Information Notice No. 86-99, Supplement 1: Degradation of Steel Containments
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 14, 1991
Information Notice No. 86-99, SUPPLEMENT 1: DEGRADATION OF STEEL
CONTAINMENTS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This supplement to Information Notice (IN) 86-99 is intended to alert
addressees to additional information about a potential degradation problem
regarding corrosion in steel containments. It is expected that recipients
will review the information for applicability to their facilities and
consider actions, as appropriate, to avoid similar problems. However,
suggestions contained in this supplement to the information notice do not
constitute NRC requirements; therefore, no specific action or written
response is required.
Discussion:
IN 86-99 was issued on December 8, 1986, in response to the discovery of
significant corrosion on the external surface of the carbon steel drywell in
the sand bed region of the Oyster Creek plant. This supplement updates the
status of Oyster Creek containment corrosion and the licensee's mitigation
program.
Since drywell corrosion was detected in 1986, the licensee instituted
periodic wall thickness measurements by the ultrasonic testing (UT)
technique to determine corrosion rates. The most severe corrosion was found
in the sand bed region at a nominal elevation of 11'-3". The highest
corrosion rate determined was 35.2 +/- 6.8 mils per year. To mitigate the
corrosion in the sand bed region, water was drained from the sand bed and
cathodic protection (CP) was installed in the bays with the greatest wall
thinning in early 1989. Subsequent UT thickness measurements in these bays
indicated that CP was ineffective. The licensee's consultants indicated
that it would be necessary to flood the sand bed and to install CP in all
the bays to make the CP system effective. The licensee decided that large
amounts of water in the sand bed would be counterproductive.
9102080329
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IN 86-99, Supplement 1
February 14, 1991
Page 2 of 3
In the spherical portion of the drywell above the sand bed region, the
highest corrosion rate determined was 4.6 +/- 1.6 mils per year at a nominal
elevation of 51'. In the cylindrical portion of the drywell above the
spherical portion, where minor corrosion was discovered and was thought to
have originated mostly during construction, no significant wall thinning was
detected (at a nominal elevation of 87'). However, this is the region in
which the nominal thickness of the wall has the least margin, thus requiring
periodic monitoring of actual thickness.
The licensee has instituted a drywell program to arrest corrosion and to
ensure containment integrity for the full licensed term of the plant. The
licensee has taken action to investigate, identify, and correct leak paths
into the drywell gap and plans to take more action to survey leakage and
prevent it. The stainless steel liners in the refueling cavity and the
equipment pool developed cracks along the perimeter of the liner plates
where they were welded to embedded channels. For the refueling cavity, all
potential leakage pathways have been thoroughly checked and liner cracks are
sealed with adhesive stainless steel tape before a strippable coating is
applied. Since the refueling cavity is flooded only during refueling, no
leakage concerns exist at other times. At the end of an outage, the
refueling cavity is drained, and the tape and strippable coating are
removed. The licensee found leaks related to the equipment pool and stopped
them with liner weld repairs. The equipment pool also will be protected
with a strippable coating during flooded periods of operation.
The licensee believes that a thorough program has been established for
managing leakage that could affect drywell integrity due to corrosion from
moisture ingress into the drywell gap. Recent surveillance of the sand bed
drains indicates that the sand bed is free of water. To further mitigate
drywell corrosion, the licensee is considering removing the sand,
insulation, gap filler material, and corrosion film and applying a
protective coating to the exterior drywell surface. The licensee is
proceeding with the analysis, engineering and planning to support removing
the sand from the drywell sand bed region in the near future. Removal of
the insulation and gap filler material from the drywell gap is being
evaluated for future consideration.
The BWR Owners Group is surveying its members to determine whether other
plants are experiencing water leakage into the drywell gap and possible
corrosion of the exterior surfaces in the sand bed region as well as in the
spherical and cylindrical parts of the drywell.
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IN 86-99, Supplement 1
February 14, 1991
Page 3 of 3
This supplement requires no specific action or written response. If you
have any questions about the information in this notice, please contact one
of the technical contacts listed below or the appropriate NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Frank J. Witt, NRR
(301) 492-0767
C.P. Tan, NRR
(301) 492-3315
Attachment: List of Recently Issued NRC Information Notices
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