Hurricane Irma will pose the toughest test yet for US nuclear power plants since reactors strengthened their defenses against natural disasters following the Fukushima Daiichi nuclear accident in Japan in 2011.

Irma was on course to hit South Florida early on Sunday after slamming Cuba as a Category 5 storm. It weakened to a Category 3 storm with maximum sustained winds of 130 miles per hour (210 km per hour) on Saturday, but was expected to strengthen before reaching Florida, bringing a storm surge to a state home to four coastal nuclear reactors.

The National Hurricane Center’s forecast track shows Irma making landfall on the southwest side of the Florida Peninsula, west of the two nuclear reactors at the Turkey Point plant.

The operator, Florida Power & Light (FPL), has said it will shut Turkey Point well before hurricane-strength winds reach the plant. The reactors are about 30 miles (42 kilometers) south of Miami.

FPL said it will also shut the other nuclear plant in Florida at St Lucie, which also has two reactors on a barrier island on the state’s east coast, about 120 miles (193 km) north of Miami.

“We will shut the reactors down 24 hours before Category 1 force winds are forecast to hit,” FPL Chief Executive Eric Silagy told a news conference.

FPL said both Turkey Point and St Lucie were designed to withstand storms stronger than any ever recorded in the region and both plants are elevated 20 feet (6 meters) above sea level to protect against flooding and extreme storm surges.

But South Miami Mayor Philip Stoddard said he was concerned about the potential for floods to damage power generators at Turkey Point, which in turn might threaten the ability of the plant to keep spent nuclear fuel rods cool. At Fukushima in Japan, an earthquake and tsunami disrupted power supplies and caused the fuel in some units to meltdown.

“The whole site is pretty well able to handle dangerous wind, the real problem from my perspective is water,” Stoddard said. He said he was more worried about the nuclear waste than the reactors.

“The real question is can they keep the spent fuel cool.”

Peter Robbins, an FPL spokesman, said “Mayor Stoddard is wrong. Turkey Point is safe and is ready for Hurricane Irma.”

US nuclear operators have taken steps to improve preparations for disasters since Fukushima. The US Nuclear Regulatory Commission (NRC) required plants to install portable pumps and generators to keep water moving over fuel rods and the spent fuel pool even if offsite power supply was lost.

Nuclear plants also hired more staff and stored equipment needed to deal with reactor problems.

“Things are better today than in March 2011. Time will tell whether better proves good enough,” said Dave Lochbaum, director of a watchdog group, the nuclear safety project, at the Union of Concerned Scientists.

The NRC said it was posting more inspectors at the two Florida plants and was considering sending more inspectors to plants in Georgia and South Carolina should the storm head that way.

The NRC said the plants should not be compromised by Irma’s storm surge.

“The storm surge forecasts that we have seen so far do not challenge the sites’ designs,” said NRC spokesman Scott Burnell.

Both Florida plants have previously weathered major storms. Turkey Point took a hit from Hurricane Andrew, a Category 5 storm, in 1992, damaging a smokestack at fossil fuel plant at the site. Repairs cost about $90 million.

St Lucie withstood the back-to-back impact of Hurricanes Frances (Category 2) and Jeanne (Category 3) in 2004.

FPL shut St Lucie last October as Hurricane Matthew skirted the Florida coast.

Lochbaum said the NRC and the industry could do more to reduce vulnerability to flooding.

In January 2014, about 50,000 gallons of rainwater leaked into the St Lucie plant after a heavy downpour. An NRC study blamed degraded and missing flood seals that were not discovered during checks after Fukushima.

