Nebraska's two nuclear plants sit below the nation's largest reservoir system — six dams on the Missouri River.
And last year, as record runoff barreled through those dams and flooding closed in on Fort Calhoun Nuclear Station, Midlanders nervously asked, "What happens if the dams fail?"
On Tuesday, the federal Nuclear Regulatory Commission made clear that it wants that type of question answered for reactors across the country.
Regulators point to a preliminary review begun several years ago that they say suggests damage from flooding at the nation's 104 reactors could be, in some cases, "greater than previously expected."
Lara Uselding, a spokeswoman for the commission, said problems at Fort Calhoun were among the reasons the agency believes the nation needs a better understanding of flood threats to reactors.
Not only did floodwater surround Fort Calhoun last year in a way that had not happened before in the U.S., but in 2009, Fort Calhoun failed a routine flood inspection.
"What happened at Fort Calhoun underscored the importance of plants having adequate protections," Uselding said.
Nuclear plants have always had to prepare for extreme natural disasters.
In this area, that means Fort Calhoun and Nebraska's other nuclear reactor, Cooper Nuclear Station, are designed to withstand flooding, earthquakes and tornadic winds.
But the level of protection varies widely. Cooper, for example, is built higher off the floodplain than Fort Calhoun and should, according to the Nebraska Public Power District, be able to withstand flooding at a probability of one in 1 million years.
Officials at Fort Calhoun use different language to describe their level of protection. They are required to be protected to at least a 1-in-1,000-year flood threat.
Omaha Public Power District spokesman Jeff Hanson said Tuesday that Fort Calhoun's design would protect it against six times the volume of floodwater that flowed by the plant at the peak of flooding.
The river never entered the plant's critical areas, and the public faced no imminent risk, experts agree. With the physical changes required by regulators and their experiences last year, OPPD says the reconstituted Fort Calhoun could be one of the nation's most flood-ready plants.
"We are confident in our ability to protect the plant," Hanson said. "Our workers are probably the most experienced nuclear power plant workers in the country in protecting a plant from floodwaters."
A sobering lesson learned last year was that, during flooding, dam managers may be unable to take into account the needs of a downstream nuclear plant.
The Army Corps of Engineers, which operates the Missouri River reservoir system, said last year that it had little flexibility in addressing problems that might arise at Fort Calhoun. Its top priority was getting water out of its dams as safely as possible.
On Tuesday, Col. Robert Ruch, commander of the Omaha District of the corps, applauded the nuclear regulatory commission's review. Every decade the corps updates its dam failure analysis, which includes the potential for multiple dam failures at once. It last did so in 2009.
That information has been shared with nuclear regulators but is not available to the public.
Ruch reiterated Tuesday, as he did this summer, that the dams were not at risk of failing.
"Our dams along the Missouri River main stem are safe. They performed as designed under the duress of last year's high water and could take on substantially higher flows ... in the future."
One of the nation's leading advocates of tougher oversight of the nuclear industry called one part of the review particularly encouraging — that the NRC is including it in its overall, post-Fukushima analysis of nuclear power plant safety.
"That will tend to fast-track it," said David Lochbaum, director of nuclear safety for the Union of Concerned Scientists. "Generic issues (like flooding) by themselves can take decades to resolve. By linking it to Fukushima, it might get resolved quicker, which is good news."