> Nuclear is not: it needs grid power to be able to run.
But the "grid power" needed to run the reactor's control systems, etc., can almost surely be generated by the reactor itself while it's online, right?
A naval nuclear reactor needs external power only to get started; when the ship is in port, this will typically be shore power from the grid, and either in port or at sea the external power could come from the onboard emergency diesel generators. When the reactor is started up, it will eventually "go critical," i.e., achieve self-sustaining fission. After that happens, one or more in-plant, steam-turbine electrical generators will be started and brought online to provide electrical power for the ship, including the reactor control systems, etc. [0] A similar arrangement seems likely to be used for civilian power plants. [1]
> But the "grid power" needed to run the reactor's control systems, etc., can almost surely be generated by the reactor itself while it's online, right?
In theory, yes. But in practice, most nuclear power plants are required by law to trip offline if their local grid goes down or even experiences a minor disturbance [0]. Nuclear power plants always have onsite diesel generators to run pumps, operate control rods, and shut down the reactor if needed.
Nuclear plants are notoriously finicky about offsite power quality because they take a long time to ramp up and down and the consequences of e.g. losing cooling are potentially disastrous.
But the "grid power" needed to run the reactor's control systems, etc., can almost surely be generated by the reactor itself while it's online, right?
A naval nuclear reactor needs external power only to get started; when the ship is in port, this will typically be shore power from the grid, and either in port or at sea the external power could come from the onboard emergency diesel generators. When the reactor is started up, it will eventually "go critical," i.e., achieve self-sustaining fission. After that happens, one or more in-plant, steam-turbine electrical generators will be started and brought online to provide electrical power for the ship, including the reactor control systems, etc. [0] A similar arrangement seems likely to be used for civilian power plants. [1]
[0] e.g., https://fas.org/man/dod-101/sys/ship/eng/reactor.html
[1] e.g., https://www.elprocus.com/what-is-a-nuclear-power-plant-worki...