U.S. Conducts 34th Subcritical Nuclear Experiment

June 2024
By Daryl G. Kimball

The U.S. National Nuclear Security Administration (NNSA) announced that it had successfully carried out a subcritical experiment on May 14 at an underground facility at the Nevada National Security Site.

Officials at the U.S. National Nuclear Security Site answer questions from nongovernmental experts about the operation of the Cygnus subcritical experiment machine in the site’s “zero room” in November. (Photo U.S. National Nuclear Security Administration)

In a May 16 statement, the NNSA said the experiment, its 34th at the former U.S. nuclear weapons testing site, provided “valuable information to…improve our modeling and simulation capability, part of the science-based [S]tockpile [S]tewardship [P]rogram.”

The agency said “the experiment performed as predicted.” It “did not form a self-sustaining, supercritical nuclear chain reaction” and therefore was “consistent with the zero-yield standard” of the Comprehensive Test Ban Treaty (CTBT).

The United States, China, and Russia continue to engage in weapons-related activities at their former nuclear testing sites. Although the treaty’s International Monitoring System is operational and more effective than originally envisioned, very low-yield nuclear test explosions such as these subcritical experiments still can be difficult to detect without on-site inspections, which will not be in place until after the treaty’s entry into force.

To address concerns about activities at former nuclear test sites, NNSA Administrator Jill Hruby announced in June 2023 that her agency is “open to working with others to develop a regime that would allow reciprocal observation with radiation detection equipment at each other’s subcritical experiments to allow confirmation that the experiment was consistent with the CTBT.”

In November 2023, Corey Hinderstein, now NNSA acting principal deputy administrator, and Marvin Adams, NNSA deputy administrator for defense programs, told Arms Control Today that the agency has been investigating technical approaches to potential confidence-building measures that could be applied to subcritical experiments without revealing classified information. Adams suggested that the most reliable strategy for independent verification of the absence of a nuclear explosion would involve measuring for the absence of a self-sustained chain reaction. That would be indicated by a very rapid drop-off in the production of neutrons and gamma rays from the experiment, he said.

To date, no technical discussions on confidence-building measures relating to subcritical experiments have been held among U.S., Chinese, and Russian officials.