ML17059A417

Provides Response to GL 94-03, IGSCC of Core Shrouds at Bwrs. Results of Encl Safety Evaluation Determined That Continued Operation of Facility Justified Until Next Insp of Core Shroud.W/One Oversize Drawing

ML17059A417
Person / Time
Site:	Nine Mile Point
Issue date:	08/23/1994
From:	Terry C NIAGARA MOHAWK POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML17059A418	List: ML17059A417 ML18040A133
References
GL-94-03, GL-94-3, NMP2L-1489, NUDOCS 9408310284
Download: ML17059A417 (30)

Text

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ACCESSION NBR:9408310284 DOC.DATE: 94/08/23 NOTARIZED: NO DOCKET FACIL:50-410 Nine Mile Point Nuclear Station, Unit 2, Niagara Moha 05000410 AUTH.NAME AUTHOR AFFILIATION TERRY,C.D.

Niagara Mohawk Power Corp.

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~RECIP.NAME RECIPIENT AFFILIATION Document Control Branch (Document Control Desk)

SUBJECT:

Provides response to subject GL 94-03, Intergrannular Stress Corrosion Cracking Of Core Shrouds At Boiling Water Reactors.

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~0NeHA~K NIAGARAMOHAWKPOWER CORPORATION/301 PLAINFIELDROAD, SYRACUSE, N.Y. 13212/TELEPHONE (315) 474-1511 August 23, 1994 NMP2L 1489 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 Gentlemen:

RE:

Nine Mile Point Unit 2 Docket No. 50-410 NPF-6

Subject:

Response

to Generic letter 94-03, 1ntergrannular Stress Corrosion Cracking of Core Shrouds at Boiling Water Rectors This letter provides Niagara Mohawk's response to the subject generic letter for Nine Mile Point Unit 2.

Although the recommendations of GE Service Information Letter (SIL) 572, Rev. 1, did not require an inspection of the Nine Mile Point Unit 2 Core Shroud at this time, it was visually examined in October 1993.

No recordable indications were identified.

Based on the results of this examination, the response of the BWR Owners'roup to the Commission's generic questions, and the results of the safety analysis provided as Attachment 1 to this letter, Niagara Mohawk has determined that continued operation of Nine Mile Point Unit 2 is justified until the next inspection of the Core Shroud.

The next inspection of the Core Shroud willbe scheduled in accordance with the forthcoming BWRVIP inspection subcommittee recommendations, but no later than Refueling Outage Number 6 which is currently scheduled for Spring 1998.

As requested, included with this letter as Figure 1-1 of Attachment 1 is the drawing of the Core Shroud configuration for Nine Mile Point Unit 2 showing details of the Core Shroud geometry. provides the report of the previous Core Shroud inspection performed during the last refueling outage.

Niagara Mohawk willuse the guidance developed by the BWR Owner's Group to develop core shroud inspection and repair contingency plans.

As requested we willprovide the inspection plan for the core shroud three months prior to performing the next inspection.

Contingency plans for the evaluation and/or repair of the core shroud willalso be provided at that time.

The results of the inspection willbe submitted within thirty days from completion of the inspection.

Niagara Mohawk willcontinue to work closely with the BWRVIP on coordination of inspections, evaluations and repair options for all BWR internals susceptible to IGSCC.

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yours, C. D. Te Vice President Nuclear Engineering

Page 2 CDT/KBT/ksj Attachments xc:

Regional Administrator, Region I Mr. B. S. Norris, Senior Resident Inspector Mr. M. L. Boyle, Acting Director, Project Directorate I-l, NRR Mr. D. S. Brinkman, Senior Project Manager, NRR Records Management

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UNITED STATES NUCLEAR REGULATORY COIVIMISSION In the Matter of

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NIAGARAMOHAWKPOWER CORPORATION

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Nine Mile Point Nuclear Station Unit 2

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Docket No. 50%10 C. D. Terry, being duly sworn, states that he is Vice President, Nuclear Engineering of Niagara Mohawk Power Corporation; that he is authorized on the part of said Corporation to sign and file with the Nuclear Regulatory Commission the document attached hereto; and that the document is true and correct to the best of his knowledge, information, and belief.

