ML20249C823
| ML20249C823 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 06/26/1998 |
| From: | Jacob Zimmerman NRC (Affiliation Not Assigned) |
| To: | Dennis Morey SOUTHERN NUCLEAR OPERATING CO. |
| References | |
| TAC-M99575, TAC-M99839, NUDOCS 9807010258 | |
| Download: ML20249C823 (12) | |
Text
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i June 26, 1998 4
Mr. D. N. Morey
'.Vice President - Farley Project Southem Nuclear Operating Company, Inc.
Post Office Box 1295 '
Birmingham, Alabama 35201-1295
SUBJECT:
JOSEPH M. FARLEY NUCLEAR PLANT, UNITS 1 AND 2 - REVIEW OF STEAM GENERATOR 90-DAY REPORTS (TAC NOS. M99575 AND M99830)
Dear Mr. Morey:
. By letter dated August 29,1997, and supplemented by letter dated September 17,1997,
Unit 1, steam generator (SG) 90-day report, "Farley Unit 1 1997 Alte' mate Repair Criteria 90-Day Report." In addition, by letter dated March 9,1997, SNC submitted its FNP Unit 2, SG 90-day report, "Farley Unit 21997 Altemate Repair Criteria 90-Day Report." These reports summarized the results of SNC's assessments of the eddy current program with respect to the guidance established for voltage-based tube repair criteria applied to indications located at the tube support plate intersections, and attributed to outside diameter stress corrosion cracking.
. The staff has reviewed the submittals and found the assessments to be acceptable. The staffs review is provided in the enclosures.
Sincerely, ORIGINAL' SIGNED BY:
' Jacob 1. Zimmerman, Project Manager Project Directorate 11-2 Division of Reactor Projects -1/11 Office of Nuclear Reactor Regulation Docket Nos. 50-348 and 50-364 '
DISTRIBUTION Docket File LPlisco, Ril
Enclosures:
- 1. Unit 1 Review PUBLIC PSkinner, Ril 2.- Unit 2 Review PDil-2 RF JZimmerman SCoffin HBerkow cc w/encis: See next page ACRS.
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June 26,1998 Mr. D. N. Morey
.Vice President - Farley Project l
Southem Nuclear Operating Company, Inc.
Post Office Box 1295 Birmingham, Alabama 35201-1295
SUBJECT:
JOSEPH M. FARLEY NUCLEAR PLANT, UNITS 1 AND 2 - REVIEW OF STEAM GENERATOR 90-DAY REPORTS (TAC NOS. M99575 AND M99839)
Dear Mr. Morey:
I By letter dated August 29,1997, and supplemented by letter dated September 17,1997, Southem Nuclear Operating Company, Inc. (SNC), submitted its Farley Nuclear Plant (FNP),
1 Unit 1, steam generator (SG) 90-day report. "Farley Unit 1 1997 Altemate Repair Criteria 90-Day Report." In addition, by letter dated March 9,1997, SNC submitted its FNP Unit 2, SG 90-day report, "Farley Unit 21997 Altemate Repair Criteria 90-Day Report." These reports summarized the results of SNC's assessments of the eddy current program with respect to the guidance established for voltage-based tube repair criteria applied to indications located at the tube support plate intersections, and attributed to outside diameter stress corrosion cracking.
The staff has reviewed the submittals and found the assessments to be acceptable. The staff's review is provided in the enclosures.
Sincerely, M
cob 1. Zimmerman, Project Manager Project Directorate ll-2 Division of Reactor Projects -1/11 Office of Nuclear Reactor Regulation Docket Nov. 50-348 and 50-364
Enclosures:
- 1. Unit 1 Review 2.- Unit 2 Review cc w/encis:
See next page Y
Joseph M. Farley Nuclear Plant cc:
Mr. R. D. Hill, Jr.
