Nuclear reactors are a heavy operator. We must be ready to properly shutdown the plant.
If the whole system becomes corrupt, backups and shutdowns will be in place.
Yeah, but even if the reactor is shutdown by the SCRAM sequence, the core is still extremely hot (fission elements) and needs to be cooled. If the plant loses power and they can't get electricity back to the plant. None of the systems will function and they're dead in the water. This is exactly what happened at the Fukashima Daiichi nuclear power plant in Japan. Power to the plant was knocked out by the earthquake and tsunami, emergency backup systems such as the diesel generators kicked in to power the cooling pumps, then when they ran out of fuel, battery power kicked in, and when that died, the reactors boiled themselves dry, overheated, and exploded.
If you look at the design of the stricken Japanese nuke plant, and compare that to almost any other plant in the world. That scenario could theoretically happen in the US, or anywhere else in the world.
*Many of the plant's important systems and subsystems were severely damaged/destroyed by the earthquake/tsunami
*The plant lost electrical power
*The diesel and battery-powered emergency cooling pumps kicked in, but eventually ran out of fuel and power.
*Plant operators vented pure hydrogen gas from the reactors into the reactor buildings to relieve pressure, and then the gas ignited and blew the reactor buildings sky high.
*The reactors eventually lost flow of cooling water, boiled dry, the core was exposed, and melted down.
The perfect formula for a nuclear catastrophe happened in Japan.