ML092510065

Summary of Post-Submittal Public Meeting with Exelon Generation Company, LLC, Regarding Byron Station, Unit No. 2, Relief Request for Alternate Examination Frequency Under ASME Code Case N-729-1 for Reactor Vessel Head Penetration Welds

ML092510065
Person / Time
Site:	Byron
Issue date:	09/08/2009
From:	David M Plant Licensing Branch III
To:	Exelon Generation Co
Mahoney, M NRR/DORL/LPLIII- 2 415-3867
References
TAC ME1066
Download: ML092510065 (52)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 September 8, 2009 LICENSEE: Exelon Generation Company, LLC FACILITY: Byron Station, Unit NO.2

SUMMARY

OF SEPTEMBER 2,2009, POST-SUBMITTAL PUBLIC MEETING WITH EXELON GENERATION COMPANY, LLC, TO DISCUSS BYRON STATION, UNIT NO.2, RELIEF REQUEST FOR ALTERNATE EXAMINATION FREQUENCY UNDER ASME CODE CASE N-729-1 FOR REACTOR VESSEL HEAD PENETRATION WELDS (TAC NO. ME1066)

On September 2,2009, a Category 1 public meeting was held between the U.S. Nuclear Regulatory Commission (NRC) and representatives of Exelon Generation Company, LLC (the licensee) at the NRC Headquarters, One White Flint North, 11555 Rockville Pike, Rockville, Maryland. The purpose of the meeting was to discuss Byron Station (Byron), Unit No.2, Relief Request (RR) 13R-16, submitted for NRC staff review on April 2, 2009, for an alternative examination frequency under American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Code Case N-729-1 for reactor vessel head penetration welds. A list of attendees is provided as Enclosure 1.

The licensee's presentation (See Enclosure 2) elaborated on the technical justification submitted in the April 2, 2009, RR. The presentation provided background on the issue, inspection results, industry operating experience, boat sample results of a flaw found in a Byron, Unit NO.2 reactor vessel head penetration (Penetration 68), growth projections for the flaw, and a probabilistic assessment pertaining to the RR. The licensee also discussed the uniqueness of the occurrence of primary water stress-corrosion cracking in Penetration 68 based on the inspection results and boat sample evaluations, in support of the requested proposed alternative inspection frequency.

Members of the public were not in attendance. Public Meeting Feedback forms were not received.

-2 Please direct any inquiries to me at 301-415-1547, or marshall.david@nrc.gov.

Sincerely,

/t;hf{;

Marshall David, Project anager Plant Licensing Branch 111-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-455

Enclosures:

1. List of Attendees

2. Licensee Handout cc w/encl: Distribution via ListServ

LIST OF ATTENDEES SEPTEMBER 2,2009, MEETING WITH EXELON GENERATION COMPANY, LLC, TO DISCUSS BYRON STATION, UNIT NO.2, RELIEF REQUEST FOR ALTERNATE EXAMINATION FREQUENCY UNDER ASME CODE CASE N-729-1 FOR REACTOR VESSEL HEAD PENETRATION WELDS Name Affiliation Phone S. Campbell NRC/NRRIDORL 301-415-2466 M. David NRC/NRRIDORL 301-415-1547 M. Mahoney NRC/NRRIDORL 301-415-3867 E. Sullivan NRC/NRR/ADESIDCI 301-415-2796 R. Hardies NRC/NRR/ADES/DCI 301-415-5802 T. Chan NRC/NRR/ADESIDCI/CPNB 301-415-2768 J. Tsao NRC/NRR/ADES/DCI/CPNB 301-415-2702 J. Collins NRC/NRR/ADES/DCI/CPNB 301-415-4038 I K. Hoffman NRC/NRR/ADESIDCI/CPNB 301-415-1294 A. Rezai NRC/NRR/ADES/DCI/CPNB 301-415-1328 J. Wallace NRC/NRR/ADESIDCI/CPNB 301-415-3135 S. Greenlee Exelon/Byron Engineering Director 815-406-3800 R. Gesior, Exelon/Corp En9....E!"ograms Director 630-657-3801 R. Hall Exelon/Corp Asset MQmt Senior Manager 630-657-3296 P. Simpson Exelon/Manager - Licensing 630-657-2823 J. Cirilli Exelon/Senior Staff Engr - Corp Asset Mgmt 610-765-5966 G.DeBoo Exelon/Senior Staff Engr - Corp Asset Mgmt 630-657-3828 S. Koernschild Exelon/Senior Staff Eng Analyst - Byron Pro~ng 815-406-4006 L. Schofield Exelon/Senior Engr - Licensing 630-657-2815 ~

