[gunapologist99]

It seems like you might really be asking about a meltdown scenario.

The reaction can be slowed with control rods, which stops/minimizes the heat from being generated. The previously generated heat still needs to be handled, however (by evaporation).

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

A meltdown occurs when the reaction can't be slowed down through normal means because the safety systems fail.

For example, Fukushima and Chernobyl:

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

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

However, those safety systems are obviously designed expressly to prevent such a disaster. For example, control rod systems are often designed to be fall (via gravity) into the reactor in the event power fails. These failsafe systems are typically very reliable, in the absence of other external events (such as the flooding and earthquake that took place in Fukashima.)

[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 After the control rods are withdrawn it takes a few days for the poisons to be burned up and the reactor can resume power production.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.

[euroderf]

> These failsafe systems are typically very reliable

From the RISKS archives:

"A fail-safe system fails by failing to fail safe."