What any device called a reactor does. May be called "a meltdown", "destabilizing", "going critical", or something more fanciful.
The failure of an actual nuclear reactor will often be described with the actual term "meltdown", but it will not resemble any meltdown known to science. The resulting explosion will be suspiciously similar to that of an atom bomb, or at least large enough to blow the vehicle/facility in question to pieces. Alternately, if the main result is a release of radiation, it will be described by a huge red circle on a map. The size and danger level of the circle may suggest even more death than an atom bomb would cause.
Sci-fi reactors are usually based on the idea that a nuclear reactor is a continuous nuclear explosion in a really strong box. By extension, reactors in the future are a different Sealed Evil in a Can, just waiting to blow the hell out of everything once the shielding is cracked.
These reactors are almost a Chekhov's Gun situation. Calling any device a "reactor" is your cue to expect a spectacular explosion. A drive strands a ship in deep space, a generator subjects characters to an environmental hazard, a reactor removes something from the plot forever.
In real nuclear physics, "critical" means the reaction sustains itself. A reactor is critical if it's on. Relatedly, "supercritical" simply means the reaction is increasing in power. An explosive surge of power requires the reactor to go promptcritical, something that may have happened only once by accident (in the SL-1 reactor accident and maybe at Chernobyl, but the consensus is that it was more likely to have been a steam-explosion like an overloaded household water-heater but moreso). Even in that case, the reactor will explode well before the power output reaches atom bomb levels.
(Designed bombs, on the other hand, go prompt supercritical.)
To be specific, making a nuclear explosion not only requires compressing a mass of fissile material - something that emphatically does not happen in a nuclear reactor - but keeping it compressed for a long enough time, giving the runaway "prompt critical" reaction the time it needs to build up a bang. This is a very exact science: explosive lenses, drivers, and the fissile core have to be fitted perfectly, using machines so precise that they are overkill for polishing glass lenses. If anything is off by the slightest bit, you wind up squirting fissile material out of the spots of weak pressure in the detonation shockwave, which makes a radioactive mess but doesn't make a bang.
Meltdowns are just that - the fissile core melts into slag, hot enough to flash coolant into steam (wherein you get the associated bang) and possibly melt through the reactor vessel. Since reactors currently in use are designed with safe failure modes in mind (including the famous manually triggered SCRAM) the worst you really get from a on a land-based reactor meltdown is that the reactor pile becomes a pile of reactor. It makes a very expensive mess, but isn't going to destroy much of anything outside the reactor containment facility itself, much less wipe a city off the map like nuclear bombs are capable of doing. On a sea vessel, however, the contamination would be catastrophic: the reactor mass would come into direct contact with the seawater, and shatter or even be entirely vaporized to small particles (fallout) in the massive subsequent vapour flash explosion.
Radiation will be an issue inside the facility, but widespread fallout of the kind associated with nuclear war won't be a problem unless the containment systems have been ruptured... which, give that there was probably a steam explosion during the meltdown, they very well could have been — and, indeed, in both catastrophic meltdowns that happened in reality the containment was breached, resulting in massive contamination. *
In the James Bond movie The World Is Not Enough, The Dragon Renard attempts to turn the reactor in a nuclear sub critical by inserting a rod of weapons grade plutonium. In the bare reactor. With nothing more than a shirt protecting him from hard radiation. Granted, he wasn't expecting to survive the act, but Bond is there with him. This might be slightly forgivable given that the intent was to contaminate the whole area, not necessarily blow it to hell. Of course, nobody except Dr. Jones actually understands how inserting the weapons grade plutonium into the reactor would cause an 'instant, catastrophic meltdown'.
B-MovieThe Swarm had a bunch of killer bees turn a nuclear power plant into a nuclear bomb in less than a minute, somehow.
At the end of Resident Evil Apocalypse, Raccoon City in its entirety was destroyed by a tactical nuclear missile in an attempt to wipe out the T-virus outbreak. Later on, there are news broadcasts shown with evidence recorded by the protagonists discredited. One reporter even made the statement that a nearby nuclear reactor exploded by "going critical."
In The Dark Knight Rises, a physicist finds a way to turn nuclear fusion reactors into multi-megaton nuclear bombs by making them "go critical". Not only is it completely impossible in real life to do so, but the process (supposed to be complicated enough that the physicist is the only person in the world to understand it) takes at most a few minutes, on a reactor design he has no way of being familiar with, and afterwards you can extract the "core" (whatever it is) from the reactor and use it as a highly portable nuclear bomb that can't be defused. Awesome, really.