NIAGARAMOHAWKPOWER CORPORATION By C. D. Terry Vice President - Nuclear Engineering Subscribed and sworn to before me, a Notary Public in d for the State of New York and the County of

, this M>

day of

, 1994.

otary Public in and for County, New York My Commission Expires:

BEVERLY W. RIPKA ttotary Public Stateof New vora Qual.in Oswego Co. Nc. 4Q4879 My Cornrnisslon Exp.~

ATTACHMENT1 RESPONSE TO GENERIC LETTER 94-03 SAFETY ANALYSISOF THE CORE SHROUD NENE MILEPOINT UNIT 2 DOCKET NO. 50-410 NPF-69

T Ie of ont nt PAGE 1 o0 INTRODUCTION o

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o 2.0 SUSCEPTIBILITYASSESSMENT..........................

3.0 PLANT-SPECIFIC ASSESSMENT..........................

5 4o0 CONCLUSION

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5.0 REFERENCES

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7 6.0 ATTACHMENTS................,...................

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1.0 INTRODUCTION

1.1 SUMMITRY Nine Mile Point Unit ¹2 is a GE BWR 5 which received a fullpower license in July 1987.

The Unit has approximately four (4) years of on line experience.

Niagara Mohawk Power Corporation has been an active participant in both the GE BWR Owner's Group (BWROG) and the BWR Vessel Internals Program (VIP). Nine Mile Point Unit ¹2 staff have kept abreast of the latest developments in the Core Shroud cracking issue.

During the period of October 21, 1993 through October 29, 1993 (RF-03), NMPC conducted an In Vessel Visual Inspection (IVVI)under the direction of General Electric Nuclear Energy, utilizing GE's inspection procedure and GE inspection equipment.

This inspection met the requirements of GE SIL No.

572 Revision 1 (Ref. 1) and the recommended areas of inspection obtained from GE by letter dated 15 October, 1993, (Ref. 2).

Subsequent to this inspection NMPC revised the Nine MilePoint Unit ¹2 Ten Year ISI Program in accordance with the GE SIL-572 Rev. 1.

The specified locations willbe examined after Nine Mile Point Unit ¹2 has accrued eight (8) Years of power operation.

Assuming 18 month refueling cycles and 2 month refueling outage durations with no unscheduled outages, it is anticipated that the unit would accrue those eight years of power operation in September 1998.

The next exam is anticipated for the first refueling in the Second Period of the Ten-Year ISI Plan, RF-06, Spring 1998.

This schedule and inspection scope is subject to change because of Niagara Mohawk Power Corporation's commitment to follow the direction provided for inspections and assessments from the BWRVIP.

1.2 PURPOSE The purpose of this safety assessment is to respond to Generic Letter GL 94-03 which requested a safety analysis supporting the continued operation of the facility until the unit is inspected.

1.3 SCOPE This specific shroud cracking safety assessment for Nine Mile Point Unit ¹2 makes reference to the BWR Shroud Cracking Generic Safety Assessment (Ref. 3) submitted to the NRC.

As requested in Generic Letter GL 94-03, this safety assessment includes a review of the Nine MilePoint Unit ¹2 conditions that influence the probability of occurrence of cracking and the rate of crack growth.

Based on this information the likelihood of shroud cracking in excess of the required structural margins is assessed and the uncertainty in the extent of cracking is reviewed.

The safety assessment identifies that Nine Mile Point Unit ¹2 is bounded by the generic probabilistic risk assessments.

In addition, the shroud response to design basis loads assuming 360 degree through wall cracking and the ability of the plant safety features to perform their design basis function was reviewed.

This safety assessment provides the basis for

the continued operation of the Nine Mile Point Unit ¹2 Nuclear Station considering the uncertainty in the extent of cracking of core shroud welds.

1.4 SHROUD FUNCTION AND WELD DESIGNATIONS The core shroud is a stainless steel cylindrical assembly that provides a partition between the core region and the downcomer annulus, to separate the upward flow of coolant through the core from the downward components, a eoolable core geometry and a floodable region following a postulated recirculation line break.

The shroud is not a primary pressure boundary component.

The following are the shroud weld designations for Nine Mile Point Unit ¹2 and are shown in Figure 1-1 (Attachment 1).

H1, H2: Upper welds, with H1 above and H2 below the Emergency Core Cooling System (ECCS) injection.

H3: Upper weld located below the bottom of the top guide support ring;.

H4, H5: Mid-plane welds located above the core plate.

