Wackenhut Corp. chief executive officer Gary Sanders left a month after the security company lost contracts from Exelon Corp. because guards fell asleep on the job at a nuclear power plant. Mr. Sanders, 55, will be replaced by G4S PLC chief operating officer Grahame Gibson, Wackenhut said in a statement yesterday. Mr. Gibson will continue to serve as operating chief and a board member of Britain-based G4S, parent company of Wackenhut. Exelon, owner of the largest fleet of U.S. commercial reactors, said last month it was terminating all nuclear-plant security contracts with Wackenhut after guards at its Peach Bottom reactor in Pennsylvania were videotaped asleep while on duty. The videotapes were made public in September. Including the Exelon contracts, which will be phased out by the middle of this year, Wackenhut provides security service at 31 of the 65 U.S. nuclear power-plant sites. GFS (London) fell 5.25 pence to £2.15 ($4.25).