A protective berm holding back the Missouri river floodwaters from the Fort Calhoun nuclear power plant has collapsed.
Flooded Fort Calhoun site. Image from US Army Corps of Engineers
Both the Fort Calhoun nuclear power plant and the Cooper Nuclear Station are operating under ‘unusual event’ declarations, the lowest of four emergency notification levels, because of flooding along the river. Cooper is operating at full power, and sits about a metre (2.5 feet) above current flooding levels.
At Fort Calhoun, the 2,000 foot long water-filled berm provided supplemental flood protection to the plant, but was not required under NRC regulations, the Nuclear Regulatory Commission said.
Its collapse allowed floodwaters to surround the auxiliary and containment buildings, which are protected by design to a floodwater level of 1014 mean sea level. The Missouri River is currently at 1006.3 feet. River levels are not expected to exceed 1008 feet, the US NRC said. However, there are now two feet of water around many parts of the site.
The collapse of the berm also allowed floodwaters to surround the main electrical transformers. Operators transferred power from offsite sources to the emergency diesel generators as a precautionary measure due to water leakage around the concrete berm surrounding the main transformers. Efforts are underway to reconnect to offsite power once all safety checks have been completed, the NRC said.
NRC inspectors were onsite at the time of the berm collapse and responded to the event. They have verified that reactor shutdown cooling and spent fuel pool cooling remain unaffected. NRC augmented its resident inspection staff on June 6, to provide around the clock coverage of site activities. The plant has been shutdown since April 7 for a refueling outage, it said.
The Nebraska Public Power District, which operates the Cooper plant, said that staff are continuously monitoring the river’s water levels. It said that flooding preparations began on 30 May. Personnel have been proactive in preparing the station for flood conditions by filling sandbags, constructing barricades, procuring materials and supplies, and reinforcing the access road plant staff use to get to the station. More than 5,000 tons of sand was brought in for constructing barricades, such as Hesco barriers placed around the station’s switchyard of transformers and other electrical equipment.
Should the river’s level increase to 900 feet above sea level, plant personnel will also barricade internal doorways as another layer of protection for facility equipment. If the river’s level increases to 45.5 feet or 902 feet above sea level, plant operators would take the station offline as a protective safety measure. The plant was built at 903’ MSL, which is 13 feet above natural grade.
Cooper Nuclear Station is located three miles southeast of Brownville, Nebraska, near the Missouri River. It is owned and operated by the Nebraska Public Power District, with headquarters in Columbus, Nebraska.
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