I get the sense that you'd like to hide the complexity of a nuclear accident from your 7 year old, and I'm fine with that - what I'm not in line with is keeping the reality of how to respond in the event an accident occurs. Take a house fire as an example - do you have a plan for what happens if your kitchen catches fire? Have you taught your kids to get out of the house immediately, and assemble at a pre-designated place (the front yard or a neighbor's yard)? These are reasonable plans and precautions for a 7 year old. If they need to understand why they need to get out of a house on fire, explain that fire is very dangerous, it moves and gets much worse quickly, and tell them to be brave, stay or crawl low, and get OUT of the house right away.

Your local Fire Department should have lots of age appropriate instruction available to fill in the rough spots around my version and the local fire station should be happy to run an exercise for your kids or their classroom, but my point is for your 7 year old, treat a nuclear accident like a fire, which you want them to be able to relate to. Explain to them that nuclear accidents are extremely unlikely, but as we see in Japan, even the unlikely can happen, and we need to be ready. If we're ready, we can take the right action, and remain safe. The best plan for a nuclear accident is to stay in place until told to evacuate. Who will evacuate your 7 year old? Either you, a loved one, or possibly your child's school. Find out what the school's protocol is for nuclear accidents, and get involved. And assure your child that you will be there come hell or high water in any evacuation. You can make your next trip out of town a "practice evacuation", and take some of the mystery and potential anxiety out of the trip. But there's no need to keep this kind of thing from your kids.