[Boot Hill]

Prime Choice says:   "...encasing it all in a U-238 shell so that the fusion reaction will saturate the U-238 with free neutrons, thus changing it to Plutonium..."

Not going to happen. The U-239 half life is ~24 minutes and the Np-239 half life is ~2.4 days, meaning that the process of converting the U-238 to Pu-239 takes far longer to complete than the nanoseconds needed for the fission and fusion to be completed.

Wishing you a very Merry Christmas...

--Boot Hill

[Prime Choice]

I beg to differ. The first thermonuclear bomb detonated in 1954 was a three stage fission-fusion-fission bomb.

The core of this weapon was a fission bomb (A-Bomb) which served as the second stage's trigger. The second stage was the fusion (hydrogen) bomb comprised of deuterium and tritium (which, when exposed to the high temperatures of the fission reaction, yielded helium and high-energy neutrons).

The third stage was a shell of U-238 which, as a direct result of the high-speed neutrons from the fusion reaction, did undergo fission, albeit sans chain reaction.

The consequence: sufficient energy to cause fission of the uranium nuclei in the outer (third) shell increased the explosive yield and also contributed appreciably to the radioactivity of the fallout.