World War 2

How many atomic bombs did the US have in 1945?


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January 21, 2018 4:21AM

In August 1945 the US had four atomic bomb weapons.

The test bomb "Gadget" (MK-III "Fat Man" type) was detonated at "Trinity" test site northwest of Alamogordo, NM on 16 July 1945 (yield of 20 kilotons). Shortly after this test it was decided to discontinue manufacture of the MK-I uranium bomb and only use the one already made and in transit at the time to Tinian aboard the Indianapolis, as it was far less efficient in material usage.

Two weapons, code named for the types of bombs and not the weapons themselves, were "Little Boy" (MK-I) a uranium U-235 bomb dropped on Hiroshima 06 Aug 1945 (yield of 18 kilotons) and "Fat Man" (MK-III) a plutonium Pu-239 bomb dropped on Nagasaki on 09 Aug 1945 (yield of 21 kilotons). Niigata and Kokura were two other of 17 targeted cities not bombed.

One more weapon, a MK-III plutonium bomb was completed before 14 August 1945 but when it arrived in San Francisco on 18 August 1945 to be flown across the Pacific to Tinian for use in late August, it was instead returned to Los Alamos to become the first atomic bomb in the US stockpile.

Tokyo was never considered a target city since the Emperor and Imperial Army and Navy High Commands would have to survive to surrender and it had already been so heavily firebombed that classifying the damage for weapons effects studies would be impossible.

The US had intended to manufacture and assemble for delivery twenty more MK-III type atomic bombs before the end of 1945 (three in September, three in October, seven in November, seven in December), but the surrender of Japan halted production and assembly of further atomic weapons. At least one of the three Hanford, WA plutonium production reactors was shutdown for repairs and the others were operated at severely reduced power to slow the unanticipated damage to their graphite moderator due to neutron irradiation. Due to this production was severely limited, but it is possible that a small number of additional bombs were built in 1945, but cannot be confirmed.

By the end of Operation Crossroads in the summer of 1946 the US had built a grand total of nine atomic bombs (one MK-I uranium bomb & eight MK-III plutonium bombs) and exploded five of those (Trinity, Hiroshima, Nagasaki, Able, Baker) leaving only four atomic bombs in the stockpile.

Studies had already begun at Los Alamos before the end of 1945 for an improved plutonium bomb, which became the MK-4 but did not enter the stockpile until 1949..