ML20217F865

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Provides Info Requested in NRC GL 97-06, Degradation of SG Internals
ML20217F865
Person / Time
Site: San Onofre  Southern California Edison icon.png
Issue date: 03/30/1998
From: Rainsberry J
SOUTHERN CALIFORNIA EDISON CO.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-97-06, GL-97-6, NUDOCS 9804010192
Download: ML20217F865 (11)
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  .         j               SOblHIRN CA!.lf ORNIA
l. L. Rainsberry Manager. Plant Licensing An iDISU% MTLK\ATIU%Al = Cunnpany March 30, 1998 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555-0001

Subject:

NRC Generic Letter 97-06: Degradation of Steam Generator Internals San Onofre Nuclear Generating Station Units 2 and 3 Gentlemen: This letter provides, as an enclosure, the information requested in Generic Letter (GL) 97-06. Information for San Onofre Units 2 and 3 is provided which includes the results of evaluations and inspections and identifies programs and plans for monitoring of potential steam generator secondary side internals components degradation that may pose a risk to plant safety. If you have any questions or would like additional information on this subject, please let me know. Very truly yours, i i \ 

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980401'1'95t 0 980330 POR ADOCK 05000361 P PDR [ San Onofre Nuclear Generating Station O P. O. Ik?x !28 San Clernente CA 92674-0128 714 368-7420 i

Document Control Desk State of California County of San Diego On tM oAcA 30, M M before me, Acuud M _daAdi4J , personally appeared S . M. [I4 al>AA.a , V ' I personally known to me to be the person whose name is subscribed to the within instrument and acknowledged to me that he executed the same in his authorized c capacity, and that by his signature on the instrument the person, or the entity upon behalf of which the person acted, executed the instrument. WITNESS my hand and official seal. i mecausamme > camassene sseun . NoAmyMass-Oggeseg  ! annamoocessy 4 urconm esessnaamsme > Signature LAats) \Y A1A]D V Enclosure cc: E. W. Merschoff, Regional Administrator, NRC Region IV K. E. Perkins, Jr., Director, Walnut Creek Field Office, NRC Region IV J. A. Sloan, NRC Senior Resident Inspector, San Onofre Units 2 & 3 M. B. Fields, NRC Project Manager, San Onofre Units 2 and 3 l 

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ENCLOSURE Generic Letter 97-06 Response San Onofre Units 2 and 3 o l Introduction l San Onofre has addressed the requirements of Generic Letter (GL) 97-06. This response 

             . provides the results of evaluations and inspections at San Onofre, and identifies programs and plans for monitoring of potential steam generator (SG) secondary side internals components           1 degradation that may pose a risk to plant safety.                                                     j
             ~ San Onofre has participated in Combustion Engineering Owners Group (CEOG) and Nuclear Energy Institute (NEI) activities relating to steam generator internals. An integral component. in these assessments was an understanding of the. applicability of the degradation found in the French units to domestic steam generators. The Electric Power Research Institute '(EPRI) developed the       I report, GC-109558, Steam Generator Internals Degradation: Modes ofDegradation Detectedin EDF Units. The EPRI report provides evaluations of the causal factors involved in the modes of degradation experienced in the French units. This report was transmitted to the NRC via an NEI ,
              . letter, dated December 19,1997.                                                                       ,

I San Onofre has participated in industry initiatives to address steam generator internals  ;{ 1 degradation and a CEOG evaluation of steam generator internals degradation experience in EDF 

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and domestic Combustion Engineering (CE) designed units, including San Onofre. The EDF ) 

              ! steam generator internals degradation experience was discussed and an evaluation conducted by         I ABB-CE after NRC Information Notice 96-09 'was isrued. The findings of the evaluation were.           i that the specific causal factors identified by EDF were not applicable to the CE design.

