Eli5: Why do nuclear plants need external power? Couldn’t they just siphon off some of the power the plant makes to accomplish the same tasks (like an alternator running the electrical components in a car)?
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An airliner has 4 separate electric generators and an emergency battery. It’s called redundancy. Basically you calculate the worse scenario, design the plane in a way that work for that scenario, and add an extra backup system, just in case the worse scenario happens while you are one backup system short.
To guarantee you have the power to control and cool the plant in the worse scenario, you want to have at least 3 sources of electricity. One is the plant itself so the plant works even if the grid fails. One is the grid so if the plant fails the power is still there. One is a backup generator, generally driven by a diesel engine, that powers the plant if the plant AND the grid are failed.
Then each of these devices is not 100% reliable or may be down for maintenance.
So generally, you expect to have each of the 3 power sources actually subdivided in multiple sub system and redundant parts; meaning multiples ways the plant can power itself, multiple connections to the grid, and multiple generators. Last, the plant may have several overlapping/redundant electric circuits to guarantee that the available power from the 3 sources can be sent around the plant even with a damaged electric system.
You find the same concept in most man made vehicles/plants. Your car has a brake, an emergency brake, engine brake, can be steered off-road to slow down. That’s just a car and gives already 4 ways to slow down.
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