First let me cover the 4 basic effects of nuclear fission. Note: US Army doctrine is the source for much of this, so it may be dumbed down a bit. Refer to end notes.

1. Blast
   • The physical blast from a nuclear detonation accounts for roughly 50% of the energy released by a nuclear explosion. This includes the shockwave.
     • Overpressure - Condition produces a crushing effect.
     • Dynamic Pressure - This is the push-pull effect observed, caused by the vacuum created at the detonation site.

   • The shockwave accounts for most of the physical damage.

2. Thermal Energy
   • Accounts for roughly 35% of energy released.

     • Heat - Sparks fire storms, forest fires, urban fires, and miscellaneous fires.
     • Light - Light produced by explosion is several times brighter than the sun, and burns the retina causing blindness. Can also damage sensitive optics (military).

3. Nuclear Radiation
   • Accounts for 14% of the energy released.
     • Initial radiation (4%) - Emitted in the first minute after detonation. Radiation emitted is in the form of neutron and gamma radiation.
       • Gamma - Highly penetrating and damages living tissue.
       • Neutron induced radiation - Neutron radiation is highly penetrating, and causes particles it comes in contact with to also become radioactive, which become Beta particle and Gamma ray emitters.

     • Residual radiation (10%) - Present after the first minute. Consists of Alpha and Beta particles, and gamma rays.

4. Electromagnetic Pulse
   • Accounts for 1% of energy released, and damages electrical systems.

Preparing for a Nuclear Attack

Shelter - The ideal shelter is buried underground. Earth is the best insulator against all the effects of a nuclear explosion. The shelter must have an airtight door with a fortified pathway straight up to the surface. In addition, a person must remember to fortify their bunker with supplies such as food, water, and hygiene products. Other helpful items are radios and batteries, flashlights, digging tools, and at least one weapon and ammunition.

Notes on shelter: Don't dig a tunnel or stairs as an escape route, because the shockwave from the blast could damage these, trapping you inside or outside of your shelter.

Notes on supplies: Don't rely on plumbing to supply water, because these systems could be damaged. Personally, I would plan at least one month's worth of supplies. Weapons. It will be pandemonium after the detonation, and individuals can and will try to either seize your supplies, or your shelter. A loaded weapon is the best deterrent.

Equipment - In your shelter, you should place your electrical equipment in an area, together, and surround them with sandbags or other dense materials. This will help shield them from damage (refer to end notes). Store the items with the cables disconnected, and the power sources removed.

Reacting to a Nuclear Attack *

Drop!

Most likely, if you aren't killed by the thermal radiation, then you have a decent chance of surviving the initial blast.

Drop to the ground, close your eyes, and attempt to cover your exposed skin. This will prevent the thermal burns produced by light. LOOK AWAY! COVER YOUR EYES! If you happen to be looking in the direction of the explosion when it detonates, then hopefully your body will react instinctively, and fast enough.

Lay on your back, cover your head, and bring your knees to your chest. This will help protect your head and your vital organs. Your primary concern at this point is collateral damage. Debris presents a sizeable threat, and the effects of dynamic pressure will rip structures and objects to pieces. You are safest with a barrier that will deflect the blast above and away from you.

All is applicable if you survive the overpressure effects.

Once the initial wave and the dynamic pressure phases pass, you are free to seek shelter. Your shelter choice should attempt to resemble the previously established guidlines. Things to avoid - High ground with open winds. Low ground. (Fallout will settle in these areas.) Areas with open cross-winds. Your best bet is to seek shelter in an undamaged building above the first floor.

Now, the Army tells me to train soldiers to get near a wall and lie flat, but I don't. Reason being, that wall is coming down. Collateral damage is also a primary casualty producer in a nuclear explosion. I suggest finding a median area. Remember to get away from any windows or unstable structures you might be near, if possible. These will be destroyed, sending shrapnel in all directions.

Depending on the size (kilotons) of the weapon, fallout levels may not drop to non-lethal levels for sometime. Be patient*. Last as long as you can before you attempt to escape the area, because in your attempt you will expose yourself.

Nuclear Explosion Types

Things to remember!

Some forms of radiation can't penetrate even your skin. Alpha and Beta particles are too reactive to penetrate skin, but are highly dangerous when ingested. To minimize exposure, cover your mouth with a wet rag if you must expose yourself to fresh air.

Radiation builds up. A lethal dose is around 800 centigray (cGy). Although, most people don't have radiacmeters (radiation detectors) so the rule of thumb is *minimal exposure possible. Somatic effects of radiation probably won't be felt for days or weeks, depending on exposure.

After finding your shelter, thoroughly dust yourself off and discard the clothing and items you wore previously. Also, as Brain pointed out, you should attempt to wash your skin with water, preferably warm and soapy. These items might contain radioactive particles, and carrying them around is just begging for contamination. Fallout usually comes down in the form of ash and dust, giving the appearance that it is snowing. Remember that once you decontaminate yourself, leave the area and find a new shelter, perhaps in a different room or floor. You have tracked contamination in and want to avoid that as much as possible.

There are no surefire preventive measures that can be taken to prevent dying in a nuclear attack. There are too many factors to cover. However, with these preventive measures you can limit the amount of radiological contamination you receive and greatly increases your chance of survival.

Reacting to an attack* - No warning.

Sources come from US Military, particularly Army, training and doctrine on surviving a nuclear detonation.

Taliesin's Muse says "ps. if you have an airtight door, you haven't mentioned air supply, or scrubbers, O2 candles etc... Also, you need an effective faraday cage to protect against EMP - sandbags pretty useless. Had some other issues with your details, but those are the main ones."

Well, the military has their soldiers bury their equipment to shield them from EMP, after removing power sources. I think I'll stick with that. Taliesin's Muse is right about the air scrubbers and so on, but this is a practical list for the everyday person, not the guide to surviving a Nuclear Attack. ps. Thanks for the typo corrections, I'm a horrible editor.

Also PS, if I miss something please let me know, as T_M did. I'm just relaying the information that I've been taught, which is the same information I give to my fellow soldiers that I live with and fight beside. They trust me with their lives in this respect, so I tend to take NBC related material very seriously, as my MOS requires.

Refer to sievert for lethal radiation doses.

Thanks to vuo and Brain for some excellent points they raised.