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Davis-Besse lid could have burst in 1 to 2 years

02/26/03

John Mangels and John Funk
Plain Dealer Reporters

If workers hadn't accidentally discovered a rust hole atop the Davis-Besse nuclear reactor last March, the damaged steel lid could have ruptured in as few as one to two years, according to a new government analysis.

Although the resulting accident would have been the nation's most severe since the 1979 partial meltdown at Three Mile Island, the chance of substantial amounts of radiation escaping Davis-Besse's reactor building to threaten the public was relatively small, the study found.

Nevertheless, the carelessness that allowed the rust hole to grow unnoticed for as long as eight years, and the heightened risk posed by the decay of a main safety barrier, earned Davis-Besse the Nuclear Regulatory Commission's harshest assessment yesterday.

The NRC decided that the lid debacle at Davis-Besse deserved a "red" ranking, the most serious on its four-color scale for evaluating the safety significance of problems at nuclear plants. The next step for the agency is to determine whether the plant's negligence was intentional, which could lead to fines or criminal prosecution.

"Considering the situation, we expected this type of finding from the NRC," said Todd Schneider, a spokesman for Davis-Besse owner FirstEnergy Corp. "We're not proud of it, and we are making many improvements to return the plant to safe and reliable service."

"It's important to remember the plant shut down safely and there was no impact on public safety," Schneider said.

Schneider said the company will not contest the analysis and expects to be fined. Yesterday's bad mark against Davis-Besse could make any fines higher if the NRC determines misconduct at the plant was deliberate, said David Lochbaum of the Union of Concerned Scientists.

Davis-Besse was in the middle of a refueling shutdown a year ago when workers repairing a metal sleeve on the reactor's lid stumbled upon a deep hole nearby. The jagged pit, caused when corrosive reactor coolant leaked from a crack in the nozzle-like sleeve, was all the way through the 6.5-inch-thick steel lid.

Only a thin stainless steel liner at the bottom of the hole kept the radioactive, high-pressure coolant from spurting out and allowing the reactor's fuel rods to overheat and possibly melt.

The NRC's analysis, which took months to complete, involved some of the government's top science labs, and cost hundreds of thousands of dollars, was unable to come up with simple, absolute answers about what would have happened at Davis-Besse had the rust hole continued to grow.

"We spent an awful lot of effort trying to quantify the actual risk," said Jack Grobe, who heads the NRC panel overseeing Davis-Besse's rehabilitation. "The corrosion rate and mechanism are not well understood."

The inability to pin down those and other variables meant that agency analysts could only give ranges - sometimes wide ones - when trying to predict the likelihood of specific accident scenarios at the plant.

Overall, the analysis judged that the rust hole increased the risk of a core-damaging accident at the plant enough to merit the NRC's highest level of scrutiny. That overall risk is expressed as a math formula. In laymen's terms, what it means is that if 10,000 reactors operated for a year with the kind of lid corrosion that existed at Davis-Besse last March, one of them during that time probably would have an accident that harmed their volatile fuel rods.

While such a mishap would be potentially disastrous financially and a public relations nightmare, it likely would pose only a minimal risk to residents near the Toledo-area reactor, the analysis found. The huge concrete and steel containment building that houses the reactor has only a small chance of letting radioactive steam escape its confines.

And although Davis-Besse's emergency core cooling system had long-standing flaws that could have blocked the flow of water to the reactor in some situations, a rupture of the reactor's lid probably would not have caused that to happen, Grobe said.

The emergency system's sump, which collects spilled coolant and returns it to the reactor core, is vulnerable to being blocked by debris. But analysis showed that a rupture at the location where Davis-Besse's lid was rusted wouldn't blast loose enough insulation and other trash to foul the sump.

While the NRC's analysis can evaluate with relative confidence the risk posed by the lid hole as it existed last March, it gets hazier when trying to project into the future. Since the corrosion has never been seen before on a reactor lid, much less studied, scientists had difficulty judging how fast it would spread and when the steel cap might have burst if it had grown unchecked.

Further complicating the assessment: the steel liner holding back the coolant was uneven, bulging and cracked in spots, making predictions about its durability unreliable without much more study. That work is ongoing.

Computer models predicted that a pristine, uniformly thick liner at the bottom of a hole the size found last March would have withstood more than three times the operating pressure of the reactor.

But had the hole enlarged as some scenarios predicted, exposing more and more of the liner, the vulnerable steel sheet would have grown more likely with time to have burst under normal reactor conditions.

FirstEnergy's own analysis put that time to failure at four to seven years of reactor operation, using a conservative estimate of how fast the corrosion was spreading.

But lab tests done by nuclear industry researchers show that much faster corrosion rates are possible in some situations, chewing up to seven inches of metal per year. At that pace, the hole could have grown large enough in a year or two to allow the liner to rupture.

The NRC analysis couldn't determine which was the more likely scenario.

"It was still only a matter of good fortune, rather than good design or good planning, that the (liner) successfully served as the pressure boundary," the analysis said. While the attempt to judge the safety significance of the rust hole at Davis-Besse has been a long time in coming, the NRC has from the beginning acted as if the "red finding" has been in place, Grobe said.

That grade requires much greater agency oversight than well-performing plants get. Since last March, the NRC panel Grobe chairs has watched over the repairs and management changes at Davis-Besse.

Ultimately, the plant cannot resume producing electricity without the panel's OK.

Plain Dealer Washington bureau chief Stephen Koff contributed to this report.


2003 The Plain Dealer. Used with permission.
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