Nuclear research reactors serve primarily as a neutron source for science, research, development, education and training. They also produce neutrons for use in industry, medicine, agriculture and forensics, among other uses. They are small in comparison with power reactors. Reactors power rating is designated in megawatts (MW) with an output range of zero (critical assemblies) up to 200 MW (th). By comparison, power reactors can produce as much as 15 times more power, 3000MW (th). Research reactors in contrast are not used for power generation.