Subsequently, an evaluation of the broader question of all types of SG internals degradation was performed. : The evaluation identified potential degradation mechanisms by review of plant experience and dispositioned them on the basis of design, manufacturing, and operational practice. The industry programs address the susceptibility of CE designed units to identified degradation mechanisms. San Onofre has, in the context of known degradation, performed inspections and l safety assessments to ensure continued capability to maintain tube integrity and decay heat 

              ' removal. Steam Generator internals of other CE units have also been inspected. The objectives L
              . and conclusions in two CEOG reports focusing on susceptibility of CE SGs to degradation are summarized in the following section. Reports summarizing San Onofre steam generator tube support degradation have been submitted to the NRC previously. These reports are summarized
             ! below in the response to Item I of this Generic Letter.                                                l c

l l l l~  ! L - 1 M h'

l Summary of CEOG Reports Concerning CE Steam Generator Degradation Susceptibility I. CEOG Report, CE NPSD-1092, " Evaluation of Degraded Secondary Internals - Operability Assessment" f Objectives:

1. Assess the applicability of EDF damage mechanisms to the CE design.
2. Assess the applicability of tube support erosion-corrosion experience at Maine Yankee i

and San Onofre Unit 3 to other CE designed units.

3. Assess the impact of applicable degradation mechanisms on tube integrity and decay heat removal capability.

i

Conclusions:

I I 1

1. The primary damage mechanism related to the wrapper support failures in the French j units are not directly applicable to the CE designed steam generators. l
2. Support plate cracking is a residual effect of tube denting but is not detrimental to the safe operation of the steam generator.
3. Adequate margins against failure have been demonstrated for the Flow Accelerated Corrosion (FAC) damage observed in the San Onofre Unit 3 eggerates.
4. There are no reported tube wear indications directly related to tube support degradation.
5. Plants with degradation of tube supports, such as that observed a*. San Onofre Unit 3, can j continue to operate safely because adequate margins against failure exist and possible tube damage can be detected in the normal eddy-current testing (ECT) examinations.
6. Corrosion degradation of eggerates as manifested by denting, such as detected in the l lower eggerates of the MP2 original steam generators, has been determined to be acceptable by model boiler testing and analysis. Current chemistry practices can mitigate existing denting and preclude further degradation due to denting.

! 7. Units with more support degradation than San Onofre Unit 3 can continue to operate L safely because adequate margins against failure can be demonstrated and possible Flow- 1 Induced Vibration tube wear is postulated to result in Leak Before Break. I L l L

8. CE steam generators designed with stainless steel tube supports are much less susceptible to erosion-corrosion than steam generators with carbon steel tube supports.
9. None of the degradation mechanisms reviewed pose a threat to the Reactor Coolant System pressure boundary integrity or the heat removal function of the steam generator.

II. CEOG Report, CE NPSD-1103," Evaluation of Susceptibility ofInternals Degradation in CE- Designed Steam Generators" Objectives:

1. Review the history of steam generator internals degradation in CE-designed units l

l

2. Examine the susceptibility of CE-designed units to internals degradation mechanisms that have occurred in other CE-designed and EDF Steam generators.

Conclusions:

1., CE-designed steam generators have not encountered a significant amount ofinternals degradation. Of the degradation that has occurred, appropriate mitigating action has been taken to minimize the effect 'of this degradation. l

2. The most common form of steam generator internals degradation has been from waterhammer events or erosion of components within the feedwater system. However, these degradation mechanisms are not considered to be safety significant.  !
3. The steam generator internals degradation mechanisms described in GL 97-06 are i generally not applicable to CE-designed steam generators. The only degradation l mechanism applicable to the CE fleet that could have safety significance is FAC of peripheral eggerates.
4. FAC of peripheral eggerates is primarily the result of secondary fluid flow redistribution caused by severe tube bundle fouling. Use of ammonia for pH control in heavily fouled steam generators increases the susceptibility to FAC.
5. Steam generators with ferritic stainless steel eggerates (at Palo Verde and Palisades) have a chromium content of at least 10.5%, which increases the resistance to FAC by at least L an order of magnitude, and so are not considered susceptible to FAC of tube supports.

6.- Of the CE-designed steam generators with carbon steel tube supports, only those units ! with severe tube bundle fouling as indicated by significant steam generator secondaiy pressure loss may be susceptible to FAC of peripheral eggerates. c

7. No CEOG member plants have detected by non-destructive examination (NDE) or visual inspections any FAC of drilled tube support plates to date.