General Manager-Southem Nuclear Operating Company Post Omce Box 470 Ashford,' Alabama 36312 Mr. Mark Ajiuni, Licensing Manager Southem Nuclear Operating Company Post Office Box 1295 Birmingham, Alabama 35201-1295 Mr. M. Stanford Blanton i
Balch and Bingham Law Firm Post Office Box 306 1710 Sixth Avenue North Birmingham, Alabama 35201 Mr. J. D. Woodard Executive Vice President Southem Nuclear Operating Company 1
Post Office Box 1295 Birmingham, Alabama 35201 State Health Officer Alabama Department of Public Health 434 Monroe Street Montgomery, Alabama 36130-1701 Chairman Houston County Commission Post Office Box 6406 Dothan, Alabama 36302 Regional Administrator, Region ll U.S. Nuclear Regulatory Commission Atlanta Federal Center 61 Forsyth Street, S.W., Suite 23T85 Atlanta, Georgia 30303 i
Resident inspector
' U.S. Nuclear Regulatory Commission 7388 N. State Highway 95 Columbia, Alabama 36319 i
l l
1 REVIEW OF THE FARLEY UNIT 1 STEAM GENERATOR 90-DAY REPORT in a letter dated August 29,1997, and supplemented by letter dated September 17,1997, Southem Nuclear Operating Company, Inc. (SNC) submitted its steam generator (SG) 90-day report,"Farley Unit 1 1997 Altemate Repair Criteria 90-Day Report'[ Reference 1). The staff reviewed the submittal using criteria from References 2 and 3 and found SNC's assessment to be acceptable.
1.0 GENERAL PLANT DESCRIPTION Joseph M. Farley Nuclear Plant (FNP), Unit 1, has three Westinghouse model 51 SGs with 7/8-inch diameter tubes. During the refueling outage at the end-of-cycle 14 (EOC-14), SNC implemented 2.0 volt Altemate Repair Criteria (ARC) to be applied to outside diameter stress corrosion cracking (ODSCC) at the tube support plate (TSP) intersections. References 2 and 3 describe the 2.0 volt ARC methodology in more detail.
SNC used a lower repair limit of 2.0 voit and determined an upper voltage repair limit of 5.6 voit
~ to disposition ODSCC at TSP intersections. SNC left in service indications with bobbin coil voltages less than or equal to 2.0 volt. SNC removed from service indications with bobbin coil voltages greater than 5.6 volt and also indications with voltages between 2.0 and 5.6 volt if confirmed with a rotating pancake coil (RPC) probe.
2.0 STEAM GENERATOR TUBE EDDY CURRENT INSPECTION SCOPE AND RESULTS SNC inspected 100 percent of the Unit 1 SG tubes full length using a 0.720-inch diameter i
bobbin coil probe at all intersections at which the 2.0 volt ARC were applied. SNC used an RPC probe to inspect 100 percent of the indications with bobbin coil voltages greater than 4
l 2.0 volt in SGs "A" and "B" and 100 percent of the indications with bobbin coil voltages above 1.5 voit in SG "C." SNC reported a total of 106 indications with bobbin coil voltages that exceeded the 2.0 voit criteria; 85 were confirmed with the RPC probe and subsequently plugged. SNC also retumed 10 deplugged tubes to service based on the 2.0 volt repair criteria.
SNC reported a total of 3074 ODSCC indications at TSP intersections and retumed 2693 indications to service at Unit 1 (not including the 10 deplugged tubes). Of the 381 indications i
removed from service, 296 indications were in tubes plugged for degradation mechanisms other l
than ODSCC at the TSPs. The remaining 85 indications were above the 2.0 volt limit and were l
confirmed with the RPC probe. Two of the 85 indications were above the upper voltage limit of l
5.6 volt (at 6.4 and 13.7 volt).
The staff concludes that SNC's bobbin and RPC probe inspections were consistent with the guidance in References 2 and 3 and, thus, are acceptable.
l
. 3.0 PROBE WEAR l
Licensees monitor the eddy current bobbin prots wear. If a probe is found to be outside of its wear specification (*15 percent), licensees reinspect all tubes inspected since the last successful calibration with a new calibrated probs. Reference 3 permitted attematives 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 *15 percent limit, all tubes with voltage responses measured at 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 j
reinspection are used as the basis for tube repair. The NRC staff completed a review of the proposed altemative method and concluded the approach is acceptable as documented in Reference 4.
