Watch out for cosmic rays and x-rays. That will probably be as big a danger as anything else. Concrete doesn't help much versus cosmic rays. Metals will help. Aluminum shielding, according to Wiki, is the best affordable protection versus cosmic rays. I bought 84 aluminum baking sheets, 3 square feet each, with restaurant-quality thickness. Probably be wise to stay under cover for at least a year. I'm hoping Reflextix helps too. Remember Michelle Obama encouraging people to insulate their homes? Reflectix is insulation, but because of the two layers of aluminum, maybe that was a subtle way to help people survive without saying too much.