Hurricane Irma – devastation caused in pictures Show all 45 1 /45 Hurricane Irma – devastation caused in pictures Hurricane Irma – devastation caused in pictures Recently planted palm trees lie strewn across the road as Hurricane Irma passes by in Miami Beach, Fla. 10 September 2017. AP Hurricane Irma – devastation caused in pictures Damage outside the Mercure hotel in Marigot, on the Bay of Nettle, on the island of Saint-Martin AFP/Getty Images Hurricane Irma – devastation caused in pictures People pick up debris in Fajardo as Hurricane Irma howled past Puerto Rico after thrashing several smaller Caribbean islands Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures Large waves produced by Hurricane Irma crash into the end of Anglins Fishing Pier in Fort Lauderdale, Florida. The category 4 hurricane made landfall in the United States in the Florida Keys at 9:10 a.m. after raking across the north coast of Cuba. 10 September 2017 Getty Images Hurricane Irma – devastation caused in pictures A Royal Air Force Puma has been delivered to the US Virgin Islands to assist with the humanitarian efforts post Hurricane Irma. The Puma will be delivering Humanitarian Aid and Disaster Relief in support of the Department for International Development. Royal Air Force logisticians from RAF Brize Norton have assisted with the delivery of military personnel and aid cargo to the Caribbean to support disaster relief in the wake of Hurricane Irma. RAF aircraft including, C-17 A400M and Voyager are supporting a Joint Task Force of RAF, Royal Marines, Army and RN personnel who are supporting the Department for International Development as it delivers aid to stricken Caribbean Islands. MoD Hurricane Irma – devastation caused in pictures Flamingos at Zoo Miami, are shown in a temporary enclosure in a hurricane resistant structure within the zoo, Saturday, 9 September 2017 in Miami. Though most animals will reman in their secure structures, the cheetahs and some birds will ride out the storm in temporary housing. AP Hurricane Irma – devastation caused in pictures Boats are seen at a marina in South Beach as Hurricane Irma arrives at south Florida, in Miami Beach, Florida, U.S. 10 September 2017 Reuters Hurricane Irma – devastation caused in pictures Storm clouds are seen over Fisher Island as Hurricane Irma approaches on 9 September 2017 in Miami Beach, Florida. Florida is in the path of the Hurricane which may come ashore at category 4 Getty Images Hurricane Irma – devastation caused in pictures Waves crash over a seawall at the mouth of the Miami River from Biscayne Bay, Fla., as Hurricane Irma passes by. 10 September 2017 AP Hurricane Irma – devastation caused in pictures Some of the damage on Saint Martin EPA/Gerben Van Es/Dutch Department of Defence Hurricane Irma – devastation caused in pictures The skyline is seen as the outerbands of Hurricane Irma start to reach Florida on 9 September 2017 in Miami, Florida. Florida is in the path of the Hurricane which may come ashore at category 4. Getty Images Hurricane Irma – devastation caused in pictures A tree toped by hurricane Irma is seen on a empty street in Remedios, Cuba, 9 September 2017. Hurricane Irma reached Cuba bringing winds between 160 and 190 kilometers per hour. The hurricane has hit the north coast of the island. EPA Hurricane Irma – devastation caused in pictures James Constantineau loads sands bags in his truck as he prepares for the approach of Hurricane Irma Saturday, 9 September 2017, in East Palatka, Fla. Gov. Rick Scott is urging anyone living in an evacuation zone in southwest Florida to leave by noon as the threat of Hurricane Irma has shifted west. AP Hurricane Irma – devastation caused in pictures The Fort Louis Marina in Marigot is seen on 8 September 2017 in Saint-Martin island, devastated by Hurricane Irma. AFP Hurricane Irma – devastation caused in pictures Destruction in Orient Bay on the island of Saint-Martin AFP/Getty Hurricane Irma – devastation caused in pictures The wreckage in Orient Bay on the island of Saint-Martin AFP/Getty Hurricane Irma – devastation caused in pictures View of the aftermath of Hurricane Irma on Saint Martin Reuters Hurricane Irma – devastation caused in pictures A mobile network tower snapped in two by the hurricane on the island of Barbuda ABS TV Hurricane Irma – devastation caused in pictures A house reduced to rubble on the island of Saint Barthelemy AFP/Getty Images Hurricane Irma – devastation caused in pictures General view of damage on Saint Martin Reuters Hurricane Irma – devastation caused in pictures A damaged Casino Royale on Saint Martin after the passage of Hurricane Irma Anna Mazur/AFP Hurricane Irma – devastation