Fort Calhoun and Cooper nuclear power plants were both opened up to federal regulators and the media this week as part of a battle against persistent Internet rumors about their safety.

The Omaha Public Power District’s Fort Calhoun is the subject of more public concern because the floodwaters have surrounded that plant and forced workers to use raised catwalks to access the facility. Nebraska Public Power District’s Cooper plant is more elevated, so the floodwaters aren’t as close to the facility.

"There is no possibility of a meltdown," OPPD CEO Gary Gates said Monday. "The floodwaters are outside of Fort Calhoun, not inside."

Nuclear Regulatory Commission Chairman Gregory Jaczko offered support for both utilities after visiting the plants. He said both Fort Calhoun and Cooper remain safe.

But whether their statements can slow the online rumor mill remains to be seen. Some speculate that the Nebraska flooding could somehow spawn a disaster comparable to one in northeastern Japan in March, when a massive earthquake and tsunami destroyed power and cooling systems at the Fukushima Dai-ichi nuclear plant, melting reactor cores and leaking massive amount of radiation. Nebraska utility officials say that’s next to impossible.

One of the key differences between the Fukushima disaster and the Missouri River flooding is that the river flooding has progressed slowly and the utilities had several weeks to prepare.

"There is little to no chance of anything like Fukushima happening here," said Tim Nellenbach, who oversees Fort Calhoun’s nuclear operations.

Jaczko’s visit to Fort Calhoun Monday came one day after an 8-foot-tall, water-filled temporary berm protecting the plant collapsed. OPPD plans to replace the 2,000-foot berm with a similar one early next month and then pump out the floodwaters to restore a dry buffer area.

"We don’t believe the plant is posing an immediate threat to the health and safety of the public," Jaczko said.

Omaha Public Power District spokesman Jeff Hanson said pumps at Fort Calhoun were handling the problem and that "everything is secure and safe." The plant, about 20 miles north of Omaha, has been closed for refueling since April. Hanson said the berm’s collapse didn’t affect the shutdown or the spent fuel pool cooling.

Either floodwaters from the Missouri River or groundwater seeped into several of the peripheral buildings at Fort Calhoun, but Nellenbach said all of the areas containing radioactive material or crucial safety gear remained dry.

One of the biggest threats to the safety of any nuclear power plant would be a prolonged loss of electrical power because the plants need to be able to continue pumping water over the radioactive fuel to keep it cool.

A key factor in the disaster at the Fukushima Dai-ichi facility was the loss of all off-site power and emergency generators after the earthquake and tsunami struck.

Fort Calhoun has at least nine power sources in place, including six different power lines and two backup diesel generators, which were just tested Sunday after the failure of the water-filled berm.

Cooper also has two main lines of outside power, at least three generators on site and a battery system that can power the plant in an emergency.

Flooding remains a concern all along the Missouri because of the massive amounts of water the U.S. Army Corps of Engineers has released from upstream reservoirs. The river is expected to rise as much as 5 to 7 feet above flood stage in much of Nebraska and Iowa and as much as 10 feet over flood stage in parts of Missouri.

The corps expects the river to remain high at least into August because of heavy spring rains in the upper Plains and substantial Rocky Mountain snowpack melting into the river basin.

Both nuclear plants issued flooding alerts earlier this month, although they were routine as the river’s rise has been expected.

The main building at Fort Calhoun is at 1,004 feet above sea level, which is about 2 feet below the level of the Missouri River. That’s why floodwaters have been able to get so close to the plant.

The main building complex at Fort Calhoun is surrounded by floodwaters at least 2 feet deep, and employees use an elevated catwalk more than a quarter-mile-long each day to cross the flooded parking lot. But the utility has been able to keep the inside of its buildings and key equipment mostly dry with a network of flood barriers and a number of pumps.

Fort Calhoun workers can remain dry when walking into the plant, but OPPD has invested in about 300 life jackets and a couple hundred pairs of waders for times when employees must enter the water to check a flood barrier or build more scaffolding. Boats are also used to ferry equipment around the complex.

OPPD officials and Jaczko said the fact that Fort Calhoun has been shut down since April helps make the plant significantly safer during the flooding because the radioactive fuel has been cooling off since then.

"The risk is really very low at this point that anything could go wrong," Jaczko said.

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Associated Press Writers Nelson Lampe and Timberly Ross contributed to this report from Omaha.

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Online:

U.S. Nuclear Regulatory Commission: www.nrc.gov

Omaha Public Power District: www.oppd.com