The rare photos are from a press tour of PULSE, an underground laboratory at the Nevada National Security Site in Frenchman Flat. Once known as the U1a Complex, it now supports research for the U.S. nuclear stockpile—but its present-day experiments are not full-scale nuclear bomb detonations. The images, credited to NNSS/NNSA, were published with NPR reporter Geoff Brumfiel’s January 29, 2025, tour account.
What the photos show—and where they were taken
PULSE stands for the Principal Underground Laboratory for Subcritical Experimentation. It lies beneath Frenchman Flat, Nevada, within the Nevada National Security Site (NNSS). NPR’s visit offered journalists unusual access to a sensitive facility; it should not be read as a comprehensive view of every room, experiment, or operation there.
Some of the striking images associated with the story show the desert landscape and blast-damaged structures at Frenchman Flat. Those structures are remnants of atmospheric nuclear testing, not views of PULSE’s underground experiment chambers. The facility is below ground, at the base of the original U1a shaft and the U1h shaft, which was built in 2004. NNSS’s PULSE history describes the complex’s development and underground layout.
Why Frenchman Flat has damaged structures
Frenchman Flat was a site of atmospheric nuclear testing decades before PULSE’s current work. NPR reports that on May 8, 1953, an Air Force bomber dropped a Mk-6D nuclear bomb from 19,000 feet in the Encore test. Its reported yield was 27 kilotons of TNT. The resulting shockwave damaged mock civilian structures, including a bridge, a bank vault, and the entrance to an underground parking garage.
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These structures help make the history of testing visible above ground, but they are not part of a modern experiment at PULSE. NNSS’s Guide to Frenchman Flat covers 14 atmospheric tests conducted there between 1951 and 1962.
How the site became an underground laboratory
The U1a borehole was excavated in the 1960s for an underground nuclear test that was later canceled. The U1a shaft was constructed in 1988. In 1990, the Ledoux nuclear test took place in a horizontal tunnel mined south from the shaft’s base, 955 feet below ground, according to NNSS’s PULSE history.
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David Funk, who helps oversee underground work at NNSS, told NPR: “This was designed to be a nuclear test location originally.” Underground testing began in the early 1960s, NPR reports, as a way to protect people and the environment from radioactive fallout. PULSE’s former name, U1a Complex, reflects that history; its current role is different.
What PULSE does today
PULSE supports the National Nuclear Security Administration’s Stockpile Stewardship Program. Its scientists conduct subcritical and physics experiments to gather technical information about the U.S. nuclear stockpile. A subcritical experiment does not produce a self-sustaining nuclear chain reaction, so it is not a nuclear explosive test.
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NNSS describes PULSE as the only place in the United States where subcritical experiments using plutonium in weapons-relevant quantities can be conducted. NPR’s tour account explains that stockpile stewardship combines large-scale computer simulations with experimental data from real-world work. The purpose of the experiments is to provide information for that effort without crossing into a nuclear chain reaction.
A publicly announced example: NIMBLE
NNSS reported that the first experiment in the NIMBLE series was conducted at PULSE in May 2024, in partnership with Lawrence Livermore National Laboratory and with support from Los Alamos National Laboratory. NNSS describes NIMBLE as a multi-year series intended to improve modeling and simulation for stockpile stewardship without nuclear explosive testing. That announcement documents one specific experiment; it does not establish what may have happened at the site after the announcement.
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Historical nuclear tests and current subcritical work compared
| Work | What happened | What it means |
|---|---|---|
| Encore, Frenchman Flat, 1953 | NPR reports an airborne Mk-6D detonation with a yield of 27 kilotons of TNT; its shockwave damaged mock structures. | A full nuclear explosive test in the atmospheric-testing era. |
| Ledoux, U1a, 1990 | NNSS says the test took place in a horizontal tunnel 955 feet below ground. | An underground nuclear test associated with the U1a shaft. |
| Subcritical experiments at PULSE | Experiments provide technical data for stockpile stewardship without a self-sustaining nuclear chain reaction. | Current research, not a full-scale nuclear bomb detonation. |
Does the U.S. still test atomic bombs at this lab?
Not in the sense suggested by a present-day bomb detonation. PULSE conducts subcritical research, which does not set off a self-sustaining nuclear chain reaction. The public sources cited here establish that the NIMBLE series was underway at PULSE in May 2024, but they do not provide a complete account of classified or later activity.
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Sources
- NPR: Geoff Brumfiel’s report from the PULSE tour, including photographs credited to NNSS/NNSA
- NNSS: announcement of the first NIMBLE experiment
- NNSS: PULSE history
- NNSS: Guide to Frenchman Flat
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