01/18/03

A Tennessee nuclear power plant has found slight damage on its reactor lid from leaking coolant, prompting federal regulators to again warn the nation's nuclear fleet yesterday about the threat from corrosion.

The Nuclear Regulatory Commission's heightened concern about the destructive but poorly understood potential of coolant leaks is largely caused by an incident last year at Ohio's Davis-Besse plant. In March, workers discovered that corrosion from leaking coolant had over several years eaten a pineapple-sized hole completely through the reactor's 6½-inch-thick steel lid. Only a thin liner kept the highly radioactive coolant from spilling out of the reactor and severely testing the plant's emergency systems.

The Davis-Besse debacle has sparked several investigations and a rethinking of how the NRC handles its oversight duties, particularly the thoroughness and frequency of the inspections it requires of nuclear plants to spot leaks and corrosion.

The damage found Dec. 26 on the lid of one of two reactors at the Tennessee Valley Authority's Sequoyah plant near Chattanooga is minor compared with that found at Davis-Besse. The Toledo-area reactor lost a 35-pound hunk of its lid to the corrosive effects of the boric acid that is a vital ingredient in its coolant. FirstEnergy Corp. had to buy a replacement lid, part of the more than $400 million the company is spending to repair the idled plant.

By contrast, acid from leaking coolant etched just a slight indentation in the Sequoyah lid, a groove about 4½ inches long, less than a half-inch wide and one-tenth inch deep - not enough to warrant lid repair.

"It looks like it was roughed up a little," said Bill Beckner of the NRC's division of nuclear reactor regulation.

Nevertheless, the unusual circumstances in which the damage occurred are cause for concern at the NRC and are behind yesterday's cautionary notice to the nation's 69 pressurized-water reactors. Such alerts are only informational and, unlike the NRC's higher-priority bulletins, do not require specific action.

Like the bulletins and information notices related to Davis-Besse, yesterday's advisory warned reactor operators not to be complacent about even the smallest of coolant leaks.

"This is certainly something new," Beckner said, and "challenges current assumptions" that small leaks are relatively benign.

Reactors like Davis-Besse operate at high pressure so the coolant bathing the super-hot reactor core doesn't boil away and let the radioactive fuel rods melt. Because of the pressure, it isn't uncommon for coolant to seep out at pipe joints.

When leaking coolant hits the 600-degree reactor lid, it typically flashes to steam, leaving behind popcorn-like crystals of boric acid. Until Davis-Besse's damage came to light, the conventional wisdom in the nuclear industry and at the NRC was that dried acid residue was relatively harmless to the metal lid.

But coolant oozing from tiny cracks in the nozzles that let control rods pass in and out of the lid rewetted the huge amounts of dried acid that Davis-Besse workers had let build up for years atop the reactor. It took about four years for the concentrated acid to bore through the dome.

The conditions were somewhat different at Sequoyah, where from May to December a leak in an instrument pipe fitting allowed coolant to drip through insulation and leave about 20 pounds of dried boric acid. It's unclear what caused the aggressive corrosion.

"We're still trying to figure out the details," including whether the acid deposits were moistened again, said Alex Marion, engineering director for the Nuclear Energy Institute, the industry's trade organization.

Of greater concern is how quickly the corrosion occurred, said David Lochbaum, a nuclear engineer with the Union of Concerned Scientists, a watchdog group. If corrosion can cause severe damage in less than 18 months - the typical period between inspections, which coincide with refueling shutdowns - then the NRC may have to order shorter operating cycles, Lochbaum said.