In advance of Hurricane Irma, officials from Florida Power and Light (FPL) announced on Thursday that the utility would start shutting down the state’s only two nuclear power plants—Turkey Point, just south of Miami, and St. Lucie, north of West Palm Beach—as a safety measure.

The Turkey Point plant seems to be closest to the hurricane’s probable path according to the latest models. It has two reactors, each capable of 693 megawatts of output while operational. The plant was built in 1972, so Irma won’t be its first Category 5 hurricane. In 1992, the eye of Hurricane Andrew passed right over Turkey Point.

At the time, Turkey Point didn’t sustain any structural damage to its most sensitive facilities, despite facing sustained winds up to 145 miles per hour and gusts as strong as 175mph. The Miami Herald writes that Turkey Point’s nuclear reactors “are encased in six feet of steel-reinforced concrete and sit 20 feet above sea level.” According to a Nuclear Regulatory Commission (NRC) report published in 1993, damage to the “safety-related systems” at Turkey Point during Hurricane Andrew was limited to “minor water intrusion and some damage to insulation and paint.” No radioactive release occurred, either.

But just because the important parts of the plant weren’t compromised doesn’t mean there wasn’t damage. The NRC report states that the hurricane “caused extensive onsite and offsite damage” at Turkey Point, which ultimately cost $90 million to repair. Power from the grid to the plant was down for five days, and the facility suffered a “complete loss of communication systems.” There was also “damage to the fire protection and security systems and warehouse facilities.”

The plant sustained all that damage but didn’t become a major environmental disaster due to redundancy built in the reactor system that continuously provided electricity to cool the reactors in the aftermath of the storm. That redundancy was hardened in 2001 after the September 11 attacks and then again in 2011 after the Fukushima Daiichi disaster.

Hope for the best, prepare for the worst

Per Peterson, a professor of nuclear engineering at the University of California Berkeley, told Ars in an e-mail that, although nuclear reactors including Turkey Point can be shut down immediately at any time, “It’s preferred to shut them down more slowly, as this results in less stress on equipment than an abrupt shutdown.”

FPL wouldn’t say exactly when the plants’ reactors would be shut down, but the utility was confident that it could shut down its nuclear plants early without affecting pre-storm power in the area. “I expect that, with Irma, there is a high probability that the electricity transmission system will be damaged, which is why it’s prudent to shut the plants down in advance of the high winds,” Peterson wrote. The NRC’s 1993 report estimated that an “orderly” shut down could take place in eight hours.

In a phone call with William Gross, the Director of Incident Preparedness for pro-nuclear lobbying group Nuclear Energy Institute, Gross noted that, after its shut down, Turkey Point will continue to draw power from the grid to run cooling operations until the grid fails. At that point, a battery system exists to temporarily take over (that is, for just a matter of seconds) until diesel generators can be brought up and running. The diesel generators live in bunkers near the reactors, and those bunkers also include considerable reserves of fuel. Even back in the ‘90s, those fuel reserves provided power to Turkey Point for five days in 1992.

A skeleton crew will ride out the storm at both Turkey Point and St. Lucie. During Hurricane Andrew, access roads to the plant were blocked by debris, so that skeleton crew was crucial.

In any event and at any nuclear reactor, should a piece of equipment fail, there’s a plan for that, too. “Every plant in the country has hardened warehouses full of backup generator equipment and pumping equipment, and all of that equipment is design to be shared between plants,” Gross said, indicating that, if Turkey Point needs a pump, St. Lucie would be able to provide it, or vice versa.

“If that’s not a possibility, we have two established safety response centers,” Gross said. “One is in Memphis, Tennessee, and the other is in Phoenix, Arizona. Either one of those two facilities... house five-plus-one sets of standard equipment.” At any given time, one piece of equipment out of the six can be out for repairs, Gross explained, but there will always be five sets of generator and pump equipment at these facilities. The sites were chosen because every nuclear plant in the country can be reached by either the Phoenix or the Memphis center within 24 hours.

The backup equipment was also specially designed so that no piece of equipment weighs more than 8,000 pounds, according to this informational video from nuclear energy company Exelon. If roads are blocked, the equipment is light enough to be airlifted to its destination.

The good news: Irma's arrival won’t take anyone by surprise. “Unlike some other natural disasters, onset of a hurricane is predictable and, as a result, lends itself to adequate early preparations for minimizing its effect on a facility,” the NRC wrote in 1993.