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March 27, 2002

 



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Environment | Article published Wednesday, March 27, 2002
DAVIS-BESSE NUCLEAR PLANT
Warning signs went unheeded
Corrosion was evident in '99, FirstEnergy says
Picture
Davis-Besse has been shut down since Feb. 16.
(THE BLADE)

ZOOM 1 | ZOOM 2

By TOM HENRY
BLADE STAFF WRITER


OAK HARBOR, Ohio - FirstEnergy Corp. yesterday acknowledged that employees at its Davis-Besse nuclear plant failed to pick up on signs of a corroded reactor head when evidence surfaced in 1999.

Airborne traces of rust were found trapped inside the containment structure's radiation monitors that year.

FirstEnergy did not "make the connection" between that and the possibility that the reactor's 17-foot-wide steel reactor head was rusting. Instead, it put its faith in visual inspections that were done every two years and showed nothing out of the ordinary. But the view was partially obstructed, Richard Wilkins, a spokesman for the utility, said.

"We found traces of iron oxide [i.e. rust] in the filters of air monitors. We did not make the connection between that and a nozzle leak," Mr. Wilkins said, referring to cracks found in five of the reactor head's 69 metal tubes known as control rod drive mechanism nozzles.

TROUBLE SIGNS
FirstEnergy Corp. officials put together the following timeline based on scientific evidence about the corroded reactor head at the Davis-Besse nuclear plant in Ottawa County.
1990 (plus or minus three years): Crack likely begins to form in Nozzle No. 3, the location of the reactor head where the worst corrosion exists. The device is one of 69 in the reactor head.
1994-1996: Crack likely expands through the nozzle wall, allowing boric acid from the reactor to start leaking.
1998: Visual inspection made while Davis-Besse is taken off line for refueling, but evidence of a leak is not readily apparent.
1999: First evidence of corrosion surfaces: Airborne rust particles trapped by radiation monitors in the containment area. Workers fail to notice that as a signal that a nozzle is cracked and leaking, though.
2000: Another visual inspection made while off line for refueling but no evidence of leak is apparent.
2002: Major corrosion verified near Nozzle No. 3. Additional corrosion - though not as severe - found near another cracked tube, known as Nozzle No. 2.
Source: FirstEnergy Corp.'s Root Cause Team
Evidence of the airborne rust particles was mentioned in a five-page status report issued by a team of nuclear engineers, metallurgists, and consultants the utility assembled to identify the root cause of the corrosion.

"The potential for significant corrosion of the [reactor] head as a result of accumulating boric acid and local leakage was not recognized as a safety significant issue by the staff and management of the plant," the draft report stated.

The report is being reviewed by the U.S. Nuclear Regulatory Commission, which has its own team doing a separate investigation.

Jan Strasma, a NRC spokesman, had little to say about the preliminary conclusions submitted by the utility's team. "We don't have our assessment of that at this point," he said.

But he agreed that regulators would not be pleased if FirstEnergy overlooked evidence such as airborne rust particles.

"We expect them to look not narrowly, but broadly. This may indicate their corrective action program was not working as it should have," Mr. Strasma said.

The plant, 25 miles east of Toledo, was shut down for refueling and normal inspections on Feb. 16.

Ultrasonic tests showed three of the five cracked nozzles had split all the way through. At least two of them allowed boric acid from the reactor to dribble onto the reactor head over the past few years and burn through steel.

Leakage from one tube in particular, referred to as Nozzle No. 3, has resulted in what's been described as the worst corrosion of a U.S. nuclear reactor head: Over a number of years, the acid cut through all six inches of carbon steel and was stopped only by a layer of stainless steel that's three-eighths of an inch thick.

The stainless steel had started to warp, making regulators fear that a hole could have been created in the reactor head if the corrosion had not been caught in time. That, in turn, could have allowed radioactive steam to escape from the reactor and contaminate the interior of the containment structure.

The containment structure has thick concrete walls and is intended to trap any radiation that might get loose, but it also is one of the last lines of defense for the public.

FirstEnergy routinely inspects the reactor nozzles each time the plant is shut down. The utility now acknowledges those type of checks aren't a fool-proof way of catching a corrosion problem. As much as 15 percent of the view is obstructed by various factors - the tight manner in which nozzles are packed together, plus insulation over the reactor head and residue from minor flange leaks, Mr. Wilkins said.

Salt-like boric acid crystals are found on nozzles from time to time at Davis-Besse and other nuclear plants with pressurized water reactors, the utility team said.

Usually, it's a minor problem with a flange. For something as extensive as a full-blown crack or leak, inspectors would expect to find boric acid crystals balling up into the size of popcorn kernels, Mr. Wilkins said.

Based on evidence it has gathered, the team estimated that the most-impaired tube - Nozzle No. 3 - probably started forming an interior crack around 1990, give or take three years. Sometime between 1994 and 1996, the cracking probably spread to the exterior of the tube, which would have allowed boric acid from the reactor to start escaping.

FirstEnergy still hopes to have Davis-Besse repaired and operating by late June. The NRC will have to authorize any startup.

The utility expects to spend up to $10 million to fix damage from the corrosion. It declines to say how much it will spend for a new reactor cap, although experts have pegged that at about $20 million.


Related articles »
Regulators issue acid-damage alert 03/20/2002
More damage found on Davis-Besse reactor 03/19/2002
Outage at Besse extended 2 months 03/14/2002
Boric acid leak eats 6-inch hole in cap of Davis-Besse reactor 03/12/2002
Davis-Besse, Fermi II pass safety evaluations 03/07/2002
No leaks found in Davis-Besse’s cracked nozzles 03/06/2002
Tests spot 5 defects in safety devices at Davis-Besse 03/05/2002

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