The U.S. government detonated conventional high-explosives inside P Tunnel in Area 12 of the Nevada National Security Site on Sept. 30, 2026, in an experiment designed to replicate a so-called “decoupled” nuclear detonation, according to a National Nuclear Security Administration (NNSA) press release. The test, which also used radiotracers, was conducted to help improve methods for detecting this type of nuclear testing.
The experiment comes after U.S. officials accused China of using decoupling to conceal new nuclear tests despite its stated moratorium. At the Conference on Disarmament in Geneva in February, Thomas DiNanno, Under Secretary of State for Arms Control and International Security, said China had conducted at least one such test at Lop Nur on June 22, 2020. The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) said its International Monitoring System had not detected an event matching that description at that location on that date. China has denied the accusation.
“This experiment is especially focused on further enhancing the U.S. capabilities to detect a ‘decoupled’ explosion,” the NNSA release said. “Data collected from the experiment will be used to validate scientific models and refine algorithms that are critical to U.S. nuclear explosion detection efforts.”
The release explained that in a decoupled test, a device is detonated inside a large underground chamber, reducing the explosion’s seismic signature. Researchers gathered measurements using accelerometers, seismometers, infrasound sensors, electromagnetic sensors, chemical and radiotracer samplers, and meteorological sensors.
The U.S. maintains the U.S. Atomic Energy Detection Systems (USAEDS) network, overseen by the Air Force Technical Applications Center (AFTAC), which includes more than 3,600 sensors worldwide. The network is tied into the CTBTO’s International Monitoring System. AFTAC also uses WC-135R Constant Phoenix aircraft to monitor for nuclear testing and atmospheric radiation spikes.
China is a signatory to the Comprehensive Nuclear-Test-Ban Treaty but has never ratified it. The U.S. has also signed but not ratified the treaty and has maintained a self-imposed moratorium on nuclear testing since 1992. The treaty does not ban sub-critical testing, which involves fissile material but does not produce a full nuclear reaction.
Last week, The Wall Street Journal reported that the Trump administration has been holding talks with China regarding its nuclear arsenal and the testing allegations, ahead of a state visit by Chinese President Xi Jinping to Washington.
Sources: The War Zone · Defence Blog