I was wondering why nuclear plants require so much power to start up, when really you can't "shut off" uranium's power-generation ability. I guess it would have to do with powering up all the safety and management systems around the generator?
Uranium (or other radioactive materials) don't directly generate much electricity - they mostly generate heat. Most nuclear power plants generate electricity by having that heat turn water into steam, which is then used to power steam turbines that actually drive the generator coils. Of course, in order to do this, you have to be able to pump water into the reactor to be turned into steam, and that pumping requires power. So yes, while you definitely need power for the safety/management equipment, the major bootstrap issue is almost certainly all of the power necessary for the pumps.
Generating plants using steam turbines require station service power of up to 10% of their capacity for boiler feedwater pumps, boiler forced-draft combustion air blowers, and for fuel preparation.
Modern nuclear plants have two cascaded cooling loops and run at about 33% efficiency. This means that a 1000MW plant has 3000MW of thermal energy to manage at full load and 2000MW of energy to dissipate safely. The systems to do this take a great deal of power to run, so can't be bootstrapped off of batteries.
Also interesting is that power prices will occasionally go negative in cases where there's a 'surplus' of generation. What's happening in these cases is that generation owners are making the judgement that its more cost effective to pay people to take their power than it is to shut off their plant. Shutting down a big plant is an event, and it takes a while to get them back running. Sometimes, shutting them down doesn't make sense.
Also about ten percent of the heat energy is released later on, delayed, decay heat and all that. So there is no off switch. If you were running at 3000 MWt, 5 minutes after you scram you still have to do "something" with 300 MW of heat, and you don't want to have to rely on just one diesel. So the feds require multiple sources of power to operate, one of which is usually the diesel and one is the the grid.
Lets say your control and cooling systems require 5% of the heat they handle, that means your 3 GWt nuke would require an absolute minimum of 15 MWe to spin down. Or four if you want to run off grid so you've got two completely independent sources of power each with a 24x7 hot backup (I believe no one ever took the NRC up on that offer, but I could be wrong)
