ML20199A619
| ML20199A619 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 11/10/1997 |
| From: | Sorensen J NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML19313D074 | List: |
| References | |
| TAC-M96654, TAC-M96655, NUDOCS 9711180036 | |
| Download: ML20199A619 (17) | |
Text
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i Northern States Power Company Prairie Island Nuclear Generating Plant 1717 Wakonado Dr. East Welch, Minnesota 55049 November 10,1997 10CFR Part 50 Section 50.90 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos.50 282 License Nos.DPR-42 50 306 DPR-60 Response to NRC Request For AdditionalInformat!on Regarding Application of EF* Steam Generator Tube Repair Criteria (TAC Nos. M96654 and M96655)
By letter dated February 13,1997, the NRC Staff requested additional information regarding the application of the Elevated F* (EF*) steam generator tube repair criteria in order to complete its review of our September 24,1996 License Amendment Request. The attached information is provided in response to that Request for Additional Information.
At the request of Commonwealth Edison and Northern States Power, Combustion Engineering conducted additional testing and analysis of rerolled joints to support operability of the reroll joint at iower steam pressures down to 650 psia, to evaluate structural integrity using pull tests instead of push tests, and to evah' ate reroll samples using pulled tubing material from the Zion and Prairie Island steam generators. These tests showed acceptable pull out forces using pull tests. The expected leakage from a reroll joint has been increased based on results from pulled tube testing.
The results of this testing are documented in ABB Combustion Engineering report CEN-620-P Revision 04-P, " Series 44 & 51 Design Steam Generator Tube Repair Using a Tube Rerolling Technique", dated October 1997. Proprietary and non-proprietary version of Combustion Engineering Report CEN-620-P, Revision 04-P are provided as Attachments 2 and 3 to this submittal. Typographical errors noted in CEN-620-P are discussed in Attachment 1.
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l4 9711100036 971110 gDR ATTACHMENT CON 1alNS PROPRIETARY INFORMATION TO BE WITHHELD FRO ~M PUBLIC DISCLOSURE IN ACCORDANCE i,,, c t-w
'dI WITH 10 CFR PART 2, SECTION 2.790 b!b!bfOkbfb!hh
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USNRC NORTHERN 8TATES POWER COMPANY Nov:mber 10,1997 Page 2 As tho attached revision to Combustion Engineering Report CEN 620 P (Attachment 2) contains information proprietary to Combustion Engineering, Inc., it is supported by an affidavit (Attachment 4) signed by Combustion Engineering, the owner of the inforniation. The affidavit sets forth the basis on which the information may be withheld from public disclosure by the Commission and addresses with specificity the considerations listed in Paragraph (b)(4) of 10 CFR Part 2, Section 2.790 of the Commission's regulations.
Accordingly, it is respectfully requested that the information which is proprietary to Combustion Engineering be withheld from public disclosure in accordance with 10 CFR Part 2, Section 2.790 of the Commission's regulations, in this letter we have made no new NRC commitments. Please contact Gene Eckholt (612-3881121) if you have any questions related to the attached responses.
