150 meters of air halves ionizing radiation. So 1.5 kilometers of air is 1024 times less and 3 kilometers is 1 million times less and 4.5 kilomters is 1 billion times less
Get down to a background exposure of 10 millisieverts over say one week and it will be fine. You have a whole week to move to a location a few more kilometers away
Buildings keep fallout away and provide Radiation protection factors like Sunscreen SPF
This shows that a basement of a fairly large building provide 200 times protection against fallout. It keeps the fallout away and the ground and distance from the roof provides distance and shielding. Also the interior of the building away from the roof and walls also provide 80 times protection.
Nextbigfuture has discussed civil defence protection before. It is important to make better buildings that are more resistant to catching fire or falling over. This is better for protecting people against hurricanes, nuclear weapons, tornadoes, large scale fires and other problems.
Containment and localization
Fukushima had a certain amount of radiation coming out in a form of fallout. If that was contained with a large sealed tent like cover then it would localize the material to the reactor site. Then being 3 kilometers away would mean one million times less radiation.