ML20005F157

LER 89-043-00:on 891208,discovered That Approx 50% of One Nut Ring Half Beneath Reactor Vessel Nozzle Flange Corroded Away.Caused by Gradual Deterioration of Gasket Matl.Design Change implemented.W/900108 Ltr

ML20005F157
Person / Time
Site:	Arkansas Nuclear
Issue date:	01/08/1990
From:	Ewing E, Taylor L ARKANSAS POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
1CAN019007, 1CAN19007, LER-89-043, LER-89-43, NUDOCS 9001120365
Download: ML20005F157 (4)

Text

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L I- Arkansas Power & L';ht Company

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,i..* n. _f Atkansas Nuck *ar One Route 3 Box 137 G k ". . Russelivele. AR 72801 L Tel 501 %4 3100 L

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-January 8',1990 1CAN919867 e U, S. - Nuclear Regulatory Commission Document Control Desk Mail Station P1-137 Washington, D. C. ;20555

SUBJECT:

Arkansas Nu' clear One - Unit 1 Docket No. 50-313 License No. OPR-51 Licensee Event Report 50-313/89-043-00 j Gentle' men:

In accordance with 10CFR50.73(a)(2)(ii), attached is the subject report '

. concerning the boric acid corrosion of a control rod drive mechanism flange assembly caused by a deteriorated gasket which resulted in a Reactor .

Coolant System pressure boundary degradation.

Very truly yours, E. C. Ewing General. Manager, Technical Support and Assessment ECE/RHS/sgw attachment '

cc: Regional Administrator Region IV U. 5, Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 1000 Arlington, TX 76011 >

INPO Records Center 1500 Circle 75 Parkway Atlanta, GA - 30339-3064

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9001120365 900108 PDR ADOCK 05000313 S PDC An Enteg< Compaw

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'. *. Form 1062.01A

. NRC Fons 366 U.S. Nuclear Regulatory Commission ,

(9-83) Approved OsE No. 3150-0104 Expires: 8/31/85 ,

LICENSEE EVENT REPORT (L E R)

FACILITY NAME (1) Arkansas Nuclear One Unit One 100CKET NUMBER (2) IPAGE (3) 10151010101 31 11 3l110F1013 TITLE (4) Boric Acid Corrosion of a Control Rod Drive Mechanism Flange fastening Assembly caused by a Deteriorated Gasket Results in Reactor Coolant Systee Pressure Boundary Degredation

• _ EVENT DATE (5) i LER NUMBER (6) i REPORT DATE (7) 1 OTHER FACILITIES INVOLVfD (6) 1 -l l l lSequentia11. lRevisioni l 1 l l MonthI Day lYear (Year I I Number I i Number IManth! Day l Year I Facility Names h ket Number (s) l- l l l l l l l J l 1 0151010101 6 l 11 21 01 81 81 91 81 91--I 01 4! 31- I 01 01 01 11 01 81 91 01 0151010101 1 1 OPERATING I lTHIS REPORT IS SUBMITTED PURSUANT TO THE REQVIREMENTS OF 10 CFR 5:

DODE (9) l Ni (Check one or more of the followino) (13)

POWERI 1,_,,1 20.402(b) l_l 20.405(c) l_l 50.73(a)(2)(iv) l_l 73.71(b)

LEVEL l -l_ I 20.405(a)(1)(1) l l 50.36(c)(1) l_l 50.73(a)(2)(v) l_l 73.71(c)

(10) 1010101 1 20.405(h)(1){ii) l _l 50.36(c)(2)

_ l_l SC.73(a)(2)(vil) l_l Other (Specify in l_ l 20.405(a)(1)(111) l_l 50.73(a)(2)(1) 1,,,_l 50.73(a)(2)(viii)( A)1 Abstract below and I,_,1 20.405(a)(1)(iv) IJ I 50.73(a)(2)(11) l _l 50.73(a)(2)(vif1)(B)I in Text. NRC Form i I 20.405(a)(1)(v) l l 50.73(a)(2)(iii) i I 50.73(a)(2)(x) 1 366A)  ;

LICENSEE CONTACT FOR THIS LER (12)

Name i Teleohone Number

l. lArea l Larry A, Taylor, Nuclear Safety and Licensing Specialist ICode l 1510111916141-13til010 COMPLETE ONE LINE FOR EACH COMP 0NEhl FAILURE DESCRIBED IN THIS REPORT (13) l l l lReportablel 1 i l l IReportablel l- 'CauselSysteel Component lManufacturert to NPROS I ICauselsysteel Component IManufacturert to NPRDS I

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-I I I I I I I I I I I I I I I I i 1 1 I I I I I I l l l l l l l l l l l l 1 I i lI l i I l l I I I l l l 1 l l l l 1 1 I SUPPLEMENT REPORT EXPECTED (14) l EXPECTED l Monthi Day lYear i . .

