the superbomb, performed under the operations plans approved by the First Main Directorate. More effort should be put into research in that area, and a start should be made on the work on the practical design. Vannikov set out to study deuterium and its effects. Khariton also sent his reply on 5 May 1948, which urged the Soviet Union to set up a design group. At that time, very few people knew anything about hydrogen bomb design. The scientists in the United States also did not fully understand their own designs. The Soviet Union set up a group to work on the hydrogen bomb. In August 1948, Andrei Sakharov postulated the sloyka, or layer cake method, which consisted of alternating layers of uranium and thermonuclear fuel. In early 1949, this layer-cake design was tweaked, with lithium-6 deuteride as the thermonuclear fuel. In early 1950, Klaus Fuchs was arrested in the United Kingdom, and was unable to continue his espionage activity for the Soviet Union. The Soviet scientists had the idea to increase the deuterium density. Sakharov and his team saw the possibility of detonating a smaller nuclear bomb within the layer cake. This idea was successful, and the first implementation was used on the RDS-6s. The RDS-6s paved the way for the RDS-37. By 1952, the Soviet Union began to fully consider the two-stage bomb. However, in 1954, the plan was finally realized. Prior to 1954, the thermonuclear device was not thought to be initiated by radiation, but by a shock wave. On 1 November 1952, the United States tested their first "hydrogen bomb", codenamed Ivy Mike. The design was based on the Teller–Ulam layout. Ivy Mike was not a usable weapon. It was massive in size, weighing 82 tons. On 12 August 1953, the Soviets had tested their own "hydrogen bomb" in a test code named "Joe 4", which was based on the layer-cake design. By this time no one had created a "true" hydrogen bomb. All other tests had a kiloton yield. In the spring of 1954, the US tested a series of six nuclear devices, known as Operation Castle, with each experiment being in the megaton range. The first of these was Castle Bravo, which eventually turned out as the largest detonation by the United States ever. By spring of 1954, the Soviet scientists began to understand the possibility of releasing radiation from the nuclear-bomb trigger and