This is TEOTWAWKI stuff.
The scariest thing I ever heard was talked about on Coast to Coast one evening.
All nuclear plants are required to have the ability to generate (external) power for at least 8 hours if they shut down or go off the grid. Because they need to keep pumping cooling water in until they can at least get the fuel rods withdrawn and secured.
Otherwise, we get like what happened in Japan.
OK, so far, no biggie.
Problem is most plants, no matter what, are pretty much on their own after 30 hours or so.
The real problem is the entire power grid depends on these large (90KV to 150KV) transformers.
One decent solar storm could knock out a couple hundred of these across the grid, normally there are about 3,000 online.
And we have zero backups of these transformers. Or so close to zero, it might as well be zero.
So a decent solar storm or EMP, and we have probably less than 30 hours before we have not just one event like Japan, but maybe 50 or 100 all happening at the exact same time.
To shut down a nuclear reactor you insert neutron absorbing control rods. You do not remove the fuel rods. After the control rods are inserted the fission reaction is quenched but radioactive decay continues to release heat (not at the level of full power output) which necessitates maintaining a flow of cooling water.
That is only one problem, there are "spent" fuel rods, stored on site in a water filled pool. The spent rods are still undergoing radioactive decay and are releasing heat so the pool needs to be kept at level well above the stored rods. The worst release of radioactive material at Fukushima came from the fuel storage pools when the water level dropped below the rods, which melted their casings and released decay products along with some plutonium to the air.
Regards,
GtG