Three Mile Island is a single-unit reactor, which means it has higher operating costs and is less efficient than the state’s four other nuclear plants, which each have two reactors and produce twice as much power as TMI. The report projects that Three Mile Island, which produces about 8 percent of the state’s nuclear energy, is the only nuclear plant that will lose money over the next three years. FirstEnergy Corp., which owns the twin-unit Beaver Valley Nuclear Power Station near Pittsburgh, has announced its planned retirement in 2021, unless it is rescued.