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.”
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 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.
Read more at A Hydrogen Bomb by Any Other Name