I am reading John Archibald Wheeler's autobiography and he talks about his research in nuclear fission and the Hanford Site in eastern Washington where Plutonium was manufactured on a large scale. From what I read so far, it seems that it ultimately came down to cost, and that large amounts of Plutonium were cheaper to make/process than Uranium, but I'm still not making the connection as to why and understanding the scientific reasoning behind it, can anyone explain this to me? Thanks AskHistorians!
To make a nuclear bomb you need fissile material. These are rare isotopes that are fissionable by neutrons of relatively low energies, they produce more than one "secondary" neutron as a result of fissioning on average, and the "secondary" neutrons are of an energy level that can fission more of the same isotopes. This allows for the possibility of exponential chain reactions, where each fission reactions produces subsequent fissions that produce subsequent fissions and so on.
Just prior to WWII they knew of two fissile isotopes: uranium-235 and plutonium-239. Uranium-235 makes up less than 1% of natural uranium. Most uranium as mined from the ground is uranium-238. Uranium-238 is not fissile and acts as a contaminant in a bomb. But uranium-235 and uranium-238 are chemically identical. So the effort of separating the uranium-235 from the uranium-238 — "enriching" uranium — relies on the minute difference in mass between the two (U-238 is just three neutrons heavier than U-235). To do this, the United States built several monstrously large facilities in Oak Ridge, Tennessee, that required a huge amount of electricity, a huge amount of technical work, and a huge number of workers.
At the same time, they knew that plutonium-239 could be potentially produced if they could build large nuclear reactors. Plutonium-239 is produced when uranium-238 is irradiated by low-energy neutrons in a reactor. It is chemically different from uranium so it can be separated out from the nuclear waste that is created by reactors. But it required building several industrial-sized nuclear reactors (three of them, at Hanford) at a time when at that point there had only been two small nuclear reactors created anywhere.
The point of view of the Manhattan Project was that either one of these routes to fissile material might work, but either or both of them might prove more difficult than they expected. So they decided to do both of them at the same time. As it turned out, they both worked.
Plutonium proved to be a cheaper approach during WWII, but uranium enrichment proved to be easier technically to produce en masse once they got their plants working at full efficiency, which didn't happen until after WWII. In the Cold War the US used both in its bombs.
Ah, but they did use uranium. The trinity test and Nagasaki used plutonium based bombs, while at Hiroshima it was uranium.
It's not just a question of being unstable. It's a question of which can reliably produce a chain reaction, but not go off before you are ready.
U-235 was easier to work with, once you separated it from U-238. It had better subcritical behavior, so the design used a gun that smashed two lumps together.
Plutonium 239 had a tendency to go early and fizzle, so it used a spherical design where shaped explosive around a plutonium sphere would all detonate simultaneously to bring the mass together.
Of the two, the uranium design was considered to be less risky because it didn't require the synchronized explosives. They had lots of experience setting off sacks of cordite, but not as much with shaped charges.