4 Joel P Sorensen Plant Manager Prairie Island Nuclear Generating Plant c: Regional Administrator - Region lil, NRC Senior Resident inspector, NRC NRR Project Manager, NRC Stato of Minnesota Attn: Kris Sanda (w/o proprietary attachment)
J E Silberg (w/o proprietary attachment)
Attachments:
Affidavit 1 Response to February 13,1997 NRC Request For Additional Informatic(1 2J Combustion Engineering Rerort, CEN-620-P, Revision 04 P, " Series 44 & 51 Design Steam Generator Tube Repair Using a Tube Re Rolling Technique",
October 1997 (Proprietary)
- 3. Combustion Engineering Report, CEN-620-NP, Rowlalon 04-NP, " Series 44 & 51 Design Steam Generator Tube Repair Using a Tube Re-Rolling Technique",
October 1997 (Non-Proprietary).-
- 4. Combustion Engineering Affidavit
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UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50-282 50-306 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION RELATED TO REQUEST FOR AMENDMENT TO OPERATING LICENSES DPR 42 & DPR-60 LICENSE AMENDMENT REQUEST DATED September 24,1996 Northern States Power Company, a Minnesota corporation, by this letter dated November 10,1997, with Enclosures 1,2,3 and 4 provides a response to an NRC Staff request for additional information in support of the subject License Amendment Request dated September 24,1996. Enclosure 1 provides the response to the NRC Staff Request for Additional information. Attachments 2 and 3 are the proprietary and non-proprietary versions of Revision 4 to Combustion Engineering Topical Report CEN-620. Attachment 4 is a Combustion Engineering affidavit for withholding of proprietary information.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY By 8%I P Borensen~
Plant Manager Prairie Island Nuclear Generating Plant On this lo day of M0hbw IWI before me a notary public in and for said County, personally appeared Joel P Sorensen, Plant Manager, Prairie Island Nuclear Generating Plant, and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, information, and belief the statements made in it are true and that it is not interposed for delay.
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(JSNRC November 10,1997 Page 1 of 13 ATTACHMENT 1 i
flapponse to February 13.1997_NRC Request For Additional Inforrnation The following information is provided in response to the Request for Additional Information Regarding Application of F* and EF* Steam Generator Tube Repair Criteria dated February 13,1997 received from the NRC Staff regarding the application of the EF* steam generator tube repair criteria at Prairie Island:
Discussion on MSLB leakage.
Additional testing documented in CEN 620 P Rwision 04-P (Attachment 2) has provided new leak rate data. The maximum MSLO leakage (measured at 2600 psi) for an EF* reroll was 1.16 E 3 GPH and occurred whh dry sludge. The maximum leakage of all the additional testing at 2600 psi was 1.88 E-3 GPH in a Prairie Island tube with an F* reroll. A rnaximum MSLB leakage value of 1.88 E-3 GPH is assigned for both F*
and EF* tubes based on the additional testing. The Prairie Island in situ pressure testing result of 0.03 GPH is assigned to 20% of the reroll repairs.
IyDoaraphlgpl Errors in CEN 620 P Reviajon 04-R On page 21, Section 2.1.1, the correct Secondary Operating Pressure should be 650 psla. On page 21, Section 2.1.1, the corresponding Secondary Operating Temperature is 494 'F. The limiting condition is the accident condition, it was correctly evaluated at 650 psia.
RESEQNSE TO NRC STAFF QU.ESIlQNS Question.2.a:
Provide an ascessment of the rerolled F* tubes that experienced leakage during the secondary side hydrostatic testing completed during the 19ft7 Unit 2 refueling outage. The assessment should address the issues identified in the action plan included in the document " Prairie Island Unit 2 9701 SIG Inspection Results,"
dated February 3,1997. Also summarize the scope and results of the planned in-situ pressure testing. Demonstrate that the measured leakage is bounded by the leakage assessments in the EF* license amendment request. Discuss the changes,if any, made to the reroll acceptance criteria to identify EF* tubes that are rerolled into hard packed crevices.
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USNRC November 10,1997 Page 2 of 13
Response
@ckaround The F* criterion was authorized for use at Prairie Island by License Amendments 118 for Unit i and 111 for Unit 2, dated May 15,1995 and implemented under Modification 95L486. The F* criterion in combination with an additional roll expansion (F* reroll) was applied to the tubesheet region below the centerline of the 21 inch thick tubesheet.
The ABB Combustion Engineering process was used for the additional roll expansion.'
Prior to the October,1997 outage, there were 6 tubes in service in Unit 1. There were 401 tubes in service in Unit 2 during its previous cycle (Cycle 17) using additional roll expansion and the F* criterion. There are currently 626 F* rerolls in service in Unit 2, with 418 in 21 steam generator and 208 in 22 steam generator. Primary to secondary side leakage has been stable at about 0.3 gallons per day since the Unit 2 startup in March of 1997. There will be 12 F' i trolls in service in Unit 1 following the October 1997 refueling outage.
