ML20216H980
| ML20216H980 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 03/18/1998 |
| From: | John Hickman NRC (Affiliation Not Assigned) |
| To: | Kingsley O COMMONWEALTH EDISON CO. |
| References | |
| TAC-M97186, NUDOCS 9803230259 | |
| Download: ML20216H980 (10) | |
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UNITED STATES g
j' NUCLEAR REGULATORY COMMISSION 2
WASHINGTON, D.C. ennem mes
%... j March 18, 1998 i
Mr. Oliver D. Kingsley, President 4
Nuclear Generation Group Commonwealth Edison Company Executive Towers West til i
1400 Opus Place, Suite 500 Downers Grove, IL 60515
SUBJECT:
REVIEW OF BYRON, UNIT 1, STEAM GENERATOR 90-DAY REPORT (TAC NO. M97186)
Dear Mr. Kingsley:
By letter dated September 9,1996, the Commonwealth Edison Company (Comed or the i
licensee) submitted its steam generator (SG) 90-day report, " Byron Unit-1 End-of-Cycle 7B interim Plugging Criteria Report." The report summarized the results of the licensee's assessment of the eddy current inspection results with respect to the requirements established i
for voltage-based tube repair criteria.
The cycle-specific amendment approving for the conditional probability of tube bu the use of voltage-based repair criteria for this unit included a reporting threshold of 1 x 10 -
licensee's projections for the end-of-cycle 8 (EOC-8) estimated a conditional burst probability below this threshold using NRC staff approved methodology. The estimates of the primary-to-secondary leakage during a postulated main steamline break (MSLB) for Byron, Unit 1, were below the maximum allowable accident leak rate of 25.4 gpm and were estimated by the licensee using NRC staff approved methodology. We conclude the licensee implemented the voltage-based repair criteria in accordance with its licensing basis. The staff's review of the report is enclosed.
Although the licensee implemented the voltage-based repair criteria in accordance with its licensing basis, concems with the predictive methodology were brought out during meetings between Comed and the NRC staff. Comed and the staff met on April 30,1997, to discuss various aspects of the SG eddy current inspection performed for Braidwood, Unit 1, during its spring 1997 refueling outage (EOC-6). At this meeting, Comed compared the predictions for the MSLB leak rate at the EOC-6 with calculations based on actual inspection results. Comed found the predicted MSLB leak rate was significantly nonconservative with respect to the actual
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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)
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indications. In a subsequent meeting with the staff held on July 23,1997, the licensee reported larger indications, in general, grew at rates much faster than the rest of the indications. The methodology used to predict the Braidwood, Unit 1, EOC-6 voltage distribution did not consider such voltage-based growth rate variations; this resulted in a nonconservative prediction of the EOC-6 voltage distribution. The nonconservative prediction of voltage distribution, in tum, resulted in nonconservative calculations of MSLB leak rates. Comed also considered this issue was germane to Byron, Unit 1.
Comed addressed this potential nonconservatism in the Braidwood, Unit 1, 90-day report, submitted on August 14,1997, by revising the predictive methodology to account for 9803230259 980318 RE EHE3 MW PDR ADOCK 05000454 G
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a O. Kingsley March 18, 1998 L
voltano-dependent growth rates. The licensee also used the revised methodology to update the
' EOC-8 predictions for Byron, Unit 1. The staff reviewed the revised methodology and requested additional information from the licensee on September 19,1997. The licensee responded in letters dated October 15,1997, and January 14,1998. The staffis currently reviewing the licensee's response with respect to the EOC-7 predictions at Braidwood, Unit 1. The issue, as it applies to Byron, Unit 1, has been rendered moot because the plant was shut down in November 1997, to replace its SGs.-
Comed also discussed in the July 23,1997, meeting, the assessment of the radiological consequences of an MSLB. The licensee indicated that for Byron, Unit 1, past comparisons of u
L the predicted primary-to-secondary leakage attributed to SG tube indications to the site allowable l
leak rate limit were not conservative. The former value was calculated based on models that use a room temperature volumetric leak rate measurement while the site allowable leak rate limit L
. is calculated based on a mass flow rate measurement at accident conditions (e.g., high temperature). In the Byron, Unit 1,90-day report, Comed refers to a site allowable leak rate limit of 35.7 gpm. When corrected for temperature differences, the room temperature value of the site limit is 25.4 gpm. This revised value war.; considered in the staff review of the 90-day report and did not affect our conclusions. Because this issue affected several other plants implementing the voltage-based SG tube repair criteria, the staff addressed this issue generically through issuance of Information Notice 97-79, " Potential Inconsistency in the Assessment of the Radiological Consequences of a Main Steam Line Break Associated with the implementation of Steam Generator Tube Voltage-Based Repair Critoria."
