Dr. Thomas Cochran is a senior scientist in the nuclear program at the Natural Resources Defense Council. Cochran served on the Nuclear Regulatory Commission's Advisory Committee on the Cleanup of Three Mile Island. He explains what's at stake in Japan's possible nuclear meltdown:

What caused the Japanese nuclear reactor to explode?
All reactors create energy through nuclear fission, which means that atoms split. When they split, they create heat and radioactive products. Even if you shut down the reactor immediately, as the Japanese did, it keeps producing heat, though only at 10% capacity, and it slowly fades away. That’s why you have to cool the reactor.

So the real problem happened in the cooling of the reactor?
Yes. The tsunami that followed the earthquake took out the generator that was supposed to cool the reactor, so the plant had to use battery power, which only lasted for approximately 10 hours. Then hydrogen exploded in the secondary accident. The plant workers are still fighting to prevent the radioactive fuel from leaking through the bottom of the container. There’s still a risk of this happening, but time is on the power plant’s side.

Will Japan’s neighbors have to worry about nuclear radiation?
No. The wind has been blowing in the right direction, and the impact is small compared to what it would have been if the nuclear fuel had already burned through the bottom.

How does this accident compare to Chernobyl?
It won’t be nearly as bad as Chernobyl. Chernobyl was a much larger reactor, and the core contained a lot of carbon that caught fire. The fire spewed volatile fission products out into the open. The Japanese accident is more like Three Mile Island.

Japanese reactors have a reputation for being safe. Is the accident a cause for concern in other countries with safe nuclear reactors?

It should cause concern in the United States, which has 23 reactors of the same design. Germany has a couple of such reactors, too, and Spain has one. Of course, they don’t have the same earthquake and tsunami risk as Japan.

Should tourists feel safe eating Japanese produce and fish this summer?
Yes, it’s not a problem. Today short-term visitors to the control zone in Chernobyl can even eat the local food there.