In general, fossil kills 4-6 million people per year from air pollution. Nuclear has killed ~4000 total, ever. So nuclear plants are very, very safe compared to normal energy alternatives.
Fail-safe just means that if equipment breaks or a human does something wrong, the plant goes to low power and passive natural-circulation systems kick-in that keep the low-power shutdown mode from going to temperatures high enough to break the radiation containment structures.