The CEOG evaluation provides assurance that the only credible steam generator internals components damage mechanism ofpotential safety significance is FAC of eggcrate-type tube supports. A potential susceptibility to FAC has been identified for CE-designed plants with carbon steel tube supports with heavily fouled tube bundles. San Onofre does not have drilled tube support plates, and as such is not susceptible to failure mechanisms specifically associated with those supports. FAC of eggcrates has been detected at San Onofre Unit 3, as reported in GL 97-06. In general, FAC of tube supports is possible in units with carbon steel supports if tube bundle fouling causes redistribution of flow such that FAC threshold velocities are exceeded. The CEOG evaluations indicate that FAC without substantial fouling is unlikely. Experience has indicated that the onset of substantial fouling is evidenced by a reduction in the normal plant operating steSm pressure. Experience, including at San Onofre Unit 2, has also show that plants can experience steam pressure reduction and not be impacted by FAC damage '.

  • the tube supports. In the event of a substantial reduction in steam pressure, the CEOG recommends that an inspection for the onset of FAC of tube supports be conducted at the next scheduled outage. San Onofre has performed comprehensive inspections of each Unit.

Responses to Generic Letter 97-06 Items: Item (1) Discussion ofanyprogram in place to detect degradation ofsteam generator interruds arut a description of the inspectionplans, including the inspection scope, frequency, methods, and equipment. The discussion should include thefollowing information: (a) Whether inspection records at thefacility haw been reviewedfor irufications of tube supportplate signal arwmaliesfrom eddy-current testing of the steam generator tubes , that may be indicatiw ofsupportplate damage or ligament cracking. If the addressee hasperformed such a review, include a discussion of thefindings. (b) Whether visual or video camera inspections on the secorulary side of the steam generators have beenperformed at thefacility to gain information on the condition of steam generator interruds (e.g., supportplants tube bundle wrappers, or other 

                . components). If the addressee hasperformedsuch inspections, include a discussion of Ihefirulings.

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(c) Whether degradation ofsteam generator internals has been detected at thefacility, aridhow the degradation was assessedanddispositioned. Summary of Response to item 1: San Onofre has a program in place to detect degradation of steam generator internals. Reports ofinspections and the degradt. don that has been discove ed in San Onofre steam 

        ' generators eggerate-type tube suppor.s have been submitted to the NRC by References 1,2, and 3 that are identified at the end of this enclosure. Since the CEOG evaluation and inspections of San Onofre provide assurance that the only credible steam generator internals components damage mechanism of potential safety significance is flow accelerated corrosion (FAC) of eggerate-type tube supports, only this element is considered in the following response.

Inspections: Unit 3 eggerates were inspected for degradation in the Cycle 9 refueling outage in 1997. Unit 2 eggcrates were inspected for degradation in a Mid-Cycle 9 outage in February,1998. Unit 3 eggerates were again inspected in a Mid-Cycle 9 outage in March,1998. Scope: I. The scope for the previous inspections was a comprehensive sample of the tube support structures and included all eggerate elevations at peripheral locations and at locations within the bundle. Vertical and diagonal supports tube support areas were also inspected. Frequency: - San Onofre inspections have been performed on each Unit within the past 14 months. Future inspection schedules will be determined upon completing the evaluation of the latest Unit 3 results. Methods and Equipment: Remote visual methods have been used at San Onofre for the past inspections of the eggerate structure. Video probes using a remote video camera and fiber optics have been inserted into the tube bundle to view individual eggerate lattice bars. Each inspection point was video taped for engineering review and assessment. Degradation has been mapped using a visual grading system. 5-