At the EOC-14, SNC implemented the attemate probe wear criteria at Unit 1. All tubes with indications greater than 75 percent of the lower voltage repair limit (2.0 volt) were reinspected with a probe, which satisfied the probe wear criterion. In its go-day report, SNC evaluated the altemative approach and concluded it was adequate. Voltages measured with a wom probe
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and a new probe at the same location were compared. No indications had gone undetected by the wom probe; no pluggable tube indications were missed by the wom probe.
1 4.0 COMPARISON BETWEEN ACTUAL AND PREDICTED EOC-14 CONDITIONAL PROBABILITY OF BURST AND TOTAL I FAK RATE UNDER POSTULATED MAIN _
STEAMLINE BREAK CONDITIONS In Reference 1, SNC compared the actual EOC-14 bobbin voltage distributions with the corresponding predictions for the EOC-14 that were performed at the EOC-13. In general, the actual distribution of voltages was similar to the predicted distribution of voltages. SNC over predicted the total number of indications for all three SGs. For SG "A," SNC correctly predicted l
the maximum voltage indication. For SG *B," SNC over predicted the msximum voltage
- indication. However, for SG "C," the limiting SG, SNC significantly under predicted the size of j
the largest indication.
The under prediction of the maximum voltage associated with ODSCC indications affected the prediction of the EOC-14 conditional probability of burst. SNC calculated the conditional l
probability of burst using the actual EOC-14 bobbin voltage distribution and then compared that value to the value predicted for the EOC-14. The limiting conditional tube burst probability for one tube was 5.2 x 104 based on the actual EOC-14 distribution compared with 1.4 x 104 based on the predicted EOC-14 distribution. Both values are below the technical specification -
l reporting threshold value of 1 x 104 l
The under prediction of the maximum voltage indication also affected SNC's prediction of the EOC-14 accident leak rate, but it did not result in a nonconservative prediction. SNC calculated i
a limiting main steamline break (MSLB) leak rate of 7.6 gallons per minute (gpm) based on the i
actual EOC-14 voltage distribution compared with a leak rate of 10.2 gpm based on the i
i- !
predicted EOC-14 voltage distribution. The leak rate based on the actual EOC-14 voltage
' distribution is below the maximum allowable leak rate limit of approximately 8.2 gpm (room temperature conditions). The staff notes that the leak rate of 10.2 gpm based on the predicted EOC-14 voltage distribution is above the maximum allowable leak rate limit of 8.2 gpm. This resulted from an error discovered subsequent to the submittal of the subject 90-day report. At the EOC-13, when SNC concluded an EOC-14 accident leak rate of 10.2 gpm was acceptable l
because it was smalier than the maximum allowable leak rate, SNC was using a value of
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' 11.4 gpm for the maximum allowable leak rate.
Subsequent to the submittal of the subject 90-day report, SNC reported that the site allowable -
leak rate of 11.4 gpm was based on operating conditions. The corresponding leak rate at room temperature conditions is 6.2 gpm, and it is this latter value that should have been used when i
comparing calculated leak rate values with the maximum allowable leak rate value.
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- 5.0 TUBE INTEGRITY EVALUATIONS FOR EOC-15 For the EOC-15 predictions of the voltage distribution of indications, SNC used the most limiting l
growth rates observed during the last two inspection cycles. Specifically, SNC used the cycle 14 SG "C" growth distribution to predict the limiting voltage distribution. In addition, SNC incorporated FNP Unit 2 growth rates for dep;ugged SG tunes into the applied growth rate distribution. This was in response to the accelerated growth rates associated with deplugged l
tubes described in the recent Unit 2 90-day report [ Reference 5).
l The conditional probability of burst refers to the probability that the burst pressures associated l
. 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 i
1 x 104 SNC performed this assessment using methodology previously approved by the NRC
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staff in Reference 2. SNC reported a limiting burst probability of 9.9 x 104, just below the technical specification reporting value of 1 x 10r,
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' The predicted MSLB leak rate is calculated to ensure leakage from indications under MSLB i
conditions will not result in offsite and control room dose releases that exceed the guidelines of 10 CFR Part 100 and GDC 19 and the Unit 1 site-specific limit. SNC performed this assessment using methodology previously approved by the NRC staff in Reference 2. SNC reported a limiting MSLB leak rate of 15.7 gpm, which is higher than the site's maximum allowable leak rate of 8.2 gpm (room temperature conditions). Subsequent to the submittal of 1
the subject 90-day report, the staff approved a dose equivalent iodine reduction that increased the site-specific leak rate limit at Unit 1 from 8.1 gpm to 23.8 gpm (room temperature conditions), which encompasses the EOC-15 predicted value of 15.7 gpm.
