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I suspect that these plants were all scrammed (rapidly shutdown). The biggest constraint on re-starting, I would guess, is going to be whether the grid is restored and stable for the individual plant and how long it will take to work through the necessary paperwork

When the grid fails like this, the effect on the Nuke plant is that all of a sudden they go from 99% load to 0%. Has a h**l of an effect on generators so you have to shut them down pronto to avoid damage. This means no more power from the turbines. But to stop the turbines, you have to redirect the flow of steam, thus the large blow off of steam.

Without the transfer of heat to the steam system, the reactor will rapidly overheat. To prevent this, the uranium is allowed to return to their graphite housing which breaks the nuclear reaction - scramed. This whole process can be completed in a few min.

The problem is bring stuff back on line. It is my understanding that after a scram, the inspection / certification / paperwork to reactivate can take WEEKS.

We could have a major problem restoring power.

"Without the transfer of heat to the steam system, the reactor will rapidly overheat. To prevent this, the uranium is allowed to return to their graphite housing which breaks the nuclear reaction - scramed. This whole process can be completed in a few min." ß Lets try more in the order of 7.8 to 8.2 seconds. Been there done that.

"The problem is bring stuff back on line. It is my understanding that after a scram, the inspection / certification / paperwork to reactivate can take WEEKS." ß But usually back at power the next shift.

Certainly when the grid goes away, a power plant has to shed a lot of energy (and steam in the case of a nuke or a coal-fired plant) in a hurry. The reactor pressure will initially go up but it will very quickly go down due to the rapid cooldown of the reactor coolant system. The reactor pressure control system is designed to handle all of these effects.

I'm not sure what this graphite housing is. When the reactor is tripped (shutdown), the control rods, which absorb neutrons and shutdown the chain reaction, are rapidly inserted into the core. Control rods are typically made of silver, indium, and cadmium, although some reactors use boron carbide rods (the boron is the neutron absorber in this control rod).

It does not take weeks to start back up a reactor. If you know why the reactor was tripped (in this case we do) and all the equipment is functioning, then the reactor can be very quickly re-started. I suspect that the shutdown reactors will be res-starting any time now (it's about 8:30 am, Friday as I write this).