Japanese nulcear reactor meltdown?

Well without adequate coolent, the fuel rods will start to overheat, and if it gets too hot it can start melting and possibly become molten and float to the bottom of the reactor. If it gets even hotter it can melt through the bottom of the reactor vessel and eventually through the bottom of the plants foundation and keep going through the earth until it hits water or cools down. This is known as a meltdown but this type of meltdown is considered impossible in these nuclear plants.