Made a Justified Trope with the ultimate killer of most defeated starships in Honor Harrington. It turns out that 41st century fusion reactors keep their plasma fusing by the "simple" expedient of compressing them to stellar pressures using a gravity-based Containment Field. This makes for reactors that produce much greater output than more traditional fusion reactor designs, as per several infodumps in various places. However, the nasty side effect is that even though a breached fusion reactor instantly stops reacting (just as in a real-life fusion reactor), the pre-existing heat and pressure still make a truly satisfying kablooey.
Averted in the novel The Hunt for Red October: fatigue-induced failure of an important valve in the cooling system for the reactor of a Russian submarine causes a meltdown in its reactor, which in turn creates a glob of radioactive slag that melts its way through the bottom of the sub, eventually sinking it.
Knight Rider 2008, "Knight of the Iguana". According to KITT and Doctor Graiman, being hit by the Stolen Military Uber-Missile of the week will cause a California nuclear power plant to explode — complete with The Deadliest Mushroom, turning into a giant atomic bomb. Mind you, it may have been their intent that this have something to do with the specific and unusual nature of the missile used. But it's more likely just that this show plays hard and fast with the laws of physics.
In the Torchwood episode "Exit Wounds", a bunch of bombs went off, causing all electricity to cease, which caused a reactor to go critical. In cooling it down, Owen died.
Edge Of Darkness has probably the most realistic depiction of a criticality event in any fictional work not directly based on a real one. It's outside a reactor, though.
The music video of the song "Dancing with Tears in my Eyes" by the band Ultravox features a nuclear power plant accident which should result in an explosion according to some warning signs. The explosion is implied to even hit a house which is in a rather large distance to the power plant. (And killing the family of the plant worker which is the protagonist of the video.)
In Syphon Filter: Logan's Shadow, one level has you aboard a recently sunk nuclear powered navy ship with the nuclear reactor about to "go critical" forcing you to hurry and shut it down by removing the fuel rods instead of inserting control rods as would be done in reality.
The titular house in Maniac Mansion was powered by a nuclear reactor which could explode if it overheated, if the house's power was turned off and the reactor short-circuited, or if the player pressed the bigredbutton in the pool. Probably Justified by the fact that the reactor is extremely poorly constructed due to the Big Bad having a serious budget problem, to the point where he has to use his swimming pool to cool the fuel rods.
Portal 2: The Aperture Science Enrichment Center's nuclear reactor spends, all together, well over half the game either going critical or warning you that it's about to go critical.
A major event midway through Analogue A Hate Story. When it starts, you're given an Exact Time to Failure of twenty minutes, and it's made pretty clear that this is not enough time to get out of the blast radius, despite the Player Character being in pretty much the best possible position to be doing so.
Bioforge: Stopping a moonbase nuclear reactor from meltdown is one of the things you've got to do.
G.I. Joe: Resolute does the above too, where Cobra has turned a number of nukes into a reactor. Scarlett has no idea how they did that.
Pretty much any accident or potential accident involving the Springfield Nuclear Power Plant on The Simpsons is portrayed as a nuclear explosion, including one where Professor Frink projected everyone in a large radius would be killed instantly.
In the episode "Homer Defined," the reactor begins to go critical, and Homer (of course) does not remember his training for what to do during this very event. He picks a button at random with "Eeny Meeny Miney Mo"...and luckily for him, that's the button to initiate the SCRAM procedure. Everyone makes him out to be a hero, but he feels guilty because it was nothing more than a Felix Culpa.
Another episode "King-Sized Homer" had Homer gain weight so he could be considered morbidly obese enough to work from home. But then he decides to go see a movie, leaving a drinking bird in charge of the computer...which fails, and he has to go to the plant to stop the meltdown. He nearly falls to his death trying to press the manual shutdown button, but falls into a gas vent pipe, shutting down the reactor and preventing the release of radioactive gas by plugging the hole he fell into with his girth.
Earlier in that episode, Homer's computer asks "Vent radioactive gas?", when Homer responds no the computer insists "Venting prevents explosion." Which becomes a "Funny Aneurysm" Moment when in Fukushima reactor technicians did exactly that. And it still didn't prevent the explosion.
Justified in Mobile Suit Gundam and its subsequent sequels where Mobile Suits are powered by Minovsky Particle reactors — which explode when hit by beam weapons, but not if they're destroyed by conventional ordinance (In Gundam F91 the Shot Lancer is a weapon invented specifically to be able to hit a suit's reactor without making it go up). This became a major plot element in episodes of Victory Gundam and The 08th MS Team. However, there are some instances of explosions that are just plain silly.