Steam generator tubes repaired in May 1995 by adding an additional roll expansion (F*
reroll) in the tubesheet region (first application of F*) and by applying the F* repair criteria were inspected during the Unit 2 January 1997 refueling outage. The inspection identified twelve steam generator tubes repaired by the F* reroll process in May of 1995 that had leakage or seepage past the F* reroll and through the roll transition zone cracks. These F* reroll repairs had been in service for one cycle.
Primary to secondary side leakage during the cycle had increased slowly to a range of 2 to 4 gallons per day total leakage during the operating cycle from July 4,1995 through January 24,1997. Although the additional F* reroll expansion area was expected to be nearly leak tight, it is obvious that in some cases due to sludge content, tube characteristics or tubesheet bore hole abnormalities, the F* reroll joint may not be leak tight even though the F* reroll meets process acceptance criteria.
Evaluation of the post F* rerolling eddy current examination (ECT) data showed that the indications of stress corrosion cracking at the original equipment manufacturer's (OEM) roll transition zones with leaking cracks changed during the F* rerolling process and were probably the source of the minor primary to secondary side leakage identified early in the previous cycle of operation. Observations during the reroll process indicate that some roll transition zone cracks opened up during the hydraulic expansion process (water was observed after the hydraulic expansion step).
The two tubes with quantifiable leakage were R18C44 in 21 steam generator and R16C38 in 22 steam generator, Under a secondary side pressure of 740 psig, R18C44 leaked 2 drops per minute (~1.6E-3 gallons per hour (GPH))and R16C38 laaked at one drop per 3 minutes (-2.6E-4 GPH). During in situ pressure testing at Main Steam Line Break (MSLB) conditions, the minimum measured leak rate was.024 GPH for R18C44 and 0.057 GPH for R16C38. Accurate measurement of low levels of leakage during an in situ pressure test depends on the entire in situ pressure test system being leak tight.
Possible sources of system leakage are the relief valve, system connections, and the
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USNRC N2v:mt>er 10,1997 Pag) 3 of 13 t
i sealing bladders. Because of these sources of leakage in addition to the degradation l
in the tube, the final leakage value will be conservative and the final leakage value selected can be the minimum value obtained when several tests are done on the same l
tube. When in situ pressure tests are done to only Main Steam Line Break pressere, the morphology of the leaking indication is not significantly changed thus allowing i
repetitive testing at the same conditions or pressures. Tests on the two tubes with measurable leakage, R18C44 and R16C38, were repeated at least once. The leakage value assigned to R18C44 is 0.03 GPH and the leakage value assigned to R16C38 is 0.06 GPH In response to these discoveries, Modification 95L486 Addendum 1 was implemented which added an additional roll expansion over the original roll transition zone called an
'RTZ roll", added additional acceptance criteria for profilometry of the F* reroll and added a post maintenance leak check with the secondary side pressurized. These additional measures resulted in reduced steam generator primary to secondary leakage since the Unit 2 startup in March of 1997.
Action Plan - Examine Installation Records investigation into the possible causes of this leakage has been completed. Evaluation parameters and testing results are provided in Table 1: " Prairie Island Unit 2 Evaluation Parameters for Selected F* Rerolls". No inconsistencies with the installation procedures and process qualification were identified. However, measurements of the profilometry of the F* rerolls indicates an additional screening criteria based on profilometry is appropriate. Several areas were investigated:
- 1. Evaluation of the previous installation toraue traces: A review of the previous field data indicated no torque traces similar to the dry sludge conditions tested in the qualification process. None of the previous field torque traces were as smooth as the wet sludge qualification test samples nor as inconsistent as the dry sludge qualification test samples. There were no distinguishing features between the torque traces of the leaking tubes and a sample of tubes with no leakage. Thus, there appears to be no correlation between the tubes with signs of leakage and the torque trace characteristics. None of the torque traces are unacceptable when compared to the dry sludge qualification tests.