W. Bamford Westinghouse 412-374-5858 I B. Bishop Westinghouse 412-374-4636 J. Brihmadesam Westinghouse 662-320-3944 J. Lareau Westinghouse 860-731-1605 Enclosure 1

Exelon@

Nuclear Exelon Nuclear Post Subl11ittal Meeting Proposed Relief Request from ASME Code Case N-729-1 Inspection Frequency Byron Station Unit 2 September 2, 2009 Enclosure 2

Exelon@

Nuclear Agenda Introduction - Patrick Simpson Background and Inspection Results - Scot Greenlee Industry Operating Experience and Boat Sample Results - Jim Cirilli Growth Projections and Probabilistic Assessment - Guy DeBoo Closing Remarks - Scot Greenlee

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Nuclear Meeting Purpose

./' Present technical basis for Exelon Nuclear's Byron 2 Relief Request for alternate examination frequency under ASME Code Case N-729-1

./' Obtain NRC feedback

Exelon8 Nuclear Technical Basis Conclusions

~ Byron 2 reactor pressure vessel (RPV) head Penetration 68 flaw required welding defects, present from fabrication, to initiate primary water stress corrosion cracking (PWSCC) in the tube

~ PWSCC growth studies determined a minimum of 9 years or 6 fuel cycles is needed for a postulated flaw like the one found in Penetration 68 to initiate a leak path

~ The proposed inspection technique and frequency is sufficient to detect flaws prior to initiating a leak path

~ 2008 head inspections demonstrate no additional PWSCC in Byron 2

~ No similar issues identified in Byron 1 or Braidwood 1 & 2

Exelone Nuclear Request for Relief

~ Relief requested with a proposed alternative inspection frequency based on the uniqueness of the occurrence of PWSCC in Penetration 68, specifically:

• Perform volumetric and/or surface examinations of all penetrations as identified by Table 1 of ASME Code Case N-729-1 at a frequency of once every 4th refueling outage or 6 calendar years, whichever is less

• Except for Penetration 68, which will be volumetrically and/or surface examined each refueling outage

• In addition, bare metal visual (BMV) examinations of the RPV head will occur every 3 rd refueling outage or 5 calendar years, whichever is less

~ Approval is requested before April 2, 2010 outage

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Nuclear Byron 2 Background

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./ Westinghouse 4-loop NSSS design

• RPV head penetration nozzles were provided by B&W Tubular Products

• RPV head fabrication and penetration nozzle installation were by B&W

./ Commercial operation - August 1987

./ T-Cold Head <<550°F)

• 2.2 Effective Degradation Years (EDY) at time of 2007 inspection

./ Prior to the Spring 2007 refueling outage (B2R13), BMV examinations were performed in:

• Fall 2002 (B2R10)

• Fall 2005 (B2R12)

Exelon@

Nuclear Spring 2007 Byron 2 Inspection Res y" Penetration 68 results - Spring 2007

• 100% volumetric ultrasonic testing (UT)

• UT exam revealed 500/0 through-wall axial indication -0.52" long

• Subsequent dye penetrant (PT) exam of J-groove weld identified one rounded and one linear indication

• Leak path assessment did not detect any leaks

Exelone Nuclear Fall 2008 Inspection at Byron 2

~ Inspected in Fall 2008 (B2R14)

~ In accordance with NRC Order EA-03-009, volumetric examination of all 79 penetrations

• Leak path assessment of the RPV penetration to RPV low-alloy steel annulus

• Surface examination (dye penetrant) examination of the Penetration 68 weld overlay

• 100 % BMV of the external RPV head surface

~ No indications

Exelone Nuclear Summary

~ Byron and Braidwood Stations - Overall Inspection Results

• 100 % BMV - no indications in Byron 1 or Braidwood 1 & 2 and no additional indications in Byron 2

