A nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency[1] or by the U.S. Nuclear Regulatory Commission.[2] However, it has been defined to mean the accidental melting of the core of a nuclear reactor,[3] and is in common usage a reference to the core's either complete or partial collapse. "Core melt accident" and "partial core melt" are the analogous technical terms
“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.”