_, 1 SUBMISSION l l 1 l

l l Yes (If yes, complete Exoected Submission Date) IXl No l DATE (15) l I I I I i l A85 TRACT (Limit to 1400 spaces, i.e. , approxteately fif teen single-space typewritten lines) (16)

[ On December 6,1989, while removing the nut ring from beneath the reactor vessel (RV) nozzle flange at Control Rod Drive Mechanism (CRDM) location L-2, plant maintenance personnel discovered that approximately 50 percent of one of the nut ring halves had corroded away and that two of the four bolt holes in the l corroded nut ring half were degraded to the point there there was no bolt / thread engagement. A total

- of six CRDM flanges had been identified to be potentially leaking during a video camera inspection on November 28, 1969 while the plant was in hot shutdown. After the plant was taken to entit shutdown, maintenance personnel disassembled and inspected the six CRDM flanges which had been identified during this inspection. All of the gasket seating surfaces were found to be undamaged with the exception of L-2. The RV nozzle flange at L-2 was eroded and pitted. An inspection of the flanges and spiral wound gaskets which were removed from between the flanges revealed that the cause of the leaks was the gradual deterioration of the caskets with age. An Engineering evaluation concluded that the L-2 nozzle flange was acceptable for use. A replacement CRDM was installed at location L-2. Additionally, the gaskets on the six CRDMs were replaced with new destyn graphite type gaskets.

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Form 1062.018 NRC Fom 366A - U.S. Nuclear Regulatory Commission (9-83)- Approved OMB No. 3150-0104 >

Expires: 8/31/85 LICENSEE EVfMT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) ICOCKET NLMBER (2) l LER NUISER (6) l PAGE (3) >

l l l lSequentiell IRevisioni Arkansas Nuclear One, Unit One l l Yearl ! Number Number l 10151010101 31 11 31 81 91--! 01 41 3 -- 01 Ol01210F1013 TEXT (If more space is required, use additional NRC Fom 366A's) (17)

A. Plant Status At the time of discovery of this condition, Arkansas Nuclear One Unit 1 (AND-1) was in the cold shutdown condition. Midcycle outage 1H89 was in progress.

B. Event Description On December 8, 1989, while removing the nut ring from beneath the reactor vessel (RV) nozzle flange at Control Rod Drive Mechanism (CRDM) location L-2, plant maintenance personnel discovered that approximately 50 percent of one of the nut ring halves had corroded away and that two of the four bolt holes in the corroded nut ring half were degraded to the point where there was no bolt / thread engagement.

The CROMs are attached to the RV by eight hold down bolts which pass through the CRDM flange and the RV nozzle flange and thread into a nut ring which is positioned below the RV nozzle flange.

The nut ring is a two picco carbon steel ring with four threaded holes in each section.

The disassembly of the L-2 CROM flange was baing Performed as a result of its being identified as leaking on November 28 while the plant wat in the hot shutdown condition. At that time, a video camera inspection of a RV level monitoring instrumentation (Rad Cal) flange which had been repaired j in May, 1989 was being conducted. During this inspection, a small amount of water was observed to i- be dripping from flange L-2. Upon discovery of this leak, a 100 percent inspection of the CRDM i

flanges was conducted with the video camera. Boron deposite were observeJ on five additional CRDM flanges which indicated that they could possibly have been leaking. The Red Cal flange was also observed to be leaking.

After the plant was taken to cold shutdown, maintenance personnel disassembled and inspected the flanges of the six CRDMs which had been identified as potential leakers. The location numbers of  ;

these flanges are; E-5, D-8, G-9, H-8, N-8 and L-2. The RV nozzle flange at L-2 was eroded and pitted. All other inspected flanges were undamaged.

