Nuclear energy has insane energy density in terms of MJ/kg (something like 3.9 x 10^6 ) versus chemical fuels (4.5 x 10^1), but it’s grossly inefficient because most of the output is waste heat and “hot” isotopes-- the last things we need. I don’t have hard numbers on hand but I wouldn’t say nuclear is more than a few tens of percent efficiency. Then there’s the capital costs to build, maintain and operate plants PLUS costs to source, refine, transport, and store the fuel, and then transport and discard (contain) waste product. Not worth it at scale.
Versus Solar, Wind and Tidal which are far less energy dense per unit mass of working fluid¹, but enjoy up to 80% efficiency, and are relatively easy to scale.
Nuclear still makes sense, I think, in interior areas like the American Midwest where wind and solar are fickle, and transportation (transmission) costs for tidal would be unsustainable.
¹ Not a fair comparison because solar efficiency is quantized on intensity x area / time, while wind and tidal would quantized on flux density, or (mass / area) x velocity (over time?).