“It is not clear North Korea has that capability yet,’’ said Siegfried S. Hecker, the former director of Los Alamos National Laboratory and the nuclear weapons expert the North Koreans let in to see their uranium enrichment plants years ago, when they wanted to make clear to the Obama administration that their atomic weapons program was moving ahead, unimpeded by sanctions.
“Besides,” said Dr. Hecker, now a professor at Stanford University, “a live missile test — one loaded with an H-bomb — poses enormous risk.” He recalled that when the United States performed such tests in the early days of the Cold War, “one blew up on the launchpad and one had to be destroyed right after launch, creating significant radioactive contamination.”
The North Koreans have studied this history, too, according to current and former American intelligence officials. But the appeal of an atmospheric test is obvious: It would create a sense of fear that an explosion deep inside a tunnel in North Korea does not. The underground tests are detected on a Richter scale; an atmospheric test, like the kind the United States conducted at Bikini Atoll starting in 1948, creates a terrifying mushroom cloud.
The largest of those, a 1954 test code-named Castle Bravo, turned out to be roughly three times larger than American bomb designers anticipated. They had made a mathematical miscalculation about the power of one of the nuclear fuels contained in the weapon, and the explosion spread radioactive material across the globe. Ultimately, Castle Bravo helped fuel the call for a ban on atmospheric tests.
No one knows what kind of test the North Koreans have in mind; the country’s foreign minister, Ri Yong Ho, did not specify when he raised the possibility when talking to reporters at the United Nations on Thursday. “This could probably mean the strongest hydrogen bomb test over the Pacific Ocean,” he said. “Regarding which measures to take, I don’t really know since it is what Kim Jong-un does.”
But the presumption is that if Mr. Kim decided to go ahead, the North would attempt to conduct the test by firing it on a missile, presumably to an empty spot in the Pacific. The goal would be to demonstrate that it had solved all the technological issues involved in delivering a nuclear weapon to an American city.
But that form of testing — putting a live weapon on a missile — is particularly risky. Other countries have blanched at the potential for disaster, Dr. Hecker noted, including the Chinese, who conducted one missile launch with a live nuclear weapon in the warhead. It worked as planned, he said, but “the Chinese considered the risks unacceptable” and never tried it again. In the hands of the North Koreans, some say, it would be even riskier.
“This would be a regional nightmare” for East Asia, said Heather Conley, a former senior State Department official, now at the Center for Strategic and International Studies in Washington.
It is possible the threat will never come to fruition. Detonating a weapon inside a missile warhead, or even from a ship or barge, would be far more difficult for the North than setting one off inside a mountain, where engineers have months to wire up the weapon, and no time pressure.
It would require what experts call a “weaponized device” that could survive shocks, stresses and, if launched from a missile, the heat of re-entry into the atmosphere, something North Korea has never demonstrated it can handle.
“The DPRK would be taking a big risk — missile tests fail,” said Philip E. Coyle III, a nuclear scientist and former head of the Pentagon’s weapons testing. The live nuclear warhead could come down on a neighboring country, or if the missile blew up on the launchpad — as has been known to happen — set off the nuclear warhead in North Korea.
The transportation risks would be enormous, including the chance of an accidental detonation before the nuclear device reached the target zone. And while the world’s best missiles fail roughly once in every 100 flights, the failure rate for the North’s missiles is much higher. Last year, one type of missile failed seven out of eight times, perhaps in part because it had been targeted by a series of cyber attacks ordered by President Barack Obama. Since then, the North has ceased testing that type of missile and been more successful with others.
And even if one of the North’s missiles succeeded in lofting a nuclear weapon, the bigger challenge would be bringing it back down during the fiery re-entry. The heat, pressures and forces of deceleration are enormous. To date, evidence from the North’s test launches suggests it is still in the beginning stages of learning how to build a survivable warhead.
It would be far easier for the North to entrust a nuclear weapon to a plane or a boat. But it has few with the long-range capability for the job, and the chances that the United States or its allies would detect it in transit are considerable.
It would also break a taboo. It has been 37 years since any nation tested a nuclear weapon in the planet’s atmosphere. And given what is now known about the effects that radioactive fallout from such tests has on human health and the environment, one now would only intensify the international opprobrium Mr. Kim already faces.
According to one estimate by a physicians group opposed to nuclear weapons, 2.4 million people could die from cancer caused by the radioactivity from the more than 2,000 known tests that have already taken place.
The last atmospheric test took place on Oct. 16, 1980, when China fired what experts believed to be a nuclear-tipped ballistic missiles into a desert salt flat along what was once the Silk Road, more than 1,300 miles west of Beijing.
The United States attempted a missile-launched nuclear test so only once — on May 6, 1962 — during a frenzy of Cold War tests. A submerged submarine, the Ethan Allen, fired a Polaris A-2 missile in the direction of Christmas Island in the Indian Ocean. After traveling more than 1,200 miles, its warhead exploded at an altitude exceeding 10,000 feet.
That test helped spur negotiations that ultimately led to a treaty banning tests in the atmosphere, outer space or underwater. It was in signed in 1963 by the United States, the Soviet Union and Britain.
In 1996, a far broader agreement to ban all nuclear testing, the Comprehensive Test Ban Treaty, was adopted at the United Nations and has been ratified by 166 states. The United States, China and North Korea are among the holdouts, along with Egypt, India, Israel, Iran and Pakistan.
An effort by the Clinton administration to ratify the treaty failed; Mr. Obama promised to resubmit it for ratification but never did, fearing a second defeat. The United States and China have adhered to its restrictions, even if neither has ratified it.