H6: Weld located just above the core plate.

H7: Weld located just below the core plate.

H8: Lower cylinder weld.

2.0 SUSCEPTIBILITY ASSESSMENT 2.1 OVERVIEW The BWR Shroud Cracking Generic Safety Assessment (Ref. 3) provides a discussion of a discussion of the numerous factors which contribute to the susceptibility of a core shroud to stress corrosion cracking (SCC).

The susceptibility criteria applied in BWR Shroud Cracking Generic Safety Assessment (Ref. 3), SIL 0572 Rev.

1 and the BWROG BWR Core Shroud Evaluation (Rev. 4) are Water Chemistry, Material Carbon Content, Fabrication History, Neutron Fluence and Hot Operating Time.

The generic assessment recognized that factors such as the degree of cold work and weld residual stress are significant factors affecting susceptibility; however, since quantitative information was not available these factors were not included in the plant specific review of susceptibility.

A review of the plant specific construction records and core shroud visual inspection results are considered in this assessment.

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2.2 DETAILS OF CONDITIONS THATINFLUENCE CRACKS 2.2.1 Material Type The core shroud at Nine Mile Point Unit ¹2 was constructed in accordance with GE Purchase Specification (¹21A3319 Rev. 5) using Austenitic Stainless Steel ASME SA-240, Type 304L plates purchased in the solution heat treated condition (Ref. 5).

Other than very minor (approximately 1 inch vertical) crack lengths detected at Fermi-2, inspected L-grade shrouds with as much as 8.4 on-line years have shown no cracking.

Significant (but (180 degree) SCC in L-grade shrouds has been reported at two plants where the on-line time equalled or exceeded 10 years.

2.2.2 Form The shroud was fabricated by Sun Ship Building and Drydock Co. and shipped to the site in 1978.

Each cylinder is made from two plate segments formed and welded according to drawing requirements, 2.2.3 Water Chemistry l

Extensive SCC testing as well as theoretical modeling has shown that SCC initiation and growth are strongly dependent on water chemistry.

Nine Mile Point Unit ¹2 has maintained excellent reactor water chemistry during it's operation.

The unit has a 5 cycle conductivity of 0.129 micro seimens per cm., placing it second best in the fleet.

Per Table 3 (Response to NRC Questions) (Ref. 6) the value of conductivity above which more extensive cracking ()200 inches) was detected was 0.37 uS/cm.

Below a conductivity value of 0.22 uS/cm, cracking detected to date has been limited to less than 30 inches in length.

The generic assessment (1) identifies the mean conductivity for the first five cycles as a factor for susceptibility grouping.

Figure 2-1 (Attachment 2) provides the Nine Mile Point Unit ¹2 specific cycle mean conductivities.

This places Nine Mile Point Unit ¹2 in the lowered susceptibility category for SCC.

2.2.4 Fluence The estimated fast neutron fluence for Nine Mile Point Unit ¹2 as of December 1993 was 2.5e20 n/cm2.

This is below the IGSCC threshold value of 3e20.

Results from various plants inspected so far do not indicted that fluence is a primary contributor to extensive cracking.

However, fluence effect on cracking susceptibility is expected and has been considered in the overall shroud cracking evaluation (Ref. 7) ~

2.2.5 Carbon Content The Nine Mile Point Unit ¹2 shroud was constructed of 304L stainless steel material with the shroud and flange plate having carbon content of < 0.03%C.

The specific material carbon content is identified on drawing Figure 1-1 (Attachment 1).

Although cracking has been detected in core shroud material having a range of reported carbon contents from greater than 0.06% to as low as 0.023%, all incidents where cracking has exceeded 180 degrees of circumference have occurred in ring plate material with carbon content equal to or greater than 0.06%C.

This is consistent with current understanding of weld HAZ degree of sensitization and resultant expected IGSCC susceptibility.

Other than very minor (-1 inch) vertical crack lengths detected at Fermi-2, inspected L-grade shrouds with as much as 8.4 on-line years have shown no cracking.

Figure 2-2 of GENE-5230-A107P-0794, (Ref. 3), based on plant data available, provides a graph of extent of cracking verses on-line years of operation.

The plant indicates that cracking in excess of 180 degrees is unlikely until a plant accumulates 10 years of operation history.

