So it looks like there is one more tragedy on the front for the people of Japan, it looks like the Fukushima Daiichi nuclear power plant is potentially melting down. A large explosion in the plant occurred as plant officials tried to cool down the core so I meltdown is currently fear to be in progress.
So what is a nuclear meltdown, essentially when a nuclear power plant can no longer cool the nuclear power rods the system can heat up to the point of melting the core of reactor. Most modern reactors have a containment dome that keeps the radiation from being released into the environment.
There are some fears that the containment dome at the plant in Japan may have been damaged during the earthquake and tsunami. The natural disaster was the cause of the potential meltdown as the quake and the tsunami lead to an explosion that disrupted the cooling system for the plant. The large fear of course is that a large amount of radiation may be released if the dome has been damaged bringing up flashbacks of Chernobyl and Three Mile Island. The reality is even in a worst case scenario the release will be less than Chernobyl and worse than Three Mile Island.