Nicely averted in Cannon God Exaxxion; one particularly graphic scene features fusion-powered machinery that has been damaged by an explosion. Instead of blowing up real good, the stuff starts leaking hot plasma & horrifically burning anybody who gets near it.
Third-generation Arm Slaves in Full Metal Panic are said to use palladium reactors (older ones run on diesel/gas turbines). Judging from the Helmajistan ambush, these things pack quite a punch. One might even mistake the self-destructing Codarl at the end of the first season as a meltdown but he explicitly states that he packed a few hundred kilos of high explosive to make sure he can pull off a Taking You with Me. On the other hand, the onboard AI warned him that if he starts the sequence, there's no cancelling it. So he might have gone for a straight overload spiced with some extra HE for a bigger boom.
Palladium reactors are a real-life contraptions — they were proposed as vessels for the (sadly debunked) cold fusion. The fusion being cold, their best effort in blowing up probably would've been no more than a tank worth of gas. Enough to kill a person, but really nothing to write home about.
In an issue of The DCU comic book miniseries Identity Crisis, Firestorm is skewered by a sword, and is told to fly off for the safety of others as "everybody knows what happens if you puncture a reactor".
In the Star Wars series, the Death Star's reactor caused a space station the size of a small moon to explode like a plastic model full of gunpowder. Although considering the energy output of the thing was enough to blow up an actual planet just as violently, maybe that's not unreasonable.
The arc reactor in Iron Man 1 isn't nuclear (in fact, it's safe enough that Tony Stark can walk around with a miniature one implanted in his chest), but under the right circumstances, can be triggered to produce a very satisfying boom.
Blowups Happen is a science fiction short story by Robert A. Heinlein. The story is about a nuclear reactor which not only is in danger of exploding at any moment but is discovered to be capable of destroying all life on Earth by having such a massive explosion that the Earth's atmosphere is blown away.
We also see this in BattleTech fiction, where BattleMechs are routinely powered by fusion engines that sometimes explode from taking sufficient damage — the explosion isn't actually described as of nuke-caliber, but can inflict quite some damage on anything nearby. One rulebooks hangs a lampshade on this by patiently explaining (as part of a in-universe lecture on 'Mech systems) that such explosions are not, in fact, nuclear. Just how much more plausible the suggested alternative of air rushing into the breached reaction chamber, promptly getting heated up to several thousand degrees, and thereby causing the observed 'fireball' is a mystery.
"Stackpoling" is the term in the fandom for a Battlemech's reactor doing this, due to author Michael Stackpole using this trope repeatedly in his novels.
As part of a military operation in one of the New Jedi Order novels, Admiral Kre'fey programs the reactor on a decommissioned and unmanned Interdictor cruiser to go supercritical once its shields go below 20%. Justified in that it was intentional, and also possibly justified because most ships in the Star Wars galaxy use hypermatter reactors, which we have no idea how they work.
In Star Trek, most reactors contain anti-matter. Should the containment fail, the anti-matter would contact normal matter. This results in a massive explosion. Aside from the Romulans (see below), the Borg are the only race who don't use M/AM reactors for power. Their power sources run on pure technobabble, it seems.
The Romulans in Star Trek use captured quantum singularities (black holes). Whether matter/antimatter is safer or not is something of a moot point.
In addition to the anti-matter used in warp reactors, impulse (STL) drives are fusion-powered. Apparently, their containment systems are a lot better than what the FTL drives use, as impulse reactors never seem to explode (unless deliberately rigged)...
Fusion powered reactors can't blow up. However—depending on the design—if they lose containment on the plasma, it will escape. Generally the reaction will cease, since every form of fusion needs to put the plasma under massive pressure...but the plasma might well vaporize the ship as it escapes.
In "The Doomsday Machine" (episode of Star Trek: The Original Series), Scotty jury-rigs the badly damaged U.S.S. Defiant so that its impulse drive can function. He says repeatedly that keeping it from exploding is taking all of his effort.
Discussed and averted in the Stargate SG-1 episode "Learning Curve." When a prototype naquadah reactor is powered up, it causes a harmless distortion which sets off an alarm.
Hammond: In the future, Major, before you activate any device that includes the word "reactor," I would appreciate it if you would notify me.
Stargate Atlantis, "Trinity": The team discovers the Ancient equivalent of the Manhattan Project. Rodney tries to make it work, but it fails miserably.
Weir: You destroyed three-quarters of a solar system!
Mc Kay: Five-sixths, but it's not an exact science.
It becomes a Running Gag:
Lt. Col. Sheppard: It took Dr. Mc Kay years to figure out all things Ancient and he still doesn't completely understand.
Dr. Mc Kay: [defensively] I have a very firm grasp of Ancient technology.