• 100 % Volumetric Inspection - no indications in Byron 1 or Braidwood 1 & 2 and no additional indications in Byron 2

- UT and eddy current exam methods employed

./ Inspection results support uniqueness of Byron 2 RPV head Penetration 68 indication

./ Industry operating experience (international and domestic) also supports conclusions

Exelon@

Nuclear Industry Operating Experience and Boat Sample Results Jim Cirilli Senior Staff Engineer Corporate Asset Management

Exelon~

Nuclear Industry Operating Experience

./ Domestic Operating Experience

• All RPV upper heads inspected (hot & cold)

- Represents over 3000 cold head penetrations inspected

- No defects found in cold heads o Byron 2 Penetration 68 is the only domestic cold head penetration with PWSCC

• PWSCC flaws found in high temperature upper heads

• Greater than 1000 RPV bottom mounted penetrations inspected

• Operating experience related to weld defects

- Fall 2008 - SONGS 3 Penetration 64 indication found in embedded repair weld metal

- South Texas Project bottom mounted penetrations - Initiated from pre-existing weld defect, similar to Byron 2 Penetration 68

Exelon8 Nuclear Industry Operating Experience (continued)

./ International Operating Experience

• EdF reported cracks in CRDM nozzles with first at Bugey 3 in 1991

• Zorita (Spain) reported intergranular cracking due to high sulfate levels in 1994 (not PWSCC)

• OH I 3 CRDM indication (2004) - surface preparation issue

• Greater than 1600 bottom mounted penetrations inspected

./ Sources: EPRI, Westinghouse, AREVA

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Nuclear CRDM 68 PT Results - Before Boat Sample

Exelon8 Nuclear CRDM 68 PT Results - After Boat Sample

~ Subsurface linear defect is connected to the lack-of-fusion defect

~ Evidence indicates a high probability that the rounded PT indication not captured by the boat sample was connected below the surface to the lack-of fusion defect

~ A surface flaw the size of the rounded indication would have been considered acceptable by ASME Code of fabrication for Byron Station

~ Heavy grinding in this area may

1. Rounded PT indication have masked the indication

2. Subsurface portion of axial indication during fabrication exams 3. Subsurface linear defect connected to lack of fusion

Exelone Nuclear Boat Sample Results - Summary Cut surface of boat sam

./ Rounded subsurface defect captured by the boat sample identified as lack of fusion between the weld and tube surfaces

• Incipient cracks were observed emanating from the defect

• Weld defect produced during original fabrication process

./ Linear indication exhibited multiple defecUcrack morphologies including lack of fusion, hot cracking, and PWSCC

• In the weld, the direction of PWSCC propagation was from the subsurface location toward the wetted surface

• In the tube material, none of the PWSCC was connected to the outer surface of the tube below the J-groove and/or fillet weld SEM Image of inset area above

Exelon@

Nuclear Section C

./ Section C - Mounted along vertical cut face

• Boat sample contains tube material and last two weld passes

• Composition of Alloy 600 tube material and Alloy 182 weld metal consistent with specifications

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Exelone Nuclear Section B -- Metallography I ..

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.~B' 100 urn Intergranular Cracking in Tube Metal Intergranular Cracking (Unetched) in Tube and Weld Metals (Electrolytic Phosphoric-Nital Dual Etch)

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Intergranular Cracking in Tube Metal Susceptibility of Alloy 600 MA Tube Material (Electrolytic Phosphoric-Nital Dual Etch) (Electrolytic Phosphoric Etch)

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Exelone Nuclear Section A 1 -- Crack Surface Ductile Tearing at Wetted Surface Hot Cracking in Weld

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Nuclear Section A2 -- Cut Surface Cut Line Located Adjacent to Rounded Section A2A Ground to Indication on EDM Surface Reveal Rounded Indication

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Nuclear Section A2A .. Weld Defect Lack-of-Fusion Defect (Electrolytic Phosphoric-Nital Dual Etch)

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Exelon8 Nuclear Section A2A .. Defect and Inclusion

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./ Inclusions contained Ti, N, and 0

