Federal officials have launched an investigation of a radiation leak at the Three Mile Island nuclear power plant in Pennsylvania on Saturday. The Nuclear Regulatory Commission said about 175 workers were sent home when the contamination was detected. Some were exposed to low levels of radiation. Tests showed the contamination was confined to surfaces inside the plant, which is owned by Exelon, the nation’s largest generator of nuclear power. In 1979, a partial meltdown occurred in Three Mile Island’s Unit 2 reactor.