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According to various supplementary sources, the atmosphere processor on Acheron was powered by a fusion reactor. As far as I know, it is not physically possible for a fusion reactor to explode like a nuclear bomb unless it was using antimatter. The supplementary materials give absolutely no evidence that antimatter reactions were involved.

Given the above, how did the nuclear reactor explode like a nuclear bomb?

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    Because writers aren't nuclear physicists. – TheLethalCarrot Jun 21 '18 at 14:19
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    Since the sort of high-energy fusion power seen in Aliens is an entirely fictional concept, who's to say what is and isn't possible? – Valorum Jun 21 '18 at 14:25
  • @Valorum: fusion power is not a fictional concept. We know the science behind it, we just have yet to find a practical way to produce it. – Anonymous Jun 21 '18 at 14:36
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    Did you fully read the reference for fusion reactors? It is an article acknowledging about fictional science, and in the fusion reactor explosion section, the author's exact words are "More energy would be added by the fusion plasma, although I have no idea how much that would be" – as.beaulieu Jun 21 '18 at 14:46
  • Try this reference from ITER, an organization that is actively involved with fusion reactors, and hosts one themselves: iter.org/sci/Fusion – as.beaulieu Jun 21 '18 at 14:49
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It's clearly spelled out in the movie1:

  1. They went into the alien nest, which was built by the aliens right up against the primary heat exchangers.
  2. Ripley noticed that and pointed out to Lt. Gorman that if they fire their explosive-tipped 10mm caseless light armor piercing rounds from the M41A pulse rifles, they might rupture the cooling system.
  3. Burke confirmed that a rupture of the cooling system would lead to a thermonuclear explosion and it's 'adios muchachos'.
  4. Lt. Gorman ordered all magazines (and M56 smartgun batteries) be confiscated, but PFC Vasquez had two extra batteries which allowed her and PFC Drake's smartguns to be powered after surrendering their primary batteries.
  5. The aliens attacked the platoon after members of the platoon killed a chestburster.
  6. The platoon members fired on the aliens with flamethrowers, pistols, a shotgun, and both M56 smartguns, the latter almost certainly ruptured the cooling system.
  7. Cooling system rupture was later confirmed by the synthetic (sorry, artificial person) Bishop, who noticed emergency venting.

As to the mechanism, one way to have a fusion reactor is to put a fusion explosion in an electromagnetic bottle. The electromagnetic system could be built using superconductors, which would have to be kept cold. Failure of the electromagnetic bottle would release the fusion process, which could also rapidly consume its own fuel supply if that is nearby.

In addition to the 1 TW fusion reactor that powers it, the atmosphere processing plant (APP) processes the atmosphere by first converting it to a plasma at temperatures around 5000K. That plasma is also kept in a magnetic bottle and then magnetically separated into component elements which are then either removed as waste or recomposed and released into the atmosphere.2 The combined failure of both magnetic bottles at the same time in presence of potentially large amounts of H2 (produced in the terraforming process) would seem to be the cause of such an explosion.

  1. http://www.imsdb.com/scripts/Aliens.html
  2. http://avp.wikia.com/wiki/Atmosphere_Processing_Plant
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    Fusion reactors don't need cooling systems, unlike fission reactors that need cooling systems to prevent the heat generated by fission from melting the reactor. Fusion reactors need heating systems to heat the plasma to the needed temperatures for fusion to occur as well as containment systems to keep the plasma at the high pressures necessary for fusion. If any part of the fusion reactor fails, fusion will instantly cease. Therefore, it is considered to be scientifically impossible for a fusion reactor to explode. – M. A. Golding Jun 21 '18 at 15:25
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    Okay, fusion reactors do require cooling to keep superconducting containment magnets at the correct temperature. But if the superconducting containment magnets overheat and fail the small amounts of plasma will simply disperse and lose the density required for fusion, and fusion will cease. There won't be an atomic explosion. A fusion reactor has to constantly maintain the conditions necessary for fusion and if anything fails, fusion ceases. "Do we know how such a reactor would work?". Only by atomic physics calculations. – M. A. Golding Jun 21 '18 at 15:45
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    @M.A.Golding Not a real fusion reactor. Facts about real fusion reactors don't apply. – Todd Wilcox Jun 21 '18 at 15:46
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    so if the Alien francise has different laws of nuclear physics, how do stars shine similarly to stars in our universe, and how can there be a species that seems to similar to Earth humans with a similar society to ours? – M. A. Golding Jun 21 '18 at 15:54
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    @M.A.Golding If it has the same laws, how is there even a 1 TW fusion reactor? We don't know. Since even a 1 W (net usable power output) fusion reactor doesn't exist, we can't say at all what a 1 TW reactor would be like. One tiny example set of problems to be solved for a 1 TW reactor: Where's the deuterium coming from? What about tritium? If tritium isn't used, how is the deuterium only reaction made efficient? What's happening to the neutron streams - lithium shielding? Where's the lithium disposed of? etc., etc. – Todd Wilcox Jun 21 '18 at 16:16
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Short Answer: This explosion was exaggerated by fiction and action. A real fusion reactor with the knowledge we have to date suggests that a nuclear fusion reactor cannot explode.

According to ITER, which is described as by their description in Wikipedia:

ITER is an international nuclear fusion research and engineering megaproject, which will be the world's largest magnetic confinement plasma physics experiment.

On ITER's website, they list several advantages for fusion reactors over fission reactors. As for risk of meltdown this is their view:

No risk of meltdown: A Fukushima-type nuclear accident is not possible in a tokamak fusion device. It is difficult enough to reach and maintain the precise conditions necessary for fusion—if any disturbance occurs, the plasma cools within seconds and the reaction stops. The quantity of fuel present in the vessel at any one time is enough for a few seconds only and there is no risk of a chain reaction.

So according to their explanation, the plasma is concentrated for effect with magnetic fields. In reality, once the magnetic field loses containment, it disperses and loses effect within seconds. Even if the fuel supply is kept exclusively nearby, the fuel itself is inert in terms of nuclear fusion under normal conditions, and the plasma would already begin to disperse before travelling far enough towards the additional fuel. To further this argument, here is a video of a journalist visiting the JET reactor at Culham Centre for Fusion Energy. There, they talk about the difficulties of obtaining that plasma state in the first place, let alone maintaining it.

So there you have it, while we are all relived that the Xenomorphs were vaporized in a fictional fireball. The terrifying reality is that LV-426 would still be infested with aliens.

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