First a fallacy to be destroyed. Nuclear power plants don't explode. In the worst case scenario (and it is worst case {Only almost happened once}), a melt down could occur. In a melt down situation a runaway reactor loses coolant of it's radio active core and the nuclear reaction continues to create heat in such a manner that nothing can stop the heat gain. With this heat gain the mass of fuel melts and eats its way through the containing structures. Eventually the mass will encounter a volume of water sufficient to cool this mass. The resultant effect will be large masses of uncontrolled steam escaping into the atmosphere with radioactive particles. This steam will become widespread and uncontrolled will cause radiation over a large area.
As I said this almost happened once (Chernobyl). In Chernobyl the resultant partial meltdown caused radioactive spray over a relatively small area.
The type of reactor at TMI and the ones in Japan are PWR reactors and use water as a coolant and have a large availability of water for cooling. The reactor in Chernobyl used a solid coolant and was much more volatile.
Nuclear power is needed because of the large demands for electricity we have placed upon ourselves. The effects of alternatives to nuclear power (fossil fuels) are now viewed as bad guys as well (greenhouse gases). There are problems with all forms of mass power generation. If we want our computers, air conditioners, ovens, and washing machines, we need some large sources of energy.
But first relax. The nuclear power plant will not explode.