"I'm going to go out to the site and test the nuclear device which will launch the rocket [because it might fail]. If you see a Hiroshima Mushroom you do not come to the site; instead, turn around and leave immediately."A discussion of what nuclear weapons actually do. For this discussion, we will detonate a 1 megaton nuclear weapon in Los Angeles. 500 South Buena Vista Street, Burbank, California (Lat 34.155744, Lon -118.326766) will be ground zero and the bomb will detonate on the ground.note
— Robert A. Heinlein's Rocket Ship Galileo
The FlashThe first thing you get with a nuclear explosion is the light flash. This is very, very bright and if you are close enough (21 km (13 miles) on a clear day and 85 km (53 miles) at night for a 1 megaton nuke) will cause at least temporary flash blindness. Unless you're Sarah Jane Smith and probably for her too, this is not good at all. But heaven help you if you look up at the sky while the bomb goes off. That's not the worst of the problems, there's heat as well. If you're within 11 km (7 miles) (Beverly Hills), you're going to get a bad sunburn on exposed skin. 9 km (6 miles), permanent scars. 8 km (5 miles), third degree burns. In and around ground zero itself, an area roughly a half-mile across, the temperatures will (for a few seconds) be hotter than the sun! If you're sunbathing on the beach, you're toast in both senses of the word. It's been estimated 50% of deaths at Hiroshima and Nagasaki were due to burns. It should be noted that the thermal pulse will travel in a straight line. If you are standing behind a concrete wall at, say, 5 kilometers and somehow are unscratched by the shockwave (see below) then it may feel like you are in a oven for awhile but you will come out just fine. You will have a problem if you are standing behind something more combustible like, say, a heavy curtain since the object you are standing behind absorbs the heat from the pulse instead. The problem consists of the fact the object you were standing behind is now on fire with all that entails.
OverpressureThen you'll get the blast wave. This is measured in terms of the sudden increase in pressure. 10 psi of overpressure equals your building getting hit by a 470 km/h (294 mph) wind. This is the big problem. The following things will probably happen:
The Mushroom Cloud and FalloutAfter the fireball disperses, you will see the mushroom cloud start to form from condensing vapor. This contains water, debris and general radioactive nastiness. It's not just a nuclear explosion thing: any large explosion will produce one, as well as volcanic eruptions or a meteorite impact. There's a 1937 description of an explosion in Shanghai that references a mushroom — 8 years before the first nuclear explosions. Furthermore, one account of the eruption of Mount Vesuvius in Italy in 79 AD described it as having the shape of a pine tree; pine trees in Italy have a similar shape to mushrooms◊. In Robert A. Heinlein's story, The Moon Is a Harsh Mistress, the Lunar colonists fighting the earth's governments drop a large steel-encased rock - say the equivalent of something the size of a Greyhound bus - on a site in the middle of the desert. The impact, when it lands, generates a huge mushroom cloud. Someone at a meeting of the rebel government asks Manny Garcia, the protagonist of the story, why the colony violated civilized behavior and used nuclear weapons. Manny explains that it wasn't nuclear at all; it was simply the force of impact of a multi-ton unbraked object dropped on earth at the speed of gravity, 32 feet per second per second. It's the same thing when you strike a hammer on an anvil, you get a spark. A mushroom cloud is just the (enormously enlarged) equivalent of a spark. Just the biggest spark ever created by mankind. You hit anything with a big enough impact - nuclear or non-nuclear - you will get a mushroom cloud. This mushroom cloud disperses in a matter of hours. During that time, fallout starts raining down on the ground. Radioactive fallout consists of what's left of the bomb, stuff caught in the fireball that's been made radioactive by the bomb's intense neutron radiation, and a whole host of new and exciting isotopes created in the explosion itself. Most of it has half-lives short enough to disappear within hours, days or weeks. This is the idea behind fallout shelters - not to spend the rest of your life down there, but to wait in a shelter for a couple of days until the worst of the fallout has disappeared. However, stuff like strontium-90 (half-life of 29 years) or caesium-137 (30 years) have half-lives short enough to be really radioactive, but long enough to stick around and cause trouble for decades. It's worth noting people in the radiation biz generally use seven half lives as the rule of thumb when getting to 'zero' radiation, and that's not counting radioactive daughter products. Let's explain the term 'half-life' which you've probably heard many times but aren't sure what it means. It's the amount of time it takes for half of something radioactive to disappear.note Say for example that you have a radioactive sample with a half-life of one month that would kill you after an hour's exposure (1,000 rems (10 Sv) within an hour is a guaranteed fatal dose of radiation). Then, after 30 days, only half of the sample would remain and it would now need two hours of exposure to kill you. After 60 days, four hours, after 90 days, eight hours, after 120 days, sixteen hours and so on. Now, if its half-life is ten years, then it's going to take an awfully long time before it's safe to be anywhere near it (unless in lead suits, that is). On the plus side, the longer the half-life, the less radioactive something is. The initial radiation may well kill you, but at the distance it would, you're dead anyway from the other effects. The fallout stuff can cause hair loss, infertility, cataracts, tumors, heart failure and generally a nasty death, much earlier than planned. To get something of an idea, watch Threads. When it comes to radiation, it depends on the size of the nuclear weapon. For smaller nukes, especially rather small tactical devices, immediate radiation accounts for much more of the damage they inflict than the (still very substantial) explosive effects, although delayed radiation from fallout is insignificant. For