No I am not going to tell you to clear out a lead lined refrigerator and walk out unharmed. I also I am not saying that this is guaranteed survival this will just greatly improve chances of survival with a lesser chance of permanent health problems. Also do not attempt to see if this works by blowing up your city.

Step 1: The Explosion

When a bomb detonates or any other form of a nuclear explosion it releases a lot of energy.
These come in 3 forms:

light and heat
blast
radiation

The amount depends on the size. One megaton would wipe out most american cities. This instructable deals with an approximate one megaton blast (one million tons of TNT). How it effects people depends on where it is detonated. If in the air it produces more fire and blast damage, if near the ground makes a big crater and more radioactive fallout. Of course the weather, terrain, and other conditions effect it.

Step 2: Light and Heat

Light and Heat

The flash produced by the bomb is brighter than the sun and lasts for about ten seconds. If you don't shield your eyes temporary blindness and eye injury can result.

The heat from the explosion travel at about 300,000 km per second. It can cause fires up to six miles away. Fires can be caused when the heat pulse comes through a window and ignites flammable things inside or wooden extoriors.

Here are some effects on unprotected skin.

Skin is badly burned up to eight km (five miles) from the explosion.
Skin is blistered up to 10 km ( 6 miles ) from the explosion.
Sunburn-type burns occur up to 11km ( 7 miles ) from the explosion.

If it is in the air are more likely to cause worse burns. Clothes will give some protection, but the best would we some sort of barrier between you and the light will protect you you against almost all burns.

Step 3: The Blast

The Blast

The blast wave is obviously a lot slower than the light and heat flash. You won't be hit by the blast until a couple seconds letter. It would be about like the amount of time between the lightning and thunder. If you are 10 km ( 6 miles ) away it would take 30 seconds for the blast to hit you. Now this is possibly the most important part. YOU NEED TO FIND COVER! Roll into a ditch or some other place that will provide protection. You want to stay low. Otherwilse you will likely be hurt from being thrown about by the blast. Act like it's an earthquake. The greatest danger is from flying debris. The blast could injure you you as far away as 15 km (nine miles).

You will want to get in low lying areas of the ground outside. If there is a culvert ( underneath a bridge ) nearby that is also a good place to hide, If you are in a car get low so the glass and other things don't hit you. If you are at home the best place would be a bomb shelter obviously but a basement would also do.

Structure Damage

see diagram

Step 4: Fallout

Now first off let me make something clear radioactive fallout IS NOT a clear gas that seeps into everything. Radioactive fall out is the thing that makes nukes deadly and what most people think of when they here about atomic bombs and the such.

When a nuclear weapon goes off thousands of tonnes of earth and debris is drawn up into the fireball ( the mushroom cloud) and become radioactive. This is carried off by the wind until it settles to the ground. This is called fallout.

Sometimes you can see the fallout sometimes you can't. The radiation, however, you can not see or smell. It is like a fine coarse sand. The fallout can seriously affect an area of 6,000 sq. km (2,346 sq. miles). If nothing is done to protect you against this radiation your life will be in very grave danger.

The factors that determine how much you are affected by the radiation are:

The time that has passed
How long you are exposed
The distance you are from it
The shielding between you and it

Radioactive fallout weakens very fast in the first hours. This is called decay.

after 7 hours it loses 90 per cent of it's strength it had one hour after the explosion
after 2 days it has lost 99 per cent
in two weeks 99.9 per cent of its strength is gone

If there is even one tenth of the per cent of its strength could be harmful so you should stay in your shelter until the authorities tell you it is safe to come out.

Step 5: Protection Against Radiation

I need to say something about this shelter I am describing. IT IS NOT A BOMB SHELTER. It is simply a fallout shelter to protect against radiation. If you want a bomb shelter it would have to be much stronger then this. This would be better protection than nothing, but it would probably not be the best.

You will want to put as much solid material between you and and the outside. If you are out in the open you are dead. In a house is better. If there is a basement you'll want to go there. Once again a bomb shelter is the best protection.

If you are in a house you'll want to be as close to the center as possible. Pile objects on the floor above you and pile them next to the walls around the room so you have as much material between you and the outside. The more the better.

Use this as a guide for the distance you need between yourself and the outside. This will block 99 percent of the radiation.

30 cm (1 foot ) of solid brick;
30 cm (1 foot ) of cinder blocks filled with mortar or sand;
60 cm (2 feet ) of packed earth - 90 cm (3 feet ) if loose;
12 cm (5 inches ) of steel;
7 cm (3 inches ) of lead;
1 m (3 feet ) of water;

Yeah, I wondered about the water too not sure when you would make an underwater bomb shelter?

Step 6: What to Have With You in a Shelter

One thing that is absolutely necessary to have is a portable radio and extra batteries or a cell phone. You need some kind of communication with the outside world. It will tell you when it is safe to come out or advice on how to better protect yourself and your family.

You also need to calculate to have enough space and and supplies for each person. At the minimum you need at least 14 days worth of supplies for each member in your group. I will not go into detail about this because it would take up an insane amount of space. I will say that you need to make sure that you have water and plenty of it stored otherwise if this ever happened and you didn't you'd be hurtin' pretty bad.

Step 7: Preparing

The best way you can prepare yourself is to learn all you can about it. Basic first aid is a must along with basic firefighting skills and sanitation. Also whole lot of common sense. Learn how to properly take care of waste in a situation like this.

Your family needs a plan. Planning is a must run through your plans several times. Plan plan and plan again. A plan is one of the most important things you can have. You always need to be prepared.

Step 8: Extra Information and Sources

I got almost all my information from one book. It is a good read if you have the time. Also I am not canadian. It is titled 11 Steps To Survival by the Canadian government. It is some old cold war era book. You can find it here: http://www.4shared.com/document/DYUme_1K/11_Steps_to_Survival.html if you just want to view and not download it on scribd go here:http://www.scribd.com/doc/68305408/11-Steps-to-Survival.