What’s happening in Japan aren’t always clear about what a "nuclear meltdown" is, and isn’t. Is it an explosion? Will it burn a hole to the center of the earth? Does it spray radioactive stuff into the air, poisoning the surrounding landscape? it’s not an explosion, though there can be some explosive side effects
The world has seen two big nuclear reactor scares — in 1986 at the Chernobyl plant in Russia, and in 1979 at the Three Mile Island plant in Pennsylvania.
Thyroid cancer is the most immediate risk, and the Japanese government is handing out pills to help prevent it. Worse case scenarios — lots of radioactive fallout — can lead to other cancers down the road.
The problem with building nuclear power plants is not that they are likely to have an accident -- they aren't. In that sense, they're relatively safe. But the magnitude and consequences of even a single such accident are simply too large to warrant even a small risk. Several hundred miles of Japan's coastline were totally devastated by the tsunami. But the big worry facing the country, and the world, today is confined to two tiny sections of that devastation-the nuclear power plants
