ML20198S587

Forwards Evaluation of Unit 1 end-of-cycle 6 Steam Generator 90-day Report

ML20198S587
Person / Time
Site:	Braidwood
Issue date:	10/27/1997
From:	Dick G NRC (Affiliation Not Assigned)
To:	Johnson I COMMONWEALTH EDISON CO.
References
TAC-M95339, NUDOCS 9711140062
Download: ML20198S587 (10)

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-October 27, 1997 j

1 Ms. Irene Johnson, Acting Manager i

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Commonwealth Edison Company __

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Downers Grove,IL 60515 i

SUBJECT:

EVALUATION OF BRAIDWOOD, UNIT 1, END OF-CYCLE 6 STEAM GENERATOR 90-DAY REPORT (TAC NO. M95339)

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Dear Ms. Johnson:

By letter dated March 5,1996, as supplemented by letter dated March 8,1996, the Commonwealth Edison Company (Comed, the licensee) submitted its steam generator (SG)

W 90 day report, "Braidwood Unit-1 Cycle 6 Interim Plugging Criteria 90 Day Report." The report summarized the results of the licensee's a sessment of the oddy current (EC) inspection results with tospect to the requirements established for voltage-based tube repair criteria.

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-- included a reporting threshold of 1 x 10 g the use of voltage based repair crite The cycle specific amendment approvin for the conditional probability of tube burst. The licensee's projections estimated a conditional burst probability below this threshold using NRC' staff approved methodology. The estimates of the primary to secondary leak rate during a postulated main steamline break (MSLB) for Braidwood, Unit 1, were below the maximum allowable accident leak rate of 26.8 gpm and were estimated by the licensee using NRC staff

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approved methodology. The staff concludes the licensee implemented the voltage-based repair i

criteria in accordance with its licensing basis. The staffs evaluation of the report is enclosed.

Although the licensee implemented the voltage-based repair criteria in accordance with its L

- licensing basis, concems with the predictive methodology were brought out during recent meetings between Comed and the NRC staff. Comed and the staff met April 30,1997, to l discuss various aspects of the SG EC inspection performed during the Braidwood, Unit 1, end-of- /.

cycle 6 (EOC-6) spring 1997 refueling outage, At this meeting, Comed compared the predictions

. for MSLB leak rate at the EOC-6 (these predictions are discussed in the enclosed evaluation) 4 with calculations based on actual inspection results. Comed found the predicted MSLB leak rate wts nonconservative with respect to the actual calculated leak rate although still within site allowable leak rate limits; Comed attributed the nonconservative prediction to an unexpectedly high number of large (i.e., greater than 5 volt) outside diameter stress corrosion cracking

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(ODSCC) indications.1 in a subsequent meeting with the staff held July 23,1997, the licensee e

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reported larger indications, in general, grew at rates much faster than the rest of the voltage population. The methodology used to predict the EOC-6 voltage distribution did not consider

such voltage-based growth rate variations; resulting in a nonconservative prediction of the EOC 6 voltage distribution., in tum, the nonconservative prediction of voltage distribution resulted in a nonconservative calculation of the MSLB leak rate.

_In the same July 23,1997,' meeting, Comed also discussed the assessment of the radiological consequences of an MSLB. The licensee indicated that for Braidwood, Unit 1, past comparisons

-- were not conservative because the primary to-secondary leakage attributed to SG tube 9711140062 971027 DR s_

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-2 October 27, 1997 Indications is calculated based on models that use a room temperature volumetric leak rate measurement while the site allowable leak rate limit is calculated based on a mass flow rate measurement at accident conditions (e.g., high temporature). Because this issue potentially affects several other plants implementing the voltage-based SG tube repair criteria, the staff plans to address this issue generically.

As discussed in the July 23,1997, meeting, Comed included its resolution of these issues in the Braidwood, Unit 1, EOC 7 SG 90-day report, which was submitted on August 14,1997. The staff plans to review the EOC-7 reporiin a manner similar to the enclosed evaluation, with particular emphasis on Comed's evaluation and corrective action with respect to the predictive methodology and assessment of the radiological consequences of an MSLB.