- 2. Evaluation of the previous calibration records: The reroll control system was calibrated per the requirements of the installation procedures. No anomalies were identified.
- 3. Evaluation of Roll Expander Usaae and Lubrication: There was not a correlation between roll expander usage and the leaking or seeping F* reroll tubes. The suspect tubes were all done within the first 3 to 105 uses of the maximum allowable of 200 uses of the roll expander. The roll expanders were lubricated as required by procedure.
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USNRC Nav:mber 10,1997 Page 4 Of 13
- 4. Evaluation of toraue values: The torque values for all of the leaking tubes were within the acceptance criteria established during the qualification program and there was no correlation witi moist tubes. The torque value attained was within the acceptance band of 100 to 170 in-:bs.
Table 1: Prairie island Unit 2 Evaluation Parameters for $ elected F* Rerolls in Sdu Results i
1995 olM-OEM Applied F*R R, RTZ Normal Operating Torque, in.
Diameter Reason 80 ROW COL VOLTS CALL Pressure MSLB lbs LOCATION LOTH Ml!Is 14TO 1.i 0)
$3 RKk 21 18 44 lTT74iE613 to 063Jph.024 s 6}7gph 137_ 1BH LIAK 22 16 30 18 2
_ M Al 0004 to.023ph OSto.11oph 132gBH 1 TTO-12 0$
15 6 Seep 21 18 21 16 8 MAI O
O 126i1BH 13TO 0 9 04 31
. Seep li 11 64 23 IlAl 0
_0 ifiTBH 13TO. 1.0 61 49, 14TO-10 04 30 Seep 21 7
3b 15 0 ~ MAI O
O 121 1BH Seep 21 11 61 17.9 mal 0
0 110 1BH 14TO 1.1 03 44 123]1BH 130 hH 1 bi6 0 8 0t 43 kiep 21 1
20 26]lAl 0
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Seep 21 6
20 272 TAI O
Ol' 1$TO 09 06 38 1 $TO. 0 9 06 23 Beep 21 9
41 33 2 mal 0
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124 1BH fe~ep 21 16 41 136 MAI O
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124 1BH 16f6 10 06 33' Seep 21 3
43 30 5
_ MAI OCD48 gqh' 0l 120 1BH 16TO-1.2 04 49 Seep 22 13 46 38 8 MAf 00064 gph 01 136 1BH 1BTO 09 09 81 0
0 132TBH 1 $TO 0 7 08 25
,Large Volt 21 23 _ 32 61 4 MAI 0{
1 STO 0 7 08
$6 0
117 1BH Large Volt 21 14
$5 62.7
$Al mal e mutilple astal_ indication f
$_Ahtingle axial endication l _.
