ML20043D712

LER 90-006-00:on 900226,discovered Abnormal Degradation on Steel Containment Vessel.Corrosion Caused by Design Deficiency Caused by Unanticipated Environ Interaction. Detailed Insps conducted.W/900529 Ltr

ML20043D712
Person / Time
Site:	McGuire
Issue date:	05/30/1990
From:	Mcconnell T, Sipe A DUKE POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-006-02, LER-90-6-2, NUDOCS 9006110098
Download: ML20043D712 (11)

Text

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!!iOO Hagers hny Abad  ;

Huntersville. NC bviwx5 DUKEPOWER May 29,1990 )

U.S. Nuclear Regulatory Commission Document Control Desk  ;

Washington, D.C. 20555 l

Subject:

McGuire Nuclear Station Unit 1 and 2 '

Docket No. 50-369 Licensee Event Report 369/90-06, Voluntary Report  :

Gentlemen:

Pursuant to 10 CFR 50.73 Sections (a)(1) and (d), attached is Licensee Event f Report 369/90-06 concerning corrosion on the steel containment vessel that was ,

caused by a Design deficiency. This report is being submitted as a Voluntary '

Report. This event is considered to be of no significance with respect to the health and safety of the public.

Very truly yours, ,

M #{6 T.L.gMcConnell DVE/ADJ/cbl Attachment >

i xc: Mr. S.D. Ebneter American Nuclear Insurers Administrator, Region II c/o Dottie Sherman, ANI Library U.S. Nuclear Regulatory Commission The Exchange, Suit 245 101 Marietta St., NW, Suite 2900 270 Farmington Avenue ,

Atlanta, GA 30323 Farmington, CT 06032 INPO Records Center Mr. Darl Hood Suite 1500 U.S. Nuclear Regulatory Commission 1100 Circle 75 Parkway Office of Nuclear Reactor Regulation Atlanta, GA 30339 Washington, D.C. 20555 M&M Nuclear Consultants Mr. P.K. Van Doorn 1221 Avenue of the Americas NRC Resident Inspector New York, NY 10020 McGuire Nuclear Station 900611oope 900530

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LER 369/90-06, V:1unt:ry Report page 2 May 29, 1990 bxc B.W. Bline L.G. Bost J.S. Warren I- R.L. Gill R.M. Glover (CNS)

T.L. Curtis (ONS)

P.R. Herran S.S. Kilborn (W)

R.E. Lopez-Ibanez M.A. Mullen R.O. Sharpe (MNS)

G.B. Swindlehurst K.D. Thomas M.S. Tuckman L.E. Weaver R.L. Weber J.D. Wylie (PSD)

J.W. Willis QA Tech. Services NRC Coordinator (EC 12/55)

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'" Corrosion Occurred On The Steel Containment Vessel Because Of Design Deficiency Caused Br Unanticiosted Environmental Interaction (V98tf Daf t (Si Llh heutAD4 A tel htPO81 Daf t (Fi OTHER f ActLittil tNVOLVtD 141 MONTM Day vtAR Ytah I'(* ' ',a 6 me eM MONTM DAV vtAn paCIUTVmawel DOCalt huw$tmigi McGuire Unit 2 016 l 0 l 0 l 013 I 710 Oj 2 2l6 90 9l0 0 l 0l6 0 l0 0l5 3l0 9l0 o,5,o,o,o, , ,

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ConsPLlit ONt LtNG FOR S ACM COMPottNT f AILutt DESC AlttD IN TMit hisofit H3s CAV$t Svlt t W COMPONENT

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I I I I I I I I I I I I I I Sve+LIMENT AL atP0Rt IMPICTED lien uoNTM Dev viasi 6vevisSION 4(S itt ven. tempien ER9tCilO SL'0GISSION CA Tth NO l l l A86T R ACT ILeest se 1400 speces # e , emer.semes'y frffesa s'ap e aosco h pe *,rma s ne.1 Hop On February 26, 1990, Design Engineering personnel discovered abnormal degradation of the Unit 1 Steel Containment Vessel (SCV). The corrosion was caused by a Design Deficiency caused by unanticipated environmental interaction. The discovery was made while performing a preliminary inspection of the SCV prior to Integrated Leak Rate Testing. The corrosion was caused by moisture being retained in the area

' where a cork material serves as an expsnsion joint between the interior structural concrete and the SCV. The coating on t.ie SCV had failed in numerous locations

! allowing SCV base metal corrosion. The loss of metal is estimated to be as much as l 0.045 inches at isolated areas. Unit I wLs in No Mode (fuel removed from the core) l and Unit 2 was in Mode 1 (Power Operation) at 100% power at the time of discovery.

l Design Engineering personnel evaluated the extent of the degradation and provided I an operability determination. The SCV for each unit will be repaired based on comprehensive action plans currently under derelopment. This report is being

! submitted as a voluntary Licensee Event Report.

