expected. During this time the United States were focused on the Alarm Clock, while the Soviet Union were focusing on the Sloyka method. The alarm-clock dilemma lasted until 1951, when Ulam came up with the idea of compressing a thermonuclear secondary with the hydrodynamic shock produced by a primary fission bomb. Teller agreed with this method and even altered it by using the pressure from the radiation from the primary, rather than hydrodynamic shock. After Teller had finally accepted this method, the question remained. Which thermonuclear fuel would be involved. The three main choices were lithium deuteride, deuterated ammonia and liquid deuterium. "Each had its advantages and disadvantages, lithium deuteride would be the simplest material to engineer because it was solid at room temperature, but breeding tritium within the bomb from lithium required a complex chain of thermonuclear reactions that involved only one of lithium’s several isotopes." Deuterated ammonia could be kept in the liquid phase with moderate cooling or under mild pressure, but its physical properties were not well known at that point. The problem with liquid deuterium was the technology to transfer and store it in bulk quantities was not yet developed. The United States decided to choose liquid deuterium as their thermonuclear fuel. This was the premise behind the Ivy Mike bomb. The detonation of Ivy Mike by the United States prompted Soviet retaliation, and the Soviets quickly attempted to catch up. Although the Soviet Union had detonated their RDS-6 around that same time, the RDS-6 was initiated by high-powered explosives, while Ivy Mike was initiated by radiation method. The Soviets then abandoned their layered-cake method and focused on a two-stage bomb method. The hydrogen bomb primarily has 2 units: a nuclear bomb, which was the primary unit, and a secondary energy unit. The first stage of the hydrogen bomb resembled the layer-cake design, except the main difference is that the initiation is carried out by a nuclear device, rather than a conventional explosive. This design was initially postulated by Enrico Fermi and Edward Teller in 1941. Teller insisted that they should ignite deuterium by some fission weapon. The hydrogen bomb was a challenge, and would be more powerful and destructive than the nuclear bomb. The fusion cell itself was not very powerful, coming out to about 17.6 MeV per reaction, but the quantity of hydrogen fuel can be scaled up to make the weapon as large