Omaha Public Power District's Fort Calhoun nuclear unit near Omaha, Nebraska, was at 6% power early Monday after operators manually tripped the unit during startup on Sunday.
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An event report filed with the US Nucleaer Regulatory Commission on Sunday said the reactor had achieved criticality when plant personnel observed a "deviation" between control rods.
When attempting to level control rods, one rod could not insert [in the core] to the level of the rest of the group," OPPD said in the report. OPPD shut the reactor Thursday after one of its six sluice gates could not close due to ice buildup as a result of the sub-freezing temperatures the Midwest.
An OPPD spokesman said in an emailed response to questions Monday that Fort Calhoun went critical that morning but had yet to reconnect to the electricity grid. "This process generally takes a few days to get to full power," he said.