Simplify the following text for a Grade 11 native English-speaking person.
Original: The nuclear weapon was first detonated on 6 August 1945 , when the United States dropped a uranium gun-type device on the Japanese city of Hiroshima .
Simplified: The first event occurred on the morning of 6 August 1945 , when the United States dropped a uranium gun-type device code-named `` Little Boy '' on the Japanese city of Hiroshima .