Response to item (a) - Review ofInspection Records: The design of the eggcrate tube supports does not lend itself to examination by eddy current I testing in a fashion similar to the drilled support plate design. Hence, no review of previous eddy-current inspection information was conducted. During the 1997 Unit 3 inspection, Southern California Edison (SCE) worked with our inspection vendor in an attempt to develop an eddy-current and/or ultrasonic technique that could reliably detect eggerate degradation. This effort was not successful. Response to item (b)-Inspections Performed: San Onofre has performed comprehensive visual inspections of the steam generator tube  ; support structure on both units. These inspections included the tube support eggerate r>tructures at all eggerate elevations, the batwing diagonal supports, and venical suppons in the steam generator tube U-bends. The inspections utilized remote video camera and fiber optics. Other elements of the steam generator internals, such as eggerate suppon rings', were are also inspected. The initial visual inspection was performed on Unit 2 in December of 1996 as a corollary operation to chemical cleaning of the steam generators. This was a sample inspection of the upper eggerates to view the level of fouling prior to chemical cleaning. No eggerate wastage was noted in this inspection. A similar visual inspection was performed for Unit 3 in April,1997, Wastage of ponions of the peripheral eggcrate lattice bars was noted in this inspection, and the inspection scope was expanded to all eggerate elevations to determine the amount and extent of eggerate wastage. The results of this inspection were submitted by References 1 and 2 and are discussed in the Response to Item (c)(below). In February 1998, an inspection was performed on Unit 2 that was designed to be sufficiently - comprehensive to determine if eggerate wastage was present in Uni.t 2, and if so, the extent of any wastage. The results of this inspection were submitted by a March 10,1998 letter from SCE to the NRC (Reference 3) and are discussed in the Response to Item (c) (below). A second inspection was performed on Unit 3 in March,1998. The objectives of this inspection were to determine if additional wastage of the type noted in the previous inspection had occurred and to validate the root cause determination that was previously performed. 1 i 1 a 1 

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Response to Item (c)- Assessment and Disposition of Degradation: 

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Unit 3 l 1 I During the Cycle 9 refueling outage for San Onofre Unit 3, degradation of periphery portions of several eggerate supports was identified in both steam generators. The degradation was noted during pre-chemical cleaning assessments of the condition of the eggerate supports and was unexpected. As a result of this discovery, SCE performed a comprehensive inspection of , the internals of both Unit 3 steam generators. This inspection confirmed the degradation was primarily limited to the upper eggerates and was confined to their peripheral portions. To a lesser extent, the stay cylinder, blowdown lane areas, and cold leg porticas of the eggerate periphery were also affected. A report dated June 5,1997 (Reference 1) was submitted to the NRC that provided SCE's assessment of the cause of the degradation, its impact on continued operation, the actions needed to arrest it, and the plans for periodic monitoring. A revision to this report, dated October 17,1997 (Reference 2), was submitted to the NRC. This revision provided editorial corrections, expanded on the original report to include a final Unit 2 eggerate operability assessment, and updated the 10CFR50.59 evaluation. The degradation was caused by a form of FAC, a general term describing processes which use assistance from fluid flow to remove the protective oxide layer from base material. j Removal of the protective oxide layer exposes the base material to the fluid environment, allowing further material removal via corrosion and/or erosion processes. Relatively rapid material removal rates may occur in the presence ofcertain combinations of fluid and material propeities. The carbon steel eggerate material can be susceptible to FAC in the presence of sufficiently high-fluid velocities in combination with high-steam quality. The FAC apparently occurred during recent operation of Unit 3 as a result of steam generator secondary side fluid parameter changes caused by the buildup of deposits on the steam generator tubes. This buildup led to increased fluid velocities and steam quality, particularly in the tube bundle periphery areas. After sufficient buildup, secondary fluid properties reached levels where FAC occurs. The chemical cleaning of all steam generators restores fluid properties to nominai conditions by removing the deposit buildup. Under nominal conditions, fluid parameters are not conducive to FAC and the condition is arrested. Due to improvements in controlling the sources of deposits, the rate of deposit buildup will be much less in the future; consequently, fluid properties conducive to FAC are not expected to recur. An extensive re-analysis of the eggerates in their degraded condition was conducted to assess tube integrity during postulated accidents, steam generator tube bundle support during postulated accidents, and the potential for excessive flow-induced vibration of tubes given l the absence of support at certain locations. Since the upper-most (10th) eggerate was found 

