« Meltdown » means « the accidental melting of the core of a nuclear reactor », and literally after that, a large displacement of radiation throughout the entire atmosphere. Here is how it works (VIDEO):
Japanese officials have just confirmed a radiation leak at Fukushima Dai-Ichi nuclear power plant. A radiation level of 1,015 Î¼Sv/h (mircrosieverts per hour) has been measured near the plant before the blast. The situation is deemed almost as serious as the Three Mile Island partial reactor meltdown in the 70s, and the Chernobyl disaster in 1986. Cesium would have been detected. Therefore, the reactor number one might be having a meltdown.
The U.S. is sending special coolant. Four workers have been injured. The cordoned-off area has been widened to a 20km-radius zone. The roof of the reactor would have collapsed in the aftermath of the explosion. A scram (shutdown) might not be achieved on another reactor because of a glitch in the emergency cooling system.