Nuclear fission is actually by definition the least renewable energy source. Even coal and oil are renewable on long enough time scales. But there will never be more uranium than there is right now.
We actually don't have that much of it if we consider the long term future, only a thousand years or so. So nuclear is intended to be a bridge to eventual full renewable power generation and storage, an essential component in the present day but it's still a bridge.
Another thing to consider is that nuclear is the only power source that works in deep space away from the Sun. So if we're serious about exploring the solar system or further, we'd be best not to burn up all of our fissionable material right away.
The production of the uranium fuel, the gigantic building itself, the transport (the fule gets shipped around the world), the storage after its depleted.
Its definitely better than any Combustion fuels, but not at all better than actual renewables.
