(Middletown) -- Workers have started replacing the warning siren system around the Three Mile Island nuclear power plant in Dauphin County. All 96 sirens in the 10 mile radius around TMI will be swapped out for new models that include a battery back-up system in case of a blackout, if an emergency occurs at the plant. The $2.1 million project is expected to be completed early next year. Exelon Nuclear, the company that operates the plant, says the upgrade is part of a $9 million plan to replace 400 sirens at its power stations across the mid-Atlantic. The company also runs Peach Bottom Atomic Power Station in York County.