Among the places surrounded by floodwaters in the American Midwest: Nebraska's Fort Calhoun Nuclear Power Station. And when I say "surrounded," I mean literally. The power plant is a berm- and sandbag-protected island in the middle of temporary lake. This has not ever happened to an American nuclear power plant before. So far, it sounds like things are going fine (though, of course, the power plant's owners are the primary source of information here, along with government regulators). Overall, there are some good reasons to be nervous, and some good reasons not to be too terrified.
First, on the positive side, the power plant's single reactor has been in cold shutdown since April for maintenance. But that's not a guarantee against problems. After all, Fukushima Daiichi's Reactor 4 was also down for maintenance, and the spent fuel in its cooling ponds still overheated and caused problems with hydrogen explosions and fires. That said, a reactor in cold shutdown is significantly less vulnerable than one that's operating.
Second, on the downside, the power plant got into trouble with federal regulators last year, because its flood defenses weren't up to standards. But, on the positive side, that's ended up meaning that the flood defenses that Fort Calhoun is currently dependent upon are newly improved and inspected—the results of mandated upgrades.
Floods also happen at a slower place than earthquakes and tsunamis, and Fort Calhoun has had time to really double down on preparedness. They've built dams around not only the plant itself, but also the electrical substations that supply its primary source of power. And they've stockpiled weeks worth of fuel for the backup generators, so that in case those power lines go down the fuel rods will continue to be cooled.
On the other hand, all that preparedness is kind of dependent on conditions. According to the Omaha World Herald, The Army Corps of Engineers expects the river to crest no higher than 1,008 feet elevation, and the flood barriers would protect the power plant to 1,010 feet. But that doesn't leave a lot of margin for error. If rainfall becomes extraordinarily heavy again, the river could crest higher. If that happened, Fort Calhoun would be at much greater risk. Hopefully, the container around its reactor would be as watertight as advertised, and the water wouldn't reach the spent fuel pool, which is on higher ground at 1,038.5 feet.
There've also been a couple of small accidents. On June 7th, the cooling pools lost power for an hour and a half because of an electrical fire. And, on the 26th, one segment of secondary flood berm collapsed. The berm was water-filled, and so its collapse caused some flooding in the plant, even though the floodwaters, proper, remained at bay. That accident forced a temporary switch to backup power.
Shorter version: From the information available, it sounds like things are currently under control and that the power plants owners are prepared for the situation they're dealing with. But it's also too soon to know how this will play out, or whether "prepared for the situation they're dealing with" is the same as "prepared for a worse-but-plausible scenario."
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