“If you see a flash that’s brighter than anything else that you’ve ever seen and it feels like the sun, that’s probably a nuclear explosion. There aren’t that many things that fit that category,” says nuclear historian Alex Wellerstein of the Stevens Institute of Technology. “Don’t stand there and look dumbfounded because you may have about 10 to 15 seconds to do something. And what you do in that 10 to 15 seconds may actually save your life.”
According to the Department of Homeland Security, if you can see the nuclear flash or if you have enough warning beforehand, take shelter immediately. Even a blast shelter would not be able to keep you safe from a direct hit by a nuclear weapon. But if you’re far enough away from the center of the explosion, sheltering in place can keep you shielded from flying glass or falling objects. If you’re not in immediate danger, Wellerstein says the next thing to do is to move to a sheltered space or as far underground as you can, because after the explosion comes the fallout.
During a nuclear explosion, dirt, debris and other particles are forced upward into the atmosphere in a giant cloud. As wind pushes the cloud away from the blast site, radioactive ash falls out of the cloud. Fallout can arrive at Ground Zero within an hour, and it’s most dangerous within the first 48 hours of detonation. But its radiation decays exponentially, which means it loses its intensity fairly rapidly. After two weeks, the radiation from the explosion is about 1 percent of its initial level. It’s important to take shelter immediately to keep yourself from being exposed to high doses of radiation at the beginning of the explosion.