Most of the nuclear plants that make up that base rate were built when nuclear regulations were much much more lax than they are now, and yet Nuclear ends up with the safest track record out of all energy sources.
Because it only looks at what has happened, not what could happen. And what could happen is the interesting thing.
Before Chernobyl happened, the statistics didn't tell us it could happen. Before Fukushima happened, the statistics didn't tell is it could happen.
They happened anyway. The next thing that happens will similarly be completely independent from anything that happened before.
So - what happened before is irrelevant. What COULD happen is relevant.