Castle Bravo mushroom cloud
|Test series||Operation Castle|
|Test site||Bikini Atoll|
|Date||March 1, 1954|
Castle Bravo was the code name given to the first United States test of a dry fuel, hydrogen bomb, detonated on March 1, 1954, at Bikini Atoll, Marshall Islands, as the first test of Operation Castle. Castle Bravo was the most powerful nuclear device ever detonated by the United States (and just under one-third the energy of the Tsar Bomba, the most powerful device ever detonated), with a yield of 15 megatons of TNT. That yield, far exceeding the expected yield of 4 to 8 megatons (6 Mt predicted), combined with other factors, led to the most significant accidental radioactive contamination ever caused by the United States.
Fallout from the detonation—intended to be a secret test—fell on residents of Rongelap and Utirik atolls and spread around the world. The islanders were not evacuated until three days later and suffered radiation sickness. They were returned to the islands three years later but were removed again when their island was found to be unsafe. The crew of the Japanese fishing vessel Daigo Fukuryū Maru ("Lucky Dragon No. 5"), was also contaminated by fallout, killing one crew member. The blast created an international reaction about atmospheric thermonuclear testing.
The device was a very large cylinder weighing 23,500 pounds (10.7 t) and measuring 179.5 inches (4.56 m) in length and 53.9 inches (1.37 m) in diameter. It was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in the Bikini Atoll. A sizable array of diagnostic instruments was trained on it, including high-speed cameras trained through an arc of mirror towers around the shot cab.
Coordinates for Bravo Crater are. The coordinates of the remains of the Castle Bravo causeway are .
The Shrimp device in its shot cab.
|Type||Teller-Ulam design Thermonuclear weapon|
|Designer||Ben Diven-project engineer|
|Designed||24 February 1954 (GMT)|
|Manufacturer||Los Alamos National Laboratory|
|Weight||10,659 kilograms (23,499 lb)|
|Length||455.93 centimeters (179.50 in)|
|Diameter||136.90 centimeters (53.90 in)|
|Blast yield||15 megatons of TNT (63 PJ)|
The device detonated for the test was named "Shrimp" and was the same basic configuration as the experimental Ivy Mike device, except with a different type of fusion fuel. The Shrimp used lithium deuteride, which is solid at room temperature; Ivy Mike used cryogenic liquid deuterium, which required elaborate cooling equipment. Castle Bravo was the first test by the United States of a practical deliverable fusion bomb (hydrogen bomb). The successful test rendered the cryogenic design used by Ivy Mike and its derivative, the Mark 16 nuclear bomb, which was to be tested as Castle Echo, obsolete.
Inside the cylindrical case was a smaller cylinder of lithium deuteride fusion fuel (the secondary) with a fission atomic bomb (the primary) at one end, the latter employed to create the conditions needed to start the fusion reaction. Running down the center of the secondary, was a cylindrical rod of plutonium (the sparkplug), which 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 to the secondary; the space is filled with plastic which turns to plasma from the x-rays, which in turn compress the secondary (see Teller-Ulam design) externally, increasing the density and temperature of the fusion fuel to the level needed to sustain a thermonuclear reaction. (See nuclear weapon design.)
It was practically identical to the "Runt" device later detonated in Castle Romeo, but used partially enriched lithium in the fusion fuel. Natural lithium is a mixture of lithium-6 and lithium-7 isotopes (with 7.5% of the former). The enriched lithium used in Bravo was approximately 40% lithium-6. The primary was a RACER IV tritium-boosted atomic bomb, specifically designed as a thermonuclear primary.
When Bravo was detonated, it formed a fireball almost four and a half miles (roughly 7 km) across within a second. This fireball was visible on Kwajalein atoll over 250 miles (400 km) away. The explosion left a crater 6,500 feet (2,000 m) in diameter and 250 feet (76 m) in depth. The mushroom cloud reached a height of 47,000 feet (14,000 m) and a diameter of 7 miles (11 km) in about a minute; it then reached a height of 130,000 feet (40 km) and 62 miles (100 km) in diameter in less than 10 minutes and was expanding at more than 100 meters per second (360 km/h; 220 mph). As a result of the blast, the cloud contaminated more than seven thousand square miles of the surrounding Pacific Ocean including some of the surrounding small islands like Rongerik, Rongelap and Utirik.
In terms of TNT tonnage equivalence, Castle Bravo was about 1,000 times more powerful than each of the atomic bombs which were dropped on Hiroshima and Nagasaki during World War II. The largest nuclear explosion ever produced was a test conducted by the Soviet Union seven and a half years later, the 50 Mt Tsar Bomba. Castle Bravo is the fifth largest nuclear explosion in history, exceeded by the Soviet tests of Tsar Bomba at 50.6 Mt, Test 219 (24.2 Mt), and two other ~20 Mt Soviet tests in 1962 at Novaya Zemlya.
