- " or when using any other high-temperature heat source, like a nuclear reactor. Because the steam generation happens at a constant temperature, which must be relatively low for reasonable pressures, the heat transfer from a high-temperature source is inefficient"
Nuclear (fission) reactors really aren't high-temperature heat sources. The working fluid inside reactors is itself liquid water, which doubles as a neutron moderator. That severely limits their temperature range (<374° C), so, there's no downside to limiting yourself to steam on the energy conversion side as well.
The linked articles make brief mention of a nuclear/sCO2 combination, but, to be clear, they're talking about radically different types of nuclear reactors. Not types that are currently commercialized/mature technology. Types where you replace the working fluid on the nuclear side with higher-temperature compatible substances—molten metals, molten fluoride salts, or inert gases like CO2 or helium.
edit: Also applies to nuclear fusion, I guess. IIRC, the proposed working fluids for those are molten lead/lithium, or molten lithium fluoride—both match with the sCO2 temperature range. (Lithium is the common factor, because the overriding concern of the working fluid is to transmute lithium into tritium, using the fusion reactor's neutron flux, to hopefully allow a sustainable fuel cycle).
Most existing nuclear reactors are cooled with water as you say, and they use steam turbines.
Nevertheless, for the future reactors it is desirable to use higher temperatures in order to increase their energy efficiency. This requires the use of other cooling fluids, as you also mention, and for such high-temperature nuclear reactors sCO2 turbines are preferable to steam turbines.
Nuclear (fission) reactors really aren't high-temperature heat sources. The working fluid inside reactors is itself liquid water, which doubles as a neutron moderator. That severely limits their temperature range (<374° C), so, there's no downside to limiting yourself to steam on the energy conversion side as well.
The linked articles make brief mention of a nuclear/sCO2 combination, but, to be clear, they're talking about radically different types of nuclear reactors. Not types that are currently commercialized/mature technology. Types where you replace the working fluid on the nuclear side with higher-temperature compatible substances—molten metals, molten fluoride salts, or inert gases like CO2 or helium.
edit: Also applies to nuclear fusion, I guess. IIRC, the proposed working fluids for those are molten lead/lithium, or molten lithium fluoride—both match with the sCO2 temperature range. (Lithium is the common factor, because the overriding concern of the working fluid is to transmute lithium into tritium, using the fusion reactor's neutron flux, to hopefully allow a sustainable fuel cycle).