Members of the Defense Nuclear Facilities Safety Board told the 2026 Seismological Society of America Annual Meeting that extensive faulting and associated rock displacement have been documented throughout the underground drifts that host U.S. nuclear subcritical experiments, about 1000 feet below the surface.
Although faults in the facility have been recognized since at least 1989, the Board noted in a 2024 letter to U.S. Secretary of Energy Jennifer Granholm that neither the National Nuclear Security Administration nor Nevada National Security Site personnel have evaluated whether the structures are active or capable of rupture during an earthquake.
Subcritical experiments at PULSE mate special nuclear material with high explosives in configurations designed to avoid a self sustaining chain reaction while yielding data on the reliability and performance of stockpiled nuclear weapons.
A 2024 safety review, prompted by plans to enlarge the PULSE laboratory and expand its diagnostic capabilities, concluded that existing and proposed seismic safety controls do not explicitly consider shaking and displacement from faults passing through the facility.
An uncontrolled detonation of an experimental package could release radioactive material at levels that threaten workers in the underground lab and surrounding Nevada communities, the Board warned.
"Our main concern is if there is an earthquake and it causes something to topple on top of the experimental package when outside of a container, setting it off," said Austin Powers, an engineer at the Defense Nuclear Facilities Safety Board.
"The infrastructure and overhead equipment in the experimental operations area need to be designed so they are seismically qualified to not fail," he added, arguing that any evidence that the faults are active should feed into design criteria.
In a December 2024 response, the U.S. Department of Energy told the Board that the National Nuclear Security Administration is developing plans to investigate the historic seismic activity of faults near PULSE, but the Board said no substantive action has yet been taken.
Senior scientist Yong Li and colleagues at the Board inspected the tunnels and drifts and reported fault traces, rock displacement and slickensides in many locations, indicating repeated relative motion of rock surfaces along fault planes.
Based on observed offsets that exceed the 17 foot drift height, the researchers suggest that displacement during past events could have reached 17 feet or more, though they said detailed analysis is needed to estimate potential future slip.
Li said earlier mapping campaigns established the distribution, geometry and orientation of PULSE faults and considered how they relate to regional fault systems, but those studies did not address the age of the structures or their Quaternary activity.
The U.S. Geological Survey's Quaternary Fault and Fold Database includes faults with evidence of movement in the past 1.6 million years and potential to generate magnitude 6 or larger earthquakes, yet the Department of Energy has stated that PULSE faults do not currently meet criteria for inclusion.
Displacement along active faults could damage tunnel linings and underground infrastructure directly, Li noted, while seismic shaking from nearby earthquakes could introduce additional vibrational loads that must be reflected in seismic design and qualification of experimental systems.
Powers said the annual number of subcritical experiments at PULSE has varied over time, but expansion of the underground facility is expected to enable more frequent testing, increasing the urgency of resolving outstanding questions about fault behavior beneath the lab.
Research Report:NNSS PULSE ECSE PDSAs and Seismic Faults
Related Links
Principal Underground Laboratory for Subcritical Experimentation
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