bomb test, codenamed 'Project 596,' occurs at Lop Nur, making China the fifth nuclear-armed state.
Discussed by: NORSAR analysts, arms control researchers at James Martin Center for Nonproliferation Studies
Further seismic analysis could confirm an explosion occurred but find insufficient evidence to determine it was nuclear in origin. The event may have been a large conventional explosion, an industrial accident, or a subcritical experiment—all of which fall outside CTBT prohibitions. This would leave the dispute unresolved, with the US maintaining its accusations and China its denials, but without definitive proof either way.
Discussed by: Arms Control Association, Union of Concerned Scientists, European Leadership Network
The Trump administration could cite Chinese violations as justification for resuming US nuclear testing, invoking the 1992 Hatfield-Exon-Mitchell Amendment which allows testing if another country tests first. This scenario would end the 34-year global testing moratorium and potentially trigger a cascade of tests by Russia, China, and other nuclear powers.
Discussed by: Council on Foreign Relations, Chatham House, Bulletin of the Atomic Scientists
The allegations become a persistent irritant in US-China relations without definitive resolution. China continues expanding its nuclear arsenal while the US points to Lop Nur as evidence of bad faith. Arms control negotiations remain frozen. The dispute joins other unresolved friction points in the deteriorating bilateral relationship.
Discussed by: CTBTO officials, nuclear verification experts
The CTBTO's International Monitoring System conducts a detailed review of all available data—seismic, infrasound, and radionuclide—and reaches a conclusive determination about whether a nuclear explosion occurred. This outcome is constrained by technical limitations: the detected events were extremely small, and without radionuclide evidence, certainty may be impossible.
Defense Intelligence Agency Director Robert Ashley accused Russia of 'probably' violating its CTBT commitment by conducting low-yield nuclear tests at Novaya Zemlya. The allegations were based on classified intelligence suggesting Russia was not adhering to a zero-yield standard.
Russia rejected the allegations as 'completely unacceptable.' Independent analysts noted Russia had built a new facility at Novaya Zemlya but believed it was for subcritical testing, which is permitted.
The allegations remained unresolved and contributed to deteriorating US-Russia arms control relations. Russia subsequently suspended participation in New START in 2023.
The current China allegations follow the same pattern: US intelligence claims based on ambiguous seismic data, categorical denials from the accused, and independent experts unable to confirm or deny. The 2019 episode shows how such disputes tend to remain unresolved.
India conducted five nuclear tests on May 11 and 13, followed by Pakistan's six tests on May 28 and 30. These were the first acknowledged nuclear tests by any country since China's 1996 moratorium, catching the US intelligence community largely by surprise.
The UN Security Council passed Resolution 1172 condemning both countries. The US imposed mandatory economic sanctions, and the G-8 postponed over $1 billion in international lending to India.
Sanctions were largely lifted by 2001. Neither country signed the CTBT, and both retain their nuclear arsenals. The episode demonstrated both the limits of international pressure and the durability of the testing taboo once broken.
The 1998 tests showed that nations willing to accept international consequences can conduct tests openly. China's alleged use of 'decoupling' to hide a test would represent a different approach—attempting to maintain the appearance of compliance while secretly testing.
The US detonated a nuclear device inside a large underground salt cavern in Mississippi to test whether such 'decoupled' explosions could evade seismic detection. The experiment demonstrated that a cavern could reduce seismic signals by a factor of 70 compared to a fully contained underground test.
The experiment confirmed that decoupling was technically feasible, complicating verification efforts for any future test ban treaty.
Decoupling became a persistent concern in CTBT negotiations. Experts debate whether large-scale decoupling is practically achievable for higher-yield weapons.
US officials allege China used decoupling techniques to mask the 2020 test. The Sterling experiment established that such concealment is technically possible, but independent experts note the challenges of constructing and maintaining caverns large enough for significant yields.