Nuclear fission the Manhattan Project during World War II (September 1, 1939 – September 2, 1945) carried out most of the early scientific work on fission chain reactions, culminating in the three events involving fission bombs that occurred during the war. The first fission bomb, codenamed "The Gadget", was detonated during the Trinity Test in the desert of New Mexico on July 16, 1945. Two other fission bombs, codenamed "Little Boy" and "Fat Man", were used in combat against the Japanese cities of Hiroshima and Nagasaki in on August 6 and 9, 1945 respectively. Even the first fission bombs were thousands of
Nuclear arms race on Bikini Atoll. Scientists significantly underestimated the size of the bomb, thinking it would yield 5 megatons. However, it yielded 14.8 megatons, which is the largest nuclear explosion tested by the U.S. The explosion was so large the nuclear fallout exposed residents up to 300 miles away to significant amounts of radiation. They were eventually evacuated, but most of them experienced radiation poisoning and resulted in one death from a crew member of a fishing boat 90 miles from the explosion. The Soviet Union detonated its first "true" hydrogen bomb on November 22, 1955, which had a yield of 1.6