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l C. Safety Significance i An Engineering evaluation was conducted which determined that in its as-found condition, with two of the eight bolts carrying no load, the L-2 CRDM flange and nut ring assembly was within code allowable stress limits and would have withstood the worst case faulted condition loads. However, the ring nut assembly at L-2 was severely corroded and did represent a significant degradation of a Reactor Coolant System (RCS) [AB) pressure boundary. Had this condition gone undetected, the nut ring would have continued to corrode and could have resulted in a significant non-isolable RCS leak. It is unlikely, however, that the leak rate could have become significant without being l detected due to the various RCS leakage detection methods / systems used at ANO. During power i

operation, RCS leakage is monitored by the Inventory Balance Systee, the Reactor Building (RB)

( Sump Monitoring System, and the RB Air Monitoring System. If the total RCS leakage exceeds 10 l gallons per minute (gpe) or if unidentified RCS leakage exceeds 1 gpe, the plant's Technical Specifications require that the reactor be shut down.

In addition, prior to depressurizing in preparation for outages, and after repressurization before resuming power operativn, the RCS is inspected for leaks. This inspection includes the RV head area. Any identified R*S leaks are evaluated for boric acid corrosion effects.

D. Root Cause An inspection of the flanges and the spiral wound gaskets which were removed from between the flanges revealed that the cause of the leaks was the gradual deterioration of the gasket material with age. The gasket at L-2 had been in place since initial plant operation. The initial leakage flowpath at L-2 was across the face of the flange, down the hold down bolts and on to the carbon .

steel nut ring. This leakage flowpath had not been previously anticipated. Since the leakage did not travel to the outside edge of the flange and bewuase the nut ring area is not easily viewed, it was not detected during previous routine inspections of the RV head area. The five other CRDM flange leaks were the result of minor gasket deterioretion. The Rad Cal leak was caused by a defective seal.

7 4 O #

Form 1062.018 NRC form'366A - Lt.5. Nuclear Regulatory Commission

'(9-83) Approved OMB No. 3150-0104 Expires 8/31/85 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) l DOCKET NUMBER (2) l LER NUMBER (6) l PAGE (3) l l l 15equentiall IRevision!

' Arkansas Nuclear One, Unit One l l Yearl I Number l l Number l 10151010101 31 Il 31 81 91--I 01 41 31--! 01 Ol01310F1013 TEXT (If more space is required, use additional NRC Form 366A's) (17)

E. Basis For Reportability

$1nce this condition represented a significant degradation of an RCS pressure boundary, it is considered reportable pursuast to 10CFR50.73 (a)(2)(11).

This condition was also reported in accordance with 10CFR50.72 on December 12, 1989, via the Emergency Notification System.

F. Corrective Actions An Engineering evaluation was performed by Babcock and Wilcox (B&W) which concluded that, although the L-2 RV nozzle flange was eroded and pitted, it was acceptable for use. B&W is presently developing a method by which the flange can be remachined. After development of this method, the L-2 flange will be renachined during the first outage of sufficient duration. The RV dome area and the RV head bolts were visually inspected for boric acid corrosion. This inspection revealed no significant corrosion or metal loss in this area.

A design change was implemented which installed new design graphite type gaskets, which were recommended by the vendor, in the six CRDM flanges and the Red Cal flange.

A replacement CRDM was installed in location L-2. Additionally, the nut rings, bolts and washers were replaced on all six of the CRDH flanges and on the Red Cal flange, I

The defective Red Cal seal design was modified to prevent leakage.

The " Inspection and Evaluation of Boric Acid Leaks" procedure will be revised by June 15, 1990, I to require the inspectjon of the nut rings and associated hardware whenever a CRDM flange leak is identified. There are no RV head area inspections anticipated prior to this date.

The new design graphite gaskets which have been installed will be evaluated to determine if they are more reliable than the spiral wound gaskets which were originally installed. If the graphite gaskets prove to be more reliable, the spiral wound gaskets will be replaced, as necessary.

' G. Additional Information Similar events in which abnormal degradation of RCS pressure boundary components resulted from boric acid corrosion were reported in LERs 50-313/86-006-00 and 50-368/87-003-01.

Energy Industry Identification System (EIIS) codes are indicated in the text as {XX).  !

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