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t _, l tp be in good condition, and the other tube support members (batwings and vertical strips) were un'affected by the FAC process, the analysis results were acceptable for all accident 1 conditions. The analysis also concluded that for most locations in the tube bundle excessive l flow-induced vibration would not occur until support at three or more consecutive eggerate j elevations wquid be lost. Although excessive flow-induced vibration has not occurred, as j evidenced by the absence of tube wear at degraded eggerate locations, SCE has conservatively plugged those tubes with two or more consecutive uncredited eggerate supports. I15 tubes in the two steam generators were plugged in the Cycle 9 refueling outage as a result. Internal stabilizers were added to each affected tube prior to plugging to ensure tube vibration would not affect adjacent in-service tubes.- The analysis showed that substantial margin remains despite the eggerate degradation. i SCE performed a written safety evaluation to verify that an unreviewed safety question would not result from having degraded eggerate supports in the Unit 3 steam generators. l To confirm that removal of deposits through chemical cleaning has returned the steam generator to nominal thermohydraulic conditions, arrested further FAC, and stabilized the eggerate degradation, SCE performed a special inspection of the Unit 3 eggcrates at approximately the mid point of the current Cycle 9 operating cycle. Results of these inspections will be provided in separate correspondence. If effectiveness is confirmed based  ; on this mid-cycle inspection, sufficient basis will exist to monitor any progression of the l degradation on a refueling interval basis. i Unit 2 Although no flow accelerated corrosion degradation was noted during the limited visual inspection conducted after chemical cleaning during the last Unit 2 refueling outage, an eddy-current inspection for tube degradation was performed during a Mid-Cycle 9 outage. SCE elected to conduct a more extensive visual inspection of the Unit 2 steam generators in parallel with the eddy-current inspection. Based on the degradation observed during the Unit i 3 refueling outage, a survey of 20% (about 50 tubes) of the Unit 2 tubes on the hot leg l periphery was concluded to be appropriate. A copy of the Engineering Guide for Steam Generator Secondary Side Inspection (SSI) prepared for this inspection was provided with Reference 3. The actual mid-cycle inspections performed exceeded the scope in the Inspection Guide. The original scope was to inspect 10 locations on the hot leg periphery of each steam generator.  ! In actuality,17 locations (75 tubes) were inspected in steam generator 88, and 18 locations (90 tubes) were inspected in steam generator 89. In general, the secondary side of both steam generators was concluded to be in good to excellent condition. Minor isolated l instances of eggerate lattice bar thinning were observed as expected, but overall, the thinning was an order of magnitude less than that observed on Unit 3. No repairs were required. I I i

Reference 3 provided the results of the inspection using the grading criteria in the Inspection Guide. The observed minor areas of eggerate lattice bar thinning do not affect the ability of the tubes or the supports to meet their design basis. Inspections of the secondary side of the Unit 2 steam generators will continue to be performed during future refueling outages until it is established that further inspections are not required. The results of the Mid-Cycle 9 inspection were reported to the NRC by Reference 3. Item (2) If the addressee currently has no program in place to detect degradation ofsteam generator internals, include a discussion aruljustification of the plans and schedulefor establishing such a program, or why noprogram is needed. ! Response to item 2: Item 2 is not applicable to San Onofre. 1

References:

l 1. June 5,1997 letter from Dwight Nunn (SCE) to the Document Control Desk (NRC), l

Subject:

Docket No. 50-362, Steam Generators, San Onofre Nuclear Generating Station, 

               . Unit 3.
2. ' October 17,1997 letter from Dwight Nunn (SCE) to Document Control Desk (NRC),

Subject:

Docket No. 50-362, Steam Generatcrs, San Onofre Nuclear Generating Station, Unit 3.

3. March 10,1998 letter from J. L. Rainsberry (SCE) to Document Control Desk (NRC),

Subject:

Docket Nos 50-361 and 50-362, Steam Generator Secondary Side Inspection (SSI), San Onofre Nuclear Generating Station, Unit 2. l 

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