Until this weekend, you were more likely to see the term "meltdown" in a story about Charlie Sheen than just about anywhere else. But with the earthquake and tsunami striking Japan late last week and setting off crisis situations at several nuclear reactors, the term is now spread across the headlines in a much more worrisome context. But what is a nuclear meltdown?
Let's start with the basics of how a nuclear power plant works: At its heart is the reactor, in which are tubes made of zirconium alloy and filled with pellets of uranium. When a neutron hits an atom of uranium, it sets off a chain reaction, splitting the uranium and releasing energy in the form of heat. That heat is used to drive a turbine that creates electricity.
The reactor can be shut down by moving control rods into place around the fuel. The control rods absorb neutrons before they can reach the uranium. Because the fuel rods will give off heat even when the reactor is in shut-down mode, this whole apparatus is kept under water. That water can heat up so much that it turns to steam, but a cooling system replaces it with fresh, cool water.
If the fuel rods become exposed to air--- as has happened with some of the Japanese nuclear reactors when their cooling systems became disabled---they can quickly heat up, cracking the zirconium casing and releasing radioactive gases. If the fuel gets hot enough, the uranium can melt, eventually falling to the bottom of the reactor and even burning through it. This is a meltdown. If it is bad enough, the molten, radioactive uranium could burn through all the protective layers surrounding the reactor and get released into the surrounding environs.
The most famous nuclear accident here in the United States, Three Mile Island in 1979, is called a partial meltdown because the fuel rods were only partially exposed, though melting did occur.
For more information (including some great graphics):
New York Times: How a Reactor Shuts Down and What Happens in a Meltdown and A Look at the Mechanics of a Partial Meltdown
Popular Science: How Nuclear Reactors Work, And Fail
Washington Post: How the nuclear emergency unfolded
Scientific American: Nuclear Experts Explain Worst-Case Scenario at Fukushima Power Plant
Slate: What's in the radioactive vapors?
A primer on nuclear reactors from whatisnuclear.com