This completes the staff's review of the subject report.

Sincerely, ORIGINAL SIGNED BY:

George F. Dick, Jr., Senior Project Manager Project Directorate 111-2 Division of Reactor Projects - til/IV Office of Nuclear Reactor Regulation Docket No. STN 50-456

Enclosure:

Evaluation cc w/ encl: see next page DISTRIBUTION:

Docket File PUBLIC PDill-2 r/f E. Adensam, EGA1 R. Capra C. Moore S. Bailey G. Dick M. Lynch S. Coffin E. Sullivan OGC, 015B18 ACRS, T2E26 R. Lanksbury, Rlli

• concurrence provided by memo dated 8/13/97 DOCUMENT NAME: G:\\CMNTJR\\BRAIDpR95339.LTR T4 receive a copy of this document, indicath I thx: "C" a C_opy without enclosures *E" n Copy wth enclosures *N" = No copy OFFICE PM:PDill-2 _ l 6 b:03 81-2 lb PM:PDill-2'.l/E D:PDill-2 le DE:EMCB l

NAME SBAILEY6tN DMODME GDICK MU RCAPRA T* -

ESULLIVAN*

DATE 10/1997 10/[797 10//(/97 10p/97 10/ /97 OFFICIAL RECORD COPY

i-l, l.Johtion 2

October 27. 1997 Indications is calculated based on mudels that use a room temperature volumetric leak rate measurement while the site allowable leak rate limit is calculated based on a mass flow rate measurement at accident conditions (e.g., high temperature). Because this issue potentially affects several other plants implementing the voltage-based SG tube repair criteria, the staff plans to address this issue generically.

As discussed in the July 23,1997, meeting, Comed included its resolution of these issues in the Braidwood, Unit 1, EOC-7 SG 90-day report, which was submitted on August 14,1997, The staff plans to review the EOC 7 report in a manner similar to the enclosed evaluation, with particular emphasis on Comed's evaluation and corrective action with respect to the predictive i-methodology and assessment of the radiological consequences of an MSLB.

This completes the staffs review of the subject report.

l Sincerely, ORIGINAL SIGNED BY:

George F. Dick, Jr., Senior Project Manager Project Directorato Ill 2 1

Division of Reactor Projects - lit /IV Office of Nuclear Reactor Regulation Docket No. STN 50-456

Enclosure:

Evaluation cc w/ encl: see next page DISTRIBUTION:

j Docket File PUBLIC -

PDill 2 r/f E. Adensam, EGA1 R. Capra C. Moore S. Bailey G. Dick M. Lynch S. Coffin E. Sullivan OGC,015B18 ACRS, T2E26 R. Lanksbury, Rlll

• concurrence provided by memo dated 8/13/97 DOCUMENT NAME: G:\\CMNTJR\\ BRAID R95339.LTR To receive a copy of this document.

a: "C" = Copy without enclosures "E" = Copy with enclosures "N*

• No cop r OFFICE PM:PDill 2 _

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DE:EMCB NAME SBAILEY6W DMOORE GDICK fjLO RCAPRA >

ESULLIVAN*

DATE 10/lif97 10/ff97 10kB97 '

10D/97 10/ /97 i

OFFICIAL RECORD COPY

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indications is calculated based on models that use a room temperature volumetric leak rate measurement while the site allowable leak rate limit 8s calculated based on a mass flow rate measurement at accioent conditions (e.g., high temprature). Because this issue potentially affects several other plants implementing the voltage based SG tube repair criteria, the staff plans to address this issue generically.

As discussed in the July 23,1997, meeting, Comed included its resolution of these issues in the Braidwood, Unit 1, EOC-7 SG 90-day report, which was submitted on Augue: 14,1997. The staff plans to review to EOC-7 report in a manner similar to the enclosed evaluation, with particular emphasis on G:,1Ed's evaluation and corrective action with respect to the predictive methodology and assessment of the radiological consequences of an MSLB.

This completes the staffs review of the subject report.

