This page shows a simulation of the 20 kt nuclear bomb (20 kt) detonated over Nagasaki, Japan (1945). The map displays the estimated fireball, the zones of heavy, moderate and light blast damage, thermal radiation and ionizing radiation. Click «What does that mean?» for a detailed legend, or run your own detonation on the main page.
Nagasaki, 9 August 1945
The plutonium implosion bomb Fat Man was more powerful than the Hiroshima weapon — about 20–21 kilotons — yet it killed fewer people, roughly 40,000 on the day and 70,000 by year's end. The reason is visible on this map. Cloud cover forced the crew away from the planned aim point in the commercial centre, and the bomb detonated at about 500 metres over the Urakami valley, 3 km to the north-west, above the Mitsubishi steel works and the largest Catholic cathedral in East Asia.
The hills that line the valley on both sides acted as a shield. Within the valley, destruction was complete for about a kilometre and houses collapsed well beyond that — on this map the 20 psi ring is about 0.8 km and the 5 psi ring about 1.9 km — but the old city centre behind the ridge suffered far less than the rings alone would suggest. Nagasaki is the textbook example of how terrain reshapes nuclear effects, which is why the simulator's terrain-shadow option exists.
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