FILE - In this March 23, 1979, file photo Pennsylvania state policeman and plant security guards stand outside the closed front gate to the Metropolitan Edison nuclear power plant on Three Mile Island near Harrisburg, Pa. At Three Mile Island, efforts were concentrated on dealing with a single reactor. Its problems began at 4 a.m. on March 28 when a pressure relief valve failed and stayed open for two hours. Because operators thought it had closed, they shut off an emergency flow of water that had automatically been triggered, making the situation worse and exposing the fuel rods. (AP Photo/Paul Vathis, File)