Facing additional improvements mandated by the Nuclear Regulatory Commission, the owner of Nebraska's Fort Calhoun nuclear plant has pushed its scheduled restart into the first quarter of 2013.An Omaha Public Power District spokesman told Bloomberg that it will not make the previous restart target of December. Earlier this week, the utility received an NRC confirmatory action letter requiring further work at the plant, including flood barrier improvements, upgraded diesel generator equipment and maintenance related to the reactor's containment.The 500 megawatt Combustion Engineering pressurized water reactor is under intense scrutiny from federal regulators for a range of safety issues, many of which date to the summer of 2011. On June 7, 2011, a fire in a switchgear room damaged buses that temporarily cut spent fuel cooling. At the time, the plant was already down for refueling and under an unusual event declaration while the adjacent Missouri River flooded the area surrounding the plant. The fire netted an NRC red finding for OPPD, and the plant remains under the supervision of a special oversight panel.To help address issues at the plant, OPPD hired Exelon to take over the day-to-day management at Fort Calhoun earlier this year.