There are other ways to prepare besides physically preparing. In 1992, the U.S. performed their last known underground nuclear test in Nevada. Or, at least it is the last known test to me. While working at the Pentagon years later, we were able to view a video of that test. The person that presented the program, which included this video, asked us to imagine ourselves standing alongside the bomb as it was detonated. It was kind of difficult to do, trying to imagine yourself being vaporized in a flash of light. One fellow in the group said, "Now that's a real nuclear test."
Think of it this way, back in 1941 when the U.S. dropped the A-Bomb on Hiroshima, it was supposedly equal to 15,000 tons of TNT. Today, the the mostly same type of nuclear bomb is about 100 Kiloton, which is equal to 100,000 pounds of TNT. If that were to happen today, the 2 bombs that were dropped on Japan would pretty much take the island off the map.