President Trump’s surprise announcement last month that the United States would resume its testing of nuclear arms produced a blur of condemnations and contradictory statements. Lost in the uproar, some nuclear experts say, is a forgotten type of atomic testing. It produces no mushroom clouds and poses little radioactive risk to the public. But it can reframe the recent debate.
The tiny explosions are known to nuclear weaponeers as hydronuclear tests. They appear to lie behind the Trump administration’s claims that Moscow and Beijing have violated the global treaty that bars nuclear tests, and led to President Trump’s statement that Washington should respond “on an equal basis.”
Neither the federal Department of Energy nor its subordinate unit, the National Nuclear Security Administration, replied to questions on whether the low-level tests relate to Mr. Trump’s recent declarations.
Advocates of the blasts say they can provide valuable data for helping ensure the reliability of existing weapons. But many proponents of arms control warn that the small tests would break the three-decade taboo on the explosive testing of nuclear warheads and would reignite a global arms race.
What happens during a hydronuclear test?
In a hydronuclear test, conventional explosives start a nuclear chain reaction in a very small amount of plutonium. The tiny explosion gets bottled up in a container behind superstrong walls of steel.
The force of the blast equals that of a few pounds of conventional explosives — not the kilotons and megatons of the Cold War. It’s the lowest possible rung on the explosive ladder.
The tests are known as hydronuclear, after the Greek word for water. The name derives from how conventional explosives at the start of a test squeeze atomic fuel so violently that it exhibits fluid-like behavior. All atomic bombs go through this stage. It’s the moment a lump of dense metal turns into a supercritical mass.
To cut the reaction short, hydronuclear tests use an amount of atomic fuel “far below” that needed for a nuclear detonation, Robert N. Thorn and Donald R. Westervelt wrote in a 1987 report from the Los Alamos nuclear lab in New Mexico, the birthplace of the atomic bomb.
How do hydronuclear tests differ from other kinds?
All nuclear bomb tests, regardless of size, have in common some type of criticality, the state in which atoms split apart in stable runs of chain reactions, as in nuclear reactors.
On the one hand, there are the subcritical tests, in which the chain reactions in the nuclear fuel quickly die out, producing no explosion. Experts from Los Alamos routinely conduct subcritical tests at an underground test site in Nevada as part of the United States’ ongoing evaluations of its nuclear stockpile.
These tests are permitted under the Comprehensive Nuclear Test Ban Treaty, which the United States signed in 1996 but has never ratified.
Supercritical tests mark the other extreme. Their chain reactions increase so rapidly in number that they produce nuclear detonations, as happened over Hiroshima and Nagasaki, then thousands of times at test sites during the Cold War.
Scientists think of hydronuclear tests as a forgotten middle child — reaching the supercritical stage, but snuffed out quickly before they can progress to giant detonations.
Are hydronuclear tests happening now?
President Bill Clinton prohibited the method in the 1990s during nuclear test ban negotiations. The tiny blasts were seen as producing some “yield,” or explosive force, and Mr. Clinton’s stance was that the treaty should hew to the principle of “zero-yield.”
Although the 1996 global test ban ruled out hydronuclear methods, suspicions of their covert use have grown in recent years.
In 2019, Lt. Gen. Robert P. Ashley Jr., the director of the Pentagon’s intelligence arm, said Russia “is probably not adhering to the nuclear testing moratorium in a manner consistent with the zero-yield standard.”
Recently, the charges grew louder and gained political traction. Mr. Trump said he ordered a restart of American testing because “other countries” were doing so.
John Ratcliffe, the C.I.A. director, posted on social media that Mr. Trump “was right.” He pointed to General Ashley’s 2019 statement and a 2020 article on possible secret Chinese tests.
Senator Tom Cotton, the Arkansas Republican who leads the Senate Intelligence Committee, went further. In a social media post, he said the C.I.A told him that Russia and China had conducted “super-critical nuclear weapons tests” that violated the zero-yield standard. “These tests are not historic,” Mr. Cotton emphasized.
His statement contained a likely imprecision, as Jeffrey Lewis, a nuclear expert at the James Martin Center for Nonproliferation Studies in California, pointed out on social media.
What Mr. Cotton probably meant, Mr. Lewis argued, was the forgotten class of “slightly supercritical” tests. In an interview, he said a number of officials had, in recently misstating the nature of hydronuclear testing, suggested an unfamiliarity with basic types of nuclear testing.
What makes hydronuclear tests useful?
Some nuclear experts see hydronuclear testing as helping probe a potential vulnerability of America’s nuclear arsenal, which is primarily made up of hydrogen bombs. They are far more powerful than the atom bomb that leveled Hiroshima. The core of a hydrogen bomb holds a small atomic weapon made of plutonium that works like a match to ignite the hydrogen fuel.
Washington has long worried that the aging of these matches could result in failure. The oldest are said to be pushing 50 years. Hydronuclear backers such as Robert Chrien, a scientist retired from the Los Alamos lab, argue that the low-level tests could improve assessments of whether the aging might damage or disable weapons.
“Minor modifications,” he said, could turn a subcritical type of Los Alamos experiment that stops short of being hydronuclear into “a valuable nuclear test.”
This kind of health check might have been part of the rationale when the United States conducted more than 40 hydronuclear tests in secret from 1958 to 1961 during a joint U.S.-Soviet nuclear moratorium.
The Soviet Union also tested covertly during the hiatus, according to Nicole Grajewski, a fellow at the Carnegie Endowment for International Peace. In a recent paper, she told of the detonation of dozens of hydronuclear tests, which Soviet scientists later identified as “nonexplosive chain reactions.”
Jill Hruby, a former administrator of the National Nuclear Security Administration, which runs Los Alamos and the nation’s other nuclear labs, downplayed the hydronuclear rationale. Today, Dr. Hruby said, the low-level tests could provide some information “that’s hard to learn.” But she argued that other tools could achieve the same ends.
The nation’s complex for arms maintenance and evaluation includes hundreds of devices and thousands of workers. The costly tools include room-size supercomputers, the world’s most powerful X-ray machine and a laser system the size of a sports stadium. No other nation has such an extensive array of equipment for the nonnuclear testing of nuclear arms.
Daryl G. Kimball, executive director of the Arms Control Association in Washington, said the risks of any restart of American nuclear testing, including the hydronuclear type, far outweighed any possible benefits.
Such explosions, he argued, “would set off a dangerous chain reaction” of tests by other countries that could severely damage “U.S. and international security.”
William J. Broad has reported on science at The Times since 1983. He is based in New York.
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