Russia Has Restarted Low-Yield Nuclear Tests, U.S. Believes

WASHINGTON — The Trump administration believes Russia has restarted very low-yield nuclear tests, officials said on Wednesday in a finding that could be used to renew in earnest the arms race between Moscow and Washington.

But the significance of the statements by the director of the Defense Intelligence Agency and a senior National Security Council official was immediately debated by nuclear weapons experts.

Some experts said claims of low-yield tests would be nothing new. Intelligence officials and nuclear analysts in Washington have long raised the possibility of such violations going back nearly two decades, to when Russia ratified the Comprehensive Nuclear Test Ban Treaty in 2000.

Other nuclear weapons experts have argued that significant Russian cheating on the treaty is unlikely because the designs of the country’s nuclear warheads tend to be very robust. The small returns, they have said, would make the geopolitical costs of getting caught prohibitively high.

Lt. Gen. Robert P. Ashley Jr., the director of the Pentagon’s intelligence arm, said Russia appeared to have enhanced its ability to develop new nonstrategic nuclear weapons and to manage its existing stockpile by its testing capability.

“The United States believes Russia is probably not adhering to the nuclear testing moratorium in a manner consistent with the zero-yield standard,” General Ashley said in prepared remarks at the Hudson Institute.

In a question-and-answer session afterward, he appeared to soften that statement, saying only that Russia “has the capability” to conduct a test with a low nuclear yield.

But in a later panel, Tim Morrison, a senior director at the National Security Council, said Russia had already violated the test ban treaty.

“General Ashley was clear that we believe that Russia has taken actions to improve its nuclear weapons capability that run contrary to the scope of its obligations under the treaty,” Mr. Morrison said.

The government officials said Russia had been developing a range of new tactical nuclear weapons — including low-yield arms meant to be used on conventional battlefields — that the tests could help make more reliable.

Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico and now a Stanford professor, said he was skeptical that Russia was conducting low-yield tests to create new weapons.

He said any low-yield testing by Russia at Novaya Zemlya, an Arctic Ocean archipelago where Moscow conducted nuclear tests during the Cold War, would most likely relate to experiments to enhance the safety and reliability of Russia’s nuclear arsenal — not trying to develop new kinds of warheads.

If the test yield is verified as low, Dr. Hecker said, “my answer is no, I don’t think it’s militarily significant.”

Mr. Morrison said officials were being careful in describing the tests because much of what the United States knows about Russian actions remains classified. It is also not clear how the American intelligence agencies learned about the possible Russian tests.

The kind of underground nuclear tests that North Korea, for example, has conducted in recent years are revealed by radiation-detecting planes and seismological readings. By contrast, low-yield nuclear tests would be much harder to detect, and would require other sources of intelligence to confirm.

Senator Edward J. Markey, Democrat of Massachusetts, said the Trump administration must provide detailed information about any new nuclear activities by Russia.

“If these reports constitute new activity, the administration should initiate test-site visits allowed by the treaty and hold Russia accountable for any malign behavior,” Mr. Markey said. “But we should not be pushed into shortsighted moves, like resuming nuclear testing, abandoning arms control agreements or developing more low-yield weapons.”

Nuclear tensions with Russia have been rising. The United States is set to exit the Intermediate-Range Nuclear Forces Treaty in August, a move the Trump administration took after it repeatedly accused Moscow of violating that 1987 pact by deploying new nuclear-capable missiles.

American diplomats have long maintained that the 2000 test ban treaty prohibits any experimentation that creates a nuclear yield, no matter how small.

The Senate never ratified the test ban treaty, and the United States is not bound by international law to follow its provisions. Still, the United States has not conducted a nuclear test since 1992.

Members of the Trump administration, in particular John R. Bolton, the national security adviser, have long been critics of the test ban treaty. Republicans have criticized it for failing to adequately define what constitutes a nuclear test.

Fueling the criticism of arms control treaties that bind Washington but are violated by Moscow, one official raised the possibility that the United States could resume its own low-yield tests.

“We have confidence in the existing capabilities of the nuclear arsenal,” said James H. Anderson, an assistant secretary of defense. “All that said, we keep a vigilant eye on that program and if we have to reconsider at some future point the desirability of testing, or make a recommendation thereto, we will.”

Currently, he said, the United States is testing and modernizing its nuclear arsenal only through computer modeling and simulations.

The United States and Russia have for years been unable to reach an agreement on what actions the test ban treaty prohibits, said Rebeccah L. Heinrichs, a nuclear deterrence expert at the Hudson Institute. General Ashley’s announcement that the Russians were probably testing, she said, was significant because Moscow would “have an advantage over the United States.”

“It is the final end to the Comprehensive Test Ban Treaty as a viable arms control treaty,” Ms. Heinrichs said. “The Russians see it as in their interest to improve the reliability of their warheads to leave open the option to do some degree of above zero testing.”

Others were far more skeptical.

Daryl G. Kimball, the executive director of the Arms Control Association, a private group in Washington, said an important question was whether the administration’s claims were “based on new intelligence, or whether it’s simply a new assessment by a new set of people.”

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