Flooding along the Missouri River has overspread much of one nuclear power plant's boundaries, forcing it onto emergency generators, and threatens a second plant downstream.
In both cases, regulators and operators say the plants appear to be in no danger of the kind of sequence of events – exacerbated by plant-design flaws – that led to the tsunami-spawned nuclear disaster in March at Japan's Fukushima Daiichi nuclear power plant.
The two plants, nestled along the Missouri River in Nebraska, "will be annoyed but not destroyed," adds David Lochbaum, a nuclear engineer and nuclear-safety specialist at the Union of Concerned Scientists in Washington.
At the plant facing the biggest challenge, the Fort Calhoun Nuclear Station, about 30 miles north of Omaha, the Missouri River is predicted to crest Wednesday at 33 feet above flood stage – some six feet below the level critical buildings at the plant were designed to handle. That flood crest would put the flood level roughly half an inch higher than it is currently.
Much of the plant's grounds are under at least two feet of water. Through early Saturday morning, the reactor-containment building and its adjacent auxiliary buildings were high and dry, protected by a 2,000-foot-long water-filled berm. But workers operating heavy machinery ruptured the eight-foot-high berm, allowing water to lap at these structures as well. Some water has leaked into the turbine building, which houses no nuclear material.
With the emptying of the berm, the only dry patch remaining is the plant's switch yard, which holds transformers and power lines that ship the plant's electricity to the grid, but which also receive power to operate the plant.
The switch yard is surrounded by a concrete levee. But that barrier has sprung leaks, prompting plant operators to shift to diesel generators for onsite power. Workers are looking at ways to patch the leaks, as well as repair the berm.
Fort Calhoun has been off line since April for a scheduled refueling outage, and officials with the Omaha Public Power District, which owns the plant, say they won't restart it until the flood has subsided.
If something untoward should happen, workers would have more leeway to deal with a problem because the plant is cooler than it would have been if it were online and because some of the most troubling radioactive byproducts in an accident have a quick decay time, says David Lochbaum, a nuclear engineer who tracks nuclear-safety issues for the Union of Concerned Scientists in Washington.
The danger of flooded nuclear plants was thrown into stark relief in March, when an earthquake struck off northeastern Japan, sending a tsunami crashing into the Fukushima Daiichi nuclear plant. The wave easily overtopped a seawall designed to keep tsunamis at bay. The tsunami swamped the plant's emergency generators, which had been installed on the seaward side of the facility, and swept away above-ground storage tanks holding the fuel to run them.
Power from off site also had been cut off during the quake, leaving only batteries to run vital cooling pumps – and with no means of recharging them. Absent a way to keep the reactors and their spent fuel cool, three reactors experienced full core meltdowns, and fires and explosions at the site released significant amounts of radiation into the surrounding environment. Fukushima Daiichi is the second worst nuclear disaster in history, after the Chernobyl explosion and fire in 1986 in what is now Ukraine.
For the Fort Calhoun Nuclear Station, as well as the Cooper Nuclear Station south of Omaha, the Missouri River flood has been a predictable, creeping menace, rather than an unpredictable, sudden one. As a result, operators have had time to augment and implement plans to deal with floodwaters.
Yet in Fort Calhoun's case, if this year's floods had occurred last year, the prognosis could have been worse.
Last October the US Nuclear Regulatory Agency (NRC) wrote up the plant for a "violation of substantial safety significant" related to its flood-control strategy.
Among the issues:
• The plant had stockpiled plenty of sandbags but not the sand to fill them.
• The Omaha Public Power District, which runs the plant, installed floodgates designed to keep floodwaters from overpowering the doors behind the gates. But the floodgates must be shored up on the outside – and topped – with sand bags. The support structures across the top of the gates weren't strong enough to withstand the weight of sandbags that would be place on top of them.
• Perhaps most significantly, workers upgrading the plant's cooling-water intake structure in the mid-1980s failed to seal old electrical conduits running through the structure's front wall. The structure by design sits in the river along the bank to provide cooling water to the plant. NRC inspectors noted that the unplugged conduits were below the flood height specified for the rest of the plant's critical buildings. Floodwaters jetting into the intake structure would have rendered useless pumps that are the plant's last line of defense against a loss-of-coolant accident.
The upshot: The plant was at a 100 percent risk of partial core damage if a loss-of-coolant accident occurred during a flood only two feet higher than the level projected for the current flood, according to the NRC. The company, by contrast, put the risk at between 19 and 23.9 percent.
Since then, plant workers have fixed the conduit and sand-bag problems, and the company is trying to plug the organizational gaps that allowed the problems to go unnoticed and unsolved for nearly two decades.
That still leaves room for unexpected problems, such as workers punching holes in berms.
The Fort Calhoun nuclear plant uses a single, pressurized-water reactor that delivers some 476 megawatts of power. It was commissioned in 1973. Eight years ago, the NRC granted the plant a license extension that will allow it to continue operating through 2033, instead of 2013.
The Cooper Nuclear Station is a single-reactor plant that uses technology similar to the reactors and containment buildings at Fukushima. The 810-megawatt plant plant went on-line in 1974, and, like Fort Calhoun, Cooper has received a license extension that will keep it running through 2034.
Cooper is running at full power for now, and remains dry because it sits on land that is a few feet above the river.