In a maze of tunnels 900 feet beneath the Nevada desert, US nuclear weapons scientists have since the 1990s been intermittently agitating flecks of plutonium with chemical high explosives, carefully trying to push them to the brink of a chain reaction capable of yielding nuclear force.

In a separate network of underground tunnels about 4,800 miles away, in the northern Russian archipelago of Novaya Zemlya in the Arctic Circle, Russia conducts its own such experiments, meant to model the key chemical and physical actions that occur in the run-up to a full-blown nuclear explosion, without actually causing one.

Experiments at the two sites are used by both nations to help ensure their nuclear arsenals remain viable but conducted under a blanket of secrecy. And so they’ve given rise to suspicions—and accusations—that they violate a 1996 global treaty designed to stymie nuclear weapons innovations by barring any nuclear explosions.

About This Story This article was produced in partnership with the Center for Public Integrity, a nonprofit, nonpartisan news organization.

Because the experiments are designed to closely simulate such explosions, 33 Latin American and Caribbean countries in 2016 called them violations of the “spirit and letter” of the treaty, “thereby undermining its impact as a measure of nuclear disarmament.”

Washington dismissed that claim, but on May 29, the Trump administration abruptly leveled similar accusations at Russia, when a top intelligence official vaguely accused its scientists of transgressing the test ban treaty by conducting experiments meant to be barred.

The irony of the recent charge is that it comes just as the US Energy Department’s National Nuclear Security Administration is about to step up the pace of the country’s complex and costly nuclear simulation experiments, the Center for Public Integrity has learned. The frequency will specifically be increased from an average of one every year and a half to two, and possibly to three per year, using a decade-long budgetary infusion of $1 billion meant to expand and improve the underground Nevada site.

In making the new allegation, the Trump administration did not say exactly what Russia was doing, and its statement contained a qualifier, making it fall short of a direct cheating claim. “The United States believes that Russia probably is not adhering to the nuclear testing moratorium in a manner consistent with the zero-yield standard,” said Lt. Gen. Robert Ashley, director of the Defense Intelligence Agency, during a May 29 forum on arms control at the Hudson Institute, a conservative think tank. “Our understanding of nuclear weapon development leads us to believe Russia’s testing activities would help it improve its nuclear weapon capabilities,” he added. “The United States, by contrast, has forgone such benefits by upholding a zero-yield standard.”

The Defense Intelligence Agency subsequently reasserted its claim on June 13, issuing a stronger statement in response to questions about Ashley’s remarks. It said that “the US government, including the Intelligence Community, has assessed that Russia has conducted nuclear weapons tests that have created a nuclear yield.”

The White House, the State Department, and the Pentagon have since declined to provide details or any corroborating evidence. Ashley's claim that the US, in its own experiments, has “foregone” any benefits to its nuclear arsenal appears to be contradicted by publicly available planning documents for the upgrades at the Nevada test facility, which state that new equipment and increased work will provide “vital data supporting future stockpile options.”

Pushback to Ashley’s assertion has come from multiple sources, including the Comprehensive Test Ban Treaty Organization, a Vienna-based international agency that monitors more than 300 sites around the world with dedicated seismic sensors designed to detect even small nuclear yields at sites like Novaya Zemlya.

Kirsten Gregorich Hansen, the organization’s spokeswoman, said in an email two days after Ashley’s statement that the agency has not observed anything indicating that Russia conducted a test that achieved a nuclear yield.

Ashley’s claim was also disputed by independent arms control experts, including Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies in Monterey, California, and his colleague Anne Pellegrino, a research associate there.