March 25 — Nuclear reactor operators have been ordered to check
their reactor vessels after the discovery that acid in cooling water
had eaten a hole nearly all the way through the six-inch-thick lid
of a reactor at a plant in Ohio. The corrosion left only a
stainless-steel liner less than a half-inch thick to hold in cooling
water under more than 2,200 pounds of pressure per square inch.
At the 25-year-old Ohio plant, Davis-Besse, near Toledo, the
stainless steel was bent by the pressure and would have broken if
corrosion had continued, according to the Nuclear Regulatory
Commission, where officials were surprised by the discovery. They
said they had never seen so much corrosion in a reactor vessel.
The commission, which has warned plants for years to watch for
any corrosion, has ordered all 68 other plants of similar design —
pressurized-water reactors — to check their lids. The commission is
particularly worried about a dozen of the oldest plants and ordered
them to report by early April whether they were safe enough to keep
in service. The commission told these plants to demonstrate that
technicians there would have noticed such corrosion in their normal
inspections, had it occurred.
If the liner had given way in the Ohio reactor, experts say,
there would have been an immediate release of thousands of gallons
of slightly radioactive and extremely hot water inside the reactor's
The plants have pipe systems that are meant to pump water back
into a leaking vessel, but some experts fear that if rushing steam
and water damaged thermal insulation on top of the vessel, the pipes
could clog. In that event, the reactor might have lost cooling water
and suffered core damage — possibly a meltdown — and a larger
release of radiation, at least inside the building.
Such extensive corrosion "was never considered a credible type of
concern," said Brian W. Sheron, associate director for project
licensing and technology assessment at the regulatory commission.
Small leaks of cooling water are common, Mr. Sheron said, but
engineers always thought that if cooling water leaked from the
piping above the vessel and accumulated on the vessel lid, the water
would boil away in the heat of over 500 degrees, leaving the boric
acid it contains in harmless boron powder form. At Davis-Besse,
however, it appears that the water was held close to the metal
vessel lid, or head, perhaps by insulation on top of the vessel.
Boric acid is used in cooling water to absorb surplus neutrons,
the subatomic particles that are released when an atom is split and
go on to split other atoms, sustaining the chain reaction.
Engineers are not yet certain why the corrosion occurred.
A nuclear engineer at the Union of Concerned Scientists, a
nonprofit watchdog group that is often critical of the Nuclear
Regulatory Commission, said the discovery was troubling.
"This is really something that shouldn't happen," said the
engineer, David Lochbaum. "You shouldn't get such a huge hole in a
Edwin S. Lyman, the scientific director of the Nuclear Control
Institute, an anti-proliferation group based here, said: "This is a
pretty serious issue, and it has generic implications. And it was
discovered by accident."
Workers stumbled on the problem in the process of fixing a
leaking tube that connects to the vessel head, which is 17 feet in
diameter and weighs 150 tons. The tube is part of the reactor
control system; inside it there is a control rod, which operators
can lower into the core to smother the flow of neutrons and stop the
chain reaction, or raise to allow the reactor to run.
Technicians discovered that the metal that supports the tube had
The plant owner, FirstEnergy Corporation, is hoping to patch the hole, an irregular opening
about 4 by 5 inches. But the commission is skeptical about whether
this is possible.
No one in this country has replaced a reactor vessel head,
although several plants have ordered parts to do so. FirstEnergy
ordered a new head just before the extent of the problem became
obvious. A company spokesman said the company hoped to install it in
the spring of 2004.
That date reflects how the industry, with no new reactor orders
in decades in this country, has limited production capacity for such
The plant might also be able to use a vessel head from a reactor
in Midland, Mich., that was never completed, or from a similar plant
that was retired in 1989.
Davis-Besse, which began operating in 1977, was not designed with
the idea that the head would be replaced; technicians would have to
cut a bigger hole in the steel-reinforced concrete containment
building to get the new head into it.
The company has not said what the job will cost, but Duke Power
Company, which operates three reactors similar to Davis-Besse, plans
to replace the heads of all three for about $20 million. FirstEnergy
could spend nearly that much each month for electricity from
alternative sources if it must wait for the replacement part.
Because of the discovery at Davis-Besse, the regulatory
commission ordered a dozen other plants to report back within two
weeks and prove that inspections they have done in the past would
have found any corrosion.
The inspection cannot be done while the plant is running, and if
the utilities cannot convince the commission, they presumably face
shutdowns of perhaps several weeks just for the checks.
Such shutdowns occurred intermittently in the 1970's and 80's but
have become extremely rare as reactors have improved their
The industry is hopeful, however, that inspections it began under
commission orders several years ago, to look for leaks, would have
found any similar cases. Those inspections began after the heads of
French reactors showed signs of leaks and corrosion.
"It could be something unique to Davis-Besse," said Alexander
Marion, director of engineering at the Nuclear Energy Institute, the
industry's trade association. A goal for the investigation at the
plant, he said, would be to find out not only why the corrosion
occurred but also why it was not noticed sooner.
"The plants are getting older and we're starting to see these
kinds of problems," Mr. Marion said.