Who has ever promised that nuclear plants will never pollute? That's a ludicrous statement to make, and it's almost as ludicrous to state that someone respectable has ever promised so.
Option 1: coal plant that, statistically and averaged over all plants, kills x amount of people every day of the year (through exhaust pollution, killed miners, etc.)
Option 2: nuclear plant that kills y amount of people, averaged the same; direct kills through accidents + indirect through increased cancer risks (discounted for natural occurrences of those cancers etc.)
When x > y, it's better to use nuclear. And y, despite the Fukushima fear mongering, is quite small.
The matter is not 'nuclear can fail, so we shouldn't use it'; the matter is 'which one has the lowest overall cost' (cost taking into account lives, health risks, economic risks etc); and then choosing that one.
