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.
- 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 it's explained that the system 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 retracts the modules from the tank and jams the shifting mechanisms in order to destroy it. One of the heroes manages to restore some of the modules, which brings them back on line, but not enough.
- Averted in Alien After the Nostromo's reactors are intentionally set to overheat by shutting down the cooling systems, the ship's computer repeatedly warns throughout a ten-minute countdown that after five minutes the process is irreversible. And it's telling the truth. Ripley turns the cooling units back on just a few seconds too late and the reactors detonate five minutes later.
- Also averted in the sequel. By the time Ripley and the surviving Marines notice the atmospheric reactor's emergency venting, it's already too late to shut it down and prevent meltdown. They have a few hours to find a way to evacuate before it explodes.
- The China Syndrome averts this trope. The crisis at the Ventana Nuclear Plant begins after the reactor is safely shut down. A faulty gauge leads the operators to believe the containment vessel is flooded so they open multiple relief valves to dump the coolant, resulting in an immediate Oh, Crap! when they realize too late that the water level is critically low, and they're about to incur a Title Drop when the still-hot core is exposed.
- 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: 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 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 its otherwise steady golden glow flickering on and off) to the point where they barely managed to re-enable the failsafes at the last minute before it would've detonated in an Earth-Shattering Kaboom. As soon as the failsafes were on, however, the ZPM instantly stabilized.
- 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.
- Torchwood: In the episode "Exit Wounds", bombs are detonated around the city. This causes a power loss at the nuclear power plant, causing a meltdown in the reactor (similar to what happened at Fukushima when the cooling equipment was damaged). So far so good. But then Owen fiddles with some controls in the control room that somehow "vents the radioactive coolant through the control room" (?) and causes an Instant Cooldown of the reactor. The control room wouldn't even be in the same building as the reactor.
- Chernobyl: as shown in the Real Life section, the design of the Soviet RBMK reactors actually increased the reaction for a short while when SCRAM (or AZ-5, as the Soviets called it) was initiated due to the graphite tips on the boron control rods. The flaw had been discovered ten years previously, but Soviet authorities clamped down on the knowledge and declared it a state secret, all to avoid looking bad and being forced to fix the problem. Dyatlov was being so reckless when trying to get the test completed specifically because he believed that he always had the AZ-5 failsafe in case something went wrong. So when the reactor started to overload, he ordered the AZ-5 button pushed. All the rods dropped, but the temporarily increased reaction damaged the control rod channels, keeping the graphite tips in perpetual contact with the core, accelerating the reaction. Moments later, the enormous steam pressure blew the steel lid open, and the air the rushed in caused the whole reactor to explode.
- Metroid Fusion: After Samus reactivates the Main Boiler's 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 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 Hand Wave as to how this was accomplished.
- However, this is lampshaded a bit: it's explicitly stated that the countdown is not to the point when the reactor actually melts down, but to when it becomes impossible to keep it from doing so.
- Done in Conduit 2 with the pulsing, antique runaway Nazi nuclear Doomsday Device that Prometheus disables ten seconds before it blows.
- Averted in Half-Life 2 Episode One. The Combine deliberately shut off their Dark Energy reactor's failsafe to cause it to Go Critical. At one point, Alyx and Gordon rush to reactivate the failsafe, but at no point is it suggested that this will stop the reactor: it's already too damaged. What reactivating the failsafe will do is delay the explosion a bit, allowing more time to evacuate the surrounding area.
- Zig-zagged in the MechWarrior series, where the Humongous Mecha are powered by potent nuclear fusion reactors, which build up heat during strenuous combat. Past a certain heat level, the mech begins to take internal system damage, until the reactor bursts inside its chest. Active fusion reactors can cool down quickly, but its often not fast enough to overcome the heat before it bursts, so a mech can either perform an emergency shutdown (rendering it totally helpless for a few seconds, but dramatically improving its cooling) or in some games flush excess coolant, which has no inherent downsides but has a limited supply of coolant. In some games, it's straight up impossible to die to overheating when shut down, while in others the latent heat can still kill a shut down mech.
- Nicely averted in Analogue: A Hate Story. Shutting down the Mugunghwa's failing reactor does prevent it from going critical, but still leaves a dangerous amount of built-up heat to deal with. This is ultimately solved by venting the ship's atmosphere into space as a crude heat sink.
- In 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.
- If only it worked like that in real-world nuclear reactors. In real life, a shutdown (known in the United States as a SCRAM) is largely a matter of turning an immediate and overwhelming problem into a big-but-manageable problem spread out over time. 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 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.
- Another thing to consider is that the uranium dioxide fuel itself is an extremely poor conductor of heat. During ordinary operation, the outside of the fuel pellet can reach several hundred degrees Celsius, while barely a centimeter away, the center of the pellet can be hotter than the surface of the sun. Even if there is no more fission taking place, the cooling systems have to keep operating for days before cold shutdown is achieved.
- The Chernobyl disaster managed to invert the trope, thanks to the counterintuitive Soviet control rod design. To make a long story short, in the case of a shutdown, the rods would eventually cool the core as intended- but for a brief moment, the reaction would instead accelerate. That was all the time Chernobyl needed for a runaway reaction to go completely out of control and turn from a problem to a disaster.
- Ever wondered why many nuclear power stations are built near the coast or some other large body of water? It's so that the core can be cooled with the local water source and not have to have water pumped in, and eliminates or reduces the need for massive cooling towers. However, it's also convenient because 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 unlike during normal operation, the water enters the primary coolant loop instead of going through a heat exchanger, i.e. directly interacting with the core, but it's better than going full China Syndrome.