The advice is based on recent scientific analyses showing that a nuclear attack is much more survivable if you immediately shield yourself from the lethal radiation that follows a blast, a simple tactic seen as saving hundreds of thousands of lives. Even staying in a car, the studies show, would reduce casualties by more than 50 percent; hunkering down in a basement would be better by far…

The new wave is citizen preparedness. For people who survive the initial blast, the main advice is to fight the impulse to run and instead seek shelter from lethal radioactivity. Even a few hours of protection, officials say, can greatly increase survival rates.

Administration officials argue that the cold war created an unrealistic sense of fatalism about a terrorist nuclear attack. “It’s more survivable than most people think,” said an official deeply involved in the planning, who spoke on the condition of anonymity. “The key is avoiding nuclear fallout.”