Not that I hope or plan to build one myself, or anything.
But I am curious…all the implosion fission bombs I’ve heard of use Plutonium for nuclear “fuel.” But is it possible to use Uranium for the same purpose? And, if so, why isn’t it done? Too impractical? Not efficient enough? Too expensive, by weight?
Well, thanks for your (extreme) patience,
To make a nuke go off, you want to create a supercriticality. Not just a critical mass, but 2-3 critical masses.
Problem is, in going from less than critical to 3 times critical, you have to pass through 1.1 times critical, 1.5 times critical etc. At which point your nuke can blow itself apart with the force of a feeble few pounds of TNT before it even really gets going. That’s called predetonation, or a “fizzle”.
The insertion time is the SMALL time interval that you have to arrange your sub-critical assembly into a supercritical assembly without it predetonating. Uranium has a much bigger insertion time than plutonium, so you can get away with the gun-type assembly used in the Hiroshima bomb with uranium. A gun type plutonium bomb would pre-detonate.
With plutonium, you have to use an implosion assembly. With uranium, you can use an implosion assembly, and it would get you a bigger bang for the amount of uranium used. But AFAIK nobody makes or uses uranium nukes any more - enriching it to weapons grade is so hideously difficult and slow and expensive. Instead, people use plutonium because it’s easier to manufacture and it has a smaller critical mass. The old non-implosion uranium bombs are antiques.
Well, I was going to post that the “Fat Man” bomb dropped on Nagasaki was an implosion type (as opposed to “Little Boy’s” gun type) but a quick google turned up that Fat Man used Plutonium. I could’ve swore both were Uranium nukes.
It’s probably an efficiency choice. An implosion type bomb is more complicated, since the timing is more crucial. You have to get the explosive charges around the nuclear fuel to go off just right to squeeze the mass into a critical density. So you’d probably only use this type of bomb when you had to. If you’re going to use Uranium, it’s simpler to just use the gun type for the reasons matt laid out.
Concur with previous posters. Just wanted to add that an implosion configuration can get you a higher degree of supercriticality and thus a higher yield. A gun-type device is limited to just over 2x critical mass …
Absolutely. As a matter of fact, the seventh nuke ever detonated was one of these- the Yoke test of Operation Sandstone in 1948 (Eniewetok Atoll). The sixth test was a composite plutonium/uranium core as well.
The funny thing is that at the time, plutonium was harder to come by than enriched U-235, so that’s why they went to it.