Nuclear bombs release more than one type of destructive energy. First, there is the blast itself, which immediately causes a shockwave and depending on the size bomb and fuel used, it can literally destroy everything within a certain radius. A one-megaton surface blast has a destructive radius (circle) of 1.7 miles. Nothing would be left standing within the circle; buildings would be leveled to their foundations. There would a 100 percent fatality rate within the 1.7-mile radius. Then you have direct nuclear radiation and then thermal radiation.
The fallout from direct radiation would extend up to 30 miles within the first few days. Death occurs at this distance within hours. After a few days, lethal fallout will have drifted out to 90 miles. Death occurs in seven to 10 days. At 160 miles out from the blast, exposure will cause life threatening chronic illnesses. At 250 miles from the blast area, the fallout from a one-megaton bomb would be at what is considered safe levels.
Thermal radiation will cause third-degree burns beyond five miles from the blast area. Within a five-mile radius, expect a 100 percent fatality rate. At six miles the heat causes second-degree burns and at seven miles first degree burns. Fires caused by the thermal blast will be extensive within a certain radius causing further fatalities. Additional affects include an Electromagnetic Pulse (EMP), which will damage the local electrical grid and destroy or damage communication systems.
Distance from the blast has a direct effect on how much radiation your body absorbs. Shielding is important so regardless of your distance from detonation you must shield yourself by putting objects between you and the blast by covering your body with clothes and getting inside a structure made of steel, concrete, stone or wood. Time exposed directly affects how much radiation your body absorbs so move quickly away to limit time exposed.
Water, soil and snow can also shield you so if you are home and have a basement that is below ground level go there and then seal off the doorway with plastic and tape. Seal windows and doors in the home if you do not have a basement. Use tape and plastic to cover openings where air can enter. Wear clothing that covers your entire body along with a hat and face coverings. You can pile snow or soil around outside doorways or along basement walls to protect against radiation fallout. You will not be able to stop the radiation completely but your objective is to reduce it to safe levels by blocking as much as you can. Much depends on how close you are to the blast, the air humidity, air temperature, wind direction and your shelter.
If in a vehicle, stay in it and move away from the blast area. If you have the means, evacuate from the blast area and find protection away from the urban sprawl by putting hills and heavily wooded areas between you and blast. You simply cannot get far enough away from ground zero so if evacuation in the opposite direction is an option then take it.
Keep in mind if another nation detonated the nuclear device and it was not a catastrophic accident you can expect additional bombs or an attack of some sort. This is called softening the target for hostile takeover. The next attack will quickly follow the initial detonation to prevent retaliation strikes.