6.0 TUBE PULL RESULTS SNC removed three tubes during the EOC-14 refueling outage: R2C85 from SG "C," R29C47 from SG "A," and R6C28 from SG "B" to examine eddy current indications located at the first TSP. Tube R2C85 had a 13.7 volt indication that grew from 1.4 voit over the past operating i
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i L_ ___ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
4 cycle. Tube R29C47 was called "NDD" omio detectable degradation in the field. The TSP region of R6C28 could not be burst or leak tested as one of the tube pull field cuts occurred at the edge of the TSP and was not discussed furtner by SNC. The metallurgical evaluation of the pulled tubes confirmed the nature of the eddy current indications to be axially oriented ODSCC.
SNC postulated that pressure pulse cleaning, performed prior to the field eddy current inspection, may have contributed to the excessive bobbin coil voltage increase in tube R2C85.
SNC also identified limited areas of transgranular corrosion cracking in tube R2C85 and suggested that lead, being the only known cause of transgranular cracking of alloy 600, could have contributed to the unusual degradation morphology.
l SNC completed leak and burst testing of tube R2C85. Leakage through the indication was l
measured at approximately 0.72 gpm associated with accident conditions. When the leak rate i
for tube R2C85 is added to the EOC-14 leak rate predicted by SNC (after removing tube R2C85 from the analysis), the resulting leak rate of 7.g gpm is below SNC's maximum allowable leak rate of 8.2 gpm. SNC reported a burst pressure for tube R2C85 of 3990 pounds per square inch (psi). When adjusted for operating temperature, the burst pressure would be l
3636 psi. The adjusted burst pressure meets Regulatory Guide 1.121 margins of 1.4 times the l
MSLB accident pressure differential but does not satisfy the more limiting RG 1.121 margin of L
3 times normal operating pressure differential for Unit 1.
SNC evaluated the leak and burst test results and found the inclusion of the additional data from the destructive examination will not impart significant changes to the burst pressure and probability of leak portions of the ARC database; that is, the intercept, slope and various other regression parameters are only modestly changed. However, the data from tube R2C85 did i
significantly affect the leak rate portion of the ARC database. The measured leakage was larger than expected and, thus, increased both the mean leak rate value and the associated standard deviation. SNC concluded that the Unit 1 tube pull results needed to be incorporated into the database immediately, and subsequently submitted to the staff on September 17,1997, l
a reevaluation of the EOC-15 conditions using the updated database. SNC reported an EOC-15 conditional probability of burst value of 1.2 x 102 and an accident leak rate value of 20.4 gpm. SNC performed a safety assessment and concluded that a conditional probability of burst of 1.2 x 10-8 was not safety significant. SNC adoressed the increased leak rate value by l
administratively placing a lower limit on the dose equivalent iodine level and submitting a license amendment request to incorporate the reduced dose equivalent iodine levels. The staff
- approved the request on October ig,1997, effectively increasing the site allowable leak rate j
limit from 8.2 gpm to 23.8 gpm (room temperature conditions).
]
Concerns with the potentially high growth rate of deplugged tubes, SNC's nonconservative prediction of the EOC-14 maximum voltage indication and the limited amount of margin L
available between the predicted EOC-15 accident leak rate of 20.4 gpm and the maximum.
I allowable leak rate limit of 23.8 gpm prompted the staff to perform an additional assessment of j
l the EOC-15 accident leak rate at Unit 1. For the staff's assessment, the updated database was
]
used that included the most recent tube pull results from both Farley units (including the tube pull results discussed in this review). The staff also considered that a leak rate correlation l
l
' exists based on an acceptable one-sided p-test associated with the updated database. Using i
these inputs, the staff predicted an EOC-15 accident leak rate value of 5.8 gpm. This value is j
. well below the current site allowable leak rate limit of 23.8 gpm (room temperature conditions).
e
(.
