Following an outage that spanned nearly three years, the Fort Calhoun nuclear plant has begun to power back up.The Nuclear Regulatory Commission gave it the go-ahead to restart after ensuring the plant had completed a checklist of upgrades and improvements related to flooding during a 2011 refueling outage, a fire that affected spent-fuel cooling and other issues. In a release Tuesday, the Omaha Public Power District said it spent more than 8 million man-hours completing 69,000 tasks to meet the NRC's requirements. Exelon was hired in 2012 to handle day-to-day operations at the plant.For its part, the NRC said its inspections took 23,000 hours and addressed more than 450 restart action items. The plant will remain under close NRC scrutiny, with three supplemental inspectors on site in addition to the plant's two resident inspectors. The reactor will slowly ramp up to full power, stopping at multiple points along the way for final checks of plant equipment. At full power, the Combustion Engineering pressurized water reactor can generate 478 megawatts.