Each site has a pair of reactors. Turkey Point’s date back to 1972 and 1973; St Lucie’s were commissioned in 1976. Turkey Point survived Hurricane Andrew, a Category 5 storm that in 1992 passed directly over the nuclear plant. Andrew remains the most destructive hurricane to hit the state. Forecasters say Irma will be near South Florida by Sunday morning.

“We have tried-and-true processes in place,” Eric Silagy, chief executive of NextEra Energy’s Florida Power & Light subsidiary, said.

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Silagy said that the reactors would be shut down when Irma draws closer and winds hit Category 1 levels. The shutdown, which takes about 24 hours, reduces the temperature in the reactor. Lower temperatures reduce the amount of power needed for cooling, making the plant safer and less likely to spin out of control in an emergency.

“We have safely operated these plants for over 40 years,” Silagy said.

Nuclear plants are located near lakes, rivers and oceans because they need large amounts of water to cool the reactors. Turkey Point uses an unusual cooling system, relying on canals that cover 6,800 acres.

But during storms that creates hazards. Turkey Point stands just 20 feet above sea level. Essential equipment is 22 feet above sea level, the company’s website says. St. Lucie’s elevation is 15 feet above sea level.

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Like other U.S. nuclear reactors, the Florida reactors have been required to improve and reexamine their disaster preparedness after an earthquake and tsunami in March 2011 devastated the northeast coast of Japan, knocking out essential electrical power to a fleet of nuclear reactors at Fukushima.

Flooding from the tsunami disabled backup generators and cut off power, resulting in a partial meltdown and the release of radioactive materials into the ocean and surrounding land areas. The vast majority of Japan’s nuclear fleet remains closed today.

Before Fukushima, U.S. nuclear reactors focused on other perils, such as securing radioactive materials. The vital components of Turkey Point’s reactors are in buildings with steel-reinforced concrete walls six feet thick. But Fukushima showed that electricity supply could be an Achilles’ heel for reactors. Regulators assumed plants would not need to be without power for more than eight hours, said David Lochbaum, a nuclear power plant safety expert at the Union of Concerned Scientists. However, Fukushima was without power for nine days.

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In March 2012, the Nuclear Regulatory Commission ordered plant owners to provide portable equipment that could provide power for an indefinite period.

Lochbaum also said that the NRC previously required accident response plans for the outage of a single reactor at a multiunit site such as Turkey Point. “Fukushima dispelled that notion,” Lochbaum said in an email. “Now, owners must stage sufficient equipment resources and associated staff to cope with challenges to all on-site reactors and spent fuel pools.”

Allison MacFarlane, former chair of the Nuclear Regulatory Commission, said in an email that in response to the Fukushima accident, reactor owners were required to upgrade emergency equipment, put it in “hardened buildings” to withstand storms, and bolster instrumentation on spent fuel pools.

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But Lochbaum said that the NRC does not require the testing of portable pumps and generators. On Jan. 9, seven inches of rain fell in five hours at the St. Lucie nuclear plant, flooding an auxiliary building with nearly 50,000 gallons of water that flowed through missing or degraded seals.

“Fukushima revealed a vulnerability. NRC ordered that vulnerability remedied. St. Lucie claimed to have remedied it. NRC claimed to have double-checked the remedy. And then rainfall reveals them both to be wrong,” Lochbaum said.