At ten in the morning on Tuesday, Pyongyang time, a mountain in northeastern North Korea shuddered. Seismographs in nearby countries picked up the telltale signs of moving earth. These signals, and their source, suggested to many observers that the tremor was a non-natural event. Not long afterward, the North Korean government announced that it had not only tested a nuclear weapon, as was already suspected, but that it was the country’s first “H-bomb test,” and that it had been “successfully conducted.” The skepticism from Western experts came swiftly. The power of the explosion seemed on par with the largest of North Korea’s previous tests—the equivalent of around ten kilotons of TNT. But hydrogen bombs are typically measured in the hundreds or thousands of kilotons. Was this a bluff, an exaggeration, or something else?
This isn’t the first time that experts have sparred over what is, and what isn’t, an H-bomb. In a long, dull official speech about the budget of the Soviet Union, given in early August of 1953, Premier Georgy Malenkov announced to the world that the “the U.S. has no monopoly in the production of the hydrogen bomb.” His claim was greeted with a flurry of coverage in American newspapers, and with some incredulity among American politicians and nuclear experts. Edwin Johnson, a senator from Colorado, called it “manufactured propaganda.” He added: “I would take it with a grain of salt.” But then, three days later, the Soviets set off something big. Pravda, the official Communist Party newspaper, crowed that a “mighty thermonuclear reaction” had taken place. Perhaps the Soviets had the H-bomb after all.
U.S. Air Force planes had been sniffing around the borders of the U.S.S.R. since late 1948, looking for the residues of nuclear detonations. The dust that is left behind after an explosion can reveal a good deal about how a bomb works, with different radioactive isotopes signalling different processes. For instance, it is possible to discern whether a nuclear reaction consisted of fission (the splitting of heavy atoms) or also of fusion (the merging of light atoms), whether the fissile material was uranium or plutonium, and even, if both sorts of reaction took place, whether they began physically near or apart from each other. The radioactive remains of the Soviet test, which the C.I.A. had dubbed Joe 4, were duly picked up and sent to various secret laboratories. The raw data was then passed on to a panel of A-list nuclear physicists, headed by the future Nobel Prize winner Hans Bethe, which was tasked with interpreting it. In their classified report, which was finished that September, Bethe’s scientists took issue with the characterization of the Soviet weapon as a hydrogen bomb. The device had, in truth, produced “a substantial thermonuclear reaction,” and its explosive power—equal to four hundred thousand tons of TNT—was “certainly enough to cause concern.” But it wasn’t an H-bomb, at least as the panel construed the term.
What was it, then? To answer that question requires going back a little further, to the American weapons program of the nineteen-forties. The initial idea for the H-bomb was vague. Before the attacks on Hiroshima and Nagasaki had taken place, before the United States had even built a working atomic weapon, Enrico Fermi suggested to Edward Teller, his colleague at the government’s laboratory in Los Alamos, New Mexico, that it might be possible to use a fission reaction to jump-start fusion. That idea—elements from one end of the periodic table (plutonium and uranium) manipulating an element at the other end (hydrogen)—remained a constant feature of the weapon. But Teller and his colleagues had trouble making it work in practice. Between the end of the Second World War and 1951, they developed four candidates for what might be called a hydrogen bomb. Only one, in the end, became the definitive design.
The first concept was known as the Super, eventually differentiated as the Non-Equilibrium or Classical Super. The idea was to take a very large fission bomb and attach a tube of hydrogen isotopes (deuterium and tritium) to it. The heat of the initial explosion would start a reaction at one end of the tube that would continue down its length. If the design could be made to work, the power of the weapon would depend only on how long the tube was—theoretically limitless, although dropping such a thing from an airplane might prove cumbersome. The problem, though, was that the fusion reaction lost energy too quickly and stopped. As a result, the Classical Super was shelved, in 1950.
The second concept was given the code name Alarm Clock. It was a weapon with many spherical layers, one inside the other, like an onion. One layer might consist of fissile material, such as enriched uranium, and the next might be fusion fuel, usually in the form of lithium deuteride. The next layer was more enriched uranium, and so on. The contraption was surrounded by high explosives, which, when detonated, would squeeze the entire assembly, suddenly increasing its density. This would start a fission reaction in the various uranium layers, and they might further squeeze the lithium layers, causing some nuclear fusion to take place. The design resulted in a weapon whose power could be measured in the hundreds of kilotons, although most of that energy would be from fission reactions. It was effective, up to a point, but it made for a heavy bomb.
The third weapon was called the Booster. It was a standard plutonium bomb with a hollow middle. At just the right moment, a mixture of deuterium and tritium gas would be injected into the collapsing core. This would create a very small number of fusion reactions, and the neutrons from these reactions would hit the plutonium. In essence, the Booster used fusion to increase the efficiency of the fission reaction. The idea worked, and remains a component of modern American nuclear weapons, but it couldn’t achieve high yields by itself.
