DAVIS–BESSE: The nuclear power plant near Toledo, Ohio, was shut down for two years due to an equipment failure that might have resulted in a catastrophic meltdown if it had continued to go without detection. Image: Courtesy of NRC
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[This is Part 4 of an In-Depth Report on The Future of Nuclear Power.]
On Feb. 16, 2002, the nuclear power plant called Davis–Besse on the shores of Lake Erie near Toledo, Ohio, shut down. On inspection, a pineapple-size section on the 6.63-inch- (16.84-centimeter-) thick carbon steel lid that holds in the pressurized, fission-heated water in the site's sole reactor had been entirely eaten away by boric acid formed from a leak. The only thing standing between the escape of nuclear steam and a possible chain of events leading to a meltdown was an internal liner of stainless steel just three sixteenths of an inch (0.48 centimeter) thick that had slowly bent out about an eighth of an inch (0.32 centimeter) into the cavity due to the constant 2,200 pound-per–square-inch (155-kilogram-per-square-centimeter) pressure.
That cavity of as much as 30 square inches (194 square centimeters) had formed on the Unit 1 reactor head—a hole apparently missed during prior safety inspections in 2000 and 1998 by the U.S. Nuclear Regulatory Commission (NRC), the government agency charged with monitoring the nation's nuclear reactors. The inspector noted deposits of boric acid but underestimated the potential impact, despite more than three decades of issues in nuclear reactors involving boric acid corroding metal. The hole was only discovered when a replaced nozzle tipped over during the repair process in 2002.
Circular cracks had formed around several of these steel nozzles in the corroded lid that shield the control rods—20 12-inch- (30-centimeter-) long sticks made of a silver–indium–cadmium alloy that are used to dampen or shut down a nuclear reaction—rendering them vulnerable to simply popping off. The NRC first explicitly identified this problem in 1993 (and it had been occurring in similar plants since the 1980s). But it took until 2001 for the NRC to require operators of reactors prone to the potential problem to check for it, a process that had not been completed at Davis-Besse.
As a result of that gaping cavity, the NRC shut down Davis–Besse for two years (from 2002 to 2004) in an attempt to patch the nozzle that had been leaking boric acid as well as to address design flaws that had been in place since the plant started operation in 1977 and what the agency described as a poor safety culture. In other words, an unwillingness by employees of owner Akron, Ohio–based FirstEnergy Corp. to report safety issues. All told, the repairs totaled $600 million.
A subsequent review of the entire power plant by the NRC in the wake of the near-miss revealed that its emergency cooling system—a critical line of defense in the event of a meltdown—might have failed due to clogging resulting from "generic" flaws built into the plant prior to 1977.
"The reactor vessel head could have failed between two and 14 months further out, that would have been a major loss-of-coolant accident," says NRC spokesman Scott Burnell. "There would have been a very real possibility of damaging the [nuclear] core. In terms of coming up with probabilities, the staff determined that this event was one of the most serious situations since Three Mile Island," the Pennsylvania reactor that partially melted down in 1979. And other models put the incidence of an accident as little as two weeks away, due to the cracked and buckled stainless steel.