ML20138F219

Documents Util Evaluation of Impact on Zion Station of Results of Recent Leak Tests of re-rolled SG tubes.C-E re-roll Qualification rept,CEN-620-P,will Be Revised to More Clearly State That re-rolls Are Leak Limiting.W/Encl

ML20138F219
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	05/01/1997
From:	Mueller J COMMONWEALTH EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9705050301
Download: ML20138F219 (13)

Text

Commonweahh IAlivin Compan)

Zion Generating $tation

. 1015hiloh 150ules ard

, Zion ll. 6(MP)9?"9' Tel H 47-7 4(>-208 6 May 1,1997 I

U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Attention: Document Control Desk

Subject:

Information Pertaining to Application of the F* Repair Criterion to Steam Generator Tubes at Zion Nuclear Power Station Units I and 2 Zion Nuclear Power Station Units 1 and 2 NR_Cpocket Numbers: 50-295 and 50-304

References:

1) T. Simpkin letter to the Nuclear Regulatory Commission dated May 31,1995, transmitting a request to amend Technical Specifications to allow for use of the la repair criterion at Zion Units 1 and 2 l

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2) No Significant Hazards Consideration Determination Notice dated September 5,1996 This letter documents Coinmonwealth Edison's (Comed's) evaluation of the impact on Zion

Station of the results of recent leak tests of re-rolled steam generator tubes conducted at the Prairie Island (PI) Nuclear Power Plant. This letter is being provided for information, and no NRC action is requested. -

Dackground PI Unit-2 began a refueling outage in January,1997. Primary to secondary leakage ofless than 5 gallons per day had been indicated during the previous operating cycle. As a result, a secondary i

side hydro of the generators was performed at approximately 740 psi. This test identified a number of steam generator tubes that had minor leakage. These tubes had been re-rolled in the previous refueling outage, approximately 18 months earlier.

The re-rolls used in the leaking PI tubes were performed using an ABB/ Combustion (CE)

application of the "F* Distance." The F* Distance is defined as the length of roll engagement required to resist tube pullout forces during normal operation, test, upset and faulted conditions.

This is the same process that has been used at Zion Units 1 and 2.

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The F* criterion was authorized for use at Zion Station via a Technical Specification Amendment

that was based on information supplied in Reference 1. The information supplied in Reference 1 j included WCAP-14211, "F* and L* Plugging Criteria for Tubes with Degradation in the Tubesheet Roll Expansion Region of the Zion Nuclear Generating Station Units 1 and 2 Steam

Generators" (Proprietary), and in ABB/CE Qualification Report CEN-620-P, Revision 01-P,  !

" Series 44 & 51 Design Steam Generator Tube Repair Using A Tube Re-Rolling Technique"

(Proprietary). Although minor, the leakage observed at PI for two of the re-rolled tubes was in l i excess of that documented in the ABB/CE Qualification Report, CEN-620-P, Revision 01-P.  !

j This letter provides an assessment of the impact of the PI leak test results on Zion's licensing l l bases, and an evaluation of the implications of the test results on the structural and leakage i integrity of the currently re-rolled tubes at Zion.

4 f Licensing Bases i j The Zion license was initially amended to allow use the F* repair criterion in September of 1995, ,

j via Amendment 168 and 155 for Units I and 2, respectively. The associated amendment request l i included WCAP-14211 and CEN-620-P, Revision 01-P. The affected Technical Specification  !

{ was subsequently changed in license Amendment 172 /159 (issued in November 1995) and i Amendment 174/161 (issued in November 19%). The three NRC Safety Evaluations for these i amendments accurately reflect the input provided by Comed in the associated amendment

requests and supplements. The Safety Evaluations differ in their descriptions of the ability of the re-rolled tubes to mitigate leakage. Attachment B to this letter contains excerpts from the Safety i Evaluations which illustrate this conclusion.

