One of the things that always bugs me is the contradiction between 'fusion reactors cannot explode when the plasma is vented in an uncontrolled fashion' and 'jumpjets and mech flamers work by venting fusion reactor plasma'. That contradiction sums up why I ignore fluff and just play the game.
Obligatory useless trivia:
At ten billion degrees, a gram of hydrogen plasma has a thermal energy of about a gigajoule, or nearly a ton of TNT. Assuming that at those temperatures, fusion is a billion times as powerful as the conditions at the core of the sun, that gram of plasma will have a thermal output of 7.5 kWatts (the rest being lost as neutrinos). Given the energies attributed lasers and gauss rifles and the ability to fire them every ten seconds, the rupturing of a fusion reactor really should approach Halifax blast proportions. As conversion efficiencies will be less than 100%, the contained energies will be larger.
I came to the same conclusion looking at it from just a rule perspective.
Consider:
1) A mech energy weapons doesn't use any capacitators worth notice. They are thus powered directly by the reactor.
2) Said reactor can easily power 50+ points of damage
to a distant target.
3) From the reactor to the target you have massive losses, first when you extract energy from the reactor, then when you power the weapon, and finally when you dump the energy from the weapon to the target. Being extremely optimistic that's at least 90% losses along the way.
4) Conclusion: At any instant the reactor puts out easily
500 points of damage! You think a ton of MG ammo in the CT is bad? Better hope the emergency systems manage to vent the plasma before the containment fails!