Nothing is "absolutely safe" -- certainly, fossil fuels aren't. Conventional uranium reactors aren't. They're extremely safe, far beyond what I consider a reasonable cause for the fear that they're treated with, but they are not "absolutely" safe.

Frankly I'm not really sure what "absolutely safe" would even mean; even a hydroelectric dam could break, given an earthquake or intentional sabotage.

That's not a realistic criteria for any energy source; instead, we should be talking about managing and containing risks.