So, if I wanted to make my own nuclear warhead, how much (Uranium? Plutonium?) do I actually need? Say I’m making a standard-issue nuke like what we were going to drop on the USSR back in the Reagan era. Do I need a few atoms? A few ounces? A few pounds?
And what actually happens when a nuclear warhead detonates? I don’t mean on an atomic level (nuclear chain reaction and whatnot). I mean what happens to the material itself? Do two rocks just pound together? Does some sort of electrical impulse take place? Could I replicate the experience at home by banging two rocks of (Uranium? Plutonium?) together?
DISCLAIMER: In case anyone doesn’t know, I have no intention of actually trying to manufacture my own nuclear warhead.
The Federation of American Scientists site is a pretty good general resource for such things. I’d also recommend the Tom Clancy novel The Sum of All Fears (not the movie) for a good technical primer on thermonuclear weapons (though Clancy acknowledges intentionally fudging certain details so as not to encourage terrorists). The FAS site is pretty thorough with regard to your technical questions. In a nutshell, though, in most nuclear warheads, a hollow sphere of plutonium or highly enriched uranium is encased in plastic explosives. Detonating the explosives (via a number of simultaneously triggered detonators) implodes the sphere, forms a supercritical mass, and brings about the nuclear chain reaction/explosion. An alternative, technically simpler but less efficient design, fires a mass of uranium down a “gun barrel” into another mass of uranium, thus forming the critical mass. To my knowledge, the bomb dropped on Hiroshima was the only “gun bomb” ever built or detonated. A thermonuclear warhead contains a standard “implosion” warhead, but adds tritium gas to facilitate hydrogen fusion, and usually includes an outer casing of uranium 238 to generate a secondary nuclear explosion, enhancing the yield of the bomb.
Regarding weights – This article, discussing terrorist threats, cites weights of between 30 and 150lbs, for fairly puny “tactical” weapons in the sub-kiloton to 2 kiloton yield range (and specifically cites the fairly fascinating example of the “Davy Crocket”). Bombs in this range were adequate for cold war battlefield (or modern day terrorist) purposes, but well short of the 200Kt to 350Kt range that was strategic “standard issue” during the cold war. These mass about 700 to 800lbs. The very largest US weapons, reserved for “hard targets” like command bunkers under mountains, have yields of 9Mt or more (and mass perhaps 4 tons), vs. 25Mt or more for the largest Soviet/Russian weapons. That’s 12,500 times more powerful than a 2Kt tactical nuke.
Another thing to keep in mind is the difference between atomic and thermonuclear bombs (A-Bombs vs. H-Bombs). A-Bombs have a maximum yield of perhaps 20-30Kt. H-Bombs, with yields potentially from a few Kt up into the multi-megaton range, are much more technically sophisticated than A-Bombs. North Korea, for example, is not believed capable of building an H-Bomb. I don’t believe Pakistan is either, and I’m not sure offhand of India. Any sort of “scratchbuilt” terrorist weapon would almost certainly be a “simple” atomic device (though Clancy contrives a way around the problem in Sum of All Fears).