Okay, so once in 1,000 years isn't enough. I'll buy that, especially where an accident's effect can last for up to a century.

So, how about once in 10,000 years? Once in 100,000 years? That's on the level of meteor strikes, and I can't think of any power plant that could withstand those.

I think that "never" is as impractical as "two years from now."

I'd call it safe if it only failed when it no longer mattered - if your power plant does not contain the nuclear waste in case of collision with the moon, it's a non issue, because there is nobody left alive to matter.