US conducts underground blast experiment to sharpen detection of suspected nuclear tests

The United States has conducted a non-nuclear underground experiment intended to improve its ability to detect low-yield nuclear explosions, the Department of Energy said on Wednesday. Washington explicitly linked the work to its suspicion that China carried out a covert test in 2020, an allegation Beijing denies. The National Nuclear Security Administration carried out the “subsurface, non-nuclear chemical high-explosive” experiment at the Nevada National Security Site.

The test did not involve a nuclear explosion. Officials said it generated data for validating scientific models and refining algorithms used to identify nuclear detonations. The experiment focused in part on detecting a “decoupled” explosion.

In such a scenario, a warhead is detonated inside a large underground cavity rather than directly against surrounding rock, reducing the seismic signal available to monitoring systems. The US has accused China of conducting a low-yield nuclear test at the Lop Nur site in north-western China in June 2020, potentially violating commitments under the Comprehensive Nuclear-Test-Ban Treaty. Beijing has rejected the claim and said it continues to observe the treaty’s terms.

China’s last officially acknowledged nuclear test took place at Lop Nur in July 1996. The Comprehensive Nuclear-Test-Ban Treaty has not entered into force because neither the US nor China has ratified it, although both signed it in 1996. The Comprehensive Nuclear-Test-Ban Treaty Organisation said its monitoring system did not detect an event consistent with a nuclear test on June 22, 2020, while recording two very small seismic events 12 seconds apart.

It said the available data could not establish their cause with confidence. Daryl Kimball of the Arms Control Association said the experiment could enhance US detection of illicit explosions, but argued that monitoring and deterrence also required international cooperation. Carnegie Endowment fellow Tong Zhao said improved detection could strengthen deterrence and give Washington greater insight into foreign warhead development.

The Energy Department said the exercise would improve understanding of underground blast signatures and reinforce US capabilities to identify foreign nuclear testing.