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The "No Significant Hazards Consideration Determination" (Reference 2) for the most recent amendment (174/161) pertaining to re-rolled steam generator tubes, clearly indicates that the re-

roll would not be leak tight, but would be leak limiting. For example, the criteria of 10 CFR

! 50.92(c)(1) are addressed as follows:

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! " Application of the F* Distance will not significantly increase the consequences of any

accident previously evaluated. The F* Distance ensures that sufficient length of undegraded tube exists to maintain structural integrity and preclude significant leakage.

j Due to the proximity of the tubesheet to the tube, any leakage from degradation below the i

! F* Distance would be negligible and would be well below the Technical Specification

limits established for steam generator leakage."

t "Since the proposed change only clarifies how the F* Distance is ensured at Zion Station, I the proposed change will not increase the probability of occurrence or consequences of 4

any accident previously evaluated."

Therefore the most recent "No Significant Hazards Consideration Determination" is consistent j with the PIleak test results.

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Page 3 of4 4

1 Comed considers that the validity of the documents submitted in support of the original  !

amendment request, WCAP-14211 and CEN-620-P, Revision 01-P, is unaffected by the PI leak l test results. Although the qualification of the re-roll process remains valid, Comed considers that the CEN qualification report should be revised to more clearly state that the re-rolls are leak ,

limiting, and to include leakage data gained from PI and data from new pull tests performed at the request of Comed. Therefore, the CEN qualification report will be revised accordingly. -

Finally, neither the current Technical Specifications (Sections 3.3.1.B and 4.3.1.B, " Steam i Generators" and 3.3.3.E, " Primary-to-Secondary Leakage") nor the proposed Improved Technical Specifications (Sections 3.4.13, "RCS Operational Leakage" and 5.5.7 " Steam Generator (SG) Tube Surveillance Program") nor their associated Bases, address the issue of leakage integrity specifically for re-rolled tubes. Therefore, compliance with the current Technical '

Specifications and the proposed Improved Technical Specifications is not impacted by the PI leak test results.

Implications of PI Test on Re-roll Structural and Leakage Integrity at Zion Comed's evaluation of the implications of the PI leak test results on the structural and leakage integrity of the currently re-rolled SG tubes in Units 1 and 2 is documented in Attachment A.

Conclusion Comed has concluded the following:

. The "No Significant Hazard Consideration Determination" associated with the most recent amendment is consistent with the PI leak test results.

• The previous Safety Evaluation Reports do not consistently reflect the possibility of minor leakage from re-rolled steam generator tubes.

e Zion is in compliance with the current Technical Specifications and will be in compliance with the Improved Technical Specifications when issued.

• Adequate structural and leakage integrity of re-rolled tubes at Zion will be maintained during normal operations and accident conditions, even with the leak rates seen at PI.

Commitments This correspondence contains one new commitment; the Combustion Engineering re-roll qualification report, ABB/CE Report CEN-620-P, will be revised to more clearly state that the re-rolls are leak limiting, and to include leakage data gained from PI and data from new pull tests performed at the request ofComEd.

t Page 4 of 4 Ifyou have any questions conceming this correspondence, please contact Denise Saccomando, Senior PWR Licensing Administrator at 630-663-7283.

i Sincerely, i

j hn Mueller j Site Vice President Zion Station 4

Attachments:

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i cc: NRC Regional Administrator - RIII j Zion Station Project Manager- NRR

] Senior Resident Inspector - Zion Station Office of Nuclear Facility Safety - IDNS

IDNS Resident Inspector 3 Zion NLA Master Files

. Reg. Assurance File DCD Licensing I

PageIof5 Attachment A l

l STRUCTURAL AND LEAKAGE INTEGRITY EVALUATION Current Status Zion Unit 1 currently has 799 re-rolled tubes in service, distributed as shown below.  ;

The tubes were re-rolled during the fall,1995 refueling outage. I Steam Generator Re-rolled Tubes A 149 B 79 j C 486 D 85 I

Over the last six months of operation, tne maximum calculated primary to secondary leakage for Zion Unit I was as follows:

Steam Generator Leakage (gpd)

A 19 B 8 C 32 D 11 Zion Unit 2 currently has 2183 re-rolled tubes in service, distributed as shown below.