Should you have any questions on this issue please contact me at (301) 415-3017.
Sincerely, 1
Orig. signed by John B. Hickman, Project Manager Project Directorate 111-2 l-Division of Reactor Projects - lil/IV Office of Nuclear Reactor Regulation l
Docket No. STN 50-454 3
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Enclosure:
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1 O. Kingsley voltage-dependent growth rates. The licensee also used the revised methodology to update the EOC-8 predictions for Byron, Unit 1. The staff reviewed the revised methodology and requested additional information from the licensee on September 19,1997. The licensee responded in letters dated October 15,1997, and January 14,1998. The staff is currently reviewing the licensee's response with respect to the EOC-7 predictions at Braidwood, Unit 1. The issue, as it applies to Byron, Unit 1, has been rendered moot because the plant was shut down in November 1997, to replace its SGs.
Comed also discussed in the July 23,1997, meeting, the assessment of the radiological consequences of an MSLB. The licensee indicated that for Byron, Unit 1, past comparisons of j
the predicted primary-to-secondary leakage attributed to SG tube indications to the site allowable leak rate limit were not conservative. The former value was 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 temperature). In the Byron, Unit 1,90-day report, Comed refers to a site allowable leak rate limit of 35.7 gpm. When corrected for temperature differences, the room temperature value of the site limit is 25.4 gpm. This revised value was considered in the staff review of the 90-day report and did not affect our conclusions. Because this issue affected several other plants implementing the voltage-based SG tube repair criteria, the staff addressed this issue generically through issuance of information Notice 97-79, " Potential Inconsistency in the Assessment of the Radiological Consequences of a Main Steam Line Break Associated with the implementation of Steam Generator Tube Voltage-Based Repair Criteria."
Should you have any questions on this issue please contact me at (301) 415-3017.
Sincerely,
[ John B. Hickman, Project Manager Project Directorate lll-2 Division of Reactor Projects - lil/IV Office of Nuclear Reactor Regulation Docket No. STN 50-454 s
Enclosure:
As stated cc w/ encl: See next page
O. Kingsley Byron Station Commonwealth Edison Company Units 1 and 2 cc:
Michael I. Miller, Esquire Chairman, Ogle County Board Sidley and Austin Post Office Box 357 One First National Plaza Oregon, Illinois 61061 Chicago, Illinois 60603 Mrs. Phillip B. Johnson Regional Administrator, Region ill 1907 Stratford Lane U.S. Nuclear Regulatory Commission Rockford, Illinois 61107 801 Warrenville Road Lisle, Illinois 60532-4351 Attomey General 500 S. Second Street lilinois Department of Nuclear Safety Springfield, Illinois 62701 Office of Nuclear Facility Safety 1035 Outer Park Drive Commonwealth Edison Company-Springfield, Illinois 62704 Byron Station Manager 4450 North German Church Road Document Control Desk-Licensing Byron, Illinois 61010-9794 i
