Background
Summary: What Happened at Davis-Besse
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In
March 2002 plant workers discovered a cavity in the head or top of the
reactor vessel while they were
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repairing
control rod tubes which pass through the head. The tubes, which pass
through the reactor vessel
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head,
are called control rod drive mechanism nozzles. Cracks were detected in
5 of the 69 nozzles.
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In
three of those nozzles, the cracks were all the way through the nozzle,
allowing leakage of reactor cooling water, which contains boric acid.
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Corrosion,
caused by the boric acid, damaged the vessel head next to Nozzle No. 3,
creating an irregular cavity about 4 inches by 5 inches and
approximately 6 inches deep. The cavity penetrated the carbon steel
portion of the vessel head, leaving only the stainless steel lining.
The liner thickness varies somewhat with a minimum design thickness of
1/8 inch. Subsequent examination by Framatome, FirstEnergy’s
contractor, found evidence of a series of cracks in the liner, none of
which was entirely through the liner wall. After circumferential cracks
- around the nozzle wall - were found in the control rod drive nozzles
at Unit 3 of the Oconee Nuclear Power Station in 2001, the NRC required
all pressurized water reactor (PWR) operators to report to the NRC on
structural integrity of the nozzles and their plans to inspect the
nozzles.
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Plants
with similar operating history to Oconee unit 3, including Davis-Besse,
were to inspect their reactor vessel head penetrations by December 31,
2001, or to provide a basis for concluding that there were no cracked
and leaking nozzles.
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