The tubes were re-rolled during the current refueling outage which began in September 1996.

Steam Generator Re-rolled Tubes A 936 B 332 C 399 D 516 Implications for Zion et PI Test Results The following criteria apply to Re-rolled steam generator tubes:

• Structural Integrity - The structural guidelines of draft Regulatory Guide 1.121 must be met.

• Operational Leakage -- The primay to secondary leakage must be maintained below the Technical Specification limits.

• Accident Leakage - The accident leakage must be maintained below that which would result in a dose below a small fraction of both the 10 CFR 100 limits, and the 10 CFR 50, App. A, Criterion 19 limits.

As discussed in the following sections, the re-rolled tubes at Zion will continue to meet these criteria despite the PI leak test results.

.j Page 2 of 5 StructuralIntegrity:  !

In accordance with draft Regulatory Guide 1.121, re-rolled tubes must provide adequate pullout resistance to a value of three times the normal operating differential pressure.

For Zion, this corresponds to a minimum of 2892 pounds of app!ied tensile force. i Both push tests and pull tests of re-rolled tubes have been conducted by ABB/CE. i The re-rolling qualification report, CEN-620-P, Revision 01-P, documents push testing which demonstrated that re-rolled joints that experience leakage could still withstand applied compressive axial tube loads of three times the normal operating pressure differentials. In addition. ABB/CE performed pull testing by subjecting re-rolled tube  !

joints to tensile loads. These tests were conducted at room temperature conditions. l As a result, the tests were extremely conservative since the difference between the thermal  !

expansion of the tubing material and the thermal expansion of tubesheet results in more contact pressure between the tube and tube sheet at normal operating temperature than at room temperature. The increase in contact pressure increases the frictional forces which resist axial tube movement. Even with these conservative test conditions all of the re-roll tube samples satisfactorily resisted pullout forces in excess of the required 2892 pounds.

The ability of the re-roll to achieve the resistance to pullout required by draft Regulatory Guide 1.121 would not be compromised if the leakage observed at P1 were to occur at i Zion. The information obtained from PI indicates that the leakage originated from axial j flaws in the factory roll transition. Such a leak mechanism would not affect the validity of the re-roll testing described above, since (as indicated in WCAP-14211) axial indications  ;

do not significantly decrease the structural strength of the re-roll. l I

In addition to the re-roll, the original factory roll remains available to resist pullout. The currently re-rolled tubes at Zion were re-rolled due to axial indications. There is no ,

indication that the re-roll process initiated a circumferential flaw. As indicated in WCAP- l 14211, axial indications cannot lead to tube failure, and the initial factory roll meets the criteria of draft Regulatory Guide 1.121. Therefore, even if the PI leakage mechanism were to occur at Zion, there is a high degree of assurance that the required resistance to pullout will be maintained, via the the original factory roll, the re-roll, or both.

A tube rupture in the re-roll or the original factory roll is not possible due to the i reinforcing effect of the tubesheet, which would restricting the ability of an axial flaw to grow to a rupture. The information obtained from PI does not indicate that the leakage is 1 caused by, or results in, any mechanism that would alter this reinforcing effect.

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Page 3 of 5 l StructuralIntegrity

Conclusion:

e l The leakage observed at PI would not compromise the structural integrity of the re-rolled ,

! tubes ifit occurred at Zion because the leakage does not alter the following:

! e The original factory roll and the re-roll will be capable of resisting axial loads.

i e The reinforcing effect of the tubesheet will preclude tube rupture. i

. c j Operational Leakage:  !

E l The data from the PI in-situ pressure tests of re-rolled tubes indicated a maximum leak rate value of 0.023 gph at NOP for the worst tube. The number of re-rolled tubes

, identified as either damp or wet at PI was 12 or (approximately 3%) of the 401 total tubes that have been re-rolled. Applying these parameters to Zion Unit 2, the estimated leakage during the next cycle of operation from the steam generator with the most re-rolled tubes would be:

[936 re-rolled tubes (for the most limiting S/G,2A)] * [3% failure rate] * [0.023 gph leak rate] * [24 hours / day] = 15.5 gpd potentialleakage from the most limiting S/G.