./ Crevice contained oxidized W, Fe, Ni, Cr, and Nb

./ Cracks contained Inconel 182 oxidation products

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Nuclear Boat Sample Results

~ Rounded subsurface defect captured by the boat sample identified as lack of fusion between the weld and tube surfaces

• Incipient cracks were observed emanating from the defect

• Weld defect produced during original fabrication process

~ Linear indication exhibited multiple defecUcrack morphologies including lack of fusion, hot cracking, and PWSCC

• In the weld, the direction of PWSCC propagation was from the subsurface location toward the wetted surface

• In the tube material, none of the PWSCC was connected to the outer surface of the tube below the J-groove and/or fillet weld

~ No initiation in the penetration tube material

Exelon.,

Nuclear Suml11ary

./ Three elements must be present simultaneously for PWSCC initiation

• Susceptible Metallurgical Condition

- Susceptibility is related to grain boundary carbide coverage (GBCC)

- Penetration 68 Heat 80054 considered to have good GBCC - 29 other nozzles from the same heat have been inspected with no indications

• Tensile Stress

- Includes residual welding stresses and operating pressures

- Byron 2 Penetration 68 is not the location of highest stress

• Critical Corrosive Environment

- PWSCC has strong temperature dependence

- Below 570°F (as in Byron 2), PWSCC initiation and growth are very slow processes

./ Necessary conditions for the initiation of PWSCC would not have been simultaneously met without the presence of the original fabrication weld defects, which created a critical corrosive environment

./ PWSCC initiated at pre-existing weld defect, not penetration tube material

Exelon~

Nuclear Gro\Nth Projections and Probabilistic Assessl11ent Guy DeBoo Senior Staff Engineer Corporate Asset Management

Exelon~

Nuclear Probabilistic Assessment Probabilistic evaluations using industry inspection results with Weibull analyses and Monte Carlo simulations determined:

v' Probability of a 50% throughwaU crack occurring in Byron 2 after 20 years of service is three orders of magnitude below the probability expected for flaw initiation and growth due to typical PWSCC v' The observed flaw did not occur in the most susceptible Byron 2 penetration location (i.e., Penetration 72 is 4 to 6 times more likely to initiate a flaw) v' The flaw in Penetration 68 is not due to typical flaw initiation and growth by PWSCC in the Alloy 600 base metal

• Although fabrication weld defects may exist in other nozzles, Penetration 68 is the only cold head nozzle found with a flaw in the US

Exelon8 Nuclear Grovvth Projections

,/ Performed analyses to determine the PWSCC project growth rates for RPV head penetrations /

NOZZLE RV HEAD

• Tube 00 axial flaw growth studies for five RPV head -I penetration groups (0°, 25.4°, 42.8°, 43.8° and 47°)

• Growth based on operational plus weld residual stresses

• PWSCC growth rates per MRP-55 Rev 1 for Alloy 600 tube material BOTTOM -'1", I

• OF WELD

• Postulated initial flaw sized at the limit of UT detection, 0.075" by 0.150"

• Postulated flaw located at highest stressed locations on the uphill and downhill sides of the penetration

• Growth limited to the top of the J-groove weld where pressure boundary leak would initiate

Exelon@

Nuclear Gro'ftlth Projections (continued)

~ Byron 2 RPV head operating temperature for Cycle 14 indicates head temperature is typically 545°F Byron Unit 2 RVLlS TC #1 & #2 Channels A and B 560.0 I ' i I ~ -RVLlS UNHEATED JUNCTION TC #1 CH. A 'I

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~ Postulated flaw growth projections were based on 558°F

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Exelont>

Nuclear Gro\Nth Projections (continued) 1.E-09 ,-------- - - - ------- ---- ----- ------

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• PWSCC Growth @ 600°F

--PWSCC Growth @ 558°F

~- PWSCC Growth @ 545°F 1.E-10 I

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~ Using MRP-55 Rev 1 formulation growth rates for a typical hot head at 600°F, are compared to the evaluation temperature of 558°F and the typical Byron 2 head temperature of 545°F

~ At 30 ksi"in (33 MPa"m), margin factors of 1.4 from 545°F to 558°F and 4.4 from 545°F to 600°F are seen

Exelone Nuclear Gro\Nth Projections (continued)

ANSYS 5.7 APR 12 2003

../ Typical operating plus weld residual 05,59,10 PLCYr NO. J BLEMIlNT&

hoop stress field used for crack Po_rGraphicB BPACBI'=1 MAT N1lJII growth - RPV head 47° nozzle NODllL SOLUTIO.

