Teller–Ulam Design

The Teller–Ulam design – the details of which are military secrets and known to only a handful of major nations – is believed to be used in virtually all modern nuclear weapons that make up the arsenals of the major nuclear powers.

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The Teller–Ulam breakthrough—the details of which are still classified—was apparently the separation of the fission and fusion components of the weapons, and to use the radiation produced by the fission bomb to first compress the fusion fuel before igniting it. Some sources have suggested that Ulam initially proposed compressing the secondary through the shock waves generated by the primary and that it was Teller who then realized that the radiation from the primary would be able to accomplish the task (hence "radiation implosion"). However, compression alone would not have been enough and the other crucial idea, staging the bomb by separating the primary and secondary, seems to have been exclusively contributed by Ulam. The elegance of the design impressed many scientists, to the point that some who previously wondered if it were feasible suddenly believed it was inevitable and that it would be created by both the US and the Soviet Union. Even Oppenheimer, who was originally opposed to the project, called the idea "technically sweet." The "George" shot of Operation Greenhouse in 1951 tested the basic concept for the first time on a very small scale (and the next shot in the series, "Item," was the first boosted fission weapon), raising expectations to a near certainty that the concept would work.