. References
- 1. "Farley Unit 11997 Attemate Repair Criteria 90-day Report," Westinghouse Electric Corporation, SG-97-08-004, August 1997.
- 2. Letter from J. l. Zimmerman (NRC) to D. N. Morey (SN), " Issuance of Amendment - Joseph i
M. Farley Nuclear Plant, Unit 1 (TAC NO. M97510)," dated March 24,1997.
l
- 3. Generic Letter 95-05, " Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking," August 3,1995.
. 4. Letter from B. W. Sheron (NRC) to A. Marion (NEI) dated February 9,1996.
- 5. "Farley Unit 21996 Alternate Repair Criteria 90-day Report," Westinghouse Electric Corporation, SG-97-03-001, March 1997, i
o 1
l i
l
REVIEW OF THE FARLEY UNIT 2 STEAM GENERATOR 90-DAY REPORT in a letter dated March 7,1997, Southem Nuclear Operating Company, Inc. (SNC) submitted its l
steam generator (SG) 90-day report, "Farley Unit 21996 Altemate Repair Criteria 90-day Report" [ Reference 1]. The staff reviewed the submittal using criteria from References 2 and 3 and found SNC's assessment to be acceptable.
l 1.0 GENERAL PLANT DESCRIPTION Joseph M. Farley Nuclear Plant Unit 2, has three Westinghouse model 51 SGs with 7/8-inch diameter tubes. During the last refueling outage at the end-of-cycle 11 (EOC-11), SNC implemented 2.0 voit Alternate Repair Criteria (ARC) to be applied to outside diameter stress corrosion cracking (ODSCC) at the tube support plate (TSP) intersections. References 2 and 3 describe the 2.0 volt ARC methodology in more detail.
SNC used a lower repair limit of 2.0 volt and determined an upper voltage repair limit of 5.6 volt L
to disposition ODSCC at TSP intersections. SNC left in service indications with bobbin coil voltages less than or equal to 2.0 volt. SNC removed from service indications with bobbin coil i
voltages greater than 5.6 volt and also indications with bobbin coil voltages between 2.0 and 5.6 volt if confirmed with a rotating pancake coil (RPC) probe.
I 2.0 STEAM GENERATOR TUBE EDDY CURRENT INSPECT!ON SCOPE AND RESULTS SNC inspected 100 percent of the Unit 2 SG tubes full length using a 0.720-inch diameter bobbin coil probe at all intersections at which the 2.0 volt ARC were applied. SNC used an RPC probe to inspect 100 percent of the indications with bobbin coil voltages greater than 2.0 voit in SGs "A" and "B" and 100 percent of the indications with bobbin coil voltages above 1.5 volt in SG "C." SNC reported a total of 50 indications with bobbin coil voltages that exceeded the 2.0 volt criteria; 39 were confirmed with the RPC probe and subsequently plugged. SNC also returned four deplugged tubes to service based on the 2.0 volt repair
~~ criteria.
SNC reported a total of 411 ODSCC indications at TSP intersections and retumed 357 indications to service at Unit 2 (not including the four deplugged tubes). Of the 54 indications removed from service,15 indications were in tubes plugged for degradation mechanisms other l
than ODSCC at the TSPs. The remaining 39 indications were above the 2.0 volt limit and were confirmed with the RPC probe. One of the 39 indications was above the upper voltage limit of 5.6 volt (at 6.8 volt).
l SNC has two categories for eddy current indications at the TSPs. Most are called " potential indications" and SNC dispositions those indications in accordance with the 2.0 voM ARC. A l
small group are called " unusual outside diameter phase angles" (UOAs). UOAs are eddy l
_ current indications with very large outside diameter phase angles (less than 1 percent depth).