The fourth and final candidate was the Equilibrium Super, known today as the Teller-Ulam design, after the men who conceived it, in early 1951. The basic idea is, as far as we know, as follows. Take a fission weapon—call it the primary. Take a capsule of fusionable material, cover it with depleted uranium, and call it the secondary. Take both the primary and the secondary and put them inside a radiation case—a box made of very heavy materials. When the primary detonates, radiation flows out of it, filling the case with X rays. This process, which is known as radiation implosion, will, through one mechanism or another—and while there are reams of Internet speculation as to how this works, the details are still classified—compress the secondary to very high densities, inaugurating fusion reactions on a large scale. These fusion reactions will, in turn, let off neutrons of such a high energy that they can make the normally inert depleted uranium of the secondary’s casing undergo fission. So there are really three stages of nuclear reaction in such a bomb: the primary (fission), the secondary’s internal fuel (fusion), and the secondary’s casing (more fission).
The Teller-Ulam design is the principle behind every weapon currently in the U.S. stockpile, and it is what people tend to refer to as the true hydrogen bomb. In many respects, it is superior to the other working candidates. For one thing, it’s much more flexible. If weight is no object, its yield is potentially huge. But the same design can also be used to pack a few hundred kilotons of explosive power into a warhead the size of a trash can—the sort that fits comfortably on a rocket or cruise missile. If you ditch the depleted uranium and replace it with lead, you get a bomb that results in comparatively little radioactive fallout, which is primarily the by-product of fission reactions. Make the radiation case thinner and you get a so-called neutron bomb, which releases more of its energy as radiation than blast. From the perspective of a weapons designer, then, Teller-Ulam offers a path to further innovation, whereas the other ideas are one-offs.
So what was the Soviet bomb of 1953? The Bethe panel concluded that it was what the Americans called the Alarm Clock, a device made up of layers. (The Soviets gave it a more descriptive name: Sloika, after a layered pastry.) There were fusion reactions, but they accounted for only as much as twenty per cent of the total explosive power of the bomb. Bethe would later deride it as nothing more than “a big boosted fission weapon.” In other words, not truly an H-bomb.
This might seem like hair-splitting, but it had serious Cold War political stakes. In 1949 and 1950, U.S. military scientists, including Bethe and J. Robert Oppenheimer, the wartime director of Los Alamos, had debated whether a hydrogen bomb should be built at all. Oppenheimer, in particular, put his career and reputation on the line as a primary antagonist of the H-bomb, claiming that it was not only technically unsound but also immoral. Oppenheimer’s opponents argued that he was slowing work to the point that the Soviets, who had tested their first fission weapon in 1949, might beat the United States to the punch. Details of the classified argument eventually leaked to the newspapers, and President Truman was forced to weigh in. On January 31, 1950, he announced that development of “the so-called hydrogen or superbomb” would continue. In November of 1952, a technically conservative version of the Teller-Ulam design was tested on Enewetak Atoll, in the Marshall Islands. The device was potent, with a yield equivalent to more than ten million tons of TNT, but it wasn’t a bomb in the practical sense: clocking in at eighty tons, it couldn’t be dropped from a plane. (Unlike the Sloika, which could.) The Americans managed a weaponized version of the Teller-Ulam bomb in 1954; the Soviets built their own the following year.
If what the Soviets had detonated in 1953 were considered an H-bomb, then those in the United States who had argued against Oppenheimer, warning of an ascendant Russia, would be vindicated. At worst, if what mattered was being able to use the bomb in combat, the Soviets had beaten the Americans by a year. But if the Sloika wasn’t a hydrogen bomb, and if the fact that it could be dropped from a plane didn’t matter, then the U.S. had won by a comfortable margin of three years. Bethe and others hoped that framing the issue this way might take some of the acid out of the attacks on Oppenheimer’s credibility. (In the end, Oppenheimer’s enemies still prevailed, but on other grounds.)
Which brings us back to North Korea. Did they detonate a hydrogen bomb? The answer may depend on how we define the term, in all its political messiness. Perhaps they set off a failed Teller-Ulam, Sloika, or Booster bomb. Perhaps they performed a very small-scale test of thermonuclear principles. Perhaps they have developed a low-yield Teller-Ulam bomb. (It’s not impossible, but it’s not easy.) Or perhaps this was, after all, more “manufactured propaganda” on the part of the Kim regime. If enough radioactive residues seep out of the underground test area, physicists might eventually be able to give us the answer. Either way, the contention is unlikely to dissipate entirely—unless North Korea decides to answer its critics by setting off something much bigger.
