Associated Press

Federal officials say that the Fort Calhoun nuclear plant, which is surrounded by floodwaters from the Missouri River, is safe for now, as I wrote in Tuesday’s Times. But it may be a very long way from reopening.

One reason is that its reactor was shut earlier this year for refueling and plant technicians have not finished the refueling process for safety reasons. What is more, workers have been busy building barriers and installing pumps to deal with the flooding. Engineers at the Omaha Public Power District decided that it would be safer to leave the plant in refueling mode for the time being.

In refueling mode, the reactor vessel, the giant steel pot that holds the nuclear fuel, is opened and flooded so that a crane can pluck out used fuel assemblies and insert new ones while keeping them under water. As a result, the fuel rods are sitting under an additional 23 feet of water, which comes to around 200,000 gallons.



Normal cooling continues, with a little of the water continuously piped out and pumped through a heat exchanger to keep the temperature below the boiling point. The pump runs on power from the grid. If the connection to the grid were lost, technicians would start up one of two 2.5-megawatt emergency diesel generators. They have more than two weeks’ fuel on hand.

But if the diesel generators were flooded, as the ones at the Fukushima Daiichi plant in Japan were as a result of a tsunami, the water would heat up, and in about 36 hours it would start to boil. The preliminary calculation is that it would boil off 12 gallons a minute, so that 200,000 gallons adds 11 or so days to the reactor’s safety margin, plant managers say.

The solution would be to restore normal cooling or to pump in new water, either from a 100,000-gallon storage tank nearby or from the river itself. But managers say the most likely course is that normal cooling will continue, running on grid power.

The plant faces two other challenges, however. One is that the flowing water, four feet deep in some spots around the plant, may have undermined some structures. Already, plant workers have stopped moving heavy vehicles over paved surfaces because they may have been weakened, the managers say.

And the plant suffered a small fire in early June that led to the declaration of an alert, the second-lowest of the four levels of emergency established by the Nuclear Regulatory Commission. The commission’s Web page described the circumstances in a report .

On a tour of the plant on Monday, Gregory B. Jaczko, the Nuclear Regulatory Commission’s chairman, was shown an electrical cabinet with blackened enamel, still smelling faintly of smoke. It was cordoned off with yellow warning tape, awaiting forensic analysis.

The fire broke out in a transformer that will take some time to replace. But the more time-consuming task will be to examine similar transformers and assure that they are not prone to a short circuit, plant managers said.

A second Nebraska reactor that is threatened by the Missouri flood waters, Cooper Station, about 80 miles to the south, is still operating.