The fission of uranium-235 is used exclusively in nuclear power plants located in the United States. There are many different fission reactions of uranium-235, but all the fission reactions are self-sustaining chain reactions. Explain. Differentiate between the terms critical, subcritical, and supercritical. What is the critical mass? How does a nuclear power plant produce electricity? What are the purposes of the moderator and the control rods in a fission reactor? What are some problems associated with nuclear reactors? What are breeder reactors? What are some problems associated with breeder reactors?