I saw a pretty cool demonstration of how fisson works once.  It was pretty simple too.  

It was on the floor of a room, looked like a basketball court.  In the middle spaced about 2-3 inches apart and covering about 1/4 of the court were mouse traps with 1 ping pong ball sitting on each the trap.  The person doing the demonstration tosses a ping pong ball out into the middle of the traps and within 2 - 3 seconds all the traps have been set off.  
Pretty neat.

BTT because there's a link with pictures:

[url]http://nuketesting.enviroweb.org/hew/[/url]

Yes, the hydrogen bomb weapons often use U-238-- a normally stable isotope of Uranium that can't be used to detonate a weapon--to boost the yield. While U-238 can't sustain a critical mass, it will fission and give off energy if bombarded by fast neutrons. Which is exactly what the fusion process is providing.

So they wrap the fusion stage with cheap U-238 to drive up the yield, incidentally a fairly dirty process.

Yep, Neutron. Sorry.


[url]http://www.biggerhammer.net/videos/birdman_nuke50_700k.asf[/url]

There is a theoretical limit on how large a fusion bomb can be, however it is of no consequence to humans.

Stars are simply large fusion reactions.  There is enough fusible material in stars that they undergo constant reaction for millions-billions of years.  Gravity keeps the material together and under pressure, and therefore the reaction continues until mass has been depleted enough to end the reaction.

Heavy stars burn faster, hotter and brighter, because their gravity is so much greater.  Light stars (like the sun) burn cooler, longer and more stable.

Stars with a mass of 10 - 15 suns burn fast, and explode as a supernova.  Stars 15-50 times heavier than the sun burn extremely fast and are destined to collapse under their own weight into a black hole.  It is doubtful that stars can even form if their initial mass is over 100 times that of the sun.

So I guess the theoretical limit for the mass of a H bomb would be around 50 suns and up.

In very large stars, Even the Helium by product fuses into Beryllium and Oxygen until the star doesn't have enough mass to fuse anymore.

Tom Clancey has a fairly good decripition of how these weapons work in Sum of all Fears.

[url=http://www.progressive.org/pdf/1179.pdf]Here's a link[/url] to a PDF file of the 1979 Progressive Magazine issue on how H-Bombs work that the US government tried to stop from getting published. The Progressive finally won the court case on 1st Amendment grounds. Their sources were declassified material and information available in public libraries.