SNC does not consider them to be flaws; thus, tubes with UOAs are allowed to remain in L
i
- service. SNC retumed 107 tubes with UOAs to service for cycle 12. Of those,27 had bobbin coil voltages greater than 2.0 volt. The largest voltage associated with a UOA was 3.8 volt.
j The staff concludes that SNC's bobbin and RPC probe inspections were consistent with the guidance in References 2 and 3 and, thus, are acceptable. Based on its current understanding of the nature of the indications classified as UOAs, the staff does not accept SNC's conclusion that these flaws are not degradation. In a Request for Additional Information (RAI) letter dated February 5,1998, the staff requested that SNC provide the basis for not considering UOAs to be a form of degra@cn (e.g., historical reviews of eddy current data, RPC probe inspection results, tube pulls Wnrming no degradation). The staff requested SNC respond to the issue in the next Unit 2 90-day report, which is expected in the fall of 1998.
3.0 COMPARISON BETWFFN ACTUAL AND PREDICTED EOC-11 CONDITIONAL EBOBABILITY OF BURST AND TOTAL i FAK RATE UNDER POSTULATED MAIN STEAMLINE BREAK CONDITIONS in Reference 1, SNC compared the actual EOC-11 bobbin voltage distributions with the corresponding predictions for the EOC-11 that were performed at the EOC-10. In general, the actual distribution of voltages was similar to the predicted distribution of voltages. SNC slightly over predicted the total number of indications for all three SGs. However, SNC significantly under predicted the number of indications greater than 2.0 volt as well as the actual maximum voltage value for SG "C," the limiting SG.
SNC identified one reason for the underestimation: an increased growth rate during cycle 11 as compared to the previous two cycles. SNC postulated a sodium ingress during cycle 11 may have caused the increased growth rate for cycle 11.. However, when SNC recalculated the EOC-11 voltage distribution using the actual cycle 11 growth rate, SNC still underestimated the EOC-11 voltage distribution in that the number and size of the largest (i.e., greater than 2.0 volt) indications were not predicted for SG 'C' when compared to the actual EOC-11 voltage distribution. SNC concluded from this that there was an additional problem with the predictive methodology itself; at least as it applied to Unit 2.
SNC found that tubes that were deplugged at the EOC-10 and retumed to service for cycle 11 exhibited much higher growth rates than tubes that have always been in service. SG "C" had the greatest number of deplugged tubes in service in cycle 11. Because SNC applied a single growth rate distribution to all indications (i.e., SNC assumed growth rates for tubes that have always been in service would be applicable to deplugged tubes), SNC obtained a nonconservative prediction of the EOC-11 voltage distribution for the Unit 2 SG "C."
The under prediction of the EOC-11 voltage distribution for SG "C" affected the prediction of the limiting EOC-11 conditional probability of burst and accident leak rate. SNC calculated the conditional probability of burst and the total leak rate under postulated main steamline break (MSLB) conditions using the actual EOC-11 bobbin voltage distribution and then compared these values to those predicted for the EOC-11. The limiting conditional tube burst probability for one tube was 8.2 x 10d based on the actual EOC-11 voltage distribution compared with 1.24 x 10d based on the predicted EOC-11 voltage distribution. The limiting MSLB leak rate was 2.79 gallons per minute (gpm) based on the actual EOC-11 vc'tage distribution compared l
O
- with 1.76 gpm based on the predicted EOC-11 voltage distribution. Although SNC under i
predicted the limiting conditional probability of burst and leak rate for SG "C," the actual values
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in both cases were below the technical specification reporting threshold values of 1 x 10-8 for l
- the conditional probability of burst and 8.2 gpm (room temperature conditions) for the accident leak rate.
- 4.0 TUBE INTEGRITY EVALUATIONS FOR EOC-12 For the EOC-12 predictions of the voltage distribution 'of indications, SNC used the most limiting growth rates observed during the last two inspection cycles. More specifically, SNC used the cycle 11 SG "C" growth distribution modified io more heavily weight the growth rates associated with deplugged tubes to account for the higher growth rates associated with deplugged tubes.
l In the February 5,1998, RAI letter, the staff requested that SNC provide a benchmark of the i
weighted methodology in SNC's upcoming 90-day report for Unit 2, which is expected in the fall of 1998.