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Background

The general purpose of the Containment System is to provide a barrier confining potential releases of radioactivity resulting from severe accidents. This function is accomplished by maintaining leak tightness within specified bounds. As a design feature, the Containment System is provided primarily for the protection of public health and safety. The free standing steel containment has an outer reinforced concrete Reactor Building and an annular space which is maintained at a lower-than-atmospheric pressure following a Loss Of Coolant Accident (LOCA). These structures form a double barrier to prevent the escape of fission products should a LOCA occur.

The Steel Containment Vessel [EIIS:VSL] (SCV) is a freestanding welded steel structure with a vertical cylinder, hemispherical dome and a flat base. The

' Containment shell is anchored to the Reactor Building foundation by means of anchor bolts around the circumference of the cylinder base. The base of the containment is 1/4 inch plate encased in concrete and anchored to the Reactor Building

) foundation. The base liner plate functions only as a leak tight membrane and is i not designed for structural capabilities, i

The vertical cylinder and hemispherical dome are constructed from carbon steel plate, ASKE specification SA516 GR60. The lower approximate 14 feet of the vertical cylinder is 1 inch nominal thickness. The upper cylinder is 3/4 inch nominal thickness and the dome is 11/16 inch nominal thickness.

A cork filler material is provided in the expansion joint [EIIS:EXJ] area' between the SCV and the interior concrete structure. Cork was chosen for its flexibility to accommodate the anticipated movements of the SCV. The cork serves as part of the form work for concrete pours during construction. The cork also serves to prevent miscellaneous debris from becoming lodged between the SCV and concrete during construction.

The Code of Federal Regulations, Title 10, Part 50 (10CFR50), Appendix J,Section V, Subsection A requires that a general inspection of the Containment Vessel be performed prior to each Type A test (Integrated Leak Rate Test) (ILRT). This inspection is intended to identify any evidence of structural deterioration and interferences which may affect either the containment structural integrity or leak ,

tightness. This inspection is required to be performed every fourty months plus or minus 10 months.

Technical Specification (TS) 4.6.1.6 states in part, that the structural integrity of the Containment Vessel shall be determined during the shutdown for each Type A containment leakage rate test by visual inspection of the exposed accessible interior and exterior surface of the vessel.

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0l0 0l3 0F 0 l9 ftKi w mee mese a newer, oss assomasMeer 6 w nn Description of Event On July 29 through July 31, 1986, Design Engineering personnel performed a general inspection of all accessible interior and exterior surfaces of the Unit 1 SCV to verify the structural integrity of the containment structure. This inspection is required by Appendix J to 10CFR50 prior to the scheduled Integrated Leak Rate Test.

This inspection identified no evidence of any significant structural deterioration of the SCV. ,

On February 26, 1990, Design Engineering personnel again inspected the Unit 1 SCV.

This inspection was more thorough and investigative as a result of the discovery of SCV corrosion in the annulus area of the Unit 2 SCV in July 1989. (This event is described in Licensee Event Report (LER) 369/89-20.) The inspection on Unit I revealed expansion joint epoxy compound failures and cork degradation in areas where the interior structure concrete meets the SCV. Other areas of epoxy compound failures and cork degradation were discovered on February 28, 1990.

On March 7,1990, Design Engineering personnel observed SCV corrosion at various locations where expansion joint cork degradation had occurred.

f On March 15 through 22, 1990, Design Engineering personnel-performed a preliminary I inspection of all accessible expansion joint areas. This inspection revealed general surface corrosion for the entire circumference on elevation 738 feet + 3 inches. Areas of worst case corrosion were also identified. Page 7 of 9 shows a general location of the worst case corrosion areas for elevation 738 feet + 3 inches. The inspection of elevation 766 feet + 8 inches between Azimuths 247 degrees and 302 degrees resulted in identification of this entire area as a worst case area. Page 8 of 9 shows this general location. Within the worst case areas at both elevations, general pitting of up to 0.25 inches in diameter whose depth is less than 20 mils may be observed at any location. Pitting estimated to be up to 45 mils was discovered at specific locations and areas. These areas are isolated and infrequent.

SCV surface corrosion was also found along side the vertical concrete walls between elevations 738 feet + 3 inches and 766 feet + 8 inches.

Page 9 of 9 shows a SCV cross section that is representative of worat case corrosion areas for elevations 738 feet + 3 inches and elevation 766 feet + 8 inches.

On March 16, 1990 two areas were identified for Ultrasonic Examination (UT) in an effort to support the estimated corrosion depth. The first area was a general surface corrosion area. The second area was a worst case pitted area. The two areas were wire brushed to remove excess material. The UT was performed by Quality Assurance personnel by transmitting the signal from the annulus area. The signal was transmitted through a coating and steel to a rough surface and then back through steel and the coating. The depth of the corrosion was approximately 8 mils for the general areas and 38 mils for the worst case areas. The signal scatter from the medium changes and the rough surfaces was nut within procedural guidelines to accept the results as exact but the results did provide credibility to the

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due to the limited space between the concrete structure and the SCV.

