Originally Posted by EnigmaNZ

Say you want to create a 15-20 kiloton bomb, you can do this 3 ways. One is to build a fission bomb like the ones used on Japan in 1945, Fatman used 6kg of Plutonium and 260kg of Uranium, Little boy used 60kg of uranium. These are dirty bombs in that there is a lot of fallout from the explosion. Another way is to make a salted bomb, this is a fission bomb with a fusion shell desgned to produce the neutrons to ignite a outer uranium layer, or tamper. this also makes for a dirty bomb thanks to the large use of uranium. The third way is to use the smallest ammount of uranium or plutonium possible and use it as the igniter for a fusion outer shell. I think the smallest fission yield able to be used is about 0.3 kilotons, this is enough to fuse the lithium outer shell creating most of the yield, the fission core is the sparkplug. Around 2 kg of plutonium or about 6 kg of uranium is sufficient for this. This is referred to as a clean bomb as there is little fallout compared to the other types. By adding additional fusion stages the yield can be greatly increased without using more dirty trigger material. The w-80 warhead has yields from 0.3 kiloton using just the sparkplug, to 170 kt with the outer fusion stages.