ML20198A907
| ML20198A907 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, Zion File:ZionSolutions icon.png |
| Issue date: | 12/29/1997 |
| From: | Hosmer J COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-97-01, GL-97-1, NUDOCS 9801060130 | |
| Download: ML20198A907 (15) | |
Text
Commom dth faison Company 14co Opus Place -
1A>wnm Grove, IL 60515 5701 Deccinber 29,1997 U.S. Nuclear Regulatory Commission Washington, DC 20555 Attention:
Document Control Desk
Subject:
Braidwood Station Units I and 2 Byron Station Units I and 2 Zion Station Units I and 2 NRC Docketdy_mbers: 50-29L304. 454. 455. 456 and 457 Commonwealth Edison Company (Comed) Supplemental Response to NRC Generic Letter 97 01, " Degradation of Control Rod Drive Mechanism None and Other Vessel Closure Head Penetrations" dated April I,1997
References:
1.
NRC Generic Letter 97-01, "Degradaticn of Control Rod Drive Mechanism Nonic and 01er Vessel Closure Head Penetrations" dated April 1,1997 2.
J. ISsmer letter to the Nuclear Regulatory Commission dated April 29,1997, transmitting Comed's 30 Day Response to GL 97-01 3.
J. Ilosmer letter to the Nu iur Regulatory Commission dated July 30, 1997, transmitting ComEdi 120 Day Response to GL 97-0I in Reference 1, the Nuclear Regulatory Commission (NRC) transmitted Generic Letter 97-01,
" Degradation of Control Rod Drive Mechanism Nonic and Other Vessel Closure Head Penetrations." References 2 and 3 transmitted the Commcawealth Edison Company's (Comed) 30 and 120 day response to the Generic Letter, respectively. In the 170 day response to GL 97-01, Question 1.4, Comed stated that more detailed evaluations for other vessel head penetrations (VHP), which in the Comed PWRs consist of the head vent would be providea by the end of 1997.
More detailed evaluations of the head vents for the Comed PWRs have tven ecmpleted and are provided in Attachment A of this letter (Dominion Engineering, Inc. (DEI) letter L-5061-01-08 to K. Moser, December 15,1997). As was the case for the Control Rod Drive Mechanism (CRDM) penetration evaluations previously submitted in Reference 3, Comed contracted with DEI through EPRI to perform a probabilistic assesament of the potential for primary water stress corrosion cracking (PWSCC) in Comed head vents as a function of plant life. DEI used the same methodology for the head vent evaluation as was used for the CRDM penetration cvaluation.
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NRC Document Control Desk 2-December 29,1997 The results of the head vent analysis (summarized in Attachment \\) indicate that a low probability (i.e., less than or equal to 5%) of PWSCC initiation for the Comed PWR head vents will not be exceeded before the Z2R19 refueling outage, currently projected to occur in 2005.
Comed considers the 5% probability of PWSCC initiation in the head vent to be a sufficiently conservative criterion to assure compliance with the 10 CFR 50 Appendix A Criterion 14 requirement that there be "an extremely low probability of abnormal leakage, of rapidly propagating failure, and of gross rupture."
In Comed's 120 day response to GL 97-01 (Reference 3), we committed to perform volumetric examinations of all Zion Unit 1 CRDM nozzles and other VHPs (i.e. the head vent) during ZlR17, curren1y scheduled for the year 2001. However, based on the results of the assessment provided in Attachment A to this letter, the time to reach a 5% probability of PWSCC initiation for all Comed PWR head vents is substantially beyond the aate for these Zion Unit I examinations. For this reason, examinations of Zion Unit 2, Braidwood Units I and 2, and Byron Units 1 and 2 will not be scheduled until a periodic engineering assessment of all industry data indicates that examinations are appropriate.
Comed is a participant in the WOG RPV head penetration integrated inspection program. In addition to the WOG integrated inspection program, all three PWR Owners Croups, EPRI, and NEl are ceoperatively working to compile and compare information on the estimated operating time from January 1,1997 that is needed to initiate and grow a crack 75% throughwall in PWR CRDM penetrations. As a result of the Combustion Engineering Owners Group schedule for performing the assessment needed as input to the PWR industry integrated program, the evaluation is expected to be completed by the end of the first quarter of 1998 rather than the end of 1997. The results of this industry prograin will be provided to the NRC in a supplement to this Generic Letter response when it becomes available, if you have any questions concerning this correspondence, please contact this office.
Sincerely, h r m 4 BA ohn B. Hosmer Engineering Vice President Attachment cc:
Byron /Braidwood Proj :ct Manager - NRR Zion Project Manager - NRR Senior Resident Inspector - Braidwood Senior Residen* :nspector - Byron Senior Resident Inspector - Zion Regional Administrator - Rill Office of Nuclear Safety - IDNS K: generic \\gl\\9701120s A
r ATTACHMENT A Dominion Engineering, Inc.