This value is well below the Zion's administrative limit of 150 gpd limit for any single steam generator, and very low compared to the Technical Specification limit of 500 gpd leakage per any single generator.

The estimated leakage for all Unit 2 steam generators during the next cycle of operation l from all steam generators would be:

[2183 re-rolled tubes (for all S/Gs)) * [3% failure rate] * [0.023 gph leak rate] / [60 minutes / hour) = 0.025 gpm potential leakage from all S/Gs.

This value is very low compared to the Technical Specification limit of I gpm for all steam ,

generators combined. j i

Over the last six months of operation, the maximum primary to secondary leakrate for a  !

Unit I steam generator, from all sources, was calculated to be 32 gpd, for the "C" steam generator. The source of this leakage cannot be positively attributed to leakage of re-rolled tubes, and this leak rate is well below the administrative limit and very low compared to the Technical Specification limit.

Finally, the proximity of the tube to the tubesheet inherently limits the amount of primary to secondary leakage even if a through-wall flaw occurs. This leak-limiting mechanism will remain, regardless of the leakage observed at Pl. This provides added assurance that leakage from the re-roll areas will remain well below administrative and Technical Specification limits.

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! , Opdational Leakage

Conclusion:

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• Application of the PI data to Zion, results in a leakrate well bele on's administrative limit of 150 gpd limit for any single steam generator.

' The proximity of the tube to tubesheet, limits the amount of primary to secondary leakage even if through-wall indications are present.  ;

Accident Leakage Comed has calculated a value of 5.31 gpm for the maximum primary to secondary leakage I allowable while still assuring that post accident doses would be within a small fraction of i both the 10 CFR 100 limits, and the 10 CFR 50, App. A, Criterion 19 limits. The maximum leak rate resulting from a Main Steam Line Break (MSLB) at Zion can be i estirmated by using the test results from at Prairie Island. The bounding steam generator

• for each Zion unit is as follows:

Unit 1 - SG C 486 re-rolled tubes Uni 2 - SG A 936 re-rolled tubes  !

l The MSLB accident leak rate assigned by PI to the tube with highest leak test results is 0.001 gpm. Assuming 3% of the re-rolled tubes in Zion Unit 1 - SG C were to leak at that rate, the accident leak rate value would be:

[486 re-rolled tubes] * [3%) * [0.001 gpm] = 0.015 gpm During a Unit 1 MSLB, an additional 0.049 gpm ofleakage may occur due to potential lower edge weld defects on the CE steam generator sleeves. Adding the potential leakage from the re-roll condition (0.015 gpm) to that from the lower edge weld condition (0.049 gpm) the accident leakage value for Unit 1 is determined to be 0.064 gpm.

I The accident leak rate value for Unit 2 would be:

[936 re-rolled tubes] * [3%)*[0.001 gpm] = 0.028 gpm.

The lower edge weld condition does not exist for Unit 2.

These values for both Unit I and Unit 2 would be considerably less the allowable leakage limit of 5.31 gpm that equates to a small fraction of both the 10 CFR 100 limits, and the  :

10 CFR 50, App. A, Criterion 19 limits.

The above evaluation discusses the results for the bounding steam generators in each Unit.

An even more conservative evaluation can be performed assuming: 1 i

e All tubes in a steam generator are re-rolled in the hot leg tubesheet (this would equate  :

to 3388 repairs), and l e Each of these re-rolled tubes leak at 0.001 gpm j l

The accident leak rate value would be [3388 re-rolled tubes] * [0.001 gpm] = 3.39 gpm.

This value is still less than the allowable accident leakage limit of 5.31 gpm.

P:ge 5 of 5 -

Accident Leakage Conclusion Accident leakage is not a concern for re-rolled joints because:

e Application ofPI's data to Zion results in a leakrate significantly less than the site allowable leakage limit.

