ML20029C732

LER 94-004-00:on 940221,discovered Corrosion of Three Nuts on One of Incor Instrumentation Reactor Vessel Head.Caused by Increase of Wet Boric Acid.Leaking Flanges Repaired.W/ 940422 Ltr

ML20029C732
Person / Time
Site:	Calvert Cliffs
Issue date:	04/22/1994
From:	Cruse C, Maki W BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-94-004, LER-94-4, NUDOCS 9404280293
Download: ML20029C732 (7)

Text

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s CurnLts U. Cnuse ' Baltimore Gas and Electric Company Plant General Manager Calvert Clifs Nuclear Power Plant Calvert Clifs Nuclear Power Plant '1650 Calvert Citfs Parkway .

Lusby, Maryland 2c45 7 410 586-2200 Ext.4 tor Local 41o 260-4101 Baltimore April 22, 1994 U.S. Nuclear Regulatory Commission  !

Washington, D.C. 20555 ATTENTION: Document Control Dask

SUBJECT:

Calvert Cliffs Nuclear Power Plant Unit No. 1; Docket No. 50-317; License No. DPR-53

.Licensea Event Report 94-004 Excessive Corrosion of Incore' Instrumentation Flange

,pomponents '

The attached report is being sent to you as required under 10 CFR 50.73 guidelines. Should you have any questions regarding this report, we will be.

pleased to discuss them with you.

Very truly yours,

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} b' r' CHC/WDM/bj d I

Attachment ec: D. A. Brune, Esquire J. E. Silberg, Esquire R. A. Capra,'NRC-D. G. Mcdonald, Jr., NRC T. T. Martin, NRC-P. R. Wilson, NRC R.'I. McLean, DNR'-

J. H. Walter, PSC y Director, Office of Management Information and Program Control O r) t*. . .h 1

• 9404280293 940422 [. j --

PDR ADOCK0500g7 8

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- 1 4- NRd f ORM 3ee U. S NUCLEAR REOULATORY COMMISSION gg (5 92) ESTNATED BUF0EH PER RESFONGE TO OOMRYWITH THIS NONATION i

. COLLECTION NOUEST: Sa0 MHi. F0FWAFO COMMENTS EoAF0 LNG  !

LICENSEE EVENT REPORT (LER) SJ0EN ESTME TO THE INFORMATM AND RECORDS MANMEMEW UFMNCH (MNBB 7714), uS. NUCLEAR KoULATOfff COMMSSIOK 1

J WASHINGTOM DC 20N50001, AM)TO THE PAPETWOFE EDUCTION PROJECT l

01S00104), OFTlCE OF MANAGEENT AND BUDGET,WASHINGTOR DC 20501 '

(See reverse for feauired number of diaits/chracters for each block)

I F ACillTY NAME (1) DOCKET NUMBER (2) PAGE (3)

Calvert Clif f s, Unit 1 05000 317 1 OF 06 TITLE (4) 'I Excessive Corrosion of incore Instrumentation Flange Components EVENT DATE (5) LER NUMBER (6) REPORT DATE (7) OTHER FACILITIES INVOLVED (8)

MONTH DAY S R SiON ^ "^ "II YEAR YEAR NUMBER NUMBER MONTH DAY YEAR 02 21 94 94 - 004 -

00 04 22 94 05000 OPERATlfg3 TH10 RE PORT IS SUBMITTED PURSUANT TO THE RFOUIREMENTH OF 10 CF R MCheck one or more) f t1) 5 20 402(b) 20.405(c) 50.73(a)(2)(iv)

MODE (9) 73.73j) p g 20.405(a)(1)(1) 50.36(c)(1) 50.73(a)(2)(v) 73.71(c) 7 LEVEL 0 20.405(a)(1)(ii) 50.36(c)p) Sa73(a)(2)(vii)

(10) ~ (Specify in Abstract below and in 20.405(a)(1)(lii) 50.73(a)(2)(i) 50.73(a)(2)(viii)(A) 20.405(a)(1)(iv) X 50.73(a)(2)(ii) 50.73(a)(2)(viii)(B) Text NRC Form 366A) 20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x)

LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBER (include Area Code)

Wayne D. Maki, Engineer 410 260-3651 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIDEDIN THIS REPORT (13)

CAUSE SYSTEM COMPONENT AC- WRTAM TO CAUSE TURER NPRDS SYSTEM COMPONENT TURER NPRDS X AB PSF Y SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MONTH DAY YEAR

" ' 8' "

X YES No 07 31 94 (if yes, complete EXPE'CTED SUBMISSloN DATF) DATE (15)

ABS 1RACT (LimM to 1400 spaces,ie., apprMmately15 single space typewritten lines) (16)

On February 21, 1994, we discovered higher than anticipated corrosion of three nuts on one of the Incore Instrumentation flanges on the Unit i reactor vessel head. The joints were known to be leaking slightly since 1993, but we deferred repairs until 1994 because the expected corrosion rate was very low.

