Cold War

Castle Bravo – The US’ Most Powerful Nuke

Just 9 years after the United States detonated the first atomic bomb came Castle Bravo, a thermonuclear device more than 700 times more powerful. Releasing the equivalent explosive power of 15 million tons of TNT, Castle Bravo accidentally produced far more energy than was expected, destroying much of the infrastructure around it and sending a plume of radiation over nearby islands.

This unexpected power came from a miscalculation relating to the bomb’s fuel, resulting in an explosion almost three times more powerful than had been predicted for the test. It sent a mushroom cloud 130,000 feet in the sky and left a 6,500 feet wide crater where there was once an island.

It remains to this day, the most powerful nuclear weapon ever detonated by the United States, and one of the worst nuclear disasters in the nation’s history.



Castle Bravo was part of the Castle series of nuclear tests, which occurred throughout 1954 and saw the detonations of six nuclear weapons. The purpose of these tests was to prove that a new type of nuclear device, the thermonuclear bomb, could be a practical weapon.

Trinity Explosion
The first ever atomic explosion, Trinity. It produced a yield of 25 kilotons.

The Castle series built upon the concept that had been started two years earlier with the Ivy Mike test, the first successful detonation of a proper thermonuclear device. However, Ivy Mike was a proof of concept and could not be used as deployable weapon.

What is the difference between atomic and thermonuclear bombs?

The first atomic weapons designed were fission bombs. Put very, very simply, fission bombs contain a fissile fuel that will be compressed by an immense force.

This compression squeezes the fuel until it reaches critical mass, a point where the atoms inside the fuel begin splitting apart in a process called fission. As they split apart, the atoms release unfathomable energy, creating a chain reaction that further splits more atoms. Collectively, the energy released by this sustained nuclear chain reaction is what we see as an atomic explosion.

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The other type are thermonuclear bombs, also known as fusion bombs.

Fat Man atomic bomb.
The Fat Man bomb used on Nagasaki was an implosion-type device. Essentially, a sphere of explosives surrounds the fuel, and compresses it upon detonation, triggering the fission process.

As the name indicates, fusion bombs fuse atoms together, rather than pull them apart. Under normal conditions, atoms repel away from each other, so an immense amount of energy is required to overcome this. Scientists figured that an atomic bomb was capable of providing this energy.

In a thermonuclear bomb, an atom bomb is detonated close to a fuel source. This first detonation provides the energy needed to force the atoms in the fuel together. The fusing of atoms releases more energy than spitting them apart, so thermonuclear bombs are much more powerful.

Hydrogen is an ideal fuel for the fusion process, so the bombs are often referred to as “H-Bombs”.

Modern nuclear weapons are of the thermonuclear type, as they release much more energy than their fission siblings.

Ivy Mike mushroom cloud.
Ivy Mike mushroom cloud. This explosion had a yield of 10.4 megatons.

The Castle Series

The US had been aware that a thermonuclear device was theoretically possible since the early 1940s, but it wasn’t until the Soviet Union detonated their own first atomic bomb in 1949 that it was taken seriously.

In 1952, the United States successfully detonated the first thermonuclear device with the Ivy Mike test. Ivy Mike had a yield (the explosive energy) of 10.4 million tons of TNT, 700 times more powerful than the bomb dropped on Hiroshima.

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However, this test was a proof of concept and the device was not able to be adapted into a proper weapon. To test a practical thermonuclear weapon, the Castle Series of tests were initiated.

Ivy Mike Sausage Device.
The Ivy Mike device (shown here) weighed around 80 tons and took up most of a building. Therefore, a smaller, usable test was required.

Seven detonations were planned for 1954, all occurring in the Marshall Islands in the Pacific. The pristine shores of the Marshall Islands are very remote, but they have native populations who still live a very traditional lifestyle.

All but one of the tests were to take place in Bikini Atoll, a circular coral reef with a central lagoon. The natives of Bikini had been relocated by the US in 1946 to a neighbouring island, making way for a total of 23 nuclear tests up until 1958.

Castle Bravo was first up in the series.

Castle Bravo shot cab.
The Castle Bravo “Shrimp” device was located in this building, erected in the Ivy Mike crater. The vertical pillars to the left of the building are mirrors for filming the explosion. The long pipes running from the left of the image are line-of-sight tubes. These allow radiation to pass through in the first few milliseconds after the blast to study the performance of the device.

Security zones were set up around the bomb to prevent civilians from coming near, but there was nothing stopping the nearby islands from being contaminated by radioactive materials should the weather change unexpectedly.

The nuclear devices used in these tests all received names; for Castle Bravo it was “Shrimp”. Cylindrical in shape and weighing 10,700 kg (23,500 lbs), Shrimp was expected to produce a yield of around 5-6 million tons of TNT (5-6 megatons).

It had also been specially made to be easily adaptable into a weapon should the test be successful.

Castle Bravo Shrimp device.
The Shrimp device used in the Castle Bravo test was significantly smaller than with Ivy Mike. It was designed with being air-deployable in mind.