Sincerely,

1. y )T v

GeorgeFF. Dick, Jr., Sen' r Project Manager Project Directorate ill-2 Division of Reactor Projects -lil/IV Offica of Nuclear Reactor Regulation Docktt No. STN 50-456

Enclosure:

Evaluation ec w/ encl: see next page

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1. Johnson-Braidwood Station Commonwealth Edison Company Unit Nos.1 and 2 cc:

Michael Miller, Esquire U.S. Nuclear Regulatory Commission Sidley and Austin Braidwood Resident inspectors Office One First National Plaza Rural Route #1, Box 79 Chicago, Illinois 60603 Braceville, Illinois 60407 Regional Administrator Mr. Ron Stephens i

U.S. NRC,,9egion ll1 lilinois Emergency Services 801 Warrenville Road and Disaster Agency Lisle, Illinois 60532-4351 110 East Adams Street Springfield, Illinois 62706 Illinois Department of Nuclear Safety Chairman Office of Nuclear Facility Safety Will County Board of Supervisors 1035 Outer Park Drive Will County Board Courthouse Springfield, Illinois 62704 Joliet, Illinois 60434 Document Control Desk Licensing Ms. Lorraine Creek Commonwealth Edison Company Rt.1, Box 182 1400 Opus Place, Suite 400 Manteno, Illinois 60950 Downers Grove, Illinois 60515 Attomey General Mr. William P. Poirier 500 South Second Street Westinghouse Electric Corporation Springfield, Illinois 62701 Energy Systems Business Unit Post Office Box 355, Bay 23G West George L. Edgar Pittsburgh, Pennsylvania 15230 Morgan, Lewis and Bochlus 1800 M Street, N.W.

Joseph Gallo Wasliington, DC 20036 Gat'o & Ross 1250 Eye St., N.W., Suite 302 Commonwealth Edison Company i

i Washington, DC 20005 Braidwood Station Manager Rt.1, Box 84 Ms. Bridget Little Rorem Braceville, Illinois 60497 Appleseed Coordinator 117 North Linden Street EIS Review Coordinator Essex, Illinois 60g35 U.S. Environmental Protection Agency 77 W. Jackson Blvd.

Howard A. Leamer Chicago, Illinois 60604-3590 Environmental Law and Policy Center of the Midwest Mr. H. G. Stanley 203 North LaSalle Street Site Vice President Suite 1390 Braidwood Station Chicago, Illinois 60601 Commonwealth Edison Company RR #1, Box 64 Braceville,IL 60407 l

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= WAsHINeToN D.C. 30806 4001 OFFICE OF NUCLEAR REACTOR REGULATION EVALUATION OF END-OF-CYCLE 6 STEAM QElNERATOR 90-DAY. REPORT BRAIDWOOD STATION. UNIT 1 DOCKET NO. STN 50-456 By letter dated March 5,1996, as supplemented by lottar dated March 8,1996, Commonwealth Edison Company (Comed or the licensee) submitted its steam generator (SG) 90-day report, "Brsidwood Unit 1 Cye.le 6 Interim Plugging Criteria 90 Day Rsport" [ Reference 1). The staff reviewed the submittal using criteria from References 2 anci 3 and found the licensee's

' assessment to be acceptable. Details of the review are provided below.

1.0 General Plant Description The Braidwood Nuclear Power Station, Unit 1 (Braidwood, Unit 1) has four Westinghouse model DO S.S. with 3/4-inch diameter tubes. During the last refueling outage at the end-of-cycle (EOC) 5, Comed implemented a 1.0 voit Ini.orim Plugging Criteria (IPC) to be applied to outside diameter stress corrosion cracking (ODSCC) at the tube support plate (TSP) intersections on the cold leg side of the S.S. and a 3.0 volt IPC to be applied to ODSCC at the TSP intersections on the hot leg side of the S.S., with certain exceptions. References 2 and 3 describe the 1.0 volt and 3.0 volt IPC methodologies in detail.

