On Jan. 9, a freak storm dumped a record rainfall on central Florida's Treasure Coast, inundating the St. Lucie nuclear power plant facing the Atlantic Ocean. Storm drains failed, and 50,000 gallons of water flooded the plant's Unit 1 reactor auxiliary building through improperly sealed electrical passages, disabling core cooling pumps.
Had the reactor tripped during that storm, all of the emergency core cooling pumps would have been submerged. Under that scenario, "after 24 hours, the plant would not achieve a 'safe and stable' condition and reactor core would be damaged, unless emergency recovery action succeeded," according to a Nuclear Regulatory Commission notice of violation against the St. Lucie plant owner, Florida Power & Light Co. (FPL), a subsidiary of NextEra Energy. The NRC notified FPL on Nov. 19 that it would be subject to increased safety inspections because of the violations.
The incident surprised and jarred the NRC, which has made flooding threats to U.S. reactors a top regulatory priority following the 2011 tsunami at the Fukushima Daiichi nuclear plant in Japan. Walls of water sweeping through the plant on Japan's east coast wiped out first- and second-line systems for bringing reactors to a safe shutdown, leading to an explosion of leaking hydrogen gas and three reactor meltdowns.
Following the Fukushima crisis, the NRC directed U.S. nuclear plant operators to make "walkdown" plant inspections to check for flooding vulnerabilities. FPL hired a contractor for the task, and following its inspection, assured the commission that Unit 1 was protected, the NRC said.
The walkdown inspection did not detect the missing seals in electrical conduits that were installed through the auxiliary building wall more than 30 years ago and are now significantly degraded, the NRC found. The NRC added that FPL missed opportunities to spot the violations during company inspections in 2009 and 2010, as well as the 2012 walkdown.
The worst-case scenario did not occur in January: The reactor did not trip during the storm, and the openings into the auxiliary building were promptly fixed following the flooding. The NRC did not assess a civil penalty, noting FPL's actions to remedy the leakage vulnerability. The overall risk of a worst-case scenario was rated "low to moderate" by the NRC, because operators would have had multiple opportunities to provide emergency core cooling. FPL did not contest the commission's decision.
But the St. Lucie incident highlights the NRC's concerns over flooding hazards that confront reactors sited alongside oceans or rivers -- units that will be exposed to increasing threats of storm surges or extreme high water in coming decades, according to most climate change experts.
The Union of Concerned Scientists, in an April paper, reported that some 100 power plants and substations in the contiguous United States are within 4 feet of local high tide. "As sea levels continue to rise, the risks to these facilities from storm surge and floods will also increase," it said.
U.S. reactors have withstood a barrage of hurricanes and other powerful storms over the past decade, from Hurricane Katrina to Superstorm Sandy, noted Eric Leeds, director of the NRC Office of Nuclear Reactor Regulation, speaking at a Jan. 6 NRC conference. A trio of hurricanes hit St. Lucie in 2004, but no flooding occurred, an FPL spokesman noted. (In January's storm, the drain valve failures were critical.)
But while engineers can design equipment with well-calculated margins of safety to survive predicted earthquakes, counteracting unpredictable extreme flooding threats is a far more uncertain task, senior NRC staff officials told commissioners at the January conference.
"In flooding, you get to a certain level and the switch gear gets wet. You get to a certain level and you just take out equipment that you cannot recover. You have to bring in other equipment," Leeds said.
"With electricity, it gets wet and it is over. So for flooding, personally, for flooding, it is a large concern to me. And I know that we have been very focused as an agency to make sure that these plants are prepared and can withstand that type of thing," Leeds added.
The NRC has ordered U.S. nuclear plant operators to update existing flood vulnerability assessments, which may be many decades old, to take account of the current understanding of threats. The assessments are to be completed by the end of 2016.
White paper on flooding threats
Meanwhile, NRC staff have prepared a new policy white paper that would require plant operators to integrate the updated flooding threat assessments into another new post-Fukushima directive on natural disaster planning. This second policy requires operators to create flexible, "mitigating" strategies for maintaining cooling capability for reactor cores and spent fuel pools in the face of flooding, earthquakes and other extreme storms and weather conditions that exceed the current hazard scenarios -- the "beyond design basis external events."
The paper states that the NRC staff intend to ask the commission to require plant operators to address the updated evaluations of flooding threats in creating their mitigating strategies. The new approach may require plant operators to strengthen defenses beyond current levels and develop specific, and possibly "unconventional," flooding defenses, the paper said.
The white paper will be presented Thursday to the NRC Advisory Committee on Reactor Safeguards, a group of outside nuclear experts, and will await the NRC commissioners' review.
Three senior NRC officials have criticized the "mitigation" approach called for in the staff white paper, filing a statement of "non-concurrence" with the commission. Led by Gary Holahan, deputy director of the Office of New Reactors, the statement's authors said the white paper proposal, by focusing on mitigation or response to extreme natural crises, neglects the need to strengthen nuclear plants in order to prevent crises from occurring in the first place.
The white paper's approach "would constitute a lost opportunity to identify potential plant vulnerabilities and to implement practical measures to protect key safety-related equipment," the three staff members said.
"We believe it is also necessary to conduct a thorough and systematic re-evaluation of protection" of standard equipment that forms the first line of defense in emergencies, including backup diesel generators to ensure power for core and storage pool cooling, secure electrical distribution equipment, and cooling water delivery, they said.
"Simply stated, we do not believe that mitigation is an appropriate substitute for protection," they said.
In the St. Lucie case, the plant was not protected against a flood threat even using current standards, said David Lochbaum, director of the nuclear safety project for the Union of Concerned Scientists.
Speaking to the Jan. 6 NRC conference, Lochbaum said 90 percent of the NRC's post-Fukushima walkdown inspections found "long-standing violations of existing [flood protection] regulatory requirements. How is this even possible?" Before they can adopt more stringent defenses against even more serious flooding threats, the commission and the industry need to understand why the current inspection system has not done its job, he said.
"Many, many walkdowns have been conducted by plant workers and NRC inspectors over the past four decades allegedly looking for stuff like degraded or missing seals, so why did so many attempts over so many years fail to find these numerous violations? This must not remain a rhetorical question," Lochbaum told the NRC conference.