On August 6, 1945, the US Air force dropped the fist atomic bomb ever used operationally. The target was Hiroshima, a city that had been spared fire-bombing to provide a clear picture of what the bomb could do.
70,000 - 80,000 Japanese civilians died that day, with another 20,000 to 70,000 dying from their wounds by the end of 1945.
The bomb itself was the simplest possible design. Extremely inefficient, but extremely reliable. So reliable, they didn't feel the need to test it first. To get a critical mass, they fired a hollow Uranium bullet at a cylindrical Uranium target. The idea was that if you could bring the two sub-critical masses together fast enough, then you'd get a good detonation. It worked. Only 1.38% (828 grams) of the 60kg of U238 actually underwent fission, but that was good enough for a 12-13kt explosion. 1kt = 1,000 tons of TNT. You'd need 13,000 FedEx trucks to carry the 13kt of TNT.
source: wikipedia
The gun-type bomb was crude and very dangerous for those making, handling or delivering it. Anything that would cause the bullet to travel down the barrel towards the target would result in a critical mass. The explosion would be a fizzle (North Korea's specialty). Enough to kill anything within a few hundred meters. I don't believe the US built another gun-type, switching over immediately to the plutonium implosion method. Implosion was far more efficient and much, much safer for handling and delivery.
AFAIK, the only gun-type weapons subsequently built were for specialist applications by the US Army, and by the South Africans. They built as many as five devices before abandoning their program entirely. One would expect that the weapons were rarely, if ever, fully assembled.
There's a superb book that explains the Manhattan project in clear, understandable detail. Richard Rhodes' The Making of the Atomic Bomb. A Pulitzer prize winner, easily found in most libraries.
When I was younger, it seemed obvious that the atomic bombings were war crimes. Now I've changed my mind. The terror bombings of German and Japanese cities were war crimes. As was the German bombing of Rotterdam, Warsaw, many English cities and just about everything they did in Russia. Gruesome crimes were routine by both sides in day-to-day combat in the Pacific. But the atomic bombs were different. The Americans became desperate to end the war without invading Japan. They used the bombs with the intention of provoking a political collapse in Tokyo and thus a swift end to the war. It worked.
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Note: War crimes are distinct from crimes against humanity and have a different legal basis. Genocide is a crime against humanity regardless of whether it takes place during a war. Its also worth noting that the most serious of all war crimes is not killing civilians, but engaging in a war of aggression. During WW2, the Germans, Russians, Japanese and Italians started wars of aggression. The US invasion of Iraq (but not Afghanistan) was a war of aggression. People were hung after WW2 for engaging in the same activities as the Bush administration.