Commonwealth Edison Company 1400 Opus Place, Suite 400 Commonwealth Edison Company Downers Grove, Illinois 60515 Site Vice President - Byron 4450 N. German Church Road Ms. C. Sue Hauser, Project Manager Byron, Illinois 61010-9794 Westinghouse Electric Corporation Energy Systems Business Unit Mr. David Helwig Post Office Box 355 Nuclear Services Senior Vice President Pittsburgh, Pennsylvania 15230 Commonwealth Edison Company Executive Towers West lli Joseph Gallo 1400 Opus Place, Suite 900 Gallo & Ross Downers Grove, IL 60515 1250 Eye St., N.W., Suite 302 Washington, DC 20005 Mr. Gene H. Stanley PWR's Vice President Howard A. Leamer Commonwealth Edison Company Environmentallaw and Policy Executive Towers West til Center of the Midwest 1400 Opus Place, Suite 900 203 North LaSalle Street Downers Grove,IL 60515 Suite 1390 Chicago, Illinois 60601 Mr. Steve Perry BWR's Vice President U.S. Nuclear Regulatory Commission Commonwealth Edison Company Byron Resident inspectors Office Executive Towers West lll 4448 North German Church Road 1400 Opus Place, Suite 900 Byron, Illinois 61010-9750 Downers Grove, IL 60515 Ms. Lorraine Creek RR 1, Box 182 Manteno, Illinois 60950
O. Kingsley Byron Station Commonwealth Edison Company Units 1 and 2 Mr. Dennis Farrar Regulatory Services Manager Commonwealth Edison Company Executive Towers West ll1 1400 Opus Place, Suite 500 Downers Grove,IL 60515 Ms. Irene Johnson, Licensing Director Nuclear Regulatory Services Commonwealth Edison Company Executive Towers West 111 1400 Opus Place, Suite 500 Downers Grove,IL 60515 Commonwealth Edison Company Reg. Assurance Supervisor-Byron 4450 N. German Church Road Byron, Illinois 61010-9794 Mr. Michael J. Wallace Senior Vice President Commonwealth Edison Company Executive Towers West til 1400 Opus Place, Suite 900 Downers Grove, IL 60515
Review of Byron Station, Unit 1, Steam Generator 90-Day Report in a letter dated September 9,1996, Commonwealth Edison Company (Comed or the licensee) submitted its steam generator (SG) 90-day report, " Byron Unit-1 End-Of-Cycle 7B Interim Plugging Criteria 90 Report"(Reference 1). The staff reviewed the submittal using criteria from References 2 and 3 and found the licensee's assessment to be acceptable. Details of the review are provided below.
1.0 General Plant Description Byron Station, Unit 1 (Byron-1), has four Westinghouse model D4 SGs with 3/4-inch diameter tubes. During the midcycle outage in NovemLs '1995, Comed implemented 1.0 volt interim Plugging Criteria (IPC) to be applied to outside nmeter stress corrosion cracking (ODSCC) at the tube support plate (TSP) intersections on the cold leg side of the SG and a 3.0 volt IPC to be applied to ODSCC at the TSP intersections on the hot leg side of the SG, with certain exceptions. The higher voltage repair limit is based on stabilization of the TSP by expansion of selected tubes at the support plate intersections so that the TSP could be credited as staying in place under postulated main steamline break (MSLB) conditions. References 2 and 3 describe the 1.0 volt and 3.0 voit IPC methodologies in detail.
The licensee used a lower repair limit of 1.0 voit and determined an upper voltage repair limit of 2.56 voit to disposition ODSCC at TSP intersections on the cold leg side. The staff did not review the details of Comed's basis for the upper voltage repair limit of 2.56 volt. Indications less than or equal to 1.0 volt were left in service; indications greater than 2.56 volt were removed from service; and indications with voltages between 1.0 and 2.56 volt were removed from service if confirmed with a rotating pancake coil (RPC) probe. Comed used a repair limit of 3.0 volt to disposition ODSCC at TSP intersections on the hot leg side. Indications less than or equal to 3.0 volt were left in service while indications greater than 3.0 volt were removed from service.
