On this day March 1, 1954

The Castle Bravo, a 15-megaton hydrogen bomb, is detonated on Bikini Atoll in the Pacific Ocean, resulting in the worst radioactive contamination ever caused by the United States.

castle_bravo_blastCastle Bravo was the code name given to the first U.S. test of a so-called dry fuel thermonuclear hydrogen bomb device, detonated on March 1, 1954, at Bikini Atoll, Marshall Islands, by the United States, as the first test of Operation Castle.

Fallout from the detonation—intended to be a secret test—poisoned the islanders who inhabited the test site, as well as 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–tritium used as fuel for the fusion stage of the U.S. experimental 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 design (known as the “Alarm Clock” in the U.S.), in 1953. It was not a “true” hydrogen bomb, deriving only 20% of its yield from fusion reactions, the rest from boosted fission reactions, and was limited to a yield of 400 kt.

By comparison, the Teller-Ulam-based Ivy Mike device had a yield of 10.4 MT, and Castle Bravo had an even larger yield of 15 MT. 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 compressed the fusion secondary to high densities, which could then undergo thermonuclear reactions after being heated by the exploding fission weapon. After a few years, the Soviets, led by Andrei Sakharov, independently developed and tested their first Teller-Ulam device in 1955.

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, led to the most significant radiological contamination ever caused by the United States.

In terms of TNT tonnage equivalence, Castle Bravo was about 1,200 times more powerful 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.

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