On March 22, 1990, a preliminary inspection of Unit 2 upper containment accessible expansion joint areas was performed by Design Engineering personnel. Some SCV corrosion was observed but the extent and magnitude of the corrosion was not as significant as that identified on Unit 1.

On April 3, 1990, Design Engineering personnel performed an Operability evaluation to verify the operability of the Unit 1 and Unit 2 SCV.

During April 9 through 12, 1990, additional preliminary inspections of all accessible expansion joint areas in Unit I containment were performed by Design Engineering personnel to further evaluate the extent of the problem.

Conclusion This event is assigned a cause of Design Deficiency because of unanticipated environmental interaction. The cork expansion joint between the SCV and the

' interior structure concrete had an epoxy surfacing compound applied to the surface of the cork to ensure uniform elevation with the surrounding concrete floor. The i specified coating applied to the concrete floor was also applied to the epoxy surfacing compound. This epoxy surfacing compound hardened and became rigid.

During Integrated Leak Rate Tests, the SCV experiences radial growth. The epoxy L compound cracked at its interface with the SCV or concrete floor. This resulted in l a flow path for moisture to access and penetrate the cork.

Several sources contributed to the moisture including condensation and Icakage from outage related work and condensation from thermal affects associated with the ice condensers. The cork tends to retain this moisture. Two results have been observed when this happens. First, fungi develop and degrade the cork. Second, a chemical reaction occurs which liquifies the cork as seen by a brown liquid stream stain down the side of the SCV from where the cork was degraded.

As the cork retained the liquid, the SCV coatings were continuously exposed to the l- liquid. The SCV coatings failed and corrosion of the SCV beneath the surface of I the epoxy compound began. As the cork degraded, a void developed between the cork and the epoxy compound. Only after the epoxy compound failed and fell on the j degraded cork could the SCV corrosion or cork degradation be detected by visual inspection.

The cork degradation varied from a few inches in depth to completely missing in some areas.

A review of the Operating Experience Program (OEP) Data Base for the previous twenty four months revealed one event involving Safety Barrier Degradation that was  !

attributed to unanticipated environmental interaction. LER 369/89-20 describes corrosion of the SCV on the annulus side caused in part by unanticipated environmental interaction. The discovery of the Unit 1 containment side corrosion

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0l0l6 0l0 0l5 or 0 l9 i rext ,, . = =ac gamami was partially due to the heightened awareness of the Design Engineering personnel ,

as a result of the events described in LER 369/89-20. This event is considered recurring. .

'This event is not Nuclear Plant Reliability Data System (NpRDS) reportable.

• There were no personnel injuries, radiation overexposures, or uncontrolled releases of radioactivity as a result of this event.

CORRECTIVE ACTIONS:

Immediate: None Subsequent: Detailed inspections were performed on the Unit 1 SCV by Design  ;

Engineering personnel at the areas where the expansion joints were 4 located.

l planned: Unit 1 g Design Engineering personnel will develop a comprehensive action plan that includes but is not limited to the following: a) i( prioritize areas to receive corrective action b) develop an acceptance criteria for expansion joint material and coatings c) remove and replace failed coatings and expansion joint material as deemed necessary d) perform additional pit inspections e) identify and weld repair areas of excessive corrosion.

Unit 2 l 1. Design Engineering personnel will conduct detailed inspections

) of those areas at the cork expansion joint /SCV interfaces.

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2. Develop a comprehensive action plan based on the above mentioned inspection.

SAFETY ANALYSIS:

As a result of the corrosion discovered in the Unit 1 pre-ILRT inspections the extent and magnitude of the metal loss has been determined. Design Enginetring personnel performed a calculation that evaluated the original design / analysis to establish the minimum required thickness of the SCV at elevations 738 feet + 3 inches and 766 feet + 8 inches. The McGuire Containment Vessel Stress Rept rt was reviewed for all loading combinations at these two elevations. The stress resultants were compared to ASME Code stress allowables to establish the respective minimum wall thicknesses. Appropriate stress categories as defined in the ASME Code and NRC regulatory documents were considered.

Corrosion rates were established based on measured Unit 1 metal loss assuming corrosion activity since the previous Unit 1 pre-ILRT inspection. The SCV can be considered operable if the minimum wall thickness at both elevations is greater gonu sees .v.s. v oi s.eea m es o w o

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0l 0 l6 0l0 0l6 or 0 l9 rartin .m . m w=ac _an m i i than 0.620 inches. The preliminary inspection resulted in a minimum measured thickness of the Unit 1 SCV to be 0.705 inches, which is well above the required 0.620 inch minimum wall thickness. Preliminary inspection of Unit 2 indicates corrosion of less magnitude. The projected worst case corrosion rate is 12.9 mils per year. The amount of corrosion that is projected during the delay time for implementation of corrective actions will not result in further significant loss of metal.

As discussed above, the SCV was not rendered inoperable as a result of the corrosion and subsequent loss of metal, and is capable of fulfilling its safety functions. The health and safety of the public were not affected by this event.

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