As driven largely by US Navy development work for submarine propulsion, the pressurized water cooled nuclear has become the predominate reactor used for the generation of electrical energy, with several hundred units in operation world-wide. Two variations exist: the boiling water reactor that directly produces steam and the pressurized version that indirectly produces steam using a heat exchanger. Both types use a containment building that prevents the release of radioactive steam/water/materials to the environment if pipe breaks occur.

Modern nuclear reactors, while exceptionally safe, do have a few drawbacks:    
   A. Very expensive to build.    
   B. Reactor core will melt if water cooling is lost.