Why is it impossible for nuclear power plant to explode like a nuclear bomb? ?

Their is not a high enough concentration of fissile materials. In other words, a nuclear power plant in the US has 3 to 5 % U-235. This is the fuel for the nuclear reactions. A nuclear, or atom, bomb requires at least 93% of U-235. If you want to get a really high yield, out of a given amount of U-235 or Pu-239, then you also need to have a good geometry and others materials in it to control the fissions. A nuclear plant has similar stuff, but it is set up to save on the amount of uranium required to run. Now the reactor of a power plant can get hot enough to instantaneously turn the water into steam making a explosion, which is what happen at Chernobyl, but it's nothing like a nuclear bomb still.  more