Cause of high yield
The yield of 15 megatons was three times the yield of 5 Mt predicted by its designers. The cause of the higher yield was a theoretical error made by designers of the device at Los Alamos National Laboratory. They considered only the lithium-6 isotope in the lithium deuteride secondary to be reactive; the lithium-7 isotope, accounting for 60% of the lithium content, was assumed to be inert. It was expected that lithium-6 isotope would absorb a neutron from the fissioning plutonium and emit an alpha particle and tritium in the process, of which the latter would then fuse with the deuterium and increase the yield in a predicted manner. Lithium-6 indeed reacted in this manner.
It was assumed that the lithium-7 would absorb one neutron, producing lithium-8 which decays (via beryllium-8) to a pair of alpha particles on a timescale of seconds -- vastly longer than the timescale of nuclear detonation. However, when lithium-7 is bombarded with energetic neutrons, rather than simply absorbing a neutron, it captures the neutron and decays almost instantly into an alpha particle, a tritium nucleus, and another neutron. As a result, much more tritium was produced than expected, the extra tritium fusing with deuterium and producing an extra neutron. The extra neutron produced by fusion and the extra neutron released directly by lithium-7 decay produced a much larger neutron flux. The result was greatly increased fissioning of the uranium tamper and increased yield.
This resultant extra fuel (both lithium-6 and lithium-7) contributed greatly to the fusion reactions and neutron production and in this manner greatly increased the device's explosive output. The test used lithium with a high percentage of lithium-7 only because lithium-6 was then scarce and expensive; the later Castle Union test used almost pure lithium-6. Had sufficient lithium-6 been available, the usability of the common lithium-7 might not have been discovered.
The fission reactions of the natural uranium tamper were quite dirty, producing a large amount of fallout. That, combined with the much-larger-than-expected yield and a major wind shift, produced some very serious consequences. In the de-classified film Operation Castle, task force commander Major General Percy Clarkson points to a diagram indicating that the wind shift was still in the range of "acceptable fallout", although just barely.
The decision to carry out the Bravo test under the prevailing winds was made by Dr. Alvin C. Graves, the Scientific Director of Operation Castle. Graves had total authority over detonating the weapon, above that of the military Commander of Operation Castle. Graves appears in the widely available film of the earlier 1952 test "Ivy Mike", which examines the last-minute fallout decisions. The narrator, Western actor Reed Hadley, is filmed aboard the control ship in that film, showing the final conference. Hadley points out that 20,000 people live in the potential area of the fallout. He asks the control panel scientist if the test can be aborted and is told "yes", but it would ruin all their preparations in setting up timed measuring instruments in the race against the Russians. In Mike the fallout correctly landed north of the inhabited area but, in the 1954 Bravo test, there was a lot of wind shear, and the wind that was blowing north the day before the test steadily veered towards the east. (In the 1946 Los Alamos accident Graves' personal friend, Louis Alexander Slotin, died from radiation exposure and Graves had himself received an exposure of 400 röntgens, or 3.5 grays (Gy).)
Radioactive fallout was spread eastward onto the inhabited Rongelap and Rongerik atolls, which were evacuated 48 hours after the detonation. Subsequently many Marshall Islands natives suffered from birth defects and received compensation from the U.S. federal government. A medical study, named Project 4.1, studied the effects of the fallout on the islanders.
Although the atmospheric fallout plume drifted eastward, once fallout landed in the water it was carried in several directions by ocean currents, including northwest and southwest.
A Japanese fishing boat, Daigo Fukuryu Maru (Lucky Dragon No.5), came in direct contact with the fallout, which caused many of the crew to grow ill; one died of a secondary infection. This resulted in an international incident and reignited Japanese concerns about radiation, especially as Japanese citizens were once more adversely affected by U.S. nuclear weapons. The official U.S. position had been that the growth in the strength of atomic bombs was not accompanied by an equivalent growth in radiation released. Japanese scientists who had collected data from the fishing vessel disagreed with this. Sir Joseph Rotblat, working at St Bartholomew's Hospital, London, demonstrated that the contamination caused by the fallout from the test was far greater than that stated officially. Rotblat was able to deduce that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity a thousandfold. Rotblat's paper was taken up by the media, and the outcry in Japan reached such a level that diplomatic relations became strained and the incident was even dubbed by some as "a second Hiroshima". Nevertheless, the Japanese and U.S. governments quickly reached a political settlement, with the transfer to Japan of US $15,300,000 as compensation, with the surviving victims receiving about ¥ 2 million each ($5,550 in 1954, or about $48,700 in 2014)[unreliable source?] It was also agreed that the victims would not be given Hibakusha status.
Unanticipated fallout and the radiation emitted by it also affected many of the vessels and personnel involved in the test, in some cases forcing them into bunkers for several hours. In contrast to the crew of the Lucky Dragon No. 5, who did not appreciate the hazard and therefore did not take shelter in the hold of their ship, or refrain from licking the fallout dust, the firing crew that triggered the explosion were able to safely shelter in their firing station when they noticed the wind was carrying the fallout in the unanticipated direction towards the island of Enyu on the Bikini Atoll where they were located, with the fire crew sheltering in place or "button[ing] up" until several hours had passed and the radiation levels outside had decayed to values safe enough to travel in.
