A nuclear explosion produces both immediate and delayed destructive effects. All of these are produced and cause significant destruction within seconds or minutes of a nuclear detonation. The delayed effects are principilly radioactive fallout and other possible environmental effects occurring over an extended period ranging from hours to centuries. Radioactive fallout can inflict harm some distant from the site of the detonation. Nagasaki The atomic bomb dropped on Nagasaki was code named Fat Man and used plutonium as the fissionable material.
Plutonium is a man-made element that is more efficient than uranium as a fission source.
The design is also more complicated than the Little Boy atomic device. It relies on a rapid and simultaneous implosion of a fissionable shell into a critical mass. The explosion over Nagasaki was more powerful but the terrain and layout of the city resulted in fewer deaths. The previous day Russia had declared war on Japan and launched a huge offensive involving thousands of tanks and self propelled guns.