Castle Bravo – What You Need to Know About the Largest Thermonuclear Bomb America Ever Detonated


When it comes to Nuclear bombs there’s large and then there’s Castle Bravo. On March 1st, 1954 the United States detonated the largest Thermonuclear weapon in the world birthing a blast that wouldn’t be topped for nearly 10 years. To this day, it holds the spot as the largest explosion ever caused by the United States and sits as the fifth largest man-made detonation of all time. This is the story of how the Castle Bravo bomb stunned researchers and brought about an international call to ban such thermonuclear testing.

The First Lithium Deuteride Fueled Nuke for the U.S.

The United States knew Castle Bravo would be a groundbreaking weapon. The bomb was the first of its kind in the United States Arsenal as a Lithium deuteride fueled Thermonuclear Bomb. Bravo was developed at the infamous Los Alamos Research Lab (you’ll remember them from our episode The Demon Core). Its unique fuel was solid at room temperature. A one-of-a-kind cooling system liquefied the Lithium deuteride at the time of detonation. This allowed the weapon to be more effective and lightweight. However, this fuel choice would lead to a detonation beyond anyone’s expectations.

The Detonation

Researchers planned to test the bomb on an artificial island built at Bikini Atoll, the long-running U.S. nuclear test site. A single second after detonation Castle Bravo formed a fireball 4 and a half miles in diameter. At its peak, the mushroom cloud reached a height of 130,000 feet and a diameter of 62 miles. To put the massive scale of this explosion in perspective, the cloud stretched nearly 10 miles outside of the Ozone layer and within 10 miles of exiting the stratosphere. Manhattan could stretch across the diameter of the blast 5 times over. When the dust cleared a crater 250 feet deep and 6,500 feet wide was left in the earth. Contaminants from the event would span over 7,000 square miles.

A mushroom cloud from the first atomic bomb detonated at Bikini Atoll similar to Castle Bravo
Bikini Atoll became a Nuclear Test site in 1946 when the first test was conducted on the islands.
Photo by Science in HD on Unsplash

Within seconds of the blast researchers observing the controlled detonation immediately knew that something was wrong. Scientists knew this bomb was big, but no early estimates came anywhere near the size of the blast that they had just witnessed. People tell rumors today that the rays from the bomb allowed the researchers to see the bones in the others observing the event. So, what went wrong?

It Needs to Be At Least Three Times as Big

Designers of the thermonuclear bomb expected a yield of 5 megatons from the blast. Castle Bravo bore a yield of 15 megatons, 3 times the expected blast size. This wasn’t a mathematic error, it was an underestimate of the reaction of the lithium-7 making up the lithium deuteride fuel. Scientists assumed that the lithium-7 would dissolve through the process of fission. However, rather than dissolve the element created an extra neutron. Making up nearly 60% of the fuel the reaction from the lithium-7 lead to an additional 10 Mt of explosive force.

The Fallout

Under the originally expected conditions, the fallout of this nuclear test would have been wide-ranging. The United States selected the location of the test to be far enough from any human settlement to avoid any harmful effects of radioactive fallout. The exponential increase in blast size coupled with an unexpected wind shift on the day of the detonation triggered a dangerous situation that spanned from the Marshall Islands to Japan.

In the years after the test, thousands of island people groups suffered serious health conditions as a direct result of fallout from Castle Bravo. Los Alamos conducted Operation Castle in secret but the effects of the test gained international attention quickly. Countries around the world called to ban atmospheric thermonuclear testing. The calls after the blast did not cease nuclear testing. Tests would continue for 40 years, even after the Tsar Bomba was detonated by Russia and unleashed nearly 5 times the energy of Castle Bravo sending a shockwave that circled the planet three times.


Castle Bravo was a mistake. A mistake that created a bomb significantly more powerful than anyone expected. People across the pacific suffered serious health consequences from the Castle Bravo detonation. Also, the blast destroyed miles of underwater wildlife habitat including much of the coral reefs at Bikini Atoll. Thankfully, the Comprehensive Nuclear Test Ban Treaty has banned the above-ground testing of such weapons. Hopefully, a detonation like this will never again transpire.


Castle Bravo – Wikipedia

Nuclear Testing Ban – Wikipedia

Tsar Bomba – Wikipedia

Operation Castle – Nuclear Weapon Archive

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