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 10-8 SNC performed this assessment using methodology previously approved by the NRC l
staff in Reference 2. SNC reported a limiting burst probability of 1.2 x 10-8, which is below the threshold value of 1 x 10-8 and is, therefore, acceptable. If SNC included UOAs in its calculations, the burst probability increases to 1.6 x 108, which is also acceptable because it is l
below the threshold value.
The predicted MSLB leak rate is calculated to ensure leakage from indications under worst -
l 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 and the Unit 2 site-specific limit. SNC performed this assessment using methodology previously approved by the NRC staff in Reference 2.
j SNC reported a iimiting MSLB leak rate of 5.1 gpm, which is below the site's maximum
' allowable leak rate of 8.2 gpm (room temperature conditions) and is, therefore, acceptable.' If SNC included UOAs in its calcuistions, the leak rate increases to 7.1 gpm, which is also below L
- the site's maximum allowable leak rate. The staff notes that subsequent to the submittal of the subject 90-day report, the staff approved a dose equivalent iodine reduction that increased the l
l site-specific leak rate limit at Unit 2 from 8.2 gpm to 23.8 gpm (room temperature conditions).
l Concoms with the high growth rate of deplugged tubes, the exclusions of tubes with UOAs and SNC's nonconservative predictions of the EOC-11 voltage distribution in SG "C" prompted the l
- staff to perform an additional assessment of the EOC-12 conditions at Unit 2. For the staffs assessment, an updated database was used that included the most recent tube pull results '
from both Fa.rley units. The staff included UOAs in its voltage population but did not include the growth rates for the UOAs because they were not available in the 90-day report. The staff also did not explicitly consider the higher growth rates of deplugged SG tubes. Most significantly, the staff considered that a leak rate correlation exists based on an acceptable one-sided p-test i
associated with the updated database. Using these inputs, the staff predicted an EOC-12 f
I 4
4.-
4 conditional probability of one or more tube burst at 2.76 x 10-8 and a leak rate value of 3.5 gpm.
These values are below the reporting threshold values previously discussed.
t 5.0 TUBE PULL RESULTS SNC removed one tube from SG *C" during the EOC-11 refueling outage to examine eddy
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current indications located at the TSP intersections. Tube R34C53 had a 6.8 volt indication at the first TSP; there was no detectable degradation at the other TSP intersections. SNC deplugged this tube at the EOC-10, and the ODSCC indication had a larger than normal bobbin probe voltage increase during cycle 11 operation. The metallurgical evaluation confirmed the nature of the eddy current indication to be axially oriented ODSCC.
SNC completed burst and leak rate testing. Leakage through the indication in tube R34C53 was measured at approximately 0.010 gallons / minute associated with accident conditions. This leak rate, even when combined with a calculated leak rate of 2.79 gpm, is below SNC's maximum allowable leak rate of 8.2 gpm. The adjusted burst pressure for tube R34C53 was 4628 psi. This burst pressure value is within Regulatory Guide 1.121 margins for Unit 2.
SNC evaluated the leak and burst test results, as previously discussed, and found the inclusion of the additional data will not impart significant changes in the ARC database; that is, the intercept, slope and various other regression parameters are only modestly changed. The Unit 2 tube pull results were to have been incorporated into the ARC database at the next update expected in the spring of 1998. In summary, the results of the Unit 2 tube pull appear to
. be consistent with the guidance in Reference 3. The metallurgical evaluation of the tube supported the continuing applicability of the voltage-based repair criteria to the SG tubes at Unit 2.
References
. 1.
"Farley Unit 21996 Altemate <tepair Criteria 90a$ay Report," Westinghouse Electric Corporation, SG-97-03-001, Iwarch 1997.
l-2.
Letter from J. l. Zimmerman (NRC) to D. N. Morey (SN), "!ssuance of Amendment -
.ioseph M. Farley Nuclear Plant, Unit 2 (TAC NO. M95146)," dated October 11,1996.
3.
Generic Letter 95-05, " Voltage-Based Repair Criteria for Westinghouse Steam Generator L
Tubes Affected by Outside Diameter Stress Corrosion Cracking," August 3,1995.
l
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