The licensee used a lower repair limit of 1.0 volt and determined are upper voltage repair limit of 1.99 volt to disposition ODSCC at TSP intersections on the cold leg side. The staff did not

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review the details of Comed's basis for the upper voltage repair limit of 1.99 vr'It. Indications less than or equal to 1.0 voit were left in service, indications greater than 1.99 voit were removed from service, and indications with voltages between 1.0 and 1.99 volt were removed from service if confirmed with a rotating pancake coil (RPC) probe. Comed used a repair limit i,

of 3.0 volt to disposition ODSCC at TSP intersections on the hot leg side. Indications less than i

or equal to 3.0 volt were left in service while indications greater than 3.0 volt were retroyed l-from service.

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' 2.0 Steam Generator Tube Eddy Current inspection Scope and Results l:,

Comed lospected 100 percent of its SG tubes full length using a 0.610 inch diameter bobbin coil with the exception of one dent 6d TSP ints.section located on the cold leg side. The dont has been present since initial startup. It is not adjacent to an expanded tube nor was an IPC applied to this TSP intersection.

t' i For tubes'where the 3.0 volt IPC was applied, the licensee used the RFC pmbe to inspect 20 percent of bobbin voltages between 1.0 and 3.0 volt and all bobbin voltages greater than 3.0 volt. For tubes where the 1.0 volt IPC is applied, the licensee used the RPC probe to

. inspect 100 percent of bobbin voltages greater than 1.0 volt.

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Comed also used the RPC probe to inspect all intersections with dont signals greater than 5.0 volt, a 20 percent sample of intersections with bobbin dent voltages between 2.5 and 5.0 volt, and all intersections with large mixed residuals. The licensee did not detect any indications. ComFd's RPC inspections confirmed no primary water stress corrosion cracking (PWSCC) or circumferentially oriented ODSCC ev.isted at the TSPs. All ODSCC indications were within the confines of the TSPs. Comed reported no signalinterference from copper deposits at Braidwood, Unit 1. The licensee reported no corrosion induced dents in intersections adjacent to expanded tubes.

Comed raported a total of 4136 ODSCC indications at TSP intersections and will retum 4083 Indications to service at Braidwood, Unit 1. Of the 53 indications rernoved from service, 38 indications were in tubes plugged for degradation mechanisms other than ODSCC at the TSPs. Of the remaining 15 indications,14 were in tubes near the wedge supports and the IPC was not applied. The remaining Indication was a hot leg indication above the 3.0 volt limit at 3.17 volt. There was one cold leg indication greater than 1.0 volt, but the voltage was less than the upper repair limit of 1.99 volt, and an RPC probe did not confirm the indication. Thus, Comed retumed the tube to service.

The staff concludes the licensee's bobbin and RPC probe inspections were consistent with the requirements in References 2 and 3 and, thus, are ecceptable.

3.0 Comparison Between Actual and Predicted Conditional Probability of Burst and Total Leak Rate Under Postulated MSLB Conditions The staff evaluated the ability of the predictive methodology discussed in References 2 and 3 to provide a conservative projection of the number and distribution of indications at the next EOC suc5 that the estimated conditional probability of burst and totalleak rate under postulated main steamline break (MSLB) conditions at the next EOC are conservative.

3.i Projected and Actual EOC-5B Voltage Distributions in Reference 1, Comed compared the actual EOC-5B bobbin voltage distributions with the corresponding predictions performed at the EOC 5A. (There was a midcycle inspection in February 1995. The first half of cycle 5 is referred to as cycle SA and the second half is referred to as cycle 58.) The methodology under predicted the numoer of indications lower than 0.7 volt and over predicted the number of indications greater than 0.7 voit. Since the large, voltages dominate the leak rate and burst integrity scenarios, this result is conservative. In general, the distribution shapes of the predicted versus actual results for the EOC 5B are similar (e.g.,

similar voltage peaks). Several voltages larger than 3.0 volt were predicted, but only one actual indication of 3.17 volt was reported.