caused in pictures An aerial photograph taken and released by the Dutch department of Defense shows the damage of Hurricane Irma in Philipsburg, Sint Maarten, the Dutch section of the Caribbean Island Gerben Van Es/AFP Hurricane Irma – devastation caused in pictures Flooded houses in Gustavia on the island of Saint-Barthelemy Kevin Barrallon/AFP Hurricane Irma – devastation caused in pictures The damage on the island of Saint-Martin, a day after Hurricane Irma hit AFP/Getty Images Hurricane Irma – devastation caused in pictures A man carrying an umbrella is battered by the wind in Fajardo, Puerto Rico Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures A lone police car on patrol during the passing of Hurricane Irma in Fajardo, Puerto Rico Jose Jimenez/Getty Images Hurricane Irma – devastation caused in pictures Haitian people walk through the wind and rain on a beach in Cap-Haitien on September 7 as Hurricane Irma approaches Hector Retamal/AFP/Getty Images Hurricane Irma – devastation caused in pictures A flooded street on the island of Saint Martin AFP/Getty Images Hurricane Irma – devastation caused in pictures A tree collapsed on a house in Saint Martin Hurricane Irma – devastation caused in pictures A hotel in Saint Martin is gutted by floodwater during the hurricane Guadeloupe 1ère Hurricane Irma – devastation caused in pictures Cars submerged in Saint Martin Rinsy Xieng Hurricane Irma – devastation caused in pictures Debris floats amongst the floodwater in Saint Martin @la1ere Hurricane Irma – devastation caused in pictures Household items float down the street in Gustavia, Saint-Barthélemy Carole Greaux Hurricane Irma – devastation caused in pictures The coast of Saint Martin is flooded as the hurricane hits the island Météo Express Hurricane Irma – devastation caused in pictures A whole street underwater in Saint Martin @la1ere Hurricane Irma – devastation caused in pictures A car crashes into the tree amongst the chaos in Saint Martin @Bondtehond Hurricane Irma – devastation caused in pictures A building on the Saint Martin seafront, destroyed by the hurricane @Bondtehond Hurricane Irma – devastation caused in pictures A mobile home overturned at Princess Juliana International Airport in Saint Martin @Bondtehond Hurricane Irma – devastation caused in pictures Palm trees bend in the wind in San Juan, Puerto Rico as Hurricane Irma slammed across islands in the northern Caribbean Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures A woman runs in the rain as Hurricane Irma slammed into San Juan, Puerto Rico Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures A picture taken on September 5, 2017 shows a view of the Baie Nettle beach in Marigot, with the wind blowing ahead of the arrival of Hurricane Irma AFP/Getty Images Hurricane Irma – devastation caused in pictures A man rides past a boarded up house as part of preparations ahead of the arrival of Hurricane Irma on September 5, 2017, in the French overseas island of Guadeloupe Helene Valenzuela/AFP Hurricane Irma – devastation caused in pictures Employees of the Mercure Hotel fill sand bags on the Baie Nettle beach in Marigot, as part of the preparations for the arrival of Hurricane Irma Lionel Chamoiseau/AFP Hurricane Irma – devastation caused in pictures People in line at Costco, as they find out the store has ran out of water on September 5, 2017 in North Miami Michele Eve Sandberg/AFP Hurricane Irma – devastation caused in pictures Night view of the city of Cap-Haitien, in the north of Haiti, 240 km from Port-au-Prince, on September 5, 2017 Hector Retamal/AFP

There is also spent nuclear fuel at the site of a third power plant in Florida which stopped operating in 2009. That is the Crystal River plant, owned by Duke Energy Corp (DUK.N).

Duke is in the process of transferring used fuel from the spent fuel pool at the plant to dry cask storage as part of work to decommission the plant. Once in storage, the fuel no longer needs cooling.

The NRC’s Burnell said Duke has suspended work to transfer the rods ahead of Irma. He said the fuel was safe and the plant also has backup power, even though it has been shut for years.

Irma is expected to disrupt much of Florida’s power supply. FPL, the state’s largest electric company, has warned Irma could cut service to about 4.1 million of its nearly 5 million customers. FPL is one of four large publicly traded utilities in Florida.

Other natural disasters since Fukushima have shut some plants. An earthquake in Virginia in 2011 shut Dominion Energy Inc’s (D.N) North Anna plant for about 2-1/2 months, the time it took to complete a full damage inspection. That plant sustained no major damage.