sjl Ars Praefectus
20y
3,805
Caillebotte said:
The Manhattan project built a plutonium device and tested it in New Mexico, because it was not clear that it was going to work.
Of particular note is that the uranium device - Little Boy - was not tested before it was dropped. At all. They were completely confident it would work. It was hugely inefficient - it had 64kg of uranium, but less than one kilogram underwent fission - but it was simple enough that they didn't need to test it to know it would work.

The reason plutonium needed testing is because reactor-produced plutonium will have Pu-240 as a trace contaminant. The longer the U-238 stays in the reactor before being removed to extract the plutonium, the higher the percentage of Pu-240; and Pu-240 is poison to attempts to build a nuclear weapon. It spontaneously fissions, and that causes a gun-type weapon (like Little Boy) to trigger before the critical mass is fully assembled. Hence the need for implosion, and that in turn requires testing to verify that the implosion mechanism works properly. (But the tradeoff is that the longer the U-238 stays in the reactor, the more plutonium there is to extract - a short stay gives you higher purity, but much less material to refine.)

If they'd been able to produce pure Pu-239, as the early samples were (said samples were produced in a cyclotron rather than a reactor), the gun-type weapon would have been fine. But that would have been a very slow, hideously expensive, and grossly inefficient way to produce plutonium. Reactor-produced plutonium can be made in much higher quantities; the tradeoff of impurities is manageable once you know how to make an implosion type weapon.

Of note is that weapons for nuclear submarines are made with much higher purity Pu-239 than regular nuclear weapons: Pu-240 is more radioactive, so the higher purity Pu-239 weapons are safer for the seamen who are in relatively close contact for the duration of the deployment. For aerial weapons (ballistic missiles, bombs, and similar), a lower purity is acceptable.

Boskone said:
Bit of an OT question, but how dangerous is a uranium cube to handle (assuming one doesn't drop it on an appendage or the like)?

Relatively safe from a radiation point of view. It's primarily an alpha emitter, very little beta, not much gamma, and because it has such a long half life, the daughter products aren't significant contributors to risk. As long as you don't ingest it, you'll be fine. If you do ingest it, you have to contend with the damage from alpha radiation (can be a serious cancer risk internally, no risk if external) as well as the chemical toxicity of the material. Decent gloves are generally considered adequate for handling.