3.2 Conditional Probability of Burst and Total Leak Rate Under Postulatea MSLB Conditions Comed calcalated the conditional probability of burst and the totalleak rate under posiulated MSLB conditions using the actual EOC-5B bobbin veitsge distribution and then compared these values to those predicted at the EOC-5A using the predicted EOC 5B bobbin voltage d

distribution. The limiting conditional tube burst probability for one tube was 6.49 x 10,

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1 3-compared with a predicted value of 4.94 x 'iO. The limiting MSLB leak rate was 0.07 gpm, 4

compared with a predicted value of 0.48 gpm. The comparison between values obtained using the actual voltage distributions compared with the values obtained using the predicted voltage distributions indicate the licensee used an appropriately conservative methodology.

4.0 Tube Integrity Evaluations 4.1 Projected EOC-6 Voltage Distribution Comed projected the EOC-6 voltage distribution in accordance with Generic Letter (GL) 95-05 (Reference 3). For the EOC projections, GL 95-05 requires the use of the most limiting voltage growth rates observed during the last one or two inspection cycles. The licensee used the cycle SA SG "A" distribution, which had the Nghest average growth and included the largest growth for SG "C," which had the high*M growth increment of 5.7 volt. The EOC 5A hybrid growth distribution was used at IS c OC-5A to predict EOC-58 indications and found to be conservative, as discussed in the preceding paragraph. The staff considers the Comed approach to projecting the EOC-6 voltage distribution to be within the guidelines of GL 95-05 and, thus, acceptable.

4.2 Database Comed used an updated version of the database described in Westinghouse Report SG-95.01-003," Byron Unit 1 End of-Cycle 6 Int rim Plugging Criteria Report," Westinghouse Nuclear Service Division, June 1995. The licensee indicated the updated database is in compliance with NRC guidelines for application of leak rate versus voltage correlations and for removal of data outliers in the 3/4 inch tubing burst and leak rett correlations. The staff did not review the database as part of this evaluation.

4.3 Conditional Probability of Burst The conditional probability of burst refers to the probability that the burst pressures associated with one or more indications in the faulted SG will be less than the maximum pressure differential associated with a postulated MSLB assumed to occur at EOC The staff considers an acceptable level of structural margin consistent with the applicable General Design Criteria (GDC) of 10 CFR Part 50, Appendix A, to be met with a conditional burst probability of less than 1 x 10~2. The licensee performed this assessment using methodology previously approved by the NRC staff in Reference 2. The staff did not run confirmatory calculations as part of this review. Because the TSPs are locked in place on the hot leg side, the burst probability for those tubes i: negligible. The number and size of cold leg indications projected to be in service 4

at the EOC-6 are few and small, resulting in a limiting burst probability of 6.81 x 10, far below 2

thJ threshold value of 1 x 10,

4.4 Prnjected MSLB Leak Rate The projected MSLB leak rate is calculated to ensure leakage from indications under worst case MSLB conditions will not result in offsite and control room dose releases that exceed the guidelines of 10 CFR Part 100 and GDC 19. Comed performed this assessment using

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methodology previously approved by the NRC staff in Reference 2. The staff did not run confirmatory calculations as part of this review. The limiting MSLB leak rata for projected indications at the EOC-6 is *>.99 gpm. This value is lower than the allowable leakage limit of 26.8 gpm.

5.0 Tube Pull Results Comed did not pull any tubes during the EOC-5B outage. In 1994, the licensee removed four tubes from Braidwood, Unit 1, that included several TSP intersections. The metallurgical evaluation of the tubes supported the applicability of the voltage based repair criteria to the SG tubes at Braidwood, Unit 1. A minimum of one additional tube with a minimum of two TSP intersections will be removed during the EOC-6 Braidwood, Unit 1, refueling outage (spring 1997). This schedule meets the requirements of GL 95-05 (Reference 3) and, thus, is acceptable to the staff.

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References

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Westinghouse Eiectric Corporation, NSD SGD 1204, SG-96-02-002, "Braidwood Unit-1 Cycle 6 Inte'im Plugging Criteria 90 Day Report," February 1996.

2.

Letter from M.D. Lynch (NRC) to D.L Farrar (Comed), " Issuance of Amendments," dated November 9,1995, 3.

Generic Letter 95-05, " Voltage-Based Repair Criteria for Westinghouse Steam Generator

Tubes Affected by Outside Diameter Stress Corrosion Cracking," August 3,1995.

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