In 1958, Argonne National Laboratory began the construction of several zero-power reactors (ZPRs), which are nuclear fission reactors that don't actually generate any power. Scientists developed ZPRs to assess the performance of various reactor core configurations before actually building a full nuclear reactor. A series of ZPRs were built leading up to the construction of the Experimental Breeder Reactor-II, a sodium-cooled fast reactor power plant. In this 1970 photo, an Argonne scientist is loading the matrices of the ZPR-VI reactor prior to its first operation using plutonium fuel. | Photo courtesy of the Department of Energy.