Fusion reactors do occasionally die in spectacular manners. But most of the time, the fireworks are not actually from an exploding reactor. What typically happens is that some heavy weapon
manages to puncture the reactor itself. Since the reactor interior is a vacuum, air would get sucked in and mix with the plasma, stopping the fusion reaction. Kilograms of cold air mixing with
a tiny mass of plasma…well, that’s the wet-sand-and-torch analogy again. And, no, there’s not enough hydrogen in the reactor to really explode with the oxygen.
But while the plasma is cooling down from jillions of degrees—yes, “jillion” is a technical term, my youngest son assures me—the air is heated up to thousands of degrees and will promptly burst back out the hole in a gout of white-hot flame. Since a weapon heavy enough to puncture a reactor also generally destroys the core frame of a ’Mech, you get a blinding fireball accompanied by the ’Mech falling apart. It looks like a nuclear fireball bursting out of the ’Mech’s chest, but it’s literally just a load of hot air. And that’s a brutal way to kill a fusion engine. When you let oxygen loose inside an operating reactor, the super-hot oxygen just ravages the lining of the reactor and the delicate sensors and probes in there. It gets turned into a flashrusted mess.
Now, I earlier said that a reason the reactor shielding is so heavy is that it serves as a heat sink during a hard shutdown without a functional cooling system. I also said that there isn’t
enough heat stored in the “dead” plasma to damage shielding. Well, there are circumstances where this ultimate in passive safety systems can be overwhelmed, and you can get the fabled “nuclear reactor” explosion…though it’s more like a bursting balloon than a nuclear bomb. See, reactor shielding isn’t a great thermal conductor, so it takes time for heat to soak through the shielding. That means the interior of the reactor can get very hot while it’s waiting for the heat to soak outward. Engine designers know that and allow for that, at least for reasonable levels of heat left in the plasma. Over the centuries, some clever and stupid MechWarriors have figured out that if they overcharge the engine, then kill the magnetic containment field quickly, they can dump so much heat into the reactor walls that the reactor lining explosively evaporates. This over pressurizes the reactor, which bursts and causes a respectable explosion. Again, however, the effect is
not very much like a nuclear bomb at all.
