As far as I know, these "holes" are in fact hair-thin (but deep) cracks caused by "stress corrosion cracking" in the weld material of PWR reactor vessel penetrations (nozzles) for the control rod drive mechanisms (CRDMs).
Of course the way the media report the story, you'd think its a foot-wide gaping hole ! (its possible the NRC used the word "hole" too, but they're used to seeing barely detectable microscopic fissures on x-ray film, so for them a crack several inches deep is a "hole").
This reactor vessel head nozzle cracking issue dates back to the late 1990s, and a number of US NPPs have had their vessel heads inspected since then ( for example Ginna and Oconee Unit 1), with much less severe indications than those at Davis-Besse. Analysis for those plants concluded that the results were within expected variation of models. For some reason, the situation at Davis-Besse seems to be quite a bit worse.
The inspections also revealed boric acid deposits on vessel head surfaces (boric acid is used for reactivity suppression -- enriched fuel in those reactors has lots of excess reactivity). Reactor vessel penetrations are difficult to inspect because the entire head is covered with insulation.
As for potential consequences of a leak through a crack like that, I don't really see how its different from a crack in a small pipe anywhere else in the primary heat transfer system -- an event easily dealt with using the ECC system. I believe that its certainly not nearly as severe as other design basis postulated events, such as a main line guillotine break.
It should be interesting to see more details as the situation develops....
From: Adam McLean [mailto:firstname.lastname@example.org]
Sent: Tuesday March 26, 2002 8:42 PM
To: Canadian Nuclear Discussion List
Subject: [cdn-nucl-l] Corrosion discovered at Ohio nuclear plant
Posted in the Miami Herald on March 26, 2002 and at:
Any more technical details on the corrosion found? What is in cooling
water that makes it that acidic?
Dangerous corrosion discovered at Ohio nuclear plant
BY MATTHEW L. WALD
New York Times Service
WASHINGTON - 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.
At the 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 this kind
of 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 on whether they are
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
Florida Power & Light Co.'s Turkey Point nuclear plant in southern
Miami-Dade County is one of these plants. FPL officials couldn't be
reached for comment late Monday.
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 containment
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 lose cooling water and suffer 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. Sharon, associate director for project
licensing and technology assessment at the regulatory commission.
Small leaks of cooling water are common, 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 more than 500 degrees, leaving a harmless boron powder.
But at Davis-Besse, it appears that the water was held close to the
metal vessel lid, or head, perhaps by insulation on top of the vessel.
Engineers are not yet certain what caused the corrosion.
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