The only known nuclear attack on a civilian population was the attacks on Hiroshima and Nagasaki. Collectively, 210,000 people were killed in the blasts. At least 260,000 people have been officially recognized by the Japanese government as survivor's of the blasts and at least one of those, 93 year old Tsutomu Yamaguchi, has been officially recognized as having survived both blasts.
According to officials, Mr Yamaguchi was in Hiroshima on a business trip on Aug 6, 1945, when a US B-29 dropped an atomic bomb on the city. He then returned to his home in Nagasaki just in time for the second attack. The amazing part of this tale is that he, defied all kinds of odds, and survived both blasts. Of course, his health over the long term, was almost certainly compromised, but he's still alive at 93 years old. My point? You can survive a nuclear assault.
This amazing man really endured what can only be described as hell on earth. Check out this description of the assault from the Department of Energy's website. "Those closest to the explosion died instantly, their bodies turned to black char. Nearby birds burst into flames in mid-air, and dry, combustible materials such as paper instantly ignited as far away as 6,400 feet from ground zero. The white light acted as a giant flashbulb, burning the dark patterns of clothing onto skin (right) and the shadows of bodies onto walls. Survivors outdoors close to the blast generally describe a literally blinding light combined with a sudden and overwhelming wave of heat. (The effects of radiation are usually not immediately apparent.) The blast wave followed almost instantly for those close-in, often knocking them from their feet. Those that were indoors were usually spared the flash burns, but flying glass from broken windows filled most rooms, and all but the very strongest structures collapsed. One boy was blown through the windows of his house and across the street as the house collapsed behind him. Within minutes 9 out of 10 people half a mile or less from ground zero were dead.
People farther from the point of detonation experienced first the flash and heat, followed seconds later by a deafening boom and the blast wave. Nearly every structure within one mile of ground zero was destroyed, and almost every building within three miles was damaged. Less than 10 percent of the buildings in the city survived without any damage, and the blast wave shattered glass in suburbs twelve miles away. The most common first reaction of those that were indoors even miles from ground zero was that their building had just suffered a direct hit by a bomb. Small ad hoc rescue parties soon began to operate, but roughly half of the city's population was dead or injured. In those areas most seriously affected virtually no one escaped serious injury. The numerous small fires that erupted simultaneously all around the city soon merged into one large firestorm, creating extremely strong winds that blew towards the center of the fire. The firestorm eventually engulfed 4.4 square miles of the city, killing anyone who had not escaped in the first minutes after the attack. One postwar study of the victims of Hiroshima found that less than 4.5 percent of survivors suffered leg fractures. Such injuries were not uncommon; it was just that most who could not walk were engulfed by the firestorm."
Source articles: Double Atomic Bomb Survivor found in Japan
As you can see, his survival is amazing.