Most of this power is fueled by uranium isotopes. These isotopes are highly radioactive. The isotope catches the fast moving neutrons created by the splitting atoms, it repels the slower moving protons and electrons, then gathers the neutrons and pulls them inward. While all these atoms are flying about they smash together then split many of many times, this is when the reactor grabs and pulls in the frictional energy to be processed into electrical watts. This usually causes heat or thermal energy, this must be removed by some kind of a coolant.
Most power plants use water or another type of liquid based formula.
Very few use gas related coolants in there reactors, these are known as thermal reactor based power plants. Another nuclear reactor type is a type that runs off of uranium oxide, the uranium oxide is a gas form of the solid uranium. These fuels which cause the radioactive particles usually are always highly radioactive themselves. Because of this all the power plants take high safety standards and use special shields to prevent leakage. Usually the leakage can cause nuclear contamination.