the shape of things to come

Shielding and Fusion Engine Types
Since Kaumberg has been enjoying a boom in technology over
the past generation, I’ve noticed that local attempts to build municipal
fusion power plants have run into trouble from environmentalists
and followers of Blake. Of course, the fusion industry
is hitting back with its own propaganda about what a clean and
wonderful power source fusion is.
Yes, I said “propaganda.” Fusion is a wonderful power source.
We’re on Kaumberg and a thousand other systems today rather
than Terra because of fusion power, which even made the Kearny-
Fuchida drive possible. Even through the bloody Age of War and
the Golden Age of the old Star League, fusion power did wonders
for elevating humankind away from the cold, nasty life of a caveman.
But fusion isn’t as perfect as you might expect. All fusion
reactions generate radiation. Fusion reactors irradiate their interiors,
creating an issue when the reactor is serviced or decommissioned.
As a result, radiation shielding is the largest portion of a
’Mech-scale fusion engine’s mass.
In standard fusion engines, the shielding is a very dense ceramic.
Typically, this is tungsten carbide, which is reinforced with
short ceramic fi bers mixed into the carbide. This shielding isn’t
just thick enough to stop the radiation, but also to survive battle
Fusion engine explosions: an urban legend that won’t die. Let’s
see if I can kill it on this planet, at least. Where to start?
All right. First of all, when I said earlier that the magnetic
fields of a fusion engine keep the plasma from melting the
engine, I was already anticipating this question. In fact, the
issue is actually kind of the opposite and counter-intuitive,
so I didn’t bring it up. The magnetic fields do provide some
protection to the reactor walls from the plasma, but primarily
they protect the plasma from the cold, cold walls of the reactor
chamber.
The fusion reactions in a BattleMech’s fusion engine occur
only under very narrow conditions of temperature and pressure.
Generally, the hotter and higher the pressure, the faster the
reactions, and below a certain minimum, fusion simply ceases.
If you remember your ideal gas laws from chemistry…eh…the
condensed version is that when you heat up a gas, it wants to
expand. If it can’t expand, its pressure increases. When a gas expands,
its temperature drops. Remember those rules of thumb
and if you have trouble remembering them, hit the ‘net when this
lecture is over.
When a BattleMech’s fusion reactions spike a bit, the plasma
gets hotter. More fusion reactions mean more heat means hotter
plasma. But the magnetic confi nement fi elds are not rigid.
In fact, an ancient fusion engineering description that dates to
the twentieth century says that, “Trying to hold onto plasma with
magnetic fi elds is like trying to contain a roll of jelly with rubber
bands.” When the plasma gets hotter, it pushes against the
magnetic fi elds because its pressure is rising, and the magnetic
fi elds give a bit. The expansion cools the plasma, and the reactions
drop. There’s some elbow room in the reactor chamber for
just this purpose.
Now, I said the fusion reactions drop when they get cooler.
There are ways for the plasma to cool other than expansion.
One way is when the plasma touches the relatively frigid walls of
damage and to serve as a heat sink. By this, I mean a mass meant
to soak up heat—just in case the magnetic containment bubble
fails.
In BattleMechs, you’ll fi nd a few variations on engine shielding
types. The famous extra-light (XL) engines slim down the tungsten-carbide
reactor walls and reinforce them with a crystalline
plastic similar to those of double strength heat sinks… Don’t
worry; I’ll cover those when we get to cooling systems later.
Now, trying to make large blocks of this shielding is a trial for
engine manufacturers. The scrap rate is phenomenal and accounts
for some of the hideous price XL engines command in the
open market. The newer so-called light engines use an innovative
mix of layered shielding materials and secondary magnetic
screens…
Incidentally, I recall a rumor in Lasers and Slugs that claims
the only diff erence between XLs and standard engines is the
shielding, and that salvaged standard engine cores can be easily
refi tted into XL engines. This is an exaggeration, but it’s not completely
false. In fact, while shielding is the major diff erence between
the two fusion types, the XL engines also usually replace
some components with advanced, lightweight parts. So, while a
300 Vlar and a 300 Vlar XL have a lot in common—enough to con-
FUSION ENGINE EXPLOSIONS:
the reaction chamber. If they do, the plasma will chill so rapidly
that fusion ceases instantly. That only leaves you with a puff of
hot gas, with no continuing source to damage the reactor walls.
When confi nement fails so badly that the plasma hits the walls,
the walls are usually only scuff ed.
Surprising, isn’t it? But remember, all the heat energy comes
from the fusion reactions. It’s not stored as latent heat in the
plasma. In fact, there’s so little plasma mass to store heat that the
“dead” plasma is barely able to warm up a multi-ton reactor—
even if the cooling system completely fails. You might scorch your
hand if you touched the outer casing, but it’s not enough to melt
the shielding or damage critical components.
And, no, you can’t just keep powering the fusion reaction while
it gnaws through the reactor walls. Evaporating the lining of the
reactor will mix kilograms of cold, heavy, non-fusible elements
into the plasma, which is much lighter. The eff ect would be like
dumping a ton of wet sand on a welding torch. So, the short version
of all that is that when a fusion reactor gets out of hand, it
usually shuts itself down and is unable to do more than warm up
the reactor a bit.
And you protest, “But I saw a ’Mech explode on the news in
a blinding fl ash of light! It had to be a nuke!” Or is it, “Well, what
about that MechWarrior that buried a bunch of Clanners in a canyon
with his exploding reactor?” Or would you ask, “Well, what
about Tharkad City?” Okay…
Fusion reactors do occasionally die in spectacular manners. But
most of the time, the fi reworks 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.