Castle Bravo

Castle Bravo was the code name given to the first United States test of a dry fuel thermonuclear 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 most powerful ever detonated), with a yield of 15 megatons of TNT. That yield, far exceeding the expected yield of 4 to 8 megatons (6Mt predicted), combined with other factors, led to the most significant accidental radiological contamination ever caused by the United States.

Fallout from the detonation—intended to be a secret test—fell on residents of Rongelap and Utrik 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.

Test location
The device was a very large cylinder weighing 23,500 lb and measuring 179.5 in in length and 53.9 in 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 sizable array of diagnostic instruments was trained on it, including high-speed cameras which were trained through an arc of mirror towers around the shot cab.

Coordinates for Bravo Crater are 11.69722°N, 165.27194°W. The coordinates for remains of Castle Bravo causeway are 11.70167°N, 165.28528°W.

Bomb design
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, 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 was used to ignite the fusion reaction. 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 function of the X-rays was to compress the secondary (see Teller-Ulam design), increasing the density and temperature of the deuterium to the level needed to sustain a thermonuclear reaction, and compress the sparkplug to supercritical ignition. (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 standard RACER IV fusion-boosted atomic bomb.

Detonation
The detonation took place at 06:45 on March 1, 1954 local time (18:45 on February 28 GMT).

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 mi away. The explosion left a crater 6,500 ft in diameter and 250 ft in depth. The mushroom cloud reached a height of 47000 ft and a diameter of 7 mi in about a minute; it then reached a height of 130000 ft and 62 mi in diameter in less than 10 minutes and was expanding at more than 100 m/s. 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 obeyed this assumption.

When the lithium-7 isotope is bombarded with energetic neutrons, it captures a neutron then decays yielding an alpha particle, a tritium nucleus, and the captured neutron. This means more tritium was produced than expected, and the extra tritium is fused with deuterium. In addition to tritium formation the extra neutron released from lithium-7 decay produced a larger neutron flux. This caused more 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 more lithium-6 been available, the usability of the common lithium-7 might not have been discovered.

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 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 fire the Bravo bomb under the prevailing winds was made by Dr. Alvin C. Graves, the Scientific Director of Operation Castle. Graves had total authority over firing the weapon, above that of the military Commander of Operation Castle. Graves had himself received an exposure of 400 röntgens, or 3.5 grays (Gy), in the 1946 Los Alamos accident in which his personal friend, Louis Slotin, died from radiation exposure. 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.

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. This resulted in an international uproar and reignited Japanese concerns about radiation, especially as Japanese citizens were once more adversely affected by U.S. nuclear weapons. The official U.S. line 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 a compensation of US $15,300,000, with the surviving victims receiving about ¥ 2 million each ($5,550 in 1954, or about  $0 in 2024)  It was also agreed that the victims would not be given Hibakusha status. Unanticipated fallout and the radiation that is 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 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. 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.

As a result, Nevil Shute wrote the novel On the Beach, which alarmed the public and sparked public awareness and fear. 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. The American magazine Consumer Reports warned of the contamination of milk with the radioactive isotope strontium-90, which also alarmed the public.

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.

Weapon history
The Soviet Union had previously used lithium deuteride in their 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% 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% 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, independently developed and tested their first Teller-Ulam device in 1955.

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.