A sudden flash of bright, white light and the trademark mushroom cloud are pretty clear indicators of a nuclear explosion. Anybody within 10 miles will suffer nearly instant first- to third-degree burns while those within 50 miles may or may not have flash blindness, according to Lifehacker.
Researcher Michael Dillon from the Lawrence Livermore National Laboratory told Lifehacker that survivors should seek shelter immediately within the most dense building material possible to escape nuclear fallout, the dangerous residual radioactive contamination after the blast.
Dillon told Lifehacker that if you can move to a denser shelter within five minutes of exposure to go for it. However, if it would take longer to move, stay where you are for an hour and then move to a more protected location, which would allow a good portion of the most intense fallout radiation to subside.
The EPA also suggests taking a shower (without scrubbing or scratching your skin) and stripping off your radiation-contaminated clothing. They also say to only drink bottled water or eat food from sealed containers while waiting for rescue teams to arrive.