What is a meltdown?

An aerial photo shows the quake-damaged Fukushima Dai-Ni nuclear power plant in the town of Naraha and Tomioka in the Futaba district of Fukushima prefecture on March 12, 2011.
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Japanese officials say that at least 160 people have been exposed to excessive radiation as partial meltdowns occurred at two crippled plants.

CBS News anchor Russ Mitchell reports that officials are concerned that water levels have dropped so much inside two of the reactors that parts of the uranium-filled fuel rods are exposed. In the worst-case scenario, that could contribute to a meltdown.

But what exactly does that mean?

Japan's ambassador to the United States conceded Sunday that part of the fuel rods inside the Fukushima nuclear power plant may be melting - but, he said, the reactor core is not. A big concern is that the water levels inside the reactors has sunk, leaving the uranium-filled fuel rods exposed. A full "meltdown" is, of course, something the Japanese are trying to avoid.

Even two days after they shut down, Fukushima's reactors are still generating heat.

"From the nuclear fuel roads. It's much like if you have an electric stove, and you immediately turn it off, if you put your hand on the stove, you will not be very happy," said Dale Klein, the former chairman of the Nuclear Regulatory Commission.

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Simply put, a "meltdown" is when the nuclear fuel inside the reactor gets so hot, it literally melts. Uranium pellets are inside the long fuel rods. If the reactor is not cooled properly, the tubes can fall apart, with the radioactive material falling to the bottom.

"It's like a car accident -- it can be a fender bender all the way up to a major collision," Klein said. "So when you talk about fuel melting, you can have just a few pellets melt, or you can have a large number of pellets to melt."

The key issue is whether the reactors are being adequately cooled with water. If not, melting could begin.

"When the fuel melts, it will flow like wax into the bottom of the reactor vessel head, and if there's water there, it will solidify and freeze. End of story," Andrew Kadak, a professor of nuclear science at MIT said.

Fears of a radiological release are legitimate, but Kadak says, no one should imagine a mushroom cloud.

"Of course, we're just hypothesizing, the releases will occur when the plant decides to open the reactor containment to relieve the pressure," Kadak said.

Experts say the situation at Fukushima is similar to what happened at the Three Mile Island nuclear plant, in Pennsylvania, in 1979. Kadak recalled that there was a considerable melting of the core, "but essentially all of the material was contained in the concrete containment. Namely, very little of it was released."

Unlike volcano dust, which can be carried long distances by winds, the Nuclear Regulatory Commission says the West Coast of the U.S. is not in danger.

"Even if there were a significant release, it would be dispersed probably before it came to the United States, but at the present time it seems unlikely that there will be a major radioactive release," according to Klein.

"60 Minutes" Rewind: Ten years' worth of "60 Minutes" reporting on the Chernobyl nuclear disaster, health effects on local children, and the "sarcophagus" built to contain the ruined reactor.