Is it correct that the first bomb (dropped on Hiroshima) was a uranium bomb, and the second bomb (dropped on Nagasocki) was a plutonium bomb? I had never heard this until very recently, and I guess I was under the impression they were virtually the same. - Jinx
The two bombs were very different indeed.
Little Boy, which was dropped on Hiroshima, used a “gun” design to smash two subcritical masses of U-235 together, creating a critical mass and starting a fission chain reaction.
The second bomb, Fat Man, which was dropped on Nagasaki, used conventional explosives to compress a subcritical plutonium core, causing it to go critical. This was the same type of device which was tested at the Trinity site shortly before the bombs were dropped on Japan.
You are correct - Little Boy used on Hiroshima was a uranium bomb, Fat Man used on Nagasaki was a plutonium bomb.
Two seconds with Google confirms it :wally
Producing the necessary plutonium is easier than the uranium, so they wanted to see how effective the plutonium weapon was compared to the Hiroshima blast.
That and they wanted to end the war.
My comment was about the choice of bomb, not the decision to bomb - if they simply wanted to be sure of success at Nagasaki, why not use the same uranium design which had proved itself at Hiroshima?
Probably because they were reasonably convinced that the gun-type bomb would work, and wanted to test the implosion bomb. They wanted real-world results so they could build a better bomb afterwards; keeping the easy solution forever wouldn’t have got the US anywhere.
The United States used two different types of bombs because they were using all the bombs they had. There were parallel assembly lines making uranium and plutonium bombs. The first bomb built was a plutonium bomb; it was exploded as a test. The second bomb built was a uranium bomb; it was used at Hiroshima. At approximately the same time, a second plutonium bomb was being assembled; this one was used at Nagasaki.
They didn’t have enough uranium. Even all the other bombs that would have become available through to the end of 1945 would have been plutonium based.
That the independent uranium and plutonium production timescales wound up such that there was one of each design available shortly after Trinity was largely coincidental.
All the details of this are covered quite nicely in Richard Rhodes’ book The Making of the Atomic Bomb, which is both factual and well-written. It’s a must-read for anyone interested in the bomb.
In short, scientists realized early on that a bomb would have to be made of an element that fissioned with either high-energy or low-energy neutrons. Regular U238 only fissions with low-energy neutrons; it can’t absorb high-energy neutrons and so is not split by them. Only U235 splits with neutrons of all energies.
After doing some calculations, scientists also determined that plutonium (element 240, chemical symbol Pu) would also split with neutrons of all energies.
More calculations showed that U235 could be used in a gun mechanism that fired one sub-critical mass into another to yield a chain reaction. It wouldn’t work for plutonium, which is so reactive that it would melt before the mass was assembled.
The scientists pursued U235, but also pursued plutonium. U235 was an easy bomb to build, but U235 was extremely difficult to isolate. Plutonium was easy to make, but it required an implosion bomb which was extremely difficult to design. By fortunate accident, both bombs were ready at about the same time.
The US never tested the gun bomb. The Los Alamos team was able to test it sufficiently with sub-critical experiments. The implosion bomb had to be tested, and of course it was at Alamagordo. The US used up most of its supply of U235 at Hiroshima, using the gun bomb. The Nagasaki bomb used up the current supply of plutonium, too, but while more plutonium would have been available by mid-August, more U235 would have taken several months to produce.
The US still produces U235. Submarine nuclear reactors use U238 enriched with U235 to make a small, very reactive pile. The fission intiator in our thermonuclear bombs use a U235/plutonium combination, although the initiator is still an implosion bomb. Most of the nuclear testing since 1945 has gone towards reducing the amount of U235/plutonium needed in a working fission bomb, plus a bit of perfecting of the fusion apparatus.
All of this is covered in detail quite well by Richard Rhodes’ The Making of the Atomic Bomb.
The Hiroshima mission, regardless of what you think of its morality, went flawlessly. The Nagasaki mission bordered on complete failure from start to finish. Nagasaki was a secondary target. The weather over the city was terrible, and the bomb was dropped only at the last moment, and way off target. The bomber was forced to land at Okinawa.