j 1BH is the landmark identifier for ite lower to i transtion tone of the F* rerolllocated i 6nch above the OEM RTZ T ordy 1 pump stroke met be less thm'1 detectable I
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Action Plan - Evaluate F* Reroll Bobbin Coil Profilometrv Data All of the F* rerolls installed in 1995 were examined with the bobbin coil and a profilometry standard (In the past, bobbin coil data was used only to demonstrate that the F* reroll was present and located above the OEM hard roll). It was proposed that evaluation of the profilometry data might provide insight into the behavior of the leaking tubes. The parameter chosen for evaluation was the differenco in diameter between the average OEM hard roll diameter and the minimum F* reroll diameter. Examples of this diameter measurement difference (in mills) is shown under column "OEM.F*RR" in Table 1. One correlation, R16C38, had the largest difference between the OEM hard roll and the F* reroll. Even though there was no other relationship between the suspect tubes and the profilometry data, an acceptance criteria for profilometry based on the difference in diameter between the average OEM hard roll diameter and the minimum F* reroll diameter was added to the installation process at Prairie Island for new F*
rerolls. The distribution of the partial data set for the diameter differences of the F*
l rerolls from the 1995 and 1997 outage is shown below, i
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. Attachment 1 USNRC November 10,1997 Pa9e 5 of 13 Frequency Distribution of Reroll Profilometry r
Diameter Difference 1995 1997 OEM - Rercll, mills 21 SG 22 SG 21 SG 22SG 1
5 15 3
3 2
25 18 1
4 i
3 51 11 9
16 4
71 11 30 14 5
60 13 46 17 6
23 10 33 9
1 7
7 4
11 10 8
8 9
-2 11 9
2 5
1 3
10 0
6 1
3
> 10 2
6 6
3 Total 254 108 143 93 Implementation of the profilometry acceptance criteria for both the 1995 and 1997 rerolls required repair of 17 F* rerolls by adding a reroll above the first reroll. Six of the second reroll repairs did not meet the profilometry acceptance criteria and were plugged.
Action Plan - Evaluate Process Parameters aoainst Qualification Report This evaluation was completed satisfactorily as part of the review of the installation records as discussed above, in addition, the reroll for tube R16C38 was rerolled at the allowable upper torque limit which resulted in an applied torque of 190 in-lbs, which is less than the qualifintion report upper limit. The difference between the OEM roll and the reroll diameter was reduced from 15.6 mills to 9.2 mills.- An in situ pressure test was then conducted. The MSLB leakage for R16C38 was reduced from 0.06 GPH to 0.0019 GPH by the higher torque reroll. This demonstrated that profilometry measurements can, in at least one case, identify the potential for higher leakage rerolls. For several other rerolls at higher lorque limits in tubes to be plugged, the higher torque value did not increase the d_iameter significantly, indicating the possible presence of hard sludge in the tubesheet crevice. The procedure requirements for torque value limits were not changed.
Action Plan - Determine leak rate at MSLB conditions with in situ pressure tests of selected rerolled tubes Fourteen tubes with F* rerolls installed in May 1995 were tested in situ. Leakage could only be measured in two of the tubes. For the other twelve tubes tested for at least 20 minutes, the leakage is less than 4.5X10" gallons per hour (GPH). The tubes tested are listed in Table 1.
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Page 6 of 13 The in situ pressure test system uses two bladders to isolate the degraded tubing region in order to focus testing on the region of interest. The in situ test probe contained a sufficiently long hydro chamber such that the defects at the OEM RTZ and the F* reroll /hydraulle expansion region were all pressurized to the same value. The sealing bladders are pressurized up to 6000 psig and could have some effect on the sealing properties of the F* reroll. However, the sealing bladders were positioned outside of regions of interest and therefore could not affect the F* reroll seal. The in situ pressure test is done at room temperature and therefore there is no beneficial effect from the increased radial preload at operating temperature due to the higher thermal expansion coefficient of the Alloy 600 tubing compared to the carbon steel tube sheet. Pressures were corrected for temperature effects. The minimum required pressure for the MSLB test was 2816 psig.
Accurate measurement of low levels of leakage during an in situ pressure test depends on the entire in situ pressure test system being leak tight. Sources of leakage are the relief valve, system connections, and the sealing bladders. The pump is a diaphragm pump and is not subject to piston leak by. Higher bladder pressures can assure a reasonably leak tight system. Because of these sources of leakage in addition to the tubo degradation, the final leakage value for low levels of leakage will be conservative and the final leakage value selected can be the lowest value obtained when repetitive tests are done on the same tube under the same conditions. When in situ pressure tests are done only to Main Steam Line Break pressure, and no higher, the morphology of the leaking indication is not significantly changed thus allowing repetitive testing at that same pressure. Although particles could conceivably reduce the OEM RTZ crack opening during in situ pressure testing, domineralized water is used for testing and the water is not expected to contribute to plugging of OEM RTZ cracks. In the few cases of multiple tests on the same tube, :here was no decreasing trend which would support plugging of the cracks. In situ pressure tests on the two tubes with measurable leakage, R18C44 and R16C38 were repeated. Test results for R16C38 were 0.058, 0.072,0.05, and 0.11 GPH. Test values for R18C44 were 0.024 and 0.027 GPr-l. The leakage value assigned to R18C44 is 0.03 GPH and the leakage value assigned to R16C38 is 0.06 GPH.
