The Soviets' Two-Stage Thermonuclear Bomb

Meanwhile, the nuclear-arms race continued for the Soviets. The Americans' "Mike" shot, called a "device," had used liquid deuterium containing tritium, weighed 65 to 70 tons and was not deliverable. News of Teller's tube-type of hydrogen bomb was revealed to the Soviets via espionage, and by 1953, the Soviets concluded that this approach was a dead end. Work by Zeldovich, Tamm, Landau and Pomeranchuk verified this conclusion. A pivotal event took place when physicists Zavenyagin and Frank Kamenetskii devised the two-stage thermonuclear bomb, with the primary stage being a fission bomb and the secondary stage being the fusion components, each in a massive casing. Since the bomb's development occurred before the Soviets knew about the principle of compression by radiation, gases emanating from the primary into the secondary stage would help the secondary achieve compression. When assembled, the bomb still retained the layer-cake design, but there was no tritium, only uranium-235 and a uranium-238 tamper.