Yonder also are the graves of my youth
When an atomic bomb explodes, several things happen in short order. First is a flood of “prompt” radiation created by the nuclear fission that produces the explosion. The good news — if you can call it that — is that if you are close enough to get a lethal dose of prompt radiation, you’re close enough that you’re likely to be killed by other bomb effects before it becomes an issue. Next comes the “flash,” a brilliant pulse of light created as the air around the bomb is heated to millions of degrees; this starts out as ultraviolet, falls quickly into the visible light range, and then into the heat-ray infrared range within a few seconds. The flash can blind, or burn exposed skin, and start fires. Next comes the blast, as the superheated air expands outward, initially at supersonic speeds. The blast is dangerous on its own, and also because it crushes buildings and creates clouds of flying glass and debris.
Given that light travels almost instantaneously, for everyone outside the immediate vicinity of the bomb the flash will arrive before the blast. Furthermore, the fire-setting infrared part of the flash peaks a few seconds later than the initial burst of light. So those who see a brilliant flash of light — and know what it means — have a few seconds to get under some sort of cover to protect themselves from what comes next.
After these “prompt effects” of initial radiation, flash, and blast have passed, there is an additional hazard. A nuclear explosion sucks air, dust — and, if it’s close to the ground, vaporized soil, buildings, etc. — up into the fireball, where some components are transformed into radioactive isotopes that then fall out of the cloud and back to earth over the next few hours, hence the term “fallout.” (…)
So the Obama Administration wants to encourage people to shelter in place rather than head for the hills in the event of a nuclear attack. Even sheltering for a few hours, or a couple of days, lets radiation levels fall dramatically and avoids road tie-ups for later evacuation.