The excessive corrosion rate was apparently due to the presence of wet boric acid on some of the flange components where we expected only dry boric acid.

Investigation is ongoing to determine the reasons for this increased presence of wet boric acid. There were no actual safety consequences, although the potential existed for a significant leak. Corrective action-to repair the leaking flanges is complete. Further action awaits results of our root cause analysis. This report will be supplemented when the root cause analysis is complete.

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. 1 NRD FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 0 82) EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REoVEST: 50 0 HRS. FORWARD 1 LICENSEE EVENT REPORT (LER) COMMENTS REGARDING BURDEN ESTIMATE To THE INFORMATION AND RECORDS MANAGEMENT BRANCH (MNBB U14), U 8. NUCLEAR TEXT CONTINUATION REGULATORY COMM6SION, WASMGTON, DC 20555@01, AND TO THE PAPERWORK REDUCTION PROJECT (315f}0104), orFICE oF MANAGEMENT AND BUDGET, WA9HINGTON, DC 20503 j i

FActLITY NAME (1) DOCKET NUMBER (2) LER NUMBER (3) PAGE (4) i Calvert Cliffs, Unit 1 05000 3 1 7 94 -

004 - 00 02 0F 06 i TEXT (if more space le required, use addeonal copies of NRC Form 366A) (17)

I. DESCRIPTION OF EVENT On February 21, 1994 during a refueling shutdown inspection of the Unit 1 reactor vessel head area, we discovered higher than anticipated corrosion of three nuts on one of the Incore Instrumentation (ICI) flanges. One nut had failed and two adjacent nuts were significantly corroded but had not failed.

The corrosion was caused by reactor coolant leakage past the ICI detector joint assembly, leading to a build-up of concentrated boric acid on flange components.

In March,1993, evidence of boric acid leakage was discovered on the Unit 2 ICI flanges during refueling shutdown inspections. The issue was documented via our Issue Report system and a Root Cause Analysis initiated to determine appropriate corrective actions. An operability evaluation was performed justifying the continued operation of Unit 1. The evaluation included calculation of the potential amount of wastage of the carbon steel nuts and studs, which was deemed to present an acceptably low risk. In April, 1993, the root cause analysis on the Unit 2 leakage was completed and a modification to the Unit 2 ICI flanges initiated. The cause of the leakage was determined to be a change of gasket material in 1988 which changed the crush characteristics of the gasket and allowed leakage past the ICI detector joint. The corrective modification included replacement of the gaskets with a thicker version and the placement of Belleville washers underneath the castle nut assembly to help maintain tension on the joint. This modification was implemented on Unit 2 prior to restart following the 1993 refueling outage. Thus far, we have observed no evidence of any additional leakage from the Unit 2 ICI flanges.

In June, 1993, following indications of increasing Unit 1 containment ambient temperature and decreasing performance of the containment air coolers (CACs),

the Plant General Manager directed shutdown of the unit to investigate. During this shutdown, the ICI flanges were examined and seven of eight were found to be leaking. The amount of boric acid Icakage was similar to the previously identified leakage on Unit 2. A team was assembled to address the issues and form recovery plans. On June 25, 1993, the Plant Operations and Safety Review Committee reviewed the operability determination for Unit 1. This evaluation had incorporated the following considerations: Technical Specification and Updated Final Safety Analysis Report reviews, evaluation of the effects of boric acid on other containment components and structures, effects of boric acid on the reactor vessel head, effects and extent of boric acid corrosion of the ICI flange studs and nuts, and the potential effects of a fuel failure in conjunction with the leaks. The evaluation concluded that continued operation of the unit was justified. The Plant Operations and Safety Review Committee recommended resumption of full power operation with repair to the flanges

t NRC FORM 366A . U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 asa EXPIRES 5/31/95 ESTtMATED DURDEN PER RESPONSE TO COMPLY WilH THIS ltFoRMATION COLLECTION REQUEST: 50 0 HRS. FORWARD LICENSEE EVENT REPORT (LER) coMuENrs REGArONG BURDEN ESTIMATE To THE INFoRMATION AND FIECORDS MANAGEMENT BRANCH (MNDO 7714), U S. NUCLEAR TEXT CONTINUATION REGUuTORY CoMMSSM WASHINGTON, DC 2035 0001. AND To THE PAPERWORK REDUCTION PROJECT (3150 0104), OFFICE of MANAGEMENT ANo BUDGET WASHINGTON, Oc 20503.

f'ACluTV NAME (1) DOCKET NUMBER (4 LER NUMBER (3) PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 94 -

004 - 00 03 0F 06 Text y me,. .n.c. N ,.quir.o u .aamon.i com or NRC rorm ass 4) (17) deferred until the scheduled 1994 outage. On June 28, 1993, following completion of maintenance activities and a careful review of the issue, the Plant General Manager directed that Unit 1 be returned to service.

