When a radioactive nucleus of a heavy atom (pretty much anything above iron on the periodic table) fissions – i.e. splits into two or more parts – it releases energy. That energy comes out in the form of electromagnetic radiation (gamma radiation) and neutrons. Both of these forms of energy, but especially the neutrons, can be used to heat water. There are lots of ways to design nuclear reactors to do this – pressurized water reactors, boiling water reactors, graphite-moderated reactors, and so on – but it all comes down to that basic idea.

At this point, I’m not certain about the rest of your question. Are you wondering about the development of nuclear power, which got its start in the late 1940’s and the 1950’s, or are you wondering about the development of nuclear energy, which traces back to the early 1900’s?

http://en.wikipedia.org/wiki/Nuclear_power

that includes everything about nuclear power!