The pollution from Fukushima was very minor, blown all out of proportion by the reporting. It could be detected on the other side of the Pacific because the background levels are so low. But we can detect it far, far below the point of meaningful risk.

I don't know the numbers for Fukushima, but let's consider Three Mile Island. Same basic problem--some radioactive noble gas needed to be released to avoid trouble (and they actually released it rather than panic.) You are standing at the fence line, what do you do? Let's say you evacuate....hey, there's a street here. Cross it? Nope--it was more dangerous to walk across one ordinary street than to stay put.