The Mark 21 free-fall nuclear bomb was adopted in service in 1955. It was developed on the basis of the prototype TX-21 "Shrimp", which was tested in March 1954 (the Castle Bravo test). Despite the fact that most of the tests during the Castle series were aimed at evaluating ready-made devices, the goal of Castle Bravo was to test the design of a nuclear explosive device, which allowed a significantly reduction in the size of bombs compared to the first generation of nuclear free-fall bombs (Mk 14, Mk 17 and Mk 24).
The CASTLE BRAVO nuclear test of 1 March 1954, detonated at Bikini Atoll in the Marshall Islands, was the largest-yield American nuclear explosion. Also, it was the only American nuclear test to result in prompt harm to civilian populations.
Richard Garwin is often credited as the author of Mike, the first hydrogen bomb design, on the suggestion of Edward Teller. The United States Air Force indicated the importance of lighter weapons for delivery by the B-47 Stratojet. Los Alamos National Laboratory responded with an enriched version of the RUNT scaled down to a 3/4 scale radiation-implosion system called the SHRIMP. The proposed weight reduction would provide the Air Force with a more versatile deliverable gravity bomb.
The “Shrimp” device, a TX-21 prototype, weighed approximately 23,500 pounds and was based on the Teller-Ulam thermonuclear weapon design. The Shrimp test device was basically a scaled down version of the Runt device tested in Castle Romeo, but with partially enriched lithium as fuel. Runt I was a proof test of the Mk-17 bomb (which was deployed as the emergency capability EC-17 in a matter of months). about 7 Mt of the yield was from fast fission of the natural uranium tamper.
It used a 7075 aluminium 9.5 cm thick ballistic case, and the SHRIMP was at least in theory and in many aspects identical in geometry to the RUNT and RUNT II devices later proof-fired in Castle Romeo and Castle Yankee respectively. On paper it was a version of these devices, and its origins can be traced back to the spring. The final version tested in Castle used partially enriched lithium as its fusion fuel, natural lithium is a mixture of lithium-6 and lithium-7 isotopes. The enriched lithium used in Bravo was nominally 40% lithium-6, the fuel slugs varied in enrichment from 37 to 40% in 6Li, and the slugs with lower enrichment were positioned at the end of the fusion-fuel chamber, away from the primary.
The device had the same basic configuration as the Ivy Mike "Sausage" device, though with a different type of fusion fuel. Whereas Sausage used cryogenic liquid deuterium (which required elaborate cooling equipment), Shrimp used lithium deuteride, a fuel that is is solid at room temperature. Castle Bravo was the first American test of a deliverable hydrogen bomb.
Shrimp was a very large cylinder measuring 4.56 meters in length and 1.37 meters in diameter. Inside the cylindrical case was a fission atomic bomb (the primary stage) at one end, and a smaller cylinder of lithium deuteride fusion fuel (secondary stage). Theprimary initiated the fusion reaction according to Teller-Ulam principles of staged implosion. A cylindrical rod of plutonium (the sparkplug) ran down the center of the secondary. The sparkplug fissioned with compression and neutrons from the primary, and compressed the fusion material around it from the inside. Surrounding this assembly was a uranium tamper. The space between the tamper and the case formed a radiation channel to conduct X-rays from the primary stage to the secondary. This space was filled with plastic which turned to plasma from the X-rays, thus compressing the secondary stage externally, increasing the density and temperature of the fusion fuel to the level needed to sustain a thermonuclear reaction.
The fission sphere plus fusion cylinder was the configuration published by Howard Moreland in 1979. At that time, there were few public photographs of the exterior of hydrogen bombs [apart from the bombs of Palomares, and Dr. Strangelove], and certainly none of the interior configuration [Fat Man and Little Boy were well publicized].
The World Book Encyclopedia article written by Ralph Lapp, who Moreland noted had "published more words on the H-bomb than anybody else", was accompanied by unsigned drawings of obscure origin, which Lapp had never seen, and which was a cartoon, not a workable design. The Encyclopedia Americana article by Edward Teller, father of the H-bomb, had drawings that were basically correct depictions of Teller's miniaturized H-bomb [fission and fusion spheres], but lacking in many details.
The explosion occurred at 6:45am local time. The bomb was in a form readily adaptable for delivery by an aircraft and was thus America's first weaponized hydrogen bomb.
Due to a design error, the explosion reached a yield of 15 megatons, making it two and a half times larger than expected and more than 1,000 times as powerful as the Hiroshima bomb. The miscalculation occurred because scientists did not realize that the “dry” source of fusion fuel, lithium deuteride with 40 percent content of lithium-6 isotope, would contribute so greatly to the overall yield of the detonation. The largest-ever nuclear explosion was the 1961 Tsar Bomba with 50 megatons.
Not only did the weapon have almost three times its designed yield of 6 megatons, fallout traveled off course over a larger area than estimated. Radioactive fallout arrived within hours on several nearby populated islands, necessitating emergency evacuations of Rongerik, Rongelap, and Utirik atolls, and resulting in radiation overexposures to approximately 665 island residents. Approximately an hour and a half after the Castle Bravo test, fallout reached a Japanese fishing boat named Daigo Fukuryu Maru or “Fifth Lucky Dragon,” located 80 miles east of the test site. All 23 members of the crew, as well as their catch, were exposed to radiation. One crewmember died several months later.
Radioactive fallout from the test spread over more than 11,000 square kilometres. Traces of radioactive material were detected in Australia, India, Japan, the United States and Europe. The Bikini population had been relocated to other atolls prior to the start of the U.S. nuclear testing programme in the Pacific with the Able test in 1946. Due to the unfavourable weather conditions in which the Bravo test had been conducted, the fallout also affected the inhabited atolls of Rongelap, Utrik and others.
The length of the Mk-21 was 3.81 m, diameter - 1.42 m, weight - 6800 kg, it weighed almost half the Mk-17/24. The minimum yield was 4 Mt. Serial production of Mk-21 began in December 1955 and continued until July 1956. Three modifications were made: the Mk-21C was tested during the Operation Redwing Navajo explosion, with a yield of 4.5 Mt. Beginning in June 1957, all the Mk-21 bombs were converted into more powerful Mk-36s, which were removed from service in 1962.
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