Three Mile Island Nuclear Generating Station (TMI) is a nuclear power plant located on Three Mile Island in the Susquehanna River, south of Harrisburg, Pennsylvania in Londonderry Township.It has two separate units, TMI-1 and TMI-2.he reactor core of TMI-2 has since been removed from the site, but the site has not been decommissioned. Three Mile Island is so named because it is located three miles downriver from Middletown, Pennsylvania. The plant was originally built by General Public Utilities Corporation, later renamed GPU Incorporated.