You were asking about dirty
nuclear bombs, not dirty bombs which are entirely different. Dirty bombs combine traditional explosives with radioactive material but are not nuclear bombs as there is no fission or fusion envolved.
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.
http://www.nationmaster.com/encyclop...-weapon-design The term has also been used historically to refer to certain types of nuclear weapons. Due to the inefficiency of early nuclear weapons (such as "Fat Man" and "Little Boy"), 2% or less of the nuclear material would be consumed during the explosion. Thus, they tended to disperse large amounts of unused fissile material in the form of nuclear fallout. During the 1950s, there was considerable debate over whether "clean" bombs could be produced, and these were often contrasted with "dirty" bombs. "Clean" bombs were often a stated goal, and scientists and administrators said that high-efficiency nuclear weapon design could create explosions which generated almost all of their energy in the form of nuclear fusion, which does not create harmful fission products. But the Castle Bravo accident of in 1954, in which a thermonuclear weapon produced a large amount of fallout which was dispersed among many human populations, suggested that this was not what was actually being used in modern thermonuclear weapons, which derive around half of their yield from a final fission stage. While some proposed producing "clean" weapons, other theorists note that one could make a nuclear weapon intentionally "dirty" by "salting" it with a material (most commonly a type of cobalt) which would generate large amounts of long-lasting fallout when irradiated by the weapon core. In the post-Cold War age, this usage of the term has largely fallen out of use. The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 km (11 mi) above the epicenter. ... A post-war Fat Man model. ... Little Boy bomb casing Little Boy was the codename given to the nuclear weapon dropped on Hiroshima, Japan on Monday, August 6, 1945. ... This article or section should be merged with Fissile Fissile material is composed of atoms that can undergo nuclear fission and sustain a fission chain reaction. ... Fallout is the residual radiation hazard from a nuclear explosion and is named from the fact that it falls out of the atmosphere in to which it is spread during the explosion. ... The first nuclear weapons, though large, cumbersome and inefficient, provided the basic design building blocks of all future weapons. ... The deuterium-tritium fusion reaction is considered the most promising for producing fusion power. ... Fission products are the residues of fission processes. ... Castle Bravo was the first test of a Teller_Ulam configuration thermonuclear dry fuel hydrogen bomb, detonated at the Bikini Atoll on February 28, 1954. ... http://www.nationmaster.com/encyclopedia/Dirty-bomb http://www.strategic-air-command.com...bomb_chart.htm http://www.johnstonsarchive.net/nuclear/multimeg.html