> But usually it would only be a little bit hotter in conventional use. 300C seems like a lot though. Idk anything about reactors, but I think they plunge massive insanely hot rods into water?

You generally want steam for turbines to be pretty hot, the ones I'm familiar with usually run about 500°C. There are a few reasons for this, hotter steam is generally easier (and more efficient) to extract energy from, it's also better for the turbines because droplets can cause damage and higher steam temperatures help avoid droplet formation.