Useful Notes: Nuclear Weapons
aka: The Deadliest Mushroom
"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 This is what is called a "ground burst" and is easier to estimate the damage and casualties than an air burst, but it should be noted that if nuclear weapons were to be used for military purposes (and the two times they were used) the explosions will all but certainly be air burst. But we want to give a reasonable explanation for this article, so we'll use the simpler explanation of a ground burst
— 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:
- Within 2.45 km (1.53 miles), most of Burbank, including our target, the Mt. Sinai Memorial Cemetery and a couple of parks, will just be gone. Nothing is going to survive on the surface.
- Out to 4.50 km (2.81 miles), any ordinary house is gone and reinforced structures are going to be severely damaged. This would include Universal Studios and parts of North Hollywood.
- Beyond here, you'll get damage to everything out to Beverly Hills and Panorama City. Windows will shatter as far away as Santa Monica (~18 miles away).
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. This is why the (control) Vaults in Fallout were set to open after twenty years. (Radiation in Fallout works in ways that make nuclear physicists drink, but it's still worth noting.) 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 ("Emperor Bomb"), 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
- The Tsar Bomba (RDS-220) was huge and unwieldy (27 tons, 8 m long, and so thick that the bomb bay doors of Tu-95 bomber had to be removed to fit it there) and so not a very practical weapon.
- Multiple smaller blasts are vastly more efficient at spreading the damage of a huge area, and it is likely that where blast waves met enormous forces would result. Furthermore, widespread destruction is a little more likely to result in a firestorm.
- The trouble with that is avoiding having the bombs destroy each other. But hey, it's not like this is rocket science or anything.
Casualty FiguresPrecisely how many people would die would depend on a lot on circumstances, for example:
- Time of day: If it's a weekday and people are at work, the area would have more people in it.
- Weather: sunny day, more people outside, but the blast has it easier to dissipate. Cloudy day, fewer people outside, but the heat waves may get reflected by the clouds back towards the ground, resulting in greater damage. Windy day, fewer people outside, but wider fallout. Wind direction is important too.
- Warning: a sudden strike would kill far more people than one after several days of conflict leading to a nuclear exchange. Whether air-raid sirens went off first (if they exist) or emergency broadcasts occurred first would be important too.
- A general note on warnings- for a strike launched from the USA, the USSR or PRC would get about 20 minutes warning before the first explosions - something that contributed heavily to Soviet and Chinese paranoia in the '60s in general and the Cuban Missile Crisis in particular, given that their own nuclear missiles of the time took several hours to be deployed and fueled. From the Soviet Union the People's Republic of China would have gotten about five minutes' warning (and vice versa), and from the USSR The UK would have gotten four - just enough time to make a nice cup/pot of tea. But not to drink it.
- The availability of medical personnel and assistance, although they would be overwhelmed in even the smallest exchange. A 1978 study calculated that treating the wounded in a single-weapon attack upon the USA's city of 'Detroit' would exceeed the total medical resources of North America - the study anticipating several hundred thousand victims with severe burns versus an intensive-burn-unit capacity of less than 3000 people in North America. Treating the wounded would have been further complicated by the destruction of most, if not all, Detroit's medical infrastructure in the attack.
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:
- Part 1: http://youtu.be/6kOU0uusKPE (Surviving flash heat and blast)
- Part 2: http://youtu.be/UZSTBv4EWnU (What to do immediately after)
- Part 3: http://youtu.be/QGraGWtktrk (Fallout protection)
- Part 4: http://youtu.be/B3AEQqjqr68 (Emerging from shelter)
- Part 5: http://youtu.be/nKxmqgcjzAU (Potassium Iodide)
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.
- There's also the possibility of terrorists or other fanatics gaining control of a facility. The results would likely be closer to "accidental" below.
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.
- If all electric power goes out and your car or motorcycle stalled or your battery-powered phone or laptop or radio also failed or you spotted arcing from outlets/power lines, assume EMP from airburst. This is also a sign that none of the below will help because unless the air burst itself was the accident/attack, an air burst generally means a wider attack. Your only chance at this point, if you choose that you want to try to live, is to get into a blast shelter or underground location within 20 minutes and hope it was an accident or EMP attack, wait it out, and hope it was not the aforementioned global thermonuclear war.
