Everything about Castle Bravo totally explained
Castle Bravo was the code name given to the first U.S. test of a so-called dry fuel
thermonuclear (fusion- rather than fission-based) device, detonated on
March 1,
1954 at
Bikini Atoll,
Marshall Islands, by the
United States, as the first test of
Operation Castle (a longer series of tests of various devices). Fallout from the detonation—intended to be a secret test—poisoned the crew of
Daigo Fukuryū Maru ("Lucky Dragon No. 5"), a Japanese fishing boat, and created international concern about atmospheric thermonuclear testing.
The bomb used
lithium deuteride fuel for the
fusion stage, unlike the
cryogenic liquid
deuterium used as fuel for the fusion stage of the U.S. first-generation
Ivy Mike device, which, being the size of a small office building, was an impracticable weapon for use at war. The bomb tested at Castle Bravo was the first practical deliverable
fusion bomb in the U.S. arsenal.
The
Soviet Union had previously used lithium deuteride in a
nuclear bomb, their
Sloika (also known as Alarm Clock) design, but since it relied on using the initial fission explosion to compress, inertially confine, and ignite the fusion fuel, its
yield was limited (400 kt) in comparison to the
Teller-Ulam-based Ivy Mike (10.4
Mt) and Castle Bravo (~15 Mt). Mike and Bravo both used the
Teller-Ulam design, which featured separation of the fusion device from the fission device, and used radiation pressure (or probably radiation-induced ablation of the heavy tamper surrounding the fusion device) to produce staged-radiation implosion and fusion ignition of a much greater magnitude. After a few years, the Soviets, led by
Andrei Sakharov, independently
developed and
tested their first
Teller-Ulam device in 1956.
Castle Bravo was the most powerful
nuclear device ever detonated by the United States, with a yield of 15
megatons. That yield, far exceeding the expected yield of 4 to 6 megatons, combined with other factors to produce the worst
radiological accident ever caused by the United States.
In terms of TNT tonnage equivalence, Castle Bravo was about 1,000 times more powerful (4-8 times larger, on a logarithmic scale) than 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 several years later, the ~50 Mt
Tsar Bomba.
Design and detonation
The device detonated for the test was named "Shrimp" and was the same basic configuration as the Ivy Mike device, except with a different kind of fusion fuel. This device also implemented a light case design, using
aluminum instead of the heavy
steel case used in Mike.
Inside a cylindrical case was a smaller cylinder of lithium deuteride fusion fuel (the secondary) with a regular
fission atomic bomb (the primary) at one end; the latter was used 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 was used to ignite the fusion reaction. Surrounding this assembly was a natural
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 function of the X-rays was to compress the secondary (by various means; see
Teller-Ulam design), increasing the density and temperature of the deuterium to the levels needed to sustain the thermonuclear reaction, and compressing the sparkplug to
supercriticality ignition.
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 standard
RACER IV fusion-boosted atomic bomb.
The device was a very large cylinder weighing 23,500
pounds (10.7
tonnes) and measuring 179.5
inches (4.56
m) in length and 53.9 inches (1.37 m) in width. It was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in the
Bikini Atoll. A sizeable array of diagnostic instruments were trained on it, including a number of high-speed cameras which were trained through an arc of mirror towers around the shot cab.
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 the
Kwajalein atoll over 250 miles (450 km) away. The explosion left a crater of 6,500 feet (2,000 m) in diameter and 250 feet (75 m) in depth. The
mushroom cloud reached a height of 47,000 feet (14 km) 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 6 kilometers (4 miles) per minute.
Coordinates for Bravo Crater are . The coordinates for remains of Castle Bravo causeway are .
The detonation took place at 06:45 on March 1 local time (18:45 on
February 28 GMT).
Cause of high yield
The yield of 15 megatons was two and a half times what was expected. The cause of the high yield was a laboratory error made by designers of the device at
Los Alamos National Laboratory.
It was expected that lithium-6 isotope would absorb a
neutron from the fissioning plutonium, emit an
alpha particle and
tritium in the process, of which the latter would then fuse with
deuterium (which was already present in the LiD) and increase the yield in a predicted manner.
The designers missed the fact that when the lithium-7 isotope (which was considered basically inert) is bombarded with high-energy neutrons, it absorbs a neutron then decomposes to form an alpha particle, another neutron, and a tritium
nucleus. This means that much more tritium was produced than expected, and the extra tritium in fusion with deuterium (as well as the extra neutron from lithium-7 decomposition) produced many more neutrons than expected and induced more fission of the uranium tamper, increasing 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 yield. The test used lithium with a high percentage of lithium-7 only because lithium-6 was (at the time) scarce and expensive; the later
Castle Union test used almost pure lithium-6. Had more lithium-6 been available, the usability of the common lithium-7 might not have been discovered.
Of the total 15-megaton yield, 10 megatons were from fission of the natural uranium tamper.
Fallout incident
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 a number of very serious consequences. In the de-classified film "Operation Castle", task force commander General Clarkson points to a diagram indicating that the wind shift was still in the range of "acceptable fallout", although just barely.
The decision to fire the Bravo bomb under the prevailing winds was made by Dr
Alvin C. Graves (1912-66), the Scientific Director of Operation Castle. Dr Graves had total authority over firing the weapon, above that of the military Commander of Operation Castle. Dr Graves had himself received an exposure of 400
Roentgens in the 1946 Los Alamos accident in which his personal friend, Dr
Louis Slotin, died from radiation exposure. Dr Graves appears in the widely available film of the earlier 1952 test Mike, which examines the last minute fallout decisions. The narrator (Western actor
Reed Hadley) is filmed aboard the control ship in that film which shows 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 which was blowing north the day before the test steadily veered towards the east.
Radioactive fallout was spread eastward onto the inhabited
Rongelap and
Rongerik atolls, which were soon evacuated. Many of the Marshall Islands natives have since suffered from
birth defects and have received some compensation from the U.S. Federal government. See
Project 4.1 for controversy surrounding this exposure.
A
Japanese fishing boat,
Lucky Dragon No. 5, also came into contact with the fallout which caused many of the crew to grow ill; one eventually died. This resulted in an international uproar and reignited Japanese concerns about radiation, especially in regard to the possibility of contaminated fish. The official U.S. line had been that the growth in the strength of atomic bombs wasn't 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 which gave the fishery a compensation of 2 million dollars with the surviving victims receiving between 1.91 million yen and 2.29 million yen each. It was also agreed that the victims wouldn't be given
Hibakusha status.
Unanticipated fallout and radiation also affected many of the vessels and personnel involved in the test, in some cases trapping them in bunkers. One prominent scientist later recalled that he was on a ship 30 miles away, and received 10
Röntgen of radiation as a result. Sixteen crew members of the aircraft carrier
USS Bairoko received
beta burns and there was a greatly increased cancer rate. 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 US 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. Very 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.
Later devices
The Shrimp device design later evolved into the
Mk-21 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 Mk-36 and entered into production again.
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