Sixteen crew members of the aircraft carrier USS Bairoko received beta burns and there was an increased cancer rate among its crew. Radioactive contamination also affected many of the testing facilities built on other islands of the Bikini atoll system.
The fallout spread traces of radioactive material as far as Australia, India and Japan, and even the United States and parts of Europe. Though organized as a secret test, Castle Bravo quickly became an international incident, prompting calls for a ban on the atmospheric testing of thermonuclear devices.
In addition to the radiological accident, the unexpectedly high yield of the device severely damaged many of the permanent buildings on the control site island on the far side of the atoll. Little of the desired diagnostic data on the shot was collected; many instruments designed to transmit their data back before being destroyed by the blast were instead vaporized instantly, while most of the instruments that were expected to be recovered for data retrieval were destroyed by the blast.
The fallout also affected islanders who had previously inhabited the atoll, and who returned there some time after the tests. This was found to be due to the presence of radioactive caesium in locally grown coconut milk. Plants and trees absorb potassium as part of the normal biological process, but will also readily absorb caesium if present, being of the same group on the periodic table, and therefore very similar chemically. Islanders consuming contaminated coconut milk were found to have abnormally high concentrations of caesium in their bodies and then had to be evacuated from the atoll a second time.
The Soviet Union had previously used lithium deuteride in its Sloika design (known as the "Joe-4" in the U.S.), in 1953. It was not a true hydrogen bomb; fusion provided only 15–20 percent of its yield, most coming from boosted fission reactions. Its yield was 400 kilotons, and it could not be infinitely scaled, as with a true thermonuclear device.
The Teller-Ulam-based "Ivy Mike" device had a much greater yield of 10.4 Mt, but most of this also came from fission: 77 percent of the total came from fast fission of its natural uranium tamper.
Castle Bravo had the greatest yield of any U.S. nuclear test, 15 Mt, though again, a substantial fraction came from fission. In the Teller-Ulam design, the fission and fusion stages were kept physically separate in a reflective cavity. The radiation from the exploding fission primary brought the fuel in the fusion secondary to critical density and pressure, setting off thermonuclear (fusion) chain-reactions, which in turn set off a tertiary fissioning of the bomb's outer casing. Consequently this type of bomb is also known as a "fission-fusion-fission" device. The Soviet researchers, led by Andrei Sakharov, developed and tested their first Teller-Ulam device in 1955.
The publication of the Bravo fallout analysis was a militarily sensitive issue, with Joseph Rotblat possibly deducing the staging nature of the Castle Bravo device by studying the ratio and presence of tell-tale isotopes, namely uranium-237, present in the fallout. With this information having a history of being regarded as potentially revealing the means by which megaton yield nuclear devices achieve their yield. In the History of the Teller–Ulam design, Soviet scientist Sakharov hit upon what the Soviet Union regarded as "Sakharov's third idea" during the month after the Castle Bravo test, the final piece of the puzzle being the idea that the compression of the secondary can be accomplished by the primary's x-rays before fusion began.
The Shrimp device design later evolved into the Mark 21 nuclear bomb, of which 275 units were produced, weighing 15,000 pounds (6,800 kg) and measuring 12.5 feet (3.8 m) long and 56 inches (1.4 m) in diameter. This 4-megaton bomb was produced until July 1956. In 1957, it was converted into the Mark 36 nuclear bomb and entered into production again.
As a result of the extensive fallout, Nevil Shute wrote the novel On the Beach which was released in 1957. The novel is about a war that released so much radioactive fallout that all the life in the Northern Hemisphere disappeared, while the Southern Hemisphere awaited a similar fate. However the American government voiced a criticism of this general premise, that there was a threat of extinction from nuclear war because they did not, nor have they ever, had enough nuclear weapons to cause human extinction. Similarly the premise that all of humanity would die following a nuclear war and only the "cockroaches would survive" is critically dealt with in the book Would the Insects Inherit the Earth and Other Subjects of Concern to Those Who Worry About Nuclear War. A combination of the Castle Bravo detonation, alongside the subsequent poisoning of the Lucky Dragon 5, and the resultant flashbacks to the bombings of Hiroshima and Nagasaki all culminated in the creation of what is perhaps Japan's most widely recognized media icon, Godzilla. 
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|Wikimedia Commons has media related to Castle Bravo.|
- The short film Operation Castle Commanders Report (1954) is available for free download at the Internet Archive [more]
- The short film Military Effects Studies Operation Castle (1954) is available for free download at the Internet Archive [more]
- The short film Nuclear Test Film - Operation Castle (1954) is available for free download at the Internet Archive [more]
- Operation Castle
- US tests hydrogen bomb in Bikini (BBC News)
- First-person article about conducting the test