Upon further investigation of R16C38 it was determined that the profilometry measurement identified the F* raroll in R16C38 as an outlier. With the new profilometry acceptance criteria between the OEM hard roll and the F* reroll, R16C38 would have needed to be repaired by an authorized repair or plugged. Therefore, the appropriate conservative leak rate to be assigned to future operating cycles when profilometry acceptance criterion is used is the average leak rate from R18C44 which is 0.03 GPH. For evaluation of the previous operating cycles, the leak rate used was 0.06 GPH since the profilometry acceptance criterion had not been applied in the May 1995 (or Unit i 1996) reroll installation campaign. For tubes which meet the F* criteria based on the OEM hard roll, the qualification report value of 6.87E-4 GPH can be used since the OEM hard roll was made with clean tubing and tubesheet bore holes. The profilometry data for the six F* rerolls in Unit i have been evaluated and all meet the profilometry criteria.
USNRC N:v:mber 10,1997 i
t Pa0e 7 of 13 I
i Action Plan - Assessment of Total MSLB Leak Rate for orevious Pl 2 Cycle 17 The largest number of F* reroll tubes in service in one steam generator was 288 in 21 Steam Generator during the last operating cycle. Multiplying 0.06 GPH per F* reroll tube (F* rerolls are currently only in the hot leg) by 288 rerolls equals 17.3 GPH total leak rate at MSLB conditions. This was an aceptable value compared to the USAR allowable leak rate of 300 GPH (5 GPM)(and the revised allowable leak rate of 1 GPM) for Main Steam Line Break off site dose leak rate with an RCS activity level of 1 microcurle/ gram dose equivalent 1131 which produces off site ooses eq'Jvalent to a small fraction of 10CFR100 dose limits '8 Action Plan - Assessment of Total MSLB Leak Rate for the Comoteted Unit 1 Cvele 17 Operatino Cvele -
I The largest number of F* reroll tubes in service in one steam generator in Unit 1 was 3.
Multiplying 0.06 GPH per F* reroll by 3 equals 0.18 GPH totalleak rate at MSLB conditions. This acceptable value was less than the USAR basis for MSLB of 300 GPH l
(5 GPM).
Action Plan - Assessment of Total MSLB Leak Rate for Future Operatino Cveles Since only 2 of 14 rerolls which were tested actually leaked greater than the new qualification report value of 1.88 E-3 GPH, a conservative fraction of rerolls expected to leak at the higher plant value is 20% of the installed rerolls. Thus the leakage from 3388 tubes with rerolls in each leg is 20% of 3388 tubes x 2 x.03 GPH + 80% of 3388 tubes x 2 x 1.88 E-3 GPH
= 51 GPH Since 51 GPH is less than the new MSLB leak limit of 60 GPH (1 GPM), there is no limit on the number of F* and EF* rerolls that can be in service in one steam generator in the future when utilizing the revised Prairie Island acceptance criteria
- for F* and EF*. Note that total leakage from all leak limiting repairs must be evaluated.