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' DOMINION ENGINEERING, INC.
December 15,1997 L-5061-01-08 Mr. Keith Moser Commonwealth Edison Zion Plant 101 Shiloh Boulevard Zion,IL 60099
Subject:
Head Vent PWSCC Etaluations for Zion Units 1 & 2, Byron Units 1 & 2, and Braldwood Units 1 & 2
. Attachments: (1) Comed Head Vent PWSCC Predictions
Dear Mr. Moser:
The purpose of this letter is to report the results of the head vent nozzle PWSCC crack
'n'tiation prec ictions for your,lants in support of your GL 97-01 submittal to the NRC. Head
- sent nozies were not covered )y the CRDM nozzle PWSCC predictions presented in DEI Letter L-5061-01-03, dated July 10,1997.
The analysis methods for calculating nozzle stresses and time to crack initiation and the results of these analyses for the Comed units are presented in detail in Attachment 1. Our analyses show that:
The PWSCC susceptibility of the Byron and Braidwood head vent nozzles is predicted to be very low due to low head temperature and moderate nozzle material yield strength. The time to PWSCC crack initiation for the Byron and Braidwood units is predicted to be much longer than the current license period.
The PWSCC susceptibility for the Zion Unit I head vent nozzle is moderate due to low material yield strength, moderate (590.8'F) head temperature, and moderate weld areas.- The Zion Umt I nozzle is predicted to be more susce,tible than the Byron and Braidwood nozzles due to higher head. temperature, slightly arger welds, and the use of SB-166 material, which is more susceptib:e to PWSCC than the SB-167 material used in the other five plants. The time to crack mitiation for Zion Unit 1, assuming a conservative probability of cracking of 5%, is predicted to be 23.8 effective full power years (EFPY), which is estimated to be 1.5 EFPY before the end of the expected current license period. Based on
- the predicted future outage schedule in DEI Letter L-5061-01-03, the refueling outage just preceding me predicted time of 5% probability of cracking would be Outage 23 in 2010.
. Zion Unit 2 is predicted to be the most susceptible of the six Comed units due to moderate head temperature and moderate material yield strength. Assuming a conservative probability of 5%, the time to crack initiation for Zion Unit 2-is predicted to be 20.6 EFPY, approximately five EFPY before the end of the expected curr" license period. Based on 6862 ELM STREET
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- the predicted feae outage schedule in DEI Letter L-5061-01-03, the refueling outage just
. preceding the predicted time of 5% probability of cracking would be Outage 19 in 2005.
In summary, the time to PWSCC crack initiation for the Byron and Braidwood units, with more than 95% confidence, is predicted to be greater than the expected current license period.
The time to PWSCC crack initiation for the Zion units, assumin ; a pessimistic 5% probability of cracking, is predicted to be at worst five EFPY before the enc of the expected current license pc.iod.
Please feel free to call me or Steve llunt if you have any questions.
Sincerely, I
,,;,gy Glenn White cc: Chron File 50 61 Project File Thomas Sp,ry, Comed Raj Pathama, EPRI a
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DEI Letter L-5061-01 1 l
Head Vent PWSCC Predictions Commonwealth Edison.
Zion Units 1 & 2 Byren Units 1 & 2 Braidwood Units 1 & 2 i
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Head Veat PWSCC Predictions for Commonwealth Edison Zion Units 1 & 2. Hvron Units 1 & 2. and Braidwood Units 1 & 2 The purpose of this document is to report the results of calculations of the time to PWSCC initiation for the Comed plant reactor vessel head vent nozzles, l.
Husj Vent Nozzle Dimensiqns Closure bad assembly drawings for the two Zion units (Figure 1) give the head vents as 3/4" Schedule 80 pipe. The pipe si i for the Byron and Braidwood head vents are shown in closure head assembly and vent pipe detail drawings (Figure 2) to be 1" Schedule 160 pipe.
The weld areas for the Zion units were computed from the dimensions shown on the nozzle weld machining drawings (Figure 3). Detailed CAD drawings of the Zion head vent nozzle welds were.constmeted using the dimensions shown.
The nozzle dimensions were detennined from supplemental drawing detail "A" (Figure 4).
For the Byron and Braidwood units, the weld areas were computed from dimensions shown on the nozzle weld machining drawings (Figure 5) along with the known OD and ID of the no-zle.
2.
Head Vent Nozzle Oncratingssndition Hoon Stresses DEI has performed elastic plastic fini'e element (FEA) stress analyses for a wide range of head vent nozzle geometries and weld areas. The range of cases analyzed bounds all known head vent nozzle and weld geometries in domestic PWP plants. An algorithm was developed based on the results of these FEA analyses, to predict maximtun inside surface hoop stresses and mid-wall hoop stresses as a function of average uphid and downhill cross-sectional weld areas and material yield strength. The maximum inside surface hoop stress was determined for each Comed head vent nozzle using this algorithm, the reported material yield strength and the weld areas discussed above.