larger nukes, of course, the radiation pulse is absorbed by the atmosphere before it can reach anyone who wasn't killed by the explosion, but delayed radiation is another matter entirely. For some types of small nuclear weapons, especially neutron bombs, the immediate radiation is the main kill mechanism, because the energy distribution from the blast is designed to favor that; that's another story, though. If you're unlucky enough to be targeted with a neutron bomb, the neutrons might make other material radioactive through a phenomenon called neutron activation. While this might happen with any nuclear bomb, it's only with enhanced-radiation bombs that it's likely to be a more significant problem than fallout and blast damage (for normal thermonukes, anything close enough to be neutron activated is likely to be blown apart or incinerated instead, so the small amount of matter that's made radioactive just contributes slightly to fallout). The Doomsday Device At one point, a design was on the drawing board for a nuclear device designed to produce extra fallout. It was called the cobalt bomb. The design called for the explosive core of the bomb to be surrounded by a tamper made of (non-radioactive) cobalt-59. When the bomb went off, the neutrons zipping out of the reaction would turn the cobalt-59 into radioactive cobalt-60, and then the blast would hurl these tiny fragments of cobalt-60 far and wide. It would have been a "dirty bomb" on steroids, and enough of them could have contaminated the entire surface of the Earth with radioactivity. Even its designers referred to it as a Doomsday Device. One of the most iconic images from the Fall Out video game series is a picture of Vault Boy holding his arm out and giving a thumbs up◊. He's not celebrating or approving of anything— he's testing to see if he's in the fallout radius. Convention has it that if you can hold your arm out at full length and completely hide the mushroom cloud on the horizon behind your thumb, then you're outside of the fallout zone. If you still see the cloud outside of your thumb, then you're not safe. Of course, this method of testing is depending on the idea that the size of the cloud is directly proportionate to the fallout it will create. Not a very reliable testing method in the event of an actual detonation.
Mr. EMPThere is also electromagnetic pulse. The mechanism is rather complicated, needless to say; when it comes to the effects, they can be felt with relatively low-yield and low-altitude bursts, although over a smaller area than if one were to, say, detonate a 25 Mt device 400 km over Kansas. That might knock everything that depended on electronics in orbit out of commission and destroy every unhardened electronic device in North America, actually. Your car would probably refuse to start, for instance, because the electronics it depends on to function would be fried. They found about this in the 1950s and 1960s, when they were conducting high-altitude nuclear tests. Another issue with high-altitude bursts is particle radiation becoming trapped in the Earth's magnetic field; this leads to temporary, although nasty, artificial radiation belts. The EMP occurs when the intense flux of gamma radiation from a nuclear explosion produces an ionized region in the surrounding medium via a mechanism known as Compton scattering. The gamma rays strip electrons off of things, producing Compton electrons and positively-charged cations. The electrons are much lighter than the cations and same sign charges repel; the electrons travel to the outer parts of the deposition region while the cations stay in the central part. The outer parts are negatively charged; the inner parts are positively charged. Because this deposition region is never symmetrical or spherical (it could only be so under ideal conditions) there is a net vertical electron current (that is, there's a net flow of electrons. This is in the opposite direction as the conventional current, which is positive, and obviously vertically-oriented). This produces an intense pulse of broadband electromagnetic radiation, which is the EMP, which radiates outwards at the speed of light. Electromagnetic waves have notationally infinite range, but in practice are limited by the inverse-square law and atmospheric attenuation. Anyway, this electromagnetic radiation can be picked up by conductive objects in the same manner that an antenna picks up a signal, and once transmitted to electronics, damaging them. In fact, the EMP is so intense that it can lead to very strong currents, although for only very short durations, in things that normally aren't very conductive. Close to the Earth, the ground, which conducts electricity well, allows the electrons an alternative return path to the central deposition region, which results in an intense magnetic field in the air and ground, but in those areas there's more to worry about from the actual explosion. Effects from the emitted EM radiation can be felt over a greater area. The mechanism is a little different for high-altitude bursts. The deposition region ends up being in a large region of the upper atmosphere. Only the gamma rays that travel to what becomes this region have much of an effect; otherwise, there's not much to interact with. Anyway, across this very large area Compton electrons are produced. Phillip J. Dolan, in The Effects of Nuclear Weapons, wasn't very specific; he just said that "[the] electrons are deflected by the earth's magnetic field and are forced to undergo a turning motion about the field lines...[this causes] the electrons to be subjected to a radial acceleration which results, by a complex mechanism, in the generation of an EMP that moves down toward the Earth." The electric field is rather less intense, but for obvious reasons the areas affected are much larger. This way, electronics across entire regions may be damaged. You can check the relevant publication out here, while a detailed explanation by an expert in the field is here. As implied above, you can harden electronic devices against EMP, with things like a Faraday cage. You can recognize a Faraday cage as being similar to the shield with holes on it that covers the glass window in the door of your microwave oven; if it wasn't there, the microwaves would pass through the glass and cook you.