Action Plan - Evaluate chances in ET indications due to the Hydraulic Exoansion Process l
The + Point eddy current data was evaluated for the 12 tubes with indications of-moisture. Prior to the hydraulic expansion in 1995, the average voltage of the indications was 2.34 volts. After the hydraulic expansion and the F* reroll, the average l
voltage increased to 6.7 volts. The average voltage of these same indications increased to 7.8 volts at the 1997 inspection. Average lengths of these indications increased, likewise, from.26 to.50 inches during the F* reroll process and then to 0.6 inches at the 1997 inspection. The majority of the growth occurred as a result of the F*
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USNRC N v;mber 10,1997 Page 8 of 13 reroll process. Because the May 1995 F* reroll was applied one inch above the OEM RTZ indications, it is assumed that the major change in the Indications was due to the hydraulic expansion step.
Chanaes to the Reroll Acceptance Criteria and the Roroll Process Modification 95L486 Addendum 1 added an additional acceptance criteria based on bobbin coil profilometry to insure a minimum F* reroll expansion was attained, added on additional roll expansion over the original roll transition zone, called a "RTZ roll",
and added a post maintenance leak check with the secondary side pressurized as additional measures to improve steam generator prima,y to secondary leakage reliability. All other acceptance criteria remained unchanged including the original torque value limits and torque trace requirements for the F* reroll established by ABB Combustion Engineering. These changes to the reroll process will be applied to the reroll process used for the EF* criteria.
The investigation did not identify reroll process parameters which could positively demonstrate that F* tubes are rerolled into hard packed crevices. It has been determined that the torque trace characteristics under actual steam generator conditions do not readily duplicate the qualification report characteristics. Therefore, it was necessary to identify additional measures which could provide reasonable assurance that potentialleakage from an F* rerollis minimized. In particular, as describec above, the use of bobbin coil profilometry to identify unusual crevice and or tube conditions is useful. The major change to the reroll acceptance criteria was the addition of specific profilometry acceptance criteria. An acceptance criteria was established for the difference in diameter between the OEM hard roll and the F* reroll.
This acceptance criteria resulted in 17 F* rerolls requiring repair and 6 of those reroll repairs requiring plugging. The unacceptable profilometry can be due to hard sludge interference, tube and tubesheet hole dimensional tolerances, or tubing material characteristics.
The purpose of the RTZ roll is to partially seal the roll transition zone stress corrosion cracks (SCC), prevent the hydraulic expansion from opening these known cracks, and reduce the probability of a leak developing past the F* reroll and through the existing roll transition zone cracks. This new RTZ roll is installed prior to the F* reroll and thus has no effect on the F* reroll. The RTZ roll can be added because it is located below the new pressure boundary established by the new additional hard roll installed to meet F* criteria and is not assumed to be there to meet the F* repair criteria. At the start of the 1997 F* reroll installation, a group of twenty tubes was examined by ECT a'ter each step of the new process. The average voltages are shown below.
Pro RTZ Roll Post RTZ Roll Post Hydraulic Expansion Post F* reroll 2.8 1.35 1.72 1.52 11 can be seen that the RTZ roll had the desired affect of preventing the large increase in RTZ indications identified during the F* reroll process in 1995.
Attachm:nt 1 USNRC N:v:mber 10.1997 Page 9 cf 13 A post maintenance leak check with the steam generator secondary side filled above the tube bundle and pressurized to greater than 100 psig was added to the F* rerolling process.
In summary, Modification 95L486 Addendum 1 added an additional roll expansion over the original roll transition zone, called an *RTZ roll", additional acceptance criteria for profilometry of the F* reroll and a post maintenance secondary side hydro as additional measures to improve steam generator primary to secondary leakage reliability.
In addition, the acceptable number of F* rerolls in each steam generator was established using the limiting leak rato determined from in situ pressure testing during the Prairie Island 1997 Unit 2 refueling outage. As described above, this value of leakage is small enough such that no limit is applied to the number of F* rerolls when using the fraction of tubes with leakage and the new MSLB allowable leak rate of 1 GPM established by the voltage based repair criteria licensing process.
