[thijson]

The control rods don't stop decay heat from being produced. That's what happened with Fukushima.

Also, nuclear poisons (neutron absorbers) build up after the reactor is shut down. After the control rods are withdrawn it takes a few days for the poisons to be burned up and the reactor can resume power production. I think they call that poisoning out the reactor.

https://en.wikipedia.org/wiki/Neutron_poison

[thijson]

When the Northeast power grid went down down in 2003, the Bruce Nuclear Power Development was almost entirely kicked off the grid. It was able to keep ticking over at a few percent of power output though because of a feeder line that went North. This prevented the reactors from poisoning out, and allowed them to come back faster than if they had poisoned out.