The Physics of Thermonuclear Devices
- Jay Sardesai
- Sep 11, 2020
- 5 min read
Updated: Sep 13, 2020
Thermonuclear devices are a class of nuclear weapons which involve nuclear fusion at some stage of the use of the bomb. Most thermonuclear weapons consist of two stages: a fission stage, using traditional materials (uranium-235 and plutonium-239), and a second fusion stage involving either deuterium and tritium, like in most nuclear fusion reactors, or lithium deuteride.
The idea of a thermonuclear weapon was initially proposed in May 1941 by Tokutaro Hagiwara, a physicist working in Kyoto, who stated that a nuclear fusion reaction could be caused by the intense pressures and temperatures generated by a fission reaction of uranium-235 atoms. While his research was not known to the Americans, Enrico Fermi made a similar observation in September to Edward Teller, leading to the creation of the hydrogen bomb project. Teller went on to work on his design of the "super" bomb, despite being employed by the government to work on the fission bomb at Los Alamos. The calculations and design issues for the "super" bomb ended up severely impacting his work on the fission bomb, and he often failed to do tasks which he had been set
Teller's original "super" bomb design was merely to place deuterium and tritium inside a fission bomb and hope that the intense pressure and number of neutrons released by the reaction triggered a fusion reaction. However, Stanislaw Ulam, a Polish physicist working at Los Alamos, calculated that a self-sustaining fusion reaction of deuterium would not be possible at the relatively low density in Teller's design. He therefore created, along with Teller, a new design for the hydrogen bomb, now called the Teller-Ulam design
The design consists of two stages, visible below. The primary stage resembles a traditional implosion fission bomb, boosted by fusion material. A high-explosive is detonated, compressing fission material in the centre with the help of a "pusher material/tamper" (the uranium-238), which may undergo fission later. The primary stage releases neutrons from both the fusion and fission reactions, and x-rays. The interstage (not shown on the diagram) attempts to moderate the heating of the second stage, as if it occurs too quickly, the secondary stage could be destroyed in a small conventional explosion, before the fusion reaction begins.
Little is known about the material that the interstage is from, as for obvious reasons, lots of information surrounding governmental nuclear weapons has been kept secret. However, due to one declassified government document, we know that the interstage is at least in part composed of a "toxic, brittle material", likely beryllium, for controlling neutron flux. Other leaked documents, such as one by Greenpeace showing a drawing of a British thermonuclear weapon, indicate additional parts to the interstage, but this document has no known origin, and as such is not a reliable source of information.
After the detonation of the primary stage, x-rays released are redirected by the reflective casing back at the polystyrene foam, which is irradiated by the massive dose. The foam becomes a plasma, exerting an immense pressure on the secondary stage (the hallmark of a radiation implosion device). This causes the plutonium sparkplug at the centre of the secondary stage to ignite, compressing the lithium deuteride fuel between the plasma and the fission reaction. The neutrons released by the primary and secondary stage bombard the lithium deuteride fuel, causing the lithium to split into tritium and alpha particles. The tritium and deuterium then fuse under the immense pressure and temperature, producing a very high yield explosion, far greater than a traditional fission bomb.
The first thermonuclear test was conducted by the US in 1951 as part of Operation Greenhouse. The weapon consisted of a small amount of deuterium and tritium, surrounded by fissile material. This weapon was merely a test of the principles developed by nuclear scientists and did not represent a true thermonuclear weapon, being more similar to a "boosted" device, where the yield was slightly amplified by the addition of fusion materials. Despite this, the test was an important proof of concept, as it proved that building a true thermonuclear device was possible, and such a weapon would be significantly more powerful than a traditional nuclear weapon.
This led to the creation of the first true thermonuclear weapon, "Mike", tested in 1952 on the Enewetak atoll in the Marshall Islands, as the nuclear test codenamed "Ivy Mike". The bomb was not a practical weapon, due to its size (it weighed 62 tonnes, and resembled a factory rather than a bomb). It was merely a test to see if the Teller-Ulam design would work at full scale. In that regard, Ivy Mike was a success, as the resulting explosion had a yield of 10.4-12 megatons(the "Little Boy" bomb dropped on Hiroshima had a yield of 15 kilotons). However, 77% of this yield was from fission of the uranium tamper, and extensive fallout was produced, contaminating the surrounding area. In the fallout, two new elements were discovered, fermium and einsteinium (elements 99 and 100 respectively).
The next hydrogen bomb to be tested by the US, the "Shrimp", as part of the test Castle Bravo, was also the most powerful nuclear weapon ever detonated by the US, with a yield of 15 megatons. Castle Bravo used lithium deuteride rather than cryogenic deuterium like Ivy Mike, making it a much more practical design. In addition, the "Shrimp" was much lighter than "Mike", weighing only 10.7 tonnes. However, the test, centred on Bikini Atoll, was much larger than expected, and vast quantities of radioactive fallout was released, causing the Rongelap and Rongerik atolls to be evacuated. In addition, one Japanese fisherman in the area was killed from the fallout. As a result of this test, stricter guidelines on nuclear testing were created.
The Soviets initially had a different design of the bomb, consisting of alternating layers of fission material and lithium deuteride mixed with tritium. Unlike the US design, this did not lead to a much greater yield than a traditional fission bomb(only 15-20% of the energy was from fusion). Eventually, with help from the spy Klaus Fuchs, they created a much more efficient bomb design, with a primary and secondary stage. They detonated their first two-stage weapon in 1955, with a yield of 1.5 megatons. In terms of proving the technology, the test was a success, but the weather patterns redirected the shockwave back to the ground, injuring 42 people, killing a soldier when a trench collapsed, and killing a child when the roof collapsed. The Soviets increased the % yield from fusion with their largest test ever in 1961, and indeed the largest nuclear test ever by any country, with the Tsar Bomba, which had a yield of 50 megatons, 97% of which was from fusion.
Today, all countries with nuclear weapons now possess thermonuclear weapons(9 in total), and thermonuclear weapons are a key part of all of these countries' arsenals. While the original design has been improved on, the Teller-Ulam configuration nevertheless remains, making it potentially one of the most impactful innovations ever.
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This was so interesting wow!
Great article!