Castle Bravo Test

Shrimp, placed in the top corner of Bikini Atoll, was detonated at 6:45 am on March 1, 1954.

The bomb lit up the night sky with blinding flash, blueish-white at first, then dimming into a continuous, hellish orange glow.

Within in instant, Castle Bravo had blown a crater 6,500 feet wide and 250 ft deep into the reef of Bikini Atoll, and sent a huge fireball out and upwards into the sky. The mushroom cloud reached 50,000 feet in altitude within the first minute, then passing 100,000 feet two minutes after that.

The blast was visible from 250 miles away.

Castle Bravo detonation.
Castle Bravo 3.5 seconds after detonation, viewed from 75 miles away.

It was soon clear that Castle Bravo was much, much bigger than expected, almost three times bigger in fact! Rather than produce a yield of 5-6 megatons, the device was equivalent to 15 megatons of TNT.

This was caused by the fuel within the bomb being much more reactive than expected, something it’s designers had failed to account for and only truly understood after this test.

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Because of this, the test was, in many ways, useless. Much of the instrumentation set up to measure different aspects of the tests were simply destroyed. Even buildings 40 km away on the far side of the atoll were accidentally destroyed.

Castle Bravo mushroom cloud.
The explosion, 62 seconds after detonation, viewed from 50 miles away.

Some of the observation staff on this part of the atoll were forced into reinforced shelters when they realised the test had gone awry.

However it was radioactive fallout that would become the biggest problem.

Castle Bravo Fallout

As if anything else could go wrong, the winds had also unexpectedly changed directions around the time of the test. Dr. Alvin C. Graves, the scientific director of the test, was made aware that this development increased the likelihood of fallout being spread over nearby islands, but he still gave it the green light.

This exact scenario happened almost immediately, with winds blowing the fallout east over islands 280 miles away. On some of these islands, snow-like powder began falling down within a few hours of the detonation – unknown to them at the time, this was highly contaminated particles of coral, sand, and other materials vaporised in the blast.

Over the coming days, weeks and months, radioactive fallout would reach as far as Australia and Europe, sparking significant international backlash against nuclear weapons tests.

Castle Bravo 16 minutes after detonation.
Castle Bravo 16 minutes after detonation. Winds quickly carried the radioactive cloud toward neighbouring islands.

One particularly documented occurrence was that of the Japanese fishing boat Daigo Fukuryū Maru, or “Fifth Lucky Dragon”. The Lucky Dragon had been severely contaminated by radioactive fallout just hours after Castle Bravo detonated.

They had actually been outside the US’ designated danger zone, but as this zone was for the predicted blast, it was too small for the actual yield of 15 megatons. The crew were quickly affected by radiation sickness and returned to Japan.

This alone created a significant deterioration in relations between the US and Japan because of their previous connection of atomic weapons. One of the crew members succumb to the radiation poisoning, but the rest managed to survive.

Many would later pass away from related conditions, however.

Rongelap radiation checks.
Radiation surveys on Rongelap, a neighbouring island around 100 miles from the blast. This location received some of the most significant fallout.

Wider Impacts

The metaphorical fallout from the detonation was enormous and globe-spanning. In the short term, it delayed the rest of the Castle series of tests – the next being Romeo on March 26.

Outside of this, the event greatly increased the public’s awareness of nuclear weapons, their effects, and the questionable ethics behind their testing.

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It would lead to a UN-reporting scientific committee to study the effects of radiation, and majorly contribute to the establishment of the 1963 Limited Test Ban Treaty that limited nuclear tests to underground only.

Radiation checks being carried on out Marshallese natives.
Radiation checks being carried out on Marshallese natives.

Diplomatic relations between Japan and the US would strengthen again, but the event was impactful on Japanese culture. The character of Godzilla was inspired by the The Lucky Dragon’s terrifying encounter.

But the Marshallese inhabitants were the biggest victims in all of this. They suffered significantly increased cancer rates and birth defects but were not given the medical attention or financial compensation that many would deem fair.

Many had to be evacuated from their homes for years due to heavy contamination, only to be evacuated again when they were allowed to return before it was safe. Afterward, scientific studies were carried out on the inhabitants to understand the effects of radiation, but due to language barriers and a lack of understanding, many inhabitants were not aware of what was happening to them.

The Mark 21 thermonuclear bomb.
The Mark 21 thermonuclear bomb was developed from the Shrimp device used in Castle Bravo.

In the end, Castle Bravo had successfully completed the goal of achieving a practical, aircraft deliverable thermonuclear bomb. It also marks the single largest nuclear bomb ever detonated by the US, although this is a title it achieved accidentally.

Soon after, the Shrimp device was repackaged into a usable weapon, entering service as the Mark 21, an 18 megaton thermonuclear bomb.

Despite the chaos caused by the test, the Castle Series was far from the last conducted by the US, or other nuclear powers. Just in the Marshall Islands alone, the US would conduct a total of 67 nuclear detonations.

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The effects of these bombs can still be seen to this day.