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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
Davis-Besse has
been shut down since Feb. 16.
(THE BLADE)
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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.
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