II. CAUSE OF EVENT The apparent cause of the event was an increased presence of wet boric acid rather than dry boric acid on a few of the affected ICI flange stud nuts. In deciding in 1993 to defer repairs, we assumed that the high temperature of the vessel head components would result in rapid evaporation of moisture from the leaking coolant, leaving predominantly dry boric acid. This was consistent with the observed results of leakage on Unit 2. We therefore used an expected corrosion rate of 1.6 mils per month to support the evaluations discussed above.

A root cause analysis to determine the reasons for the increased presence of wet boric acid is in progress. A supplement to this Licensee Event Report (LER) will be submitted when this analysis is complete.

III. ANALYSIS OF EVENT Each of the eight ICI flange assemblies consists of an upper and lower half secured together with eight 13 inch long 1-3/4 inch diameter studs secured by a  ;

1-3/4 inch heavy hex nut (see Figures). The nuts are made of ASTM A 194 Grade )

2H carbon steel and the studs are made of ASTM A 193 Grade B7 carbon steel. In this specific case, three adjacent studs were degraded: one was failed, and two were significantly corroded. An engineering analysis performed by the reactor  ;

vendor conservatively demonstrated that three adjacent studs would have to be l completely failed and a fourth severely degraded in order to result in a partial l failure accompanied by slight rotation of the joint. The analysis further concluded that if a loss of three adjacent studs and consequent joint rotation l were to occur, it would most likely occur such that gross leakage would be apparent for some time prior to catastrophic failure and thus alert operators of the condition in time to take mitigating action. In the unlikely event of catastrophic joint failure, the effects would be bounded by the Calvert Cliffs  ;

loss-of-coolant accident analyses. l Actual leakage in this case was within Technical Specification limits. We .

experienced no loss of pressure boundary integrity. However, the above analysis l combined with our preliminary assessment of the amount of corrosion we i experienced led us to conclude on March 25, 1994 that the potential had existed for a significant leak. We are therefore reporting this event under 10 CFR 50.73(a)(2)(ii).

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 31504104 (5 ea EXPIRES 5/31/95 EST1 MATED DURDE N PER RLSPONSE To COMPLY WrfH THIS INFORMATON COuECTION REQUC3T: 50.0 HRS. FORWARD LICENSEE EVENT REPORT (LER) COMMENTS REGAfDNG BURDEN ESTIMATE TO THE INFORMATION AND RECORDS MANAGEMENT ORANCH (MN00 7714), U S NUCLEAR TEXT CONTINUATlON REoutATORY couuiSSON. WASHINGTON. DC 20S$5&XH, AND TO THE PAPERWORK REDUCTION PFOJECT (31504104). OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.

F ACluTY MAME (1) DOCKET NUMUFR (2) LER NUMBER (3) PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 94 -

004 -

00 04 0F 06 TE XT (if mo<e space is requaed, use metaal copies of NRC Form 366A) (17)

IV. CORRECTIVE ACTIONS Corrective action for the ICI flange leakage problem was to implement the modification discussed above. The modification was implemented on Unit 2 in 1993 and thus far we have observed no evidence of further leakage on that unit.

In December, 1993, we performed a mock-up test that validated the new design.

The modification was implemented on Unit 1 during the present outage. We are conducting a root cause analysis to determine the cause of the increased presence of wet boric acid and to ascertain lessons to be incorporated into our engineering decision process with respect to boric acid leakage. The results of this analysis will be reported in a supplement to this LER.

V. ADDITIONAL INFORMATION A. Identification of components referred to in this LER:

IEEE 803 IEEE 805 Component EIIS Funct System ID ICI Flange PSF AB ICI Flange Stud PSF AB ICI Flange Hex Nut PSF AS ICI Casket PSF AB B. There have been no previous similar events.

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NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMD NO. 3150-0104 (s ert EXPlRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COMRY WITH THIS INFORMAToN COLLECTION REQUEST. 50.0 HF3. FORWARD UCENSEE EVENT REPORT (LER) CouMcNTS REoARoiNo BuRoEN EsTiMArs TO THE iNFORuAroN AND RECOROS MANAGEMENT BRANCH (MNB8 7714, U.S. NUCLEAR -

TEXT CONTINUATION REoUtATORY COMMisson, wAssiNoTON. oC sossseoi. AND TO THE PAPERWORK REQUCToN PROJECT (3t50-0104), OFF!CE OF .

MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.

F ActLITY MAME (1) DOCKET NUMBER (:4 LER NUMBER {3) PAGE (4)

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FIGURE 1 PLAN VIEW INDICATING LOCATION AND ORIENTATION OF ICI FLANGES ON PRESSURE VESSEL CLOSURE HEAD

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