NASA Nuclear Power Conversion
Heavy elements such as the uranium used in generating nuclear power are extremely concentrated sources of energy. A few pounds of nuclear fuel can produce as much as thousands of tons of coal or oil--or of high explosives, in the case of nuclear bombs. Harnessing nuclear energy for spaceflight therefore seemed a natural direction to explore.
But nuclear spaceflight is not easily accomplished. Space rockets require not just energy, but also mass, matter ejected backwards, of which nuclear fuel provides very little. The limiting factor in the operation of rockets is not a shortage of energy but the high temperature at which they operate. Ordinary rocket nozzles already run red-hot: adding extra energy to the fuel would raise the temperature, perhaps beyond what the metal can take.
See more at Far-out Pathways to Space: Nuclear Power