T:I:ME=4004 (psi) SY RSY&=l1 po_rGraphicB (AVG)

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Exelon~

Nuclear Gro\Nth Projections (continued)

./ Postulated flaw initially 0.075" by 0.150" located in the tube outer surface at the center of the J-groove weld Table 5-1 Operating Time for a Postulated Axial Flaw at the J-groove Weld to Grow Its Limit Available Operating Window Nozzle Group & Location (Fuel Cvcles)1 0.0° Nozzle 7.30 25.4° Nozzle; Downhill 9.05 25.4° Nozzle; Uphill 6.06 42.8° Nozzle; Downhill 11.69 42.8° Nozzle; Uphill 6.37 43.8° Nozzle; Downhill 12.26 43.8° Nozzle; Uphill 6.42 47.0° Nozzle; Downhill 13.75 47.0° Nozzle; Uphill 6.67 Note 1. A fuel cycle was assumed to be 18 months with a 98% capacity factor.

Hot operating time conversion is 1.5 years/fuel cycle.

Exelon8 Nuclear Gro\Nth Projections (Axial Tube ID

./ PWSCC growth projections for an inside surface, axially oriented flaw on the uphill side at the J-groove weld with 0.8 initial depth of 0.075" and fixed aspect ratio of 6

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Figure 5-7: PWSCC Growth Projections for an Inside Surface, Axially Oriented Flaw on the Uphill Side at the J-groove Weld

Exelon8 Nuclear Gro\Nth Projections (CirculI1feren

~ PWSCC growth projections for an outside surface, circumferentially oriented 09

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Figure 5-8: PWSCC Growth Projections for an Outside Surface, Circumferentially Oriented Flaw on the Downhill Side Above the J-groove Weld

Exelon~

Nuclear Grovvth Projections (SuR1R1ary) v' After 4 fuel cycles or 6 years of operation postulated flaw sizes are significantly smaller than their structural and leakage size limits v' An inspection interval of 4 fuel cycles or 6 years of operation for Byron 2 (not every refueling outage) provides adequate time to detect and repair flaws prior to initiating a leak path through the tube

Exelone Nuclear Evaluation Conclusions

./ Byron 2 head penetration flaw required welding defects, present from fabrication, to initiate PWSCC in the tube

./ UT examinations demonstrate any potential flaws in other Byron 2 penetrations are less than the threshold of detection

./ PWSCC growth studies determined a minimum of 9 years or 6 fuel cycles is needed for a postulated flaw like the one found in Penetration 68 to initiate a leak path

./ An inspection frequency of 4 fuel cycles (6 years of hot operation) provides additional flaw detection margin prior to initiating a leak path through the tube

Exelon@

Nuclear Proposed Relief Request

~ Relief requested with a proposed alternative inspection frequency based on the uniqueness of the occurrence of PWSCC in Penetration 68, specifically:

• Perform volumetric and/or surface examinations of all penetrations as identified by Table 1 of ASME Code Case N-729-1 at a frequency of once every 4 th refueling outage or 6 hot operating years whichever is less

• Except for Penetration 68, which will be volumetrically and/or surface examined each refueling outage

• In addition, BMV examinations of the RPV head will occur every 3rd refueling outage or 5 calendar years, whichever is less

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Nuclear Closing Remarks

~ Byron 2 RPV head Penetration 68 indication is unique

• Inspection results

• Boat sample evaluations

~ 2008 head inspections demonstrate no additional PWSCC in Byron 2 and no similar issues in Byron 1 or Braidwood 1 & 2

~ Proposed inspection frequency appropriate for Byron 2 RPV head consistent with intent of CC N-729-1

ML092510065 NRC-001 OFFICE DORL/LPL3-2/PE DORL/LPL3-2/PM DORLlLPL3-2/LA DORL/LPL3-2/BC NAME MMahoney MDavid THarris SCampbell DATE 09/8/09 09/8/09 09/8/09 09/8/09