2.2.6 Weld Materials Filler metal for welding the core shroud conformed to ASME SFA-5.9 ER308.

Welds contain a minimum of 5 percent ferrite (Ref. 5).

The Shroud Head flange and the Core Plant flange were manufactured from flame cut plate segments welded by the shielded metal arc welding process (SMAW) using E-308 electrodes with a carbon content of 0.08% max (Ref. 5).

2.2.7 Procedures Welding performed for the fabrication of the core shroud utilized the Submergener Arc Welding (SAW) process for shroud vertical and circumferential welds.

The Shielded Metal Arc Welding (SMAW) process was utilized for the flange segments (Ref. 5).

Weld details and joint geometry are provided on Figure 1-1 (Attach. 1).

2.2.8 Hot Operating Time In order to revise the Ten Year ISI Inspection Plan the effective on line time for Nine Mile Point Unit ¹2 was determined.

The estimate as of December 27, 1993 is given in the GE letter dated December 28, 1993 (Ref. 8).

Assuming 18 month refueling cycles with a 2 month refueling outage Nine MilePoint Unit ¹2 would accrue 8 years of power operating history in September 1998.

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3.0 PLANT-SPECIFIC ASSESSMENT The Generic Assessment (GENE-523-A107P-0794) (Ref. 3) discussed the structural margins inherent in the shroud design and noted that the 304L SS is a ductile material with high todghness properties even after accounting for the effects of neutron fluence and that only a minimal remaining ligament (5%-10% of wall) is required to maintain structural margins under post accident loads when 360 degree cracking is present.

The generic assessment applies an assumption that cracking is initiated after one fuel cycle and that crack growth can be estimated analytically using the (PLEDGE) model.

Evaluation of the Nine Mile Point Unit ¹2 parameters for operating history and mean conductivity demonstrate that the unit is bounded by the generic assessment conclusion that finding a 360 degree through wall crack with an average depth in excess of 90% before ten years of operation is unlikely. The safety consequences associated with a 360 degree through wall crack have been evaluated in the Generic Assessment (Ref. 3) for normal, transient and faulted conditions and Nine Mile Point Unit ¹2 is bounded.

Based on the analysis performed by GE for the NMP2 Core Shroud (GENE-523-147-1093)

(Ref. 9), screening criteria for flaw evaluation were developed.

Determination of effective flaw length was based on ASME Code,Section XI, Subarticle IWA-3300 (1986 Edition) proximity criteria.

Flaws for purposes of the analysis were presumed through wall. The determination of the allowable flaw size was based on both linear elastic fracture mechanics (LEFM) and limit load criteria.

Based on this evaluation the following criteria were established:

1)

For axial flaws, two neighboring flaws must be summed ifS(0.75 (Lleff+L2eff). If the longest resulting flaw is less than 63 inches, then the screening limitis met for axial flaws.

2)

For circumferential flaws, all flaws are summed in any 90 degree sector using a template.

The total flaw length in the 90 degree window must be less than 75 inches to meet the screening criteria.

The next step is the LEFM based comparison using the interaction criteria. IfS (0.75 (Lleff+L2eff),then the length L=L1eff+L2effis compared with the LEFM limitof 79 inches for circumferential flaws.

For the above evaluation, S =Distance between two indications and Lleff, L2eff+Effective flaw lengths.

3.1 INSPECTION RESULTS Niagara Mohawk Power Corporation contracted with GE and performed an inspection of the Nine Mile Point Unit ¹2 shroud during the last Refuel Outage (RF-03) October 1993.

Although NMP2 was not required by GE SIL NO. 572 Rev.

1 to inspect at that time, the inspections were conducted consistent with the recommendations of the SIL for areas of inspection identified by GE their letter dated October 15, 1993 (Ref. 2). No indications were found (Report 2-2.01-93-0453) (Ref. 10).

4.0 CONCLUSION

Nine MilePoint Unit ¹2 has been evaluated generically by General Electric as part of the BWROG work to respond to the Core Shroud Cracking Issue and by Unit specific evaluations to respond to SIL 572 Rev. 1.

Based on the IVVIexamination of the Nine Mile Point Unit ¹2 core shroud performed during RF-03, no significant cracking of the core shroud is anticipated in the near term.

Based on inspection results of 23 plants available to date when compared to the susceptibility grouping factors identified in Table 2-3, reference 3, the following conclusions are drawil; 1.