2.0 Steam Generator Tube Eddy Current inspection Scope and Results Comed inspected 100 percent of the Byron-1 SG tubes fulllength using a bobbin coil probe. For all TSP indications subject to the 1.0 or 3.0 volt IPC, the licensee used a 0.610-inch diameter bobbin coil probe as required by References 2 and 3.
For tubes where the 3.0 voit IPC were applied, the licensee used the RPC probe to inspect 20 percent of bobbin voltages between 1.0 and 3.0 voit and all bobbin voltages greater than 3.0 volt. For tubes where the 1.0 volt IPC were applied, the licensee used the RPC probe to inspect -
100 percent of bobbin voltages greater than 1.0 volt.
Comed also used an RPC probe to inspect all intersections with dent signals greater than 5.0 volt and a 20 percent sample of intersections with bobbin dent voltages between 2.5 and 5.0 volt. The licensee reported one flaw in a cold leg dent. The indication initiated on the inner diameter and was axially oriented with an RPC voltage of 1.17 volt (based on a 0.115-inch pancake coil). The tube indication started near the top of the sixth TSP and extended upwards 0.21 inch outside the TSP. The bobbin coil voltage of the dent is 15.77 volt. Comed also detected a second dent at the same intersection with a bobbin coil voltage of 7.13 volt. The RPC inspections confirmed no circumferentially oriented ODSCC existed at the TSPs, and all ODSCC indications remained within the confines of the TSPs Comed reported no mixed residuals or ENCLOSURE
. signal interference from copper deposits. With the one exception just discussed, the licensee found no indications at dented intersections and no primary water stress corrosion cracking (PWSCC).
Comed reported a total of 5719 ODSCC indications at TSP intersections and retumed 5420 indications to service at Byron-1. Of the 299 indications removed from service, 273 indications were in tubes plugged for degradation mechanisms other than ODSCC at the TSP. Of the remaining 26 indications,19 were in tubes near the wedge supports and the IPC were not applied. The remaining seven indications were hot leg indications above the 3.0 voit limit. No cold leg indications were repaired.
The staff concludes the licensee's bobbin and RPC probe inspections were consistent with the guidance in References 2 and 3 and, thus, are acceptable.
1 3.0 Probe Wear Licensees monitor the eddy current bobbin probe wear. If the probe is found to be outside of its
- 15 perceni wear specification, licensees reinspect all tubes inspected since the last successful calibration with a new, calibrated probe. Reference 3 permitted altematives to this approach subject to NRC approval.
The Nuclear Energy Institute (NEI) submitted an attemative probe wear approach to the NRC for review. The industry approach is such that if the amplitude from the probe wear standard prior to probe replacement exceeds the 115 percent limit, all tubes with voltage responses measured at i
75 percent or greater of the lower voltage repair limit must be reinspected with a bobbin probe satisfying the *15 percent wear standard criterion. The voltages from the reinspection are used I
as the basis for tube repair. The NRC staff completed a review of the proposed attemative method and concluded the approach is acceptable as documented in Reference 4.
At the end-of-cycle 78 (EOC-7B*), Byron-1 implemented the attemate probe wear criteria. All tubes with indications greater than 75 percent of the lower voltage repair limit (1.0 volt) were -
reinspected with a probe which satisfied the probe wear criterion including hot leg indications to which a 3.0 volt repair criteria was applied. In its 90-day report, Comed compared voltages measured with a wom probe and with a new probe at the same location. The licensee found a slight bias for the wom probe voltages to be higher than the new probe voltages. No indications had gone undetected by the wom probe; no pluggable tube indications were missed by the wom probe. Overall, the licensee concluded the criteria to retest tubes with wom probe voltages above 75 percent of the repair limit is adequate and moderately conservative in the sense that wom probe voltages tended to exceed new probe voltages.
