What beckoned was the idea of installing near the atom bomb a separate capsule that would hold much more thermonuclear fuel.
In 1954, on Bikini Atoll in the Pacific, the United States tried that approach. The fireball expanded for miles. The shock wave swept neighboring atolls clean of vegetation and animals. In minutes, the mushroom cloud rose some 25 miles. Slowly, its radioactivity spread around the globe.
The destructive force of that single hydrogen device turned out to be far greater than all explosives used in World War II, including the atom bombs dropped on Hiroshima and Nagasaki. The blast, code-named Bravo, was 1,000 times more powerful than the Hiroshima bomb. It was the nation’s most violent thermonuclear test ever.
But as Einstein foretold, the amount of matter that Bravo converted into energy was mind-bogglingly small — on the order of 1,500 grams, or about three pounds.
Few experts think North Korea will get close to mastering the secrets of true hydrogen bombs any time soon, if ever. But they cite a range of evidence suggesting that the isolated nation is now working hard to raise the destructive force of its nuclear arsenal with thermonuclear fire.
“It’s possible that North Korea has already boosted,” Gregory S. Jones, a scientist at the RAND Corporation, said of the first step down the thermonuclear road.
The prospect of the North making strides in missiles topped with nuclear arms that could threaten the United States has prompted the Trump administration to increase pressure on Kim Jong-un, the North’s leader.
Last month, Washington sent warships into the Sea of Japan as a deterrent to the North’s conducting a new atomic detonation. Satellite images show that preparations may be complete at Mount Mantap, the site of five previous blasts.
In South Korea, a new uncertainty is Moon Jae-in, a liberal who favors talks with the North. He recently won the race to succeed the nation’s ousted president.
Much of the technical debate over North Korea — and estimates of the global threat it poses — turn on the degree to which the nation has succeeded in miniaturizing its nuclear arms. As usual, the United States set the standard.
A hydrogen bomb derived from the Bravo test was more than 24 feet long and weighed 21 tons. That was no problem for a big aircraft. But it was way beyond the lifting capacity of any missile the military had in mind to strike distant targets.
So American experts sought to devise small, light, highly efficient hydrogen arms weighing just a few hundred pounds — not tons. Eventually, they were able to fit more than a dozen atop a single missile. In short, the size of nuclear weapons dropped significantly as their destructive power rose.
Even so, they were quite large given that the amount of matter they converted into energy was so small. Why? A main reason was that designers used massive parts to keep the exploding bomb intact as long as possible. Otherwise, the arms would tear themselves apart before much fuel got burned up.
The world’s first atom bomb, the Gadget, tested in 1945 in the New Mexican desert, had a fuel efficiency of less than 20 percent. Thereafter, over years and decades of experimentation, designers learned how to raise the burn rate. Exactly how far is a federal secret.
The North, like most countries with nuclear ambitions, has followed the American playbook. The question is how much progress it has made since its first atomic test more than a decade ago.
Two detonations last year helped clarify the picture. The first, in January, was about as powerful as the Hiroshima blast. With typical swagger, the North declared it had detonated a hydrogen bomb — a claim experts universally rejected. The explosion was far too small.
Still, emerging clues suggested the North was indeed going down the thermonuclear road — particularly in enhancing its atom bombs.
Experts found evidence that it had modified a reactor to make tritium, built a plant that could gather up the radioactive gas, and produced a thermonuclear fuel ingredient in such abundance that it was selling it online.
David Albright, a former United Nations weapons inspector and the president of the Institute for Science and International Security, a private group in Washington that tracks nuclear arms, said the findings “add credibility to North Korea’s claims that it has been developing thermonuclear or boosted-fission weapons.”
That March, another clue emerged. It was a photo of Mr. Kim and his entourage gathered around a shiny ball described as a miniaturized bomb meant to fit inside a missile warhead. Some Western analysts belittled it as the disco ball. Nonetheless, many said it appeared to be a realistic mock-up.
While propaganda photos from the North are often doctored, analysts said this one was conspicuously unretouched. The North said the shiny device was “designed for thermonuclear reaction.”
Size alone made the claim plausible. The Gadget was about five feet wide. In contrast, the disco ball was much smaller — perhaps two feet wide. That was the width of America’s first boosted atom bombs. Boosting can either raise a blast’s destructive power or lessen the need for atomic fuel, making a weapon much easier to diminish in size.
Soon after the photo emerged, American and South Korean intelligence officials concluded the isolated country had indeed finally succeeded in its efforts to shrink some of its nuclear arms.
“I think it’s pretty clear they’ve weaponized and miniaturized,” Bruce Klingner, a former head of the Central Intelligence Agency’s Korea branch, recently told a group in Washington.
The finding went to warheads for short- and medium-range missiles able to hit much of Japan and South Korea. Experts say the North still has a long way to go in perfecting warheads for its intercontinental ballistic missiles, none of which have undergone flight testing.
Last September, the North set off another blast — its fifth. By some estimates, the explosion was twice as strong as the Hiroshima bomb. That suggested its designers had used more atomic fuel, had achieved a higher rate of burning, or had engaged in thermonuclear boosting.
Mr. Albright of the Institute for Science and International Security has argued for another possibility. The North, he says, may be pursuing an intermediate stage of thermonuclear arms design known as layering.
In that step, weapon designers wrap alternating layers of thermonuclear fuel and uranium around atom bombs. That burns more hydrogen than simple boosting. When the Russians first tried that approach, Mr. Albright noted in a recent report, the test device produced a blast over 25 times stronger than the Hiroshima bomb.
All of which leads to the question of what to expect if the North decides to detonate another nuclear device — which would be its sixth.
In March, scientists at the Los Alamos weapons lab reported an expanded range of possibilities. After studying satellite images of the North’s Punggye-ri nuclear test site, they concluded that the mile-high mountain could withstand a nuclear explosion of up to roughly 20 times the Hiroshima blast. That was much larger than previous estimates.
New tests, they wrote, could in theory feature “significantly higher explosive yields,” helping North Korea advertise its possession of the world’s deadliest arms.
A month ago, 38 North, a research arm of the Johns Hopkins University School of Advanced International Studies, said the mountainous site was “primed and ready.”
Conceivably, delays in the detonation could stem from the stepped-up pressures that Beijing and Washington are trying to exert, though experts note that the North often tries to defy coercion.
Whether the nuclear test is big or small, delayed or scrapped, botched or successful, experts say the North’s program is now moving steadily beyond the rudiments of nuclear arms design, raising not only global alarms but the geopolitical stakes.
For his part, Einstein was horrified by the spread of nuclear arms and often spoke out against them. He worried that the human race had insufficient wisdom to free the primal energies.
“The unleashed power of the atom has changed everything save our modes of thinking,” he remarked, “and we thus drift toward unparalleled catastrophe.”
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