“Can they just “turn it off?””
No. If you immediately stop the nuclear reaction (fairly easy to do) there is heat generated from the short-half-life isotopes that want to warm the core up more.
You need to remove that heat just to keep the core at the same temp.
If you have lost power you rely on offsite power and diesel gensets to run the megawatt or two of very big, reliable pumps to cool the core (if it’s water cooled). Plants are designed for this, emergency cooling is a big design/construction cost and effort.
From what I’ve been reading, they have power for pumps and valves but not enough water.
“If you have lost power you rely on offsite power and diesel gensets to run the megawatt or two of very big, reliable pumps to cool the core (if it’s water cooled). Plants are designed for this, emergency cooling is a big design/construction cost and effort.”
Fox News is reporting that at least at one of the nuclear power plants the generators are failing.