RDS-37 (, CIA: JOE 19) was the Soviet Union's first two-stage thermonuclear weapon test, dropped from a Tupolev Tu-16 bomber over Semipalatinsk Test Site on 22 November 1955. The yield was 1.6 megatons, scaled down from a 3 megaton design. It was the world's first air-dropped multi-stage thermonuclear test; prior US thermonuclear tests Ivy Mike (1952) and in Operation Castle (1954) were ground-based experiments.
The Soviet nuclear weapons project had previously tested thermonuclear reactions RDS-6s in 1953. This was a 400-kiloton single-stage layer cake design (Russian: 'sloika') achieved using layers of lithium deuteride-tritide fusion fuel and natural uranium surrounding a highly enriched uranium core. Scientists expanded sloika designs, testing the tritium-free RDS-27 at 250 kilotons, and outlining a 1.8 megaton 'RDS-6sd', but the single-stage design proved very difficult to scale.
Physicists Andrei Sakharov and Yakov Zeldovich then independently invented the radiation implosion design, using the X-rays from a fission-based nuclear primary to compress a fusion-fuelled secondary, specifically a spherical sloika. Dubbed "Sakharov's Third Idea", this was equivalent to the US Teller-Ulam design.
Leading to the RDS-37
The RDS-37 was a reaction to the efforts of the United States. Previously, the Soviet Union allegedly used many of their spies in the U.S. to help them generate methods and ideas for the nuclear bomb. The creation of the hydrogen bomb required less usage of this method, although they still received help from some spies, most importantly, Klaus Fuchs.
In 1945, the Soviet Union reached a decision to work on a design for a "super bomb". Also in 1945, Enrico Fermi gave lectures at Los Alamos discussing the fusion process. At the end of his lecture he stated "so far all schemes for the initiation of the super [are] rather vague".
In the spring of 1946, Edward Teller set up a conference to assess all the information known about the hydrogen bomb. Klaus Fuchs attended this same conference. In the same year, Teller postulated a new design for the hydrogen bomb, which he called the "Alarm Clock", which he suggested would use lithium-6 deuteride instead of pure deuterium.
Klaus Fuchs had passed on information about both the nuclear bomb and the hydrogen bomb to the Soviet Union. This information resulted in the recruitment of Igor Tamm’s group, whose work helped create the hydrogen bomb. The content that Fuchs provided in 1948 was not only related to the hydrogen bomb, but also the nuclear industry as a whole. It provided detailed insight into the bomb design using a two-stage igniting block.
The designs were quickly sent to Lavrentiy Beria, who had been placed in charge of the Russian bomb program by Joseph Stalin and forwarded to Igor Kurchatov, Boris Vannikov and Yulii Khariton, to validate and assess these designs. On May 5, 1948, Vannikov and Kurchatov wrote a reply, stating: As regards the material No. 713a, the basic ideas about the role of tritium in the transfer of explosion from a uranium-235 primer to deuterium, about the necessity of careful selection of uranium primer power, and about the role of particles and photons in the transfer of the explosion to deuterium are new. These materials are valuable in that they will be helpful to Cde. Zel’dovich in his work on 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 using it to initiate the fusion part of the bomb. This idea parallels the Teller–Ulam design used in the Mike detonation. They subsequently abandoned the single-stage layer-cake and tube designs, and focused entirely on the two-stage bomb project. A report on the activity of the theoretical sector No. 1 published in 1954 states: Atomic compression is being investigated theoretically in collaboration with members of sector No. 2. The main problems associated with atomic compression are in the developmental stage. Emission of radiation from the atomic bomb used to compress the main body. Calculations show that for radiation is emitted very strongly. Conversion of radiant energy into mechanical energy to compress the main body. These principles have been developed through the efforts of Sectors No. 2. and No. 1.
On 22 November 1955, the Russians tested their first true two-stage hydrogen bomb in the megaton range, the RDS-37. This test implemented the two-stage radiation implosion. This was also the world's first air-dropped fusion-bomb test.
Foundations of RDS-37 After the Bravo Test in March 1954, Soviet scientists started to search for ways to make an effective large-yield thermonuclear bomb. After a lot of intensive research of past experience with these bombs, a new two-stage bomb was devised.
The RDS-37's thermonuclear charges are founded in fundamental scientific concepts of high-energy-density physics. The principle of radiation implosion assumes three concepts. According to Ilkaev, they are: "the predominant proportion of the energy of the explosion of the nuclear charge (the primary module) is generated in the form of X-ray radiation; the energy of the X-ray radiation is transported to the fusion module; the implosion of the fusion module using the energy of the 'delivered' X-ray radiation". Hopes for a better compression of nuclear material that could be initiated had been in discussion since the early 1950s.
Not long after, Yakov Zeldovich and Andrei Sakharov started to work on this theory. "In January 1954, Ya. B. Zeldovich and A. D. Sakharov considered in detail a device layout which incorporated the principle of a two-stage nuclear charge".
Many people questioned whether they could be successful from the very beginning. Questions concerning the two-stage nuclear charge fell into two categories.
The first set of questions concerned nuclear implosion. The first module, or fission trigger, initiated "by compression of nuclear material or fission and fusion of materials by spherical explosion of chemical explosives, in which the spherical symmetry of the implosion was dictated by the initial spherically symmetric detonation of the explosive".
There seemed to be no way in which "a heterogeneous structure composed of a primary source (or sources) and a compressible secondary module" could "maintain the spherically symmetric 'nuclear implosion".
The following is a report by Sakharov and Romanov on 6 August, with the title "Atomic Compression". "Atomic Compression is being investigated theoretically in collaboration with members of sector No. 2. The main problems associated with atomic compression are in the development stage.
(1) Emission of radiation from the atomic bomb used to compress main body. Calculations show that for [deleted] radiation is emitted very strongly...
(2) Conversion of radiant energy into mechanical energy to compress the main body. It is postulated [deleted]. These principles have been developed through the team efforts of Sectors No. 2 and No. 1 (Ya. B. Zel’dovich, Yu. A. Trutnev, and A. D. Sakharov)...".
This problem with a two-stage nuclear charge brings about two other problems. One, "what is now the carrier of the explosive energy of the original source?". Two, "how is this energy transported to the secondary module?".
The second set of questions concerns the secondary module impacted by the nuclear implosion of the fission trigger. At first, scientist thought that the energy of a nuclear initiation of the fission trigger in a two-stage charge would be transported by the flow of the products of the initiation as the shock wave spread through the heterogeneous structure of the secondary module. Zeldovich and Sakharov "decided to choose an analog of the inner element of the RDS-6s charge for the basic physical element of the secondary module, i.e. the 'layered' spherical configuration of the system".
Factors behind the design The Soviet Union was able to form some similar achievements to the United States without the help of outside information. "The active material, instead of being a solid sphere to begin with, as in the Nagasaki bomb, would be fabricated as a shell, with a 'levitated' sphere in its center. Part of the expensive plutonium was replaced with less expensive uranium-235. Levitation increased the energy yield and made it possible to reduce the size and weight of the explosive. Similar achievements were achieved without espionage by the Soviet Laboratories." The initial alarm-clock method derived by Teller was assessed by Stanislaw Ulam, who decided that it would be more difficult and costlier than 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.