USA 40's 50's

Operation Crossroads
Credit: United States Department of Defense

Operation Crossroads was a series of nuclear weapon tests conducted by the United States at Bikini Atoll in the summer of 1946. Its purpose was to test the effect of nuclear weapons on naval ships (which can be seen in this photo around the blast area). The series consisted of two detonations, each with a yield of 23 kilotons: Able was detonated at an altitude of 520 feet (158 m) on July 1, 1946; Baker (above photo) was detonated 90 feet (27 m) underwater on July 25, 1946. A third planned burst, Charlie, was cancelled.

The Crossroads tests were the fourth and fifth nuclear explosions conducted by the United States (following the Trinity test and the bombings of Hiroshima and Nagasaki). They were the first nuclear tests held in the Marshall Islands, and the first to be publicly announced beforehand and observed by an audience of invited witnesses and a large press corps.

Ultimately, the biggest news from Crossroads, not widely reported at the time, was the radioactive contamination of all the target ships by the Baker shot. It was the world's first experience with immediate, concentrated local radioactive fallout from a nuclear explosion. (The global fallout from an air burst is delayed and widely dispersed.)

Castle Bravo

Castle Bravo was the code name given to the first U.S. test of a dry fuel thermonuclear hydrogen bomb device, detonated on March 1, 1954 atBikini Atoll, Marshall Islands, as the first test of Operation Castle. 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 accidental radiological contamination ever caused by the United States. Fallout from the detonation — intended to be a secret test — poisoned the islanders who had previously inhabited the atoll and returned there afterwards,^[1] 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.^[2]