. Assuming that the entire hot leg tubesheet was re-rolled and exhibited leakage, the accident leakage would remain less than the site allowable leakage limit.

Overall Conclusion Since the requirements for structural integrity and primary to secondary leakage during both normal operating and accident conditions will continue to be met, Comed does not consider that further actions are required at this time.

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.. , Page1of4 Attachment B 1

l HISTORY OF Fa AMENDMENTS 1 l AMENDMENT 168/155  ;

! l l This amendment was issued via a letter from C. Shiraki to D. Farrar dated September 11,1995,

transmitting a Safety Evaluation for the use of F* repair criterion at Zion Units 1 and 2.

Description of the Amendment i

f This amendment revised Section 4.3.1 of the Technical Specifications to incorporate a new repair criterion for steam generator tubes. The new criterion for steam generator tubes was developed l by Westinghouse and was known as the F* Distance. The new F* criterion was to be applied to l avoid unnecessary plugging and repair of steam generator tubes.  !

i Review of Safety Evaluation  !

Section 1.0

Introduction:

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" Licensee stated that the proposed changes will provide adequate assurance of steam generator j

tube integrity because the presence of the tubesheet in conjunction with the hardroll process significantly reduces the potential for tube structural failure and/or leakage within the tubesheet compared to the free span portion of the tube. The presence of the tubesheet constrains the tube and compliments the integrity of the tube by minimizing the amount of deformation a tube can undergo beyond its expanded outside diameter. The proximity of the tube and tube sheet, due to the hardroll expansion, limits the amount of primary-to-secondary leakage."

Section 2.1 Licensee's Evaluation

" Limitation of Primarv-to-Secondan Leakage" "The presence of the elastic preload presents a significant resistance to flow of priman-to-seconday or secondary-to-primary water for degradation which has progressed fully through the thickness of the tube wall. In effect, no leakage would be expected if a sufficient length of hardroll is present."

"For t.n initial location of a leak source a distance greater than F* below the bottom of the roll transition, the feedwater line break pressure differential results in an insignificant leak rate relative to that which could be associated with normal plant operation. This is a result of the increased tube to tubesheet loading associated with the increased differential pressure. Thus, for a circumferential indication within the roll expansion that is left in service in accordance with the F*

pull out criterion, any leakage under accident conditions would be less than that experienced under normal operating conditions provided the F* distance is located in the lower half of the tubesheet thickness. Therefore, any leakage under accident conditions would be less than the existing TS leakage limit which is consistent with the accident analysis assumption."

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Page 2 of 4 Section 2.2 Staff Evaluation "In its letter dated May 31,1995, the licensee stated that it will implement re-rolled tubesheet joint expansion to obtain new leak tight re-rolled joints for tube repair."

"Although tubes with both sludge types could adequately withstand the applied axial tube loads, those tubes with rolled dry sludge showed minor leakage during testing. The licensee will remove from service those tubes which appear to have hard sludge in the crevice based on torque traces recmded during the re-rolling process."

i Condusion: j This Safety Evaluation contains verbiage that does not clearly conclude that the re-rolls are leak  !

tight or leak limiting, as evidenced by:

• A statement that clearly states no leakage would be expected if a sufficient roll length is present.

. Some statements clearly refer to the re-rolls as leak tight, e Some statements indicate that the re-rolls are leak limiting.

e The feedline break discussion implies that the leakage under this accident condition would be less than that experienced under normal operating conditions, this implies that leakage under  ;

normal operating conditions can be expected.

The Safety Evaluation contains a discussion that the presence of deposits between the outer tube wall and the tubesheet bore could result in minor leakage. To ensure that this condition would not result, Comed was committed to remove from service those tubes which appear to have hard sludge based upon the torque traces recorded during the re-roll process. Based upon PI  !

experiencing leakage in re-rolled tubes that do not have torque traces indicative of hard sludge,  ;

Comed believes that the discussion in the Safety Evaluation may lead the reader to believe that all i re-rolled tubes that could potentially leak could be identified and removed via the torque trace .

criteria. Although Comed does not believe that removing tubes with torque traces indicative of hard sludge will preclude all re-roll leakage, Comed will continue applying this criteria.

