So, where does "dirty" in a nuclear bomb come from? The left behind remnants of fission getting mixed into dust and dirt, then lofted up into the air by the for

So, where does "dirty" in a nuclear bomb come from? The left behind remnants of fission getting mixed into dust and dirt, then lofted up into the air by the force of the explosion and coming down later downwind- that's fallout, which is the "dirt." Fusion leftover products are essentially fine, and if you explode your bomb high enough that it doesn't throw much dust and dirt into the air that also will limit your fallout (obviously burying deep under the ground also works, but that is generally impossible for a weapon, only matters for tests). One more thing to explain: a typical thermonuclear weapon is actually three stages: fission (the initial atomic bomb) which creates the high temperatures under which we can get fusion, which then gives off roughly an order of magnitude more of energy (second stage) which then hits a natural uranium tamper (third stage) which essentially doubles the amount of energy, at a cost of leaving a lot more fission left-overs around and making it much more dirty. (You can continue to stage indefinitely, but that makes the bomb bigger and less practicable.) The largest weapon ever tested, the so called Czar Bomba, actually had it's third stage uranium replaced with lead, so its energy came all (~97%) from fusion, therefore it had very few fission by-products and was very very clean. This sort of weapon with no third stage tamper is also known as a Neutron Bomb- the so called 'clean bomb' that kills people (with the radiation from the fusion) but mostly leaves buildings intact, and has minimal fallout. So why were neutron bombs scary? For most of the Cold War, when NATO ran exercises, they 'lost' within 2-9 days- where lost means that Soviet armored divisions started running free and unopposed. At which point it became a political question: can NATO use nuclear weapons to wipe away those divisions before they conquer everything? Will the German government allow NATO to use nuclear weapons on German soil- or will the rest of NATO overrule the German chancellors opposition?[1] A neutron bomb, which can kill everyone in a few kilometers with minimal fallout, might be a convincing argument in that situation, and that worried a lot of people about the potential for nuclear escalation. In the post-Cold War context, it is widely believed that the most plausible first use of an American nuclear weapon would be to take out a 'hard' target- one buried deep underground that the US National Command Authority decided needed to be taken out for some reason. That sort of weapon will be extremely dirty, because it will of necessity be right up against the ground, kicking up a lot of dirt and dust and so would have a lot of fallout. Additionally, if you are trying to maximize blast-per-weight or blast-per-dollar, getting rid of that natural uranium tamper is really hard to justify, just to be a bit cleaner to the people who are receiving this weapon. [1]: The first generation French SRBM, the Pluton, had a range of 17-120km, and was based in France- meaning it could only be used to nuke West Germany.