It is expensive because we choose to make it expensive. It is regulation pushing safety levels far beyond other industries, and far beyond science (radiation is far less harmful than regulatory models suggest)
But even from a very matter of fact point of view, I'd rather have 1000 people die every year for 20 years, than have 20.000 people die on a single bad day/week. The economic and social impact is far, far bigger when it can't be "spread out" over multiple years.
We know nuclear reactor accidents could potentially kill millions of humans, even it hasn't done so yet.