1 4.0 Comparison Between Actual and Predicted Conditional Probability of Burst and Total Leak i
Rate Under Postulated MSLB Conditions The staff evaluated the licensee's ability to conservatively predict the number and distribution of indications at the next EOC such that the estimated conditional probability of burst and total leak rate under postulated MSLB conditions at the next EOC are conservative.
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o 3-4.1 Projected and Actual EOC-78 Voltage Distributions I
In Reference 1, Comed compared the actual EOC-78 bobbin voltage distributions with the corresponding predictions for EOC-78 performed at the EOC-7A. In general, the distribution shapes of the predicted versus actual results for the EOC-7B are similar (e.g., similar voltage
. peaks). The methodology underpredicted the number of indications lower than 0.4 volt and overpredicted the number of indications greater than 0.4 volt for hot leg indications. For larger indications (i.e., greater than 2.0 volt), the licensee overpredicted the total number of indications but underpredicted the actual size of the largest indications. For example, Comed predicted a total of 79.11 indications with voltages of 2.0 volt or higher, with a maximum voltage for a full tube at 3.2 volt. The licensee reported a total of 30 indications with voltages of 2.0 voit or higher, with a maximum voltage at 4.5 volt. The larger voltages dominate the leak rate and burst integrity scenarios; what bearing the aforementioned differences between predicted versus actual results have on the tube integrity is discussed in the following section.
4.2 Conditional Probability of Burst and Total Leak Rate Under Postulated MSLB Conditions Comed calculated the conditional probability of burst and the totalleak rate under postulated MSLB conditions using the actual EOC-78 bobbin voltage distribution and then compared these
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values to those predicted for EOC-78 at the EOC-7A. The limiting conditional tube burst d
probability for one tube was less than 4.0 x 10, for both actual and predicted EOC-78 voltage i
distributions. The limiting MSLB leak rate was 0.27 gpm, compared with a predicted value of 0.25 gpm. The licensee concluded a 4.5 voit indication not predicted resulted in this very minor i
underprediction. The predicted leak rates for the other three SGs were slightly conservative. In all cases, the calculated leak rate using actual EOC voltage distributions was well below the site allowable leak rate limit of 25.4 gpm (room temperature conditions). 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.
5.0 Tube Integrity Evaluations for EOC-8 5.1 Projected EOC-8 Voltage Distribution For the EOC projections, the licensee used the most limiting voltage growth rates observed during the last one or two inspection cycles. Comed used the cycle 78 SG "B" growth 64tribution, which had the highest average growth rate as well as the single highest growth increment. The staff considers the Comed approach to projecting the EOC-8 voltage distribution acceptable as it meets the guidance of Reference 2.
5.2 Database Comed used the database described in a letter from N.J. Liparuto (Westinghouse) to W.T. Russell (NRC), CAW-96-935, dated February 28,1996. 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 rate correlations. The staff did not review the database.
5.3 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 is negligible. The number and size of cold leg indications projected to be in service at the EOC-8 are few and small, resulting in a limiting burst probability of less than 9.4 x 10, far below d
the threshold value of 1 x 10 -2 and is, therefore, acceptable.
5.4 Projected MSLB Leak Rate The projected MSLB leak rate is calculated to ensure leakage from indications under MSLB conditions will not result in offsite and control room dose releases that exceed the guidelines of 10 CFR Part 100 and GDC ig as reflected in the Byron-1 site-specific limit. Comed 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. The limiting MSLB leak rate for projected indications at the EOC-8 is 1g.0 gpm. This value is lower than the allowable leak rate limit of 25.4 gpm (room f.nperature conditions) and is, therefore, acceptable.
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.5-References 1
1.
" Byron Unit-1 End-of-Cycle 7B Interim Plugging Criteria Report," dated August 1996, Westinghouse Electric Corporation, SG-96-08-005.
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," dated August 3,1995.
4.
Letter from B.W. Sheron (NRC) to A. Marion (NEI) dated February 9,1996.
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