Nuclear has a large heat source (or can make a large heat source instead of converting to electric first) to do a lot of things like this.
Nuclear reactors can produce very high temperatures, but in most reactors the heat is moved to turbines using water. Are there ways to move the heat at the high temperatures required to melt steel? (AFAIK, even molten salt is too cold.)
https://world-nuclear.org/Information-Library/Non-power-nucl...
Molten salt reactors are generally around 700C but they can go hotter.
Yes, you could potentially use waste heat from a reactor to preheat the tundish, and maybe the scrap and ladle. Nucor preheats using natural gas to save on electricity. No need to bring everything up from room temperature on electric power. A nuclear reactor immediately adjacent to a steel caster is probably not a great idea.