A subsidiary that runs FirstEnergy Corp.’s nuclear and coal-fired power plants has filed for bankruptcy. The move announced late Saturday by FirstEnergy Solutions signals the parent company’s plan to get out of the power producing business. It also comes after the utility said March 28 that it intends to shut down its three nuclear plants in Shippingport here in Beaver County and Ohio within the next three years. FirstEnergy Solutions says the filing is in the best interest of the company and creditors.