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Holes, cracks in reactor vessels raise big nuclear safety questions

The Associated Press
5/6/02 9:21 PM

WASHINGTON (AP) -- A nuclear reactor in Ohio is found to have a large hole nobody thought possible, burned almost through its six-inch protective steel cover. Cracks of a type never seen before are discovered at a reactor in South Carolina, triggering widespread inspections.

Both events caught industry leaders and government regulators by surprise, and they are fueling new questions about aging nuclear power plants and plant inspection programs.

The cracks found early last year at the Oconee Unit 3 reactor plant in South Carolina and the hole discovered in March in the steel reactor lid at the Davis-Besse plant in Oak Harbor, Ohio, were in areas thought largely impervious to such problems.

"It was material degradation that wasn't expected," acknowledges Alex Marion of the Nuclear Energy Institute, the industry's trade group.

The 25-year-old Davis-Besse reactor on the shore of Lake Erie is one of four nuclear plants owned by FirstEnergy Corp. It has been shut down since February, waiting for the hole in the reactor dome to be patched.

An inspection of most of the 68 other plants with similar designs and conditions reported no corrosion. But the regulators ordered special inspections at 14 reactors thought to be vulnerable to nozzle cracking because of their age.

Some senior officials at the Nuclear Regulatory Commission are viewing the Davis-Besse and Oconee discoveries as the most significant safety issue facing the nuclear industry since the Three Mile Island accident 23 years ago.

The steel reactor vessel, which encloses the reactor's core, has always been viewed as "a sacred component" that will not be breached, said Brian Sheron, the commission's assistant director for licensing and technology assessment. "This really challenges that assumption."

The problems at both reactors were discovered before they posed an immediate safety risk. A break through the reactor cover would have caused thousands of gallons of radioactive water to spew into the containment building, raising the risks of the core overheating and a potential meltdown and possible release of radiation into the environment.

Only a thin noncorrosive stainless steel membrane kept the hole at the Ohio reactor from bursting open. The cracks at the Oconee plant, owned by Duke Power, were less urgent. But had the crack expanded it could have caused the nozzle to separate, also causing a loss of cooling water inside the reactor, nuclear experts said.

Duke Power spokesman Tom Shiel said the cracks found in series of outages at the three Oconee reactors in late 2000 and early 2001 have been repaired. All three reactors will get new reactor vessel lids next year, he said.

Industry spokesmen said backup safety systems would have averted more serious problems, by pumping more water into the reactor than was being allowed to escape, keeping the nuclear fuel safe until the reactor could be shut down.

But that's true if everything worked perfectly, said David Lochbaum, a nuclear engineer and industry watchdog for the Union of Concerned Scientists. And that may not be the case if emergency pumping systems became clogged with debris, if other equipment is damaged, or a gauge is misread by plant operators struggling to make sure the reactor core remains covered with water, he said.

At the very least, argue nuclear industry critics, the Davis-Besse and Oconee incidents reveal shortcomings in how utilities inspect older power plants and how the NRC monitors them.

"The industry is trying to ensure safety while turning a profit, so they have competing interests that ... at times diverge," says Lochbaum.

The hole and cracks were found in largely inaccessible areas where there is substantial radiation and inspections can be done only when the plant is shut down.

The Davis-Besse corrosion was caused by a buildup of boric acid from leaking reactor cooling water dating back to the mid-90s. The first signs of corrosion appeared in 1998. Concerns about nozzle cracking were first raised in 1991 after an incident in France.

Yet their significance was not fully recognized until the recent alarms.

"If this occurred in Russia we would be saying it could never happen here," former NRC Commissioner Victor Gilinsky wrote in a recent commentary in The Washington Post on the Davis-Besse discovery. Gilinsky called it "a narrow escape" from a potential catastrophic accident.

NRC officials said inspections of other reactors have found no buildup of boron contamination. The NRC reports 62 nozzle cracks have been found at a dozen reactors, and all but 16 had been repaired as of last month. Two additional reactors, although having no cracks, are being closely watched because of their age and other characteristics, the agency said.

FirstEnergy acknowledges signs of corrosion as early as 1998 when filters at Davis-Besse became clogged with rust and some of the boron crystals were observed as turning from white to red.

"We didn't do a good job of recognizing pieces of the puzzle," says Todd Schneider, a spokesman for FirstEnergy Nuclear Operating Co., the subsidiary that runs the plant 26 miles east of Toledo.

NRC officials and industry executives say the 1991 nozzle incident in France was discounted because a test concluded that the cracking could not cause nozzle separation. A decade later at Davis-Besse, similar cracks were leaking as much as 12 gallons of water an hour.

The hole at the Ohio plant was found only because of an NRC inspection order arising from the cracking at the Oconee plant.

A nozzle supposedly affixed to the reactor dome at Davis-Besse unexpectedly moved several inches when engineers began repairing cracks in it.

Leaking borated water in itself is not a corrosion problem. But at Davis-Besse, the water, rather than evaporating, settled beneath the hardened layers of boron -- just enough moisture needed to turn the crystals back into corrosive boric acid.

This produced "a whole new phenomenon," says John Grobe, head of an NRC task force investigating the incident. "This kind of corrosion has never been seen before on a reactor pressure vessel head."


Nuclear Regulatory Commission:

Nuclear Energy Institute:

Union of Concerned Scientists:

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