21. Castle Bravo The code name Castle Bravo was given to the first United States test of a dry fuel thermonuclear hydrogen bomb. The bomb was detonated on Bikini Atoll, Marshall Isalnds on March 1, 1954, as the first test of Operation Castle and was the most powerful nuclear device ever detonated by the United States at that time. This test lead to the most significant accidental radiological contamination ever caused by the United States.