The "steam turbine attached to the gas plant" as a system benefits from the high power density of the gas turbine. For nuclear to do the same it would need to run at high temps (e.g. sodium, sodium fluoride, ...) and be coupled to a combined cycle powerset and heat recovery system...
Still needs the high temps!
Nuclear competes not just with fossil fuels but with "burn the fossil fuels, capture the carbon, inject the CO2 back into the ground option", which might not be so bad if this gets perfected
I think you misunderstand what combined cycle means.
In a gas turbine - without cogeneration - the gas turbine is driven by heating air and the expanded air spins a turbine, which spins a dynamo (as well as the compressor blades). It's like a jet engine, but hooked up to a generator. The exhaust air is hot and we do nothing with that waste heat.
Starting a couple decades ago, people started putting boilers next to the gas turbine exhaust. This boiler is heated by the gas turbine exhaust, and the generated steam drives a turbine. It's combined cycle because there's two heat engines: the jet engine which is driven by hot air, and then the steam turbine driven by steam generated from the jet engine's exhaust. It's tapping into waste heat to generate steam, and that steam drives a turbine. There's two Carnot cycles happening. One in the gas turbine, one in the steam turbine.
There's no such thing as a combined cycle nuclear plant, no matter how much thermal energy it can put out. The plant heats water which drives a turbine. If you have a reactor that generates more heat, then you can generate more steam and drive a larger turbine or additional turbines. But there's still only one heat engine, one cycle.
I guess you could use the heat exchanger as a second steam generator to drive a second turbine. But in order for that to work, the first steam turbine would have to be very inefficient and deliver a lot of waste heat to the second turbine. It'd be better to just drive two turbines in parallel or a larger turbine.
In fossil fuel use the gas turbine works by internal combustion -- heat is added by a fire inside the device and the exhaust goes out the back.
There are closed cycle gas turbines that put heat in with a heat exchanger and some have been built for non nuclear use but they haven't been competitive with alternatives (e.g. open cycle gas turbine) since they quit using mercury as the working fluid.
Something like that could heat up steam for a bottoming cycle. It's not a matter if it is possible, its a matter if it is practical.
A nuclear reactor can operate at much higher temperatures using coolants other than water, and it has been done (see the british AGR and the American FFTF) but there are a huge number of details that must be perfected to make it routine.
I think the GGP post is trying to suggest building a nuclear plant with high temperature coolant that runs a combined cycle generator. I don’t know whether this is actually cost effective.
Still needs the high temps!
Nuclear competes not just with fossil fuels but with "burn the fossil fuels, capture the carbon, inject the CO2 back into the ground option", which might not be so bad if this gets perfected
https://en.wikipedia.org/wiki/Chemical_looping_combustion