Thermonuclear weapons: U-238 tamper?

[Gilded]

One of the ways to reduce the yield of Tsar Bomba from 100Mt to 50Mt was the replacement of the depleted uranium tamper with a lead tamper. As some of you might know this was because while U-238 is not fissile it is fissionable with the high energy neutrons from the fusion reaction. This made me wonder; How much uranium can you actually wrap around a thermonuclear weapon to increase its yield? It's pretty scary actually since it doesn't just increase the yield but also contributes quite a bit to the resulting fallout.

[insane_alien]

not really no, as you can just increase the number of stages. there is no theoretical limit to the size of a thermonuclear bomb, just the fission primary.

[Gilded]

Yes I know, I was just wondering how much depleted uranium you can surround a thermonuclear bomb of a certain yield with. Like let's say you have a 20Mt thermonuclear bomb; it releases a certain amount of high energy of neutrons which can fission U-238, but the neutrons from the U-238 can't fission other U-238 nuclei so there has to be some sort of limit.

 

I suppose you could calculate the maximum increase if you know the fission energy of U-238 (which I couldn't find) and the amount of neutrons from the fusion and assume that optimally all of them fission a uranium nucleus.

[insane_alien]

well, as the fissioning U-238 will produce even more fast neutrons i would think that the limit would be a direct product of how much pressure the primary stage can dump onto the secondary stage to initiate fusion.

 

i think the correct answer here is : not enough information to make a sensible answer.

 

i think it would be affected by all sorts of details about the bombs geometry and design, i mean , there are probably better designs in use today than the typical teller-ulam design. they may be able to better utilise the fission-fusion-fission route.

[Gilded]

well, as the fissioning U-238 will produce even more fast neutrons...

 

Hmm, do the initial fissions produce neutrons fast enough to fission other U-238? That definitely complicates things.

[insane_alien]

yes they do.

 

this is exploited to make neutron bombs.

 

as i said, there are many many factors involved. there are probably many more i would never think of as well. i only know the basics of nuclear bomb design(i.e, get a supercritical mass and hold it together as long as possible)