This morning, I got an email from a BoingBoing reader, who is one of the many people worried about the damaged nuclear reactors at Fukushima, Japan. In one sentence, he managed to get right to heart of a big problem lurking behind the headlines today: “The extent of my knowledge on nuclear power plants is pretty much limited to what I’ve seen on The Simpsons“.

For the vast majority of people, nuclear power is a black box technology. Radioactive stuff goes in. Electricity (and nuclear waste) comes out. Somewhere in there, we’re aware that explosions and meltdowns can happen. Ninety-nine percent of the time, that set of information is enough to get by on. But, then, an emergency like this happens and, suddenly, keeping up-to-date on the news feels like you’ve walked in on the middle of a movie. Nobody pauses to catch you up on all the stuff you missed.

As I write this, it’s still not clear how bad, or how big, the problems at the Fukushima Daiichi power plant will be. I don’t know enough to speculate on that. I’m not sure anyone does. But I can give you a clearer picture of what’s inside the black box. That way, whatever happens at Fukushima, you’ll understand why it’s happening, and what it means.

At a basic level, nuclear energy isn’t all
that different from fossil fuel energy. The process of
generating electricity at a nuclear power plant is really all
about making heat, just as it is at a coal-fired plant. Heat
turns water to steam, steam moves turbines in the electric
generator. The only difference is where the heat comes from–to
get it, you can light coal on fire, or you can create a
controlled nuclear fission reaction.

A
fission reaction is a lot like a table filled with Jenga games,
each stack of blocks standing close to another stack. Pull out
the right block, and one Jenga stack will fall. As it does, it
collapses into the surrounding stacks. As those stacks tumble,
they crash into others. Nuclear fission works the same way–one
unstable atom breaks apart, throwing off pieces of itself,
which crash into nearby atoms and cause those to break apart,
too.

Every time one of those atoms breaks
apart, it releases a little heat. Multiply by millions of
atoms, and you have enough heat to turn water into
steam*.

In a Boiling
Water Reactor, like the ones at Fukushima, water is
pumped through the core—the central point where the
actual fission reactions happen. Along the way,
fission-produced heat boils the water, and the steam rises up
and is captured to do the work of turning turbines.

The core is
the part that really matters today.

In the
core of a nuclear reactor, you’ll find fuel
rods—tubes filled with elements whose atoms are
unstable and prone to breaking apart and starting the
Jenga-style chain reaction.

Usually, the
elements used are Uranium-238 or Uranium-235. They’re
refined and processed into little black pellets,
about the size of your thumbnail, which are poured by the
thousands, into long metal tubes. Bunches of tubes–each taller
than a basketball player–are grouped together into square
frames. These tall, skinny columns are the fuel
assemblies.

The fission reactions that
happen are all about proximity. In a fuel rod, lots of uranium
atoms can crash into each other as they break apart. Pack the
fuel rod into an assembly, and lots more atoms can affect one
another—which means the reactions can release more
energy. Put several fuel assemblies into the core of a nuclear
reactor, and the amount of energy released gets even
higher.

Proximity is also what makes the
difference between a nuclear bomb, and the controlled fission
reaction in a power plant. In the bomb, the reactions
happen—and the energy is released—very
quickly. In the power plant, that process is slowed down by
control rods. These work like putting a piece of cardboard
between two Jenga towers. The first tower falls, but it hits a
barrier instead of the next tower. Of all the atoms that could
be split, only a few are allowed to actually do it. And,
instead of an explosion, you end up with a manageable amount of
heat energy, which can be used to boil water.

Now
that you understand what’s going on inside a nuclear reactor,
you get a good idea of what happened at Fukushima. Like the
other nuclear power plants in Japan, Fukushima Daiichi got a
message from the country’s earthquake warning system, and shut
down in advance of the quake. Basically, that means that
control rods—”big metal gizmos”, as Charles
Forsberg, executive director of the MIT Nuclear Fuel
Cycle Project, described them to me—were inserted
into the fuel assemblies, cutting the fuel rods off from one
another. But, because you aren’t completely separating all the
uranium atoms from one another, shutting
down the core isn’t the same thing as flipping an “off”
switch.

When a reactor core is shut down,
its energy output drops not to zero, but about 6% of its normal
output, Forsberg told me. The reactions grind to a halt over
the next few days, as the falling Jenga towers run out of other
towers they can actually hit. In the meantime, atoms keep
breaking apart, releasing both heat and fast-moving particles
that can penetrate human skin and damage our cells. Because of
this, every nuclear reactor has ways of getting rid of the
heat, and blocking those fast-moving radioactive
particles.

When the reactor at Fukushima
shut down, it should have been kept cool by water pumped
through the core. But, because the tsunami damaged the
diesel-powered generators that pumped the water, the core kept
heating up. If that sounds like a design flaw, you’re right.
The Fukushima reactors were built in the early 1970s. In modern
nuclear reactor designs, pumps aren’t necessary to move water
through the core in an emergency shut down. Instead, the water
moves via gravity.

But, in this case, no
pumps meant no water movement. So the core got hotter, which
boiled off some of the water. The boiling caused pressure in
the core to increase. To protect the core, and prevent a bigger
problem, authorities had to vent some of that steam into the
atmosphere, which means venting some of the radioactive
particles along with them.

This is also
probably tied into the explosion that happened, according to
MIT’s Charles Forsberg.

“There’s zirconium in the fuel rods.
When you overheat the reactor core, the first thing that
happens is that the zirconium begins to react with steam or
water and forms zirconium oxide and hydrogen,” he says. “You
get a mixture of steam and hydrogen. When you release steam
into a secondary building [to decrease pressure in the core],
the steam condenses and leaves behind just the hydrogen. Then
all you need is an ignition source and you can get a hydrogen
burn. That’s what happened at Three Mile Island. I don’t know
if that’s what happened in Japan, but it’s likely to be the
source of that explosion.”

The good news is that the explosion seems to have
happened outside the core. In that case, it’s completely
reasonable that an explosion could happen without releasing
lots of radioactive material. That’s because nuclear power
plants come in layers, like an onion.

The
core is contained within a building that has solid concrete
walls, 3-to-6 feet thick. It’s meant to withstand collision
with an airplane. It’s also meant to withstand an explosion
from inside. But that bunker sits inside something a lot
flimsier—a building more akin to a metal shed. It’s
the shed that exploded today at Fukushima. Because radiation
levels didn’t rise after the explosion, we can be pretty
certain that the bunker is still intact.

This is a serious emergency, but there are some good
reasons to be hopeful. According to World
Nuclear News and Reuters,
there were seven reactors in Fukushima that were affected by
the earthquake. Of those, four have access to outside power to
run their water pumps and are stable. Three lost their power.
Out of those three, one has steady levels of water. Only two
have decreasing water levels. But, in recent hours, workers
have started pumping in seawater to one of those. Hopefully,
both can be stabilized. But it’s hard to say right
now.

And then what happens? Remember, this
is really just an emergency shutdown gone awry. The control
rods are still in place. The Jenga columns are still separated.
So, over time, the fission reactions will still slow down and
stop. As they do, heat levels will drop, and so will levels of
radiation.

Really, what we have here is a
waiting game. The goal is to keep the reactors stabilized long
enough that the shutdown can completely shut down.

For more information—and details I might
have missed—I recommend checking out a recent BBC
article, and an interview
Skepchick blogger Evelyn did with her father, a nuclear
engineer.

The
Matchbox That Ate a 40 Ton Truck