Instant Cooldown

The nuclear reactor is Going Critical! Alarms are sounding! Red lights are flashing! There is a giant timer on the wall counting down to the explosion and a filmy voice calling out the number of seconds left! Our hero must hurry! He flips the switch at the last possible second and everything is back to normal, just like that.

Wait, everything is 100% fixed? That can't be right, rewind a second. There's steam flying everywhere, pressure meters are off the scale! Look, you can even see the glass cracking! What do you mean, the reactor is perfectly fine?

That would be an Instant Cooldown.

The Instant Cooldown is the Sister Trope to Critical Existence Failure for reactors or other large explodey things. It means that the reactor is either running at perfect health or it just exploded and there is no middle ground. Security measures can indicate that something is about to go wrong but unless it actually does the device is once again fine.

The reality is, all that heat, steam and pressure takes time—days, at least—to dissipate and must be continually monitored, even under a normal, non emergency shutdown. In the wake of an emergency shutdown controlled venting required to bleed off steam pressure might be needed, and the super-high temperature of the steam, as well as to some degree the effects of radioactivity, is enough to partially break it down into oxygen and flammable hydrogen, which also needs to be dissipated.

The same idea behind Convection, Schmonvection, and very similar to Magic Antidote, but for machines rather than people. The most common aversion is when the overload/reaction/catastrophe is "too far gone" for a shutdown to stop it, which is a different kind of Hollywood Science. A Sub-Trope of Just in Time.

Film

 * Averted in Alien; when Ripley discovers the alien in the Nostromo's shuttle, she attempts to reverse the overload of the ship's engines, only to be told by the computer that it's too late. Similarly averted in Aliens when Bishop tells the Marines not to bother trying to shut down the atmosphere processing plant, because the chain reaction that will destroy it (and them, if they don't leave in time) has gone too far.
 * Return From Witch Mountain has exactly this scenario - the reactor is in the red zone (and the scientists have been saying for a while that it's almost at the point of no return); Tia mentally tries to fix the coolant system in a struggle with Tony (the needle wavers back and forth within the last quarter of the red zone), then Tia wins and the needle retreats into orange, yellow...(about 3 seconds of screen time) and the camera cuts away. It's not shown again but it's clear that things are back to normal (and even pulling up parts of the reactor room and crashing them together so that they explode doesn't disturb the reactor subsequently).
 * Spider-Man 2 both uses and subverts this trope. The first time Spidey tries to shut down Doc Ock's runaway fusion reactor, it turns off with no negative effects. The second time, it's too late. This is more justified than most, since the system is explained that it will implode on itself if it loses power before it becomes self-sufficient.
 * Played straight and justified in SF thriller Sunshine, which has a ship's computer that runs so fast it needs to be kept in a very, very cool coolant bath. The villain  One of our heroes manages to

Literature

 * Subversion: In Dan Brown's Digital Fortress an overheating super computer

Live Action TV

 * Star Trek in all of its incarnations had this happening on scales ranging from a hand phaser recently on overload to near-nova stellar events.
 * Semi-averted in the episode where the Self-Destruct Mechanism was invoked—as the countdown proceeds, Kirk states that the process will be irreversible if it reaches the five-second mark. The self-destruct is aborted just before that moment, without apparent aftereffects.
 * There's also the fact that the ship is powered by Matter-Antimatter annihilation. The Self-Destruct Mechanism isn't a reactor overload in the traditional sense, but rather a simple explosive disabling of the structures that separate the two fuel sources. The five-second mark is when the scuttling charge kicks in, blowing the safeties to hell and gone. Five seconds later is when the annihilation happens. Stopping the process any time before that point isn't 'cooling down' the engine, merely preventing the scuttling charge from detonating.
 * There was an episode of Zoey 101 that featured Miranda Cosgrove as a girl genius who had developed an alternative energy source that was tested at the school. The instafix when things go out of control is Zoey's room key.
 * Stargate Atlantis had an example where the ZPM powering the city was tampered with. Normally, the bugger has several failsafes that prevent the power grid from drawing too much power and overloading it; the Goa'uld-infested uploaded a program that disabled these then activated the city's inertial dampeners to produce the required energy drain. Cue the ZPM overloading (complete with it's otherwise steady golden glow flickering on and off) to the point where they barely managed to reenable the failsafes at the last minute before it would've detonated in an Earthshattering Kaboom. As soon as the failsafes were on however, the ZPM instantly stabilized.
 * Averted in the episode Trinity: McKay tries to shut down the Arcturus reactor but it had already gone too far.

Real Life

 * Video cards in computers cool down very quickly after exiting graphic-intensive video games, often-times managing to drop 20 degrees Celsius in less than 30 seconds. However, once you've fried a chip by overheating, it's done; might as well throw it in the trash.
 * The entire point of heatsinks is to get as close to this trope as possible; fans are used to move the heated air faster than natural convection would. A contraption like this would completely justify the trope.
 * If only it worked like that in real-world nuclear reactors. Even after shut-down, nuclear fuel still releases decay heat due to short-lived radioisotopes left over from the original chain reaction decaying, releasing radioactivity and heat. This heat that must be removed or else it will cause a meltdown. In fact, about half of a nuclear reactor's safety features are just there to allow decay heat to be vented away. Decay heat piling up after a cooling system failure was the main cause of the Three Mile Island accident and the Fukushima disaster.

Video Games

 * Subverted in Half Life 2 Episode 1: the containment field gets reactivated, but the Dark Matter reactor overheats and explodes anyway - all the field does is delay the explosion long enough for the last civilians to evacuate. It doesn't help that the Combine are trying to blow it up.
 * Subverted in Tales of Vesperia. After Rita fixes Heliord's barrier blastia at the last second, it blows up in her face. It then works perfectly again after the explosion.
 * Played straight in Metroid Fusion, after Samus reactivates the Main Boiler cooling unit, everything returns instantly back to normal.
 * At the end of Portal 2, the Big Bad's mismanagement of the Enrichment Center has brought it to the brink of a nuclear meltdown and the Final Battle is a race against the clock to put back in control before the whole place explodes. Meanwhile, the facility is breaking apart around you with flames everywhere, alarms, you name it. After the fight is won, the next scene, some hours later, shows the facility restored to normal functioning and fully repaired, with not even a Hand Wave as to how this was accomplished.
 * Done in Conduit 2 with the pulsing, antique runaway Nazi nuclear Doomsday Device that Prometheus disables ten seconds before it blows.

Web Comics

 * Averted in Schlock Mercenary, where an antimatter reactor is damaged, and its operators start shutting it down. A military idiot then orders full power, and the thing detonates.

Western Animation

 * Occurs a couple of times in The Simpsons.