There is a clear distinction that 360 degree cracking can occur in 304 welded-plate shroud rings, but is extremely unlikely, in the near term, in shrouds with 304L materials; 2.

There is a tendency in welded-plate rings for cracking extent to be greater with higher early (e.g. first 5 cycles) conductivity. NMP2 has maintained excellent water chemistry during it's operation.

The conductivity maintained at NMP2 is the second best int he BWR fleet; and 3.

360 degree cracking has not occurred in any of the eight plants inspected with less than 9.5 years of hot operation, and is therefore unlikely at NMP2 for less that 9.5 years.

These evaluations demonstrate that the Unit can safely operate until the next core shroud inspection without undue risk to the public. NMPC willcontinue it's participation in industry groups and willimplement applicable recommendations for inspections and repairs (scope and schedule) as required.

5.0 REFERENCES

1)

GE Letter dated October 27, 1993 (BJB-9340), "Review of the Nine MilePoint Unit 2 Shroud Inspection" 2)

GE Letter dated October 15, 1993, "Inspection Recommendations for the Core Shroud at Nine Mile Point Unit 2" 3)

"BWR Shroud Cracking Generic Safety Assessment",

Rev. 1, (GENE-523-A107P-0794)

August, 1994.

4)

"BWR Core Shroud Evaluation (GENE-523-148-1193)"

5)

"Shroud Fabrication and Operational History Data Nine Mile Point 2", (GENE-771 1293) December, 1993.

6)

"Responses to NRC Questions on Core Shroud and Reactor Internals", (GENE-523-A114P-0894), August 1994.

7)

GE Letter dated January 4, 1994, "Estimated Fast Neutron Fluence for NMP2".

8)

GE Letter dated December 28, 1993, "Nine MilePoint Unit 2 Effective Full Power Days" (RJB 93-013),

9)

"NMP2 Core Shroud Analysis", (Gene-523-147-1093).

10)

"Report of In Vessel Examination Results", Report 2-2.01-93-0453.

6.0 ATTACHMENTS 1)

Figure 1-1, "Nine Mile Point 2 Shroud Data" Dwg No. 105E1347A.

2)

Figure 2-1, "Reactor Water Conductivity Mean Values"

Reactor Water Conductivity Mean Values 1.00 ~

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0.10 0

a OO Data Provided by Plants /Proprietary Information Not Verified for Design

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NSSS Shroud Cracking Reported NSSS wShroud Cracking Not Reported

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NMP2 Max INk Mean VaiueiCycle NMP2 Cycle Mean Values with Std. Dev.

20 Dec 93 0.0'l 0

Fuel Cycle

ATTA HMENT 2 RESPONSE TO GENERIC LETTER 94-03 CORE SHROUD EXAMINATIONRESULTS NINEMILEPOINT UNIT 2 DOCKET NO. 50-410 NPF-69 INTRODUCTION During the period of October 21, 1993 through October 29, 1993 GE Nuclear Energy personnel performed invessel visual inspections of the Nine Mile Point Unit 2 Core Shroud to meet the recommendations of GE Service Information Letter No. 572 Revision 1, "Core Shroud Cracks".

No other inspections of the shroud have been performed since initial startup of the reactor.

EXAMINATIONTECHNIQUES The examinations were performed using remote closed circuit television equipment, underwater lighting, camera mounted lighting, and VHS videotape recording equipment in accordance with GE-VT-203 Revision 0.

Straight lenses were used on hand held cameras.

Camera resolution was verified using both a 1/32" black line and a 0.001" wire attached to a holding fixture.

Resolution was verified each time a piece of equipment was changed and on each videotape.

EXAMINATIONRESULTS No relevant indications were identified on any of the areas examined on the Core Shroud.

There were areas that contained indications that were proved to be nonrelevant.

Many of these indications can be attributed to the oxide coating of material that builds up selectively on the component surfaces.

Allindications that were suspected of being defects were re-examined using various lighting and camera angles to help resolve the indication area.

AREA EXAMINED Access Hole Covers at 0 and 180, ID of weld H3, 16 locations, 1 foot examined at each location ID of weld H4, 16 locations, 10 degree scans ID of weld H6, 16 locations, 10 degree scans OD of weld Hl accessible

areas, 130 to 170 OD of weld H2 accessible

areas, 130 to 180 OD of weld H8, 170 to 190 and 355 to 10

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