- Pay attention to emergency alerts - a North Korean attack would have warning, and an accident or terror attack might have some.
- If you hear a very loud bang or boom, even if you saw/see no flash, and there's no reasonable immediate explanation for it (e.g. a visible vehicle or aircraft crash, thunderstorms in the area, fireworks or military ordnance testing nearby, an obviously non-nuclear explosion), go indoors and check news reports/social media to find its source. Nuclear detonations can be heard anywhere from 50 to 200 miles (80 km to 320 km) away depending on size/yield, even well beyond the distance of seeing a flash, feeling heat, or even seeing the mushroom cloud itself.
- If you ever see a sudden brightening/brilliance in the sky, on the horizon or notice a flash, especially if you feel sudden heat, immediately cover your eyes with the crook of your arm and run/dive for the nearest shade or low-flammability object to shelter, not under anything made of paper or plastic or under a car because it may catch fire or melt onto you.
- Though if you are already in a vehicle stay in it, duck below the windows - the risk of the car catching fire - which it may well not as vehicle fuel tanks have sustained major heat exposures without exploding, note the cases of people driving through fires - is worth the protection from flying glass and debris and impact of being thrown that a relatively modern vehicle can provide. Also, if it's still running, don't turn off the ignition for two reasons - airbags, if the components and sensors haven't melted or been fried by EMP, usually need the vehicle running to work (and if there is a blast, the airbags may give added protection), and secondly for evacuating and gaining distance afterward because if there was EMP, it may not start again depending on what components are affected.
- If you are indoors and see suddenly bright light, do NOT run to the window to see what it was - the blast wave will break that window, if it reaches you, and you will be seriously injured or killed by the glass - as well as blinded even faster by the flash from it reflecting off the glass even if the blast wave is far away. Indoors, despite all instinct that will be telling you "what the hell was that, I've got to see," immediately run away from any windows and to the lowest or most interior room you can reach in seconds.
- Start counting - the length of the flash = size/yield of the nuke and likely source as a result. If the flash only lasts 1-3 seconds, the weapon was likely 20 kilotons or under, which would mean a terrorist device or a North Korean attack. 7 seconds or longer generally means a submarine-launched or intercontinental ballistic missile from a nuclear-capable nation and likely multiple nuclear warheads due to an all-out nuclear war. 20 seconds or more means you are in the multi-megaton range, which is almost certainly a result of all-out nuclear war (unless it was an accidental launch or accidental drop).
- If you still feel heat, that means the flash is ongoing. If things around you are catching fire, as in Threads, that would almost always mean either you're way too close to ground zero and likely dead/dying, or the flash was above 3 seconds unless the objects are highly combustible, since most things that aren't purposefully made to burn/easily burnable generally take longer than a couple seconds of even the most intense heat exposure (see above in the note on cars) to ignite.
- Protect your head with your other arm and/or anything available to do so. If possible, curl up into a fetal position until the blast wave has passed (which will arrive in anywhere from 24 seconds after the flash or less in 5 miles' (8 km) distance to 3 minutes from the flash 40 miles (64 km) away) - though if you notice no blast wave within four minutes, you were likely far enough away to be out of blast range.
- If you're lucky enough to be more than 6-25 miles (10-40 km) away (depending on nuke - being 4 to 6 miles (6 to 10 km) away would probably be enough to survive a badly-made small terrorist device, while 25 miles (40 km) at least would be needed for a modern warhead or bomb), this will save your vision (and you will need it to save your life) and reduce/prevent burns. Also, avoid glass if possible - it will not protect you and will cut you when the blast wave arrives.
- Once the flash and blast have passed, if you're still alive, you can still move and see to some degree, and if the blast was a one-off as it would be in most of these scenarios, you've survived the most difficult part to survive - next is avoiding the fallout, which, depending on how close you were, can arrive near-immediately or in an hour or more.
