Nuclear is definitely sustainable [1]. There's at least four billion tons of Uranium dissolved in seawater, enough to power humanity until the heat death of the universe [+]. We just have to figure out how to get it out. [2] The amount in the ocean is constantly replenished through geological activity so one could even make the case its renewable too.

[+] There's 5.5 million metric tons of known uranium deposits. Switching to breeder reactors, which produce more fuel than they consume, would allow that supply to match current generation capacity for 30,000 years. Then there's the 4.5 billion metric tons in seawater. That'll get us 15 million years worth of power.

[1] https://www.energy.gov/ne/articles/3-reasons-why-nuclear-cle...

[2] https://www.pnnl.gov/news/release.aspx?id=4514

Anything that helps avoiding a climate catastrophe short-term is a good idea since it buys us time to switch to truly sustainable sources.