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| Source: BBC |
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.
In the Core
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.
full article:
[ed. note. Wikipedia seems to have an amazingly detailed up-to-the-minute description of what's happening at Fukushima. Reactor 3 seems to be the most dangerous, with a MOX mixture of uranium and plutonium from recycled nuclear weapons: http://goo.gl/DKZ9e ]