Lt. Col. Sheppard: You've blown up entire planets, Rodney.
Dr. Mc Kay: That wasn't my fault!
Lt. Col. Sheppard: Well, it didn't do it by itself!
In the tabletop game BattleTech, Battlemechs with their fusion reactor shielding shot out don't explode at all, unless you're playing with optional rules to make it more 'cinematic' - though even then, the explosion is more or less like a boiler explosion than a nuclear explosion. The real explosion danger comes from ammunition explosions, from either a stray laser blowing up all your autocannon rounds, or the heat of your engine causing the ammo to 'cook off'. The amount of ammo involved usually means this will gut your 'mech. Even the humble machinegun, the weakest ballistic weapon in the game, carries at minimum a half-ton of ammo and will ruin anything that is carrying it if that ammo lights up. This adds to the competitive balance: Energy weapons fail gracefully if hit (so the only way to stop a laser-boat quickly is by managing to hit the cockpit, failing that, you're going to have to keep walloping away at it.) but promote overheating. Missile and autocannon weapons have high strength to weight, but there's always the possibility of instant death by lucky shot.
Something like this trope is invoked with Gauss weaponry. The ammo is just an inert metal slug, but the capacitors that power the gun's electromagnets are prone to catastrophic explosion if they get hit.
In Halo, the titular ring, large enough to have its own ecosystem, is broken into pieces by throwing 4 grenades into a starship's engine containment field. "Wildcat destabilization".
A fusion reactor can't go critical; rather than going out of control, the reaction just stops. However, if, while the reaction is still functioning properly, one were to remove whatever is holding it in place (probably a magnetic field—the hotter fusion rockets would have to use those, rather than a rocket made of matter, because even diamonds would vaporize on contact with the plasma), it would vaporize everything within a very large radius. Of course, it's unlikely one would have the time to get away before that happened.
In Half-Life 2, A dark matter reactor in a Doom Fortress is enough to fling cars into the air and knock a train off its tracks at least a mile away.
Justified: The Combine forces were very specifically trying to get the dark matter reactor to explode.
Used to a degree in the course of the later games in the Mech Warrior series, 3 and 4 to be precise. Previous games had Mechs either explode into pieces or be rendered a standing but inert corpse (most likely as a limitation of the early game engines). 3, however, first introduced dramatic heat induced deaths. One might expect the normal death animation to play where the 'Mech catches fire, its torso goes up in flames, and it falls over amid a shower of ruined internal structures spewing from the machine. Not so. Instead, a Mech destroyed by excess heat goes up in a highly damaging mushroom cloud,almost certainly invoking this trope.Mech Warrior 4 simply had every destroyed Mech spew streams of blue-white light from its core as it fell, before exploding into chunky rubble loosely resembling the original chassis.
The Trope is invoked by name in the opening cutscene of MechWarrior 3 as well, here. The resulting explosion's shockwave evokes comparisons to nukes proper, or at least is so violent that it flattens buildings for a considerable radius and convinces the pilot of a 100-ton Assault Mech to get out of there in a mighty big hurry.
The second game of the series did it too. There were no huge balls of blue light or mushroom clouds, but the killed mech would be torn apart by a series of fairly small explosions, implied to be caused by weapons and the reactor. Stepping into the mess would cause damage to your Mech's legs.
The Crysis Warheadmod, Mech Warrior Living Legends, takes the critical explosions of the previous games up to eleven - when a mech goes critical (20% chance when destroyed), it glows white while a sound builds up, then it explodes in a huge, blindingly white mushroom cloud, stirring up dust which obstructs the battlefield.
Element Zero core meltdowns are shown to be quite spectacular in Mass Effect 2: The Arrival. When the cooling system of an "after-market eezo core" was deactivated it could detonate with enough energy to destroy a small planet; and a Mass Relay's core being destroyed has an effect comparable to a supernova.
Mass Effect 1 got there first. On Virmire, the Salarian STGs make a bomb out of their ship's eezo core with about the same yield as the bomb dropped on Hiroshima.
This is the goal of the easy path's penultimate level in Star Fox 64. The mission is to destroy the core of the Venom defense satellite "Bolse" to make the whole satellite explode. Interestingly, the source material claims that the core uses Andross' trademark bio-mutated energy alongside traditional Nuclear power, explaining why Bolse explodes so spectacularly when you succeed in destroying it.
In The Batman episode "White Heat", when Firefly absorbed a large amount of radiation. Batman said a meltdown wouldn't be a nuclear explosion because he didn't absorb enough radiation, he would just let out enough heat and radiation to destroy most of Gotham. Then he tried to use it to set off an actual nuclear reactor.