- 3.
Calculation of Relative Susceptibility Factors and Time to Crack Initiation Susceptibility of materials to PWSCC is determined by comparing the subject nozzle to similar nozzles with known cracking. The Relative Susceptibility Factor (RSF) quantifies the susceptibility of the subject head vem nozzle material to PWSCC compared to a 1
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DOMINION ENGINEERING,INC.
Attxhment l reference nonJe with known operating temperature and inside sittface hoop stress which is located in a head which has nonjes which have experienced PWSCC. A head vent nonJe with an RSF of 1.0 would be predicted to have identical time to PWSCC initiation as the rc ference nozzle, while a head vent nonJe with an RSF ofless thr.n one would be predicted to have a longer time to PWSCC initiation than the reference nonJe. The RSF is calculated as follows:
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exp R
< %1 where owp,,up, is the maximum inside surface hoop stress for the subject head vent nozzle, a.pyr is the maximum inside surface hoop stress for the reference nozzle, Qma is n
the activetion energy for crack initiation of 50.0 kcal/mol, R is the gas constant of 1.103 E-3 kcal/mol *R, T,e is the temperature of the subject head vent nozzle (*R), and Tur is the temperature of the reference nozzle ("R).
The reference nozzle for each plant was selected such that differences in material fabrication and microstructure relative to the reference nozzle are reduced. The October 1994 Oconee Unit 2 CRDM nozzle inspections were selected as the crack initiation reference for all units except Zion Unit I since these head vents, like the Oconee 2 nozzles, were fabricated fmm SB 167 material. The selected crack initiation reference for Zion Unit I was DC Cook Unit 2 CRDM nozzle inspections since the Zion Unit I head vent nozzle was fabricated from SB-166 material as was the case for the DC Cook 2 nozzles.
SB-166 materials are predicted to be somewhat more susceptible to PWSCC than SB-167 materials, all other conditions being the same.
Using the RSFs, and a reference time to cracking for the reference plants, the time to crack initiation for various probability levels is determined through Weibull statistics as described in DEI Letter L-5057-00-1 (dated July 22,1997) and EPRI report TR-103696 "PWSCC of Alloy 600 Materials in PWP Primary System Penetrations." Table I shows the predicted times to crack initiation for 5%,16%, and 50% probability levels for each of the six Comed plant head vent nozzles.
4.
Discussion of Crack Initiation Predictions The PWSCC susceptibility of the Byron and Braidwood units is predicted to be very low based on low vessel head temperatures and moderate nozzle material yield strengths. The 2
D.OMINION EhGINEERING,INC.
del Letter L 5061-0108 Attachment I time to crack initiation for all probability levels is predicted to be much longer than the current license period at Byron and Braidwood.
The PWGCC susceptibility for Zion Unit 1 is moderate due to the low yield strength, moderate head temperature, and moderate weld areas which help to offset the expected increase in susceptibility from using DC Cook 2 SB-166 CRDM nozzle material as the crack initiation reference. The predicted time to crack initiation, assuming a conservative probability of 5%, is 23.8 EFPY, which is estimated to be 1.5 EFPY before the end of the expected current license period. Based on the predicted future outage schedule for Zion 1 in DEI Letter L-5061-01-03, the refueling outage just preceding the predicted time of 5%
probability of cracking would be Outage 23 in 2010.
The Zion Unit 2 head vent nozzle is predicted to be the most susceptible of the six Comed units even though it is fabricated froni SB-167 material. This higher PWSCC susceptibility results from a moderate material yield strength and moderate head temperature. Assuming a conservative probability of 5%, the time to crack initiation for Zion Unit 2 is predicted to be 20.6 EFPY, approximately five EFPY before the end of the expected current license period.
Based on the predicted future outage schedule for Zion 2 ir. DEI Letter L-5061-01-03, the refueling outage just preceding the predicted time of 5% probability of cracking would be Outage 19 in 2005, 5.
Crack Growth and Allowable Flaw Denth Since the conservative predicted crack initiation times for all six Comed plants are more than seven years from now, calculations were not performed to determine the crack growth rates and times to reach allowaole flaw depths, t
3
DONilNION ENGINEERING,INC.
- DEIIstter L 5061-01-08 Table 1. RPV Head Vent Penetration PWSCC Evaluation Commonwealth Edison PWR Units Predicted Time of Crack Initiation in EFPYs' Unit 5% Prob.
16% Prob.
50% Prob.
Zion 1 23.8 36 57 Zion 2 20.6 31 49 Braidwood 1 150 230 370 Braidwood 2 190 280 450 B> Ton 1 210 320 500 Byron 2 200 300 480
- Total Effective Full Power Years of operation. Braidwood and Byron EFPYs are modified to account for some differences in early head temperature.
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