AMENDMENT 172/159 This amendment was issued via a letter from C. Shiraki to D. Farrar dated November 21 1995, transmitting a Safety Evaluation for the use of F* repair criterion at Zion Units 1 and 2.

Description of the Amendment r The amendment revised the definition of the F* distance to allow a separate determination of the uncertainty term rather than including a fixed value in the specified distance. The new definition read: "The F* distance is the length of undegraded tube required to resist pullout. This distance is measured from the bottom of the upper hard roll transition toward the bottom of the tubesheet and has been conservatively determined to be 1.05 inches (not including eddy current uncertainty)." A previous footnote was deleted from the Technical Specification.

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Page 3 of 4 A new footnote, which specified the time frame in which the F* definition was valid, was added.

The Bases section was also revised to clarify the application of eddy current uncertainty.

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Review of Safety Evaluation Section 2.0 Evaluation j

. "The distance of 1.3 inches includes the length of undegraded hardrolled tubing necessary to 3

maintain adequate structural and leakage integrity as well as a length to account for measurement j uncertainty in determining the F* distance with nondestructive evaluation methods."

"The analysis to determine the F* distance in support of the licensee's initial F* amendment application dated May 31,1995, concluded that a hardroll distance of 1.05 inches is sufficient to 4

maintain structural and leakage integrity consistent with the staft's requirements."

j "Since the re-rolljoint meets the stmetural and leakage integrity requirements afforded by the F*

j criterion, the staff concludes that for re-rolled joints that have no operational service, an F*

l distance of 1.05 inches (not including eddy current uncertainty) is acceptable provided no defects j exist within the new hardroll of the tube."

3 "During the phone call, the licensee and staff agreed that the 1.05 was the distance necessary to l ensure that the structural and leakage integrity of the tube were maintained and that it did not

include eddy current test uncertainty."

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"The staff concludes that the licensee's proposed definition of the F* distance as the length of undegraded tube that is necessary to ensure adequate steam generator tube structural and leakage i integrity is acceptable." .

"The F* distance is defined as the length of undegraded tube required to resist pullout and provide adequate leakage integrity."  ;

Conclusion:

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The descriptions in the Safety Evaluation vary when referring to leakage integrity. However, 4 several statements clearly address the leakage integrity as being adequate, which implies that the re-roll is not leak tight but that a small amount ofleakage is acceptable.

j AMENDMENT 174/161 This amendment was issued via a letter from C. Shiraki to I. Johnson dated November 6,1996, transmitting a Safety Evaluation for the use of F* repair criterion at Zion Units 1 and 2.

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i i Description of the Anneadament i i

The amendment clarified Technical Specification 4.3 to remove the uncertainty term from the i specified distance and remove the footnote which specifies the time frame it is applicable.

The associated Bases were revised to clarify the methodology by which Zion will control

application of the F* distance.

i 1 Review of Safety Evaluation Section 2.0 Backaround 1

l "The F* criterion is based on the concept that a certain axial length (F* distance) of expanded 4 tube in the tubesheet is needed to restrain the tube from pullout and to prevent tube leak."

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" Based on the October 6 and November 20,1995 submittals, the staff concluded that the i configuration of the new hard rolljoint after the re-rolling has enough length to ensure adequate structural and leakage integrity for the tubes."

Conclusion:

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i l This Safety Evaluation contains a more limited discussion of leakage when compared to the

previous Safety Evaluations. This Safety Evaluation contains conflicting conclusions regarding

. the leakage tightness of the tubes. In one statement it clearly states that the F* criterion will

j. prevent tube leakage, while it also refers to the rolljoint as ensuring adequate leakage integrity.

Comed considers that " ensuring adequate leakage integrity" does not mean that the re-roll is leak j j tight but that a small amount ofleakage is acceptable during operation.

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