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Here is a recap done by a nuclear physicist friend of mine back on March 12th. I asked him about the term meltdown and how it applies in this case. and I think it will prove eerily familiar:

“This means that an entire BWR core, consisting of hundreds of fuel rods, each consisting of ceramic, uranium-oxide pellets stacked in a tube, 6 feet long, made of zirconium, has melted into a blob. The blob has not melted through the reactor vessel. Unfortunately, the core was at the end of a burn cycle, meaning that it contained a maximum load of “fission products,” all of which are wildly radioactive. Those products which will dissolve in steam are free to wander out of the vessel, through the steam-relief valves, and into the atmosphere through what used to be the roof over the reactor building. Most of the gaseous effluent will waft over the Pacific ocean and be lost forever. Some of it will rain down on the main island of Japan, which has plenty of other worries right now.

Depending on the wind-speed/direction, the fallout will be dangerous up to about 5 miles from the plant. Evacuating beyond 5 miles is being appropriately cautious. Some of the products, such as I-131, St-90, and Cs-137, are extremely dangerous. Some, like Ar-41 or Xe-131, are not.”