Information Notice No. 89-79: Degraded Coatings and Corrosion of Steel Containment Vessels
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 1, 1989
Information Notice No. 89-79: DEGRADED COATINGS AND CORROSION OF
STEEL CONTAINMENT VESSELS
Addressees:
All holders of operating licenses or construction permits for light-water
reactors.
Purpose:
This information notice is intended to alert addressees to the discovery of
severely degraded coatings and the corrosion of steel ice condenser contain-
ment vessels that are caused by boric acid and collected condensation in the
annular space between the steel shell and the surrounding concrete shield
building. 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 information
notice do not constitute NRC requirements; therefore, no specific action or
written response is required.
Description of Circumstances:
On August 24, 1989, Duke Power Company reported significant coating damage
and base metal corrosion on the outer surface of the steel shell of the
McGuire Unit 2 containment which was discovered during a pre-integrated leak
rate test inspection (as required by Appendix J to 10 CFR Part 50).
Subsequently, Duke Power identified similar degradation of the McGuire Unit
1 containment, which is essentially identical to the Unit 2 structure.
Both units have ice condenser-type containments consisting of a freestanding
steel shell surrounded by a concrete shield building. Between the shell and
the shield building is a 6-foot-wide annular space. The steel shells have a
nominal thickness near the annulus floor of 1 inch. The degraded area on
the shells of both units is limited to 30-foot circumferential sections no
higher than 1+ inches above the annulus floors. The average depth of the
corrosion is 0.1 inch with pits of up to 0.125 inch. Corrosion that is up
to 0.03 inch deep was also found in areas below the level of the annulus
floor on the Unit 2 shell, where concrete was removed to expose the shell
surface. This below-floor corrosion is due to a lack of sealant at the
interface between the shell and the annulus floor.
8911270001
.
IN 89-79
December 1, 1989
Page 2 of 3
The probable cause of the degradation is attack by condensed boric acid
coolant leaking from instrument line compression fittings. Drains are
provided in the annulus floor, but they are widely separated and the floor
is not sufficiently graded to prevent pooling of the condensate between the
drain locations. The current average thickness of the corroded areas of the
containment shells of 0.9 inch is expected to be reduced to no less than
0.85 inch for Unit 1 and 0.88 inch for Unit 2 by the time they are repaired
during the next planned outages for these units. These estimates were made
assuming the worst corrosion rates that could reasonably be expected and are
greater than the minimum thickness allowed by the applicable ASME Code.
On September 21, 1989, Duke Power Company discovered coating damage and base
metal corrosion during inspections of the steel shells of its two Catawba
containments. These containments are also of the ice condenser type and are
very similar to the McGuire containments; the nominal floor level shell
thickness is 1 inch. Here also, the degradation was located on the outer
surfaces of the shells at the intersection of the shell and the concrete
annulus floor. At Catawba the damage was less extensive and was limited to
a circumference of 15 feet, a height of 1 inch above the annulus floor, and
an average depth of 0.03 inch. The cause is also believed to be attack by
boric acid coolant, which had leaked from instrument line compression
fittings, condensed, and collected on the annulus floor.
Duke Power Company plans to repair the steel shells of all four units during
their next respective refueling outages. This will include weld repair and
recoating of the corroded areas.
Discussion:
The degradation of the containment shells at the McGuire and Catawba plants
is considered significant for several reasons. The fact that the corrosion
affects four different units indicates that other steel containments with
similar configurations may be susceptible to the same problem. Furthermore,
the observed rate of corrosion far exceeds the allowance made for corrosion
in the containment design. This condition leads to the concern that such
corrosion could result in undetected wall thinning to less than the minimum
design thickness, accompanied by a loss of leaktightness or structural inte-
grity. This problem can be prevented by a containment inservice inspection
program that is adequate to ensure early detection and the maintenance of
design margins through proper corrosion control.
The NRC regulations (Appendix J to 10 CFR Part 50) require that a general
visual inspection of the accessible surfaces in the containment be performed
before each integrated leak rate test. The purpose of this inspection is to
identify any evidence of structural deterioration or other problems that may
affect containment integrity or leaktightness. As a result of these and
other inspections, several instances of containment wall thinning due to
corrosion have been discovered during the past 3 years at operating power
reactors. However, the visual inspections done in connection with the
integrated leak
.
IN 89-79
December 1, 1989
Page 3 of 3
rate tests are only required to be performed three times in each 10-year
period. In addition, because of the physical arrangement of plant systems,
the steel surfaces in the annular spaces of some containments may not be
easily accessible to the visual inspections associated with leak tests.
Considering the frequency and severity of recent instances of containment
degradation due to corrosion, additional efforts to inspect steel
containment surfaces potentially susceptible to corrosion may be prudent.
This information notice 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: Chen P. Tan, NRR
(301) 492-0829
Keith R. Wichman, NRR
(301) 492-0908
Attachment: List of Recently Issued NRC Information Notices
.
Page Last Reviewed/Updated Tuesday, March 09, 2021