Tsutomu Yamaguchi (March 16, 1916 – January 4, 2010) worked as a draftsman for Mitsubishi Heavy Industries. He had been in Hiroshima on business and was just leaving when the bomb detonated at 8:16 a.m. on August 6, 1945.
After getting to the train station, he traveled to Nagasaki the next day. He returned to work on August 9, 1945, and told his boss about the bomb. His boss thought the story was unbelievable, but then Fat Man suddenly detonated over Nagasaki at about 11:00 a.m.
Mr. Yamaguchi is the only survivor of both bombs verified by the Japanese government.
Several of his poems about his experience were published in the volume, And the River Flowed as a Raft of Corpses.
RadioLab provides a fascinating podcast about his story and his exposure to the radiation of both bombs.