And that they were nuclear , not chemical, explosions. You aren't going to have huge mushroom clouds from a hydrogen gas explosion spreading it over a wide area.Iodine 131 has a half-life of 8 days. Wait a few months, and less than 0.01% of it is left. People can take precautions or evacuate temporarily to avoid it. The problem in Nevada was probably Johnrankin's point about 90 tests rather than just a single incident, plus a total lack of precautions.