- Once you've survived the flash and blast, you need to determine just where the blast was and the size of it. This can be done by visual confirmation to some degree - it's safe to look at this point, and if you can see the mushroom cloud at distance that will tell you where it was - if you cannot see it you are likely under it and in immediate danger from fallout and initial radiation.
- If you cannot see it, you have two options to try to save yourself from the fallout and initial radiation that you haven't already taken - neither of them really good, but they're your only chances at continued survival - either getting as far underground as you can (e.g. a subway station, a third or fourth basement of a parking structure or building, an actual fallout or blast shelter if you are in enough luck to be near one) and trying to grab whatever water or food you can carry on the way because you may be there over a week before being rescued. If you can actually see undamaged area anywhere, you have another option - get in or on whatever vehicle is available and immediately drive/tell the driver to go cross to the blast angle and prevailing winds - "up" or "down" out of the fallout angle as opposed to "across" into it and keep going that way, if possible, for as far as the vehicle can go - ideally you want to be as many miles away as you can be.
- If you can see the mushroom cloud and it's at a distance, this puts you in a better position to see what direction fallout will be - as well as possibly setting you up better for sheltering if you have the capacity for doing so. You also will have enough time that if you have a device not damaged by EMP and you can still access the internet and/or TV/radio broadcasts you can get likely better guidance than your own sight can give you.
- If the cloud or head of the cloud is lighter than the stem, or the stem is broken or nonexistent, the weapon was likely an air burst. This means two things - one possibly good for your survival, one possibly very bad. The "good" thing is that air bursts produce far less fallout (though you'll still likely have to follow the options below for some time). note . The bad thing is that an air burst almost certainly means an attack from another nuclear-capable nation and at best means a North Korean attack or that a nuclear-capable nation has accidentally used the weapon.
- If the head of the cloud is dark and/or the stem is a stick connected to the ground - the weapon was likely a ground burst. This is worse for fallout (because all that stuff on the ground that now no longer exists has been sucked up into the cloud, to rain down on everyone in range of the fallout plume later...) but it also means that the device was likely a smaller terrorist device, especially if the flash was short as well (2 seconds or less of intense flash plus ground burst, it's almost certainly a terror attack as opposed to World War III.)
- Once you're at a distance to some degree from the explosion (whether you were at the beginning, or you fled an area closer to it), you have three options, all of which can be combined, to reduce your risk of suffering immediately deadly effects from fallout (although there will likely be some raised cancer risk anyway, especially if you were closer to begin with, but right now you're concerned with surviving the next months and years: shielding, distance, and time.
- Wind direction is of extreme significance here. Fallout spreads out into a highly eccentric ellipse according to wind speed and direction. If you are upwind from the blast, simply head away from the blast, but if you are downwind, head crosswind, because this will be much more effective at avoiding fallout than heading further downwind.
- Shielding means being underground or in a reinforced room with at least 8 inches of steel/12 inches of concrete/20 inches of packed earth on all sides. It also means taking potassium iodide internally or painting an iodine solution externally on the body (if you can get either) to prevent radioactive iodine from being taken into and destroying your thyroid/causing thyroid cancer later on.
- Distance means getting as far away from the blast area and fallout cloud as is possible. It's the best option in general, especially combined with shielding - you might need less of a strong shelter the further away you are - for example, if you're over 400 miles away and out of the main fallout deposition path of a smaller/less powerful device (e.g. terrorists managed to make and use an atomic hand grenade or suitcase nuke), you may not even need to shelter, or you might only need to stay inside a building during rains. Distance also works for more powerful devices in the case of an air burst - if you are at 200-400 miles away from any given air burst and not in the direction of the wind blowing from it, you are likely safe enough to not even need to shelter. note
- Time: The nastiest stuff (e.g. the things that will give you acute radiation sickness if exposed to them even for a few minutes) tend to decay fast. The cancer-causing stuff hangs around much, much longer (it's the 10,000 year stuff) but the majority of intensity drops off after a week to two weeks. The longer you can stay sheltered and/or far away, the safer you'll be.