The nuclear reactor is GoingCritical! 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 [[JustInTime 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 SisterTrope to CriticalExistenceFailure 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 ConvectionSchmonvection, and very similar to MagicAntidote, 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 HollywoodScience. A SubTrope of JustInTime.
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!!Examples:

[[foldercontrol]]

[[folder: Film ]]

* The ''Franchise/{{Alien}}'' franchise:
** Averted in ''Film/{{Alien}}'' where the overload of the Nostromo's engine can only be reversed before the countdown's midpoint is passed; when Ripley discovers the alien in the ship's shuttle, she attempts to cancel the overload, only to be told by the computer that it's too late.
** Similarly averted in ''Film/{{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.
* Averted in ''K-19:The Widowmaker''. The primary coolant loop fails. The reactor is said to be "shut off" but still requires flowing coolant to the core.
* ''ReturnFromWitchMountain'' 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).
* ''Film/SpiderMan 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 ''Film/{{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 [[spoiler: retracts the modules from the tank and jams the shifting mechanisms in order to destroy it.]] One of the heroes manages to [[spoiler:restore some of the modules, which brings them back on line, but not enough.]]

[[/folder]]

[[folder: Literature ]]

* Subversion: In DanBrown's ''Literature/DigitalFortress'' an overheating super computer [[spoiler: is left without its cooling system on for too long and overheats even after the system was switched back on because it takes too long for the coolant to reach the circuits.]]

[[/folder]]

[[folder: Live Action TV ]]

* ''Franchise/StarTrek'' 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 SelfDestructMechanism 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. The ship is powered by Matter-Antimatter annihilation. The SelfDestructMechanism 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 ''Series/{{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.
* ''Series/StargateAtlantis'':
** 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 [[spoiler:Caldwell]] 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 [[PowerGlows 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 EarthShatteringKaboom. 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.
* In the second episode of the new Doctor Who, "The End of the Earth", the space station instantly cools off after The Doctor throws the emergency switch. The glass even self repairs after being hit with the Earth's sun going nova.

[[/folder]]

[[folder: Video Games ]]

* Subverted in ''VideoGame/HalfLife2: Episode 1'': the containment field for the Dark Matter reactor is reactivated, but it overheats and explodes anyway. The damage was already done, but the player is simply attempting to delay the explosion long enough to evacuate the city. Said situation isn't helped by the Combine trying to accelerate the explosion.
* Subverted in ''VideoGame/TalesOfVesperia''. 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 ''VideoGame/{{Portal 2}}'', the BigBad's mismanagement of the Enrichment Center has brought it to the brink of a nuclear meltdown and the FinalBattle is a race against the clock to put [[spoiler:[=GLaDOS=]]] 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 HandWave as to how this was accomplished.
** A blue screen preceding one of the test chambers in [[spoiler:Wheatley's track]] explains to [[spoiler:press any key in order to vent the radiological emissions into the atmosphere, and then consult the manual on how to repair the reactor core - something GLaDOS would naturally have in her body's memory banks.]] It's still fixed within ''minutes'' of the boss battle ending, however.
* Done in ''{{Conduit 2}}'' with the pulsing, antique runaway Nazi nuclear DoomsdayDevice that Prometheus disables ten seconds before it blows.

[[/folder]]

[[folder: Web Comics ]]

* Averted in ''Webcomic/SchlockMercenary'', where an antimatter reactor is damaged, and its operators start shutting it down. [[http://www.schlockmercenary.com/2005-06-14 A military idiot then orders full power, and the thing detonates]].

[[/folder]]

[[folder: Western Animation ]]

* Occurs a couple of times in ''WesternAnimation/TheSimpsons''.
* In ''WesternAnimation/{{Ben 10}}'' episode "Side Effects", Clancy tries to make a nuclear plant explode to wipe out the city. Grandpa Tennyson says that the reaction has gone too far to shut anything down normally. Heatblast, suffering from a cold, provides the "instant cooldown" to the reactor itself, and the rest of the plant dutifully follows.

[[/folder]]

[[folder: 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[[note]]this is useful because a larger temperature difference makes for faster heat dissipation[[/note]]. A contraption like [[http://www.newegg.com/Product/ImageGallery.aspx?CurImage=35-103-056-S02&SCList=35-103-056-S01%2c35-103-056-S02%2c35-103-056-S03%2c35-103-056-S04%2c35-103-056-S05%2c35-103-056-S06%2c35-103-056-S07%2c35-103-056-S08%2c35-103-056-S09%2c35-103-056-S10%2c35-103-056-S11&S7ImageFlag=2&Item=N82E16835103056&Depa=0&WaterMark=1&Description=COOLER%20MASTER%20Intel%20Core%20i7%20compatible%20V10%20Hybrid%20TEC%20RR-B2P-UV10-GP%20120mm%20CPU%20Cooler this]] would completely [[JustifiedTrope justify the trope]].
* [[AvertedTrope If only it worked like that]] in real-world nuclear reactors. Even after shut-down, nuclear fuel still releases [[http://en.wikipedia.org/wiki/Decay_heat 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.
* Ever wondered why many nuclear power stations are built near the coast or some other large body of water? It's so that in an emergency, they can open the sluice gates and use all those millions of gallons of water as a heat-sink. This isn't very good for the environment because the water enters the primary coolant loop, ie ''directly interacting with the core'', but it's better than going full China Syndrome.

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