Small doses of radiation are very likely harmless. We assume that they're not (linear no-threashhold model) just to be overly-cautious.
The evacuation was in case something unpredictable happened and things went very, very bad. The cancer risk for the firefighters is probably similar to if a cigarette factory had caught fire. The hydrogen explosions have always been the primary danger, though, both to the people and to the containment.
If containment had failed explosively (i.e. if they had not deliberately allowed some hydrogen explosions via venting, causing the containment to fail in one giant kaboom instead of the slight damage they think it might have), then we would be looking at troubling amounts of contamination over a moderately sized area.
We would never be in any danger in the USA, though. We just wouldn't. Also, the "kaboom" I was talking about was an ordinary explosive (hydrogen), not a nuclear one. You just can't hit criticality and get a nuclear explosion. The designers make sure it's impossible. Even back when we knew nothing and a few people died from criticality accidents (which were seriously scary in retrospect), they didn't cause a nuclear explosion. It's not that easy. You have to try pretty hard, or Iran would have managed one by now.