Ground Burst vs. Air BurstWe've had our bomb go off on the ground. If it was detonated in the air, you'd get more damage due to fewer buildings being in the way. More importantly, far less fallout is generated as a smaller fraction of the fireball will intersect the ground. If none of it intersects the ground, it is termed an air burst. For our 1 megaton bomb, this requires a detonation height of at least 3,000 feet.note Air bursts cause a phenomenon known as the Mach Effect. When you detonate a bomb in the air, it creates a spherical shock wave (the direct wave). When the wave hits the ground, it literally bounces off, creating a second shock wave that moves faster than the direct one. Chances are, this second wave will overtake the first and combine, producing a skirt around the base of the shock wave bubble where the two shock waves have combined. This skirt sweeps outward as an expanding circle along the ground with an amplified effect compared to the single shock wave produced by a ground burst. There are also nuclear warheads designed for ground penetration, i.e. to destroy missile silos. It's been estimated that the USSR pointed a gigaton worth of nuclear weapons just at Cheyenne Mountain, deciding to destroy the entire mountain. Should the US and Russia engage in a nuclear war, Stargate Command is toast. By comparison, the combined total yield of all nuclear testing to date (as of early June 2008) is 510 megatons, or 0.51 gigatons. The largest individual nuclear weapon ever, the RDS-220 or Tsar Bomba, was designed with a yield of ~100 megatons but had this reduced due to fallout concerns to a yield of 50-58 megatons (sources differ) when tested in 1961 — that is, one quarter of the Krakatoa eruption. At this point, the total yield of all the warheads targeted on Cheyenne Mountain exceeds total US strategic megatonnage. As regards to weapon yields, it is worth noting that
Casualty FiguresPrecisely how many people would die would depend on a lot on circumstances, for example:
While most of this article is concerned with a conventional nuclear war scenario, there are other scenarios that are actually more likely - and both preventable and possibly far more survivable, with the advice that would have generally been seen as worthless in a full counterforce or countervalue war between the US and Russia or the US and China actually being potentially lifesaving. Much of said advice can be found in this TED talk about "surviving a nuclear attack," and more can be found looking around online elsewhere, but it will be summarized here after a discussion of the more likely nuclear threats as of The New Tens. This video series, "How To Survive A Nuclear Bomb," is also very useful even IF the speaker looks like a Right Wing Militia Fanatic:
Nuclear terrorismThe threat mentioned in the Redlener video linked above. While it seems to be controlled/prevented by just how hazardous the sources that would build even a conventional explosive with radiation added, much less an actual nuke, are to terrorists and lunatics in general as well as the education/facilities needed to make a real nuke, it is a possibility - and likely the most survivable of all of the possibilities here if you're not at ground zero, because even if a jerry-rigged nuke made without said facilities was possible or a "suitcase bomb" or single transportable warhead was bought, it would not be on the level of a global thermonuclear war, and there are ways to prevent it - radiation scanning of large transport containers being the primary one.
Accidental launch/dropThis is the biggest threat and one that is probably the least survivable if you're anywhere near it, but likely the most survivable next to terror if you're outside of the circular error probability and blast range (which would be bigger, as this would involve an actual bomb or warhead from a nuclear-capable nation). It's also one that's very preventable although by political will more than anything else - that the US and Russia both have 7000+ of these that could go awry at any time (and that's not counting the rest of the nuclear club) is why saving your life from this is an important reason to insist on nuclear disarmament. It's also worth mentioning that mishandling of nuclear weapons usually doesn't result in full-yield explosions—modern thermonuclear designs require very precise conditions to get a full blast (like precisely timed detonation of explosive lenses within nanoseconds), making it much harder to get the weapon to detonate than not. Missile warheads/bombs also will not detonate unless the necessary accelerations/speeds and flight times for their mission have been matched; to quote a Soviet official from The Hunt for Red October, you could drop a missile warhead from the top of a skyscraper onto a plate of steel on the ground, and you won't get it to go off. Also, the weapons are designed to be resistant to fire and shock from mishandling and emergency jettisons, such that while the conventional explosives in the bomb might go off, the nuclear material itself won't react (though it will spread, posing a contamination issue) or it will just fizzle, or only partially react. On top of all this, almost all the nuclear powers keep very tight control over how nukes are deployed, with multiple safety features to prevent unauthorized launches (Permissive Action Links, the two-man rule, and so on). Obviously, nothing is perfect, but considering there hasn't been an unauthorized nuclear detonation in any of the major superpowers over the last seven decades, something appears to be working.
Single strike from a rogue nationNorth Korea goes berserk, basically. The same as above - if you're in ground zero or near it, you can't do anything, but survival increases the further away you are and you can do some things.