you are focusing on pollution in terms of sustainability, but perhaps fission is unsustainable because it relies on "fuels" which can run out? there is only a finite supply of uranium.
IIRC there is a formal guidance for what is classified as renewable. Things like oil, while originating from biological matter, obviously being considered 'not', while tree mass, plant mass, being considered so. Believe the guidance was whether the carbon emitted in its production has been or could be absorbed within a lifetime, via the means with which it was produced.
I have reservations with nuclear but I imagine it would fit within those guidelines easily.
Lithium is not in great supply comparatively; Photovoltaics depend on p/n junctions and silicon. Wind turbines depend on rare earth magnets.