^^ I assume thats a tricky one,. maybe if you vent it too quickly then the core will melt through your hull and do a big Kboomm because there's no plasma around enough to get rid of its energy but if you have a runaway M/AM reactor then you don't want it too blast a huge amount of energy into the drive plasma system because that will also make things go Kboom or melt right through your hull, they're quite a handfull those M/AM reactors aren't they?
This is not nearly as complicated as it probably seems...
Think of a "steam-powered" system. Yes, the Enterprise isn't steam-powered, so please, don't anyone try to misquote me as saying so (I hate to have to say that, but there are folks on here who'll seize onto any opportunity to do so, we've all seen it!).
In this case, let's pretend that the "plasma" is merely a conductor. An ionized, pressurized plasma would certainly be effective for that purpose.
Plasma... which is, remember, the fourth phase of matter (solid, liquid, gaseous, plasma... plasma varying from gas solely by virtue of the electrons having been stripped from the nuclei)... will not necessarily follow the
Ideal gas law (PV=nRT) perfectly, but is still going to follow the "general form" of of the law... that is, increase temperature, and you either increase volume or increase pressure (The difference between this and the classical version of the
Ideal gas law is due to the "semi-metallic" characteristics of plasma due to the free electrons providing additional attraction... whereas gas exists as uncharged atoms or molecules.)
Okay, so say you have a runaway reactor outputing more energy into the "plasma" conductor (or whatever other purpose you envision for plasma). That plasma is confined to a given volume... the insides of the containing conduits and pipelines and chambers and so forth. With "V" being constant, and T increasing... you have a direct, proportional rise in pressure under the
Ideal gas law.
Surely the ship's systems are tough, and designed to handle quite a bit of pressure, but get it hot enough and the pressure will risk rupturing the system.
How do you prevent that from happening?
Well, you either decrease "T" (which is normally not an option in the "Trekkian" situations we're talking about, being shot at by bad guys in full-on combat), or increase "V" (and how do you increase the volume of a fixed system? Short answer... you don't)... or you decrease "n" (the amount of material... not really in the form of "mass" here but proportional to mass).
How do you reduce the mass of "plasma" in the system? Simple... you "vent it to space."
This means you need to have a backup supply of this material, to "repressurize" that system once the temperature has returned to the normal range, of course... depending on whether this plasma is simple ionized hydrogen (the cheapest solution, and easily replenished) or something much more specialized (in which case, you'll need a supply of "replacement fluid" cannisters among your ship's stores), you'll still have to reintroduce mass later to get back to optimal operational conditions.
"Venting" is one of those things that really does make sense in the situations where we've seen it, given what we know.