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
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.