The document discusses nuclear fuels and nuclear chain reactions. It states that nuclear fuel is any material that can be used to derive nuclear energy through fission chain reactions, with the most common fuels being uranium-235 and plutonium-239. It explains that a nuclear chain reaction occurs when neutrons released during fission cause further fissions, releasing more neutrons and allowing the reaction to be self-sustaining. India generates over 4,000 MW of electricity from 20 nuclear reactors across 6 power plants, with 5 more plants under construction.