The secondary sido leak check for the new reroll installations was completed on February 18,1997 with no leakage identified at any of the new F* reroll tubes in Situ Pressure Test Leak Rate versus Operational Leak Rate When the two leaking tubes and the ten seeping tube leakage values at normal operating pressure are added together with a minimum detection level of 4E-4 GPH, the test leakage added up to 0.7 GPD primary to secondary side leakage at normal operational pressures. Although this is less than the measured operational leakage of 2 to 4 GPD, it is within a factor of five which can be considered reasonable. The EPRI PWR Primary to Secondary Leak Guidelines state that leakage below 5 GPD is not in a range that can be accurately monitored and consider leakage less than 5 GPD to be no leakage. Increased monitoring occurs at 5 to 30 GPD and is considered the range at which leakage can begin to be detected and reliably quantified. In addition, there was one leaking explosive plug which would have contributed a small amount of leakage.
Therefore, there is reasonable agreement between the in situ test results and the operational measurement of leakage, Question 2.b:
Estimate the total steam line break leak rate in the limiting steam generator due to bypass leakage around elevated F* tubes.
Response
Development of the leak rate per EF* rerolled tube was described in the answers to Question 2a. The assumption was made that 20% of the tubes were rerolled into hard packed crevices with through wall flaws. This is a conservative assumption since only two F* rerolls exhibited leakage and 10 exhibited seepage out of a total of 401 F*
USNRC Nov:mber 10,1997 Pag)10 of 13 rerolls during a hydrostatic test at 740 psig secondary side pressure. Using profilometry acceptance criteria, a MSLB leak rate of 0.03 GPH per rerollis assumed j
for 20% of the EF* reroll tubes in service. Using test results documented in Revision l
04 P to CEN-620-P, a MSLB leak rate of 1.88 E-3 GPH per reroll is assumed for the remaining 80% of the EF* reroll tubes in service. If rerolls were installed in both ends of all the tubes, the leak rate would be.85 GPM which is less than the new MSLB limiting leak rate of 1 GPM.
Question 2.c:
The ABB/CE report " Series 44 & 51 Design Steam Generator Tube Repair Using A Tube Rerolling Technique," dated December 1996 (proprietary) states that tubes rerolled into hard packed crevices may have much lower than expected average i
wall thinning for the rerolled joint. Combustion Engineering Report CEN 620 P demonstrated that tubes rerolled into hard sludge did not move during push testing. However, the assessment does not address the potential strengthening of the tube to tubesheet joint caused by the application of a compressive load along the axial direction of the tube. Assess the reduction in the pullout load resisiance for EF* tubes, if a reduction In the calculated pullout load is greater than that assumed in the analysis, discuss the basis for returning tubes to service that are rerolled into hard packed crevices.
Response
Pull out tests of F* rerolls installed in hard packed crevices were performed.
- Pull out load with baked on dry sludge occurred at 3470 pounds and in one case, the fixture failed at 6624 pounds force. This pull out load exceeds the pull out force of 2886 pounds which would result from three times the normal operating differential pressure acting on the tube diameter at the maximum design primary to secondary side differential pressure of 1600 psi. Based on these tests, there is still sufficient restraining force in a tube with hard packed crevices. It is proposed that leakage could occur in crevices which have a partial packing of the crevice due to channeling through the sludge in the crevice even though in all appearances the F* reroll is acceptable.
Based on the in situ tests at Prairie Island, this leahage will still be acceptable.
Excerpts from CE/ABB Test Report on the Pull Testing of Reroll Joints':
4.0 Test Program A set of reroll samples were prepared in order to cover the range of potentih! conditions observed in actual steam generators. These conditions include tube hole size, reroll joint torque values and the presence of dry sludge in the crevice region. The test samples and the associated conditions are listed below:
Samples 1 & 2:
Attm21 ment 1 USNRC Neven.ber 10,1997 o
Page 11 of 13 Smallliole Diameter liigh Torque No Sludge in Cresice Sampks 3 & 8:
Large llole Diameter Low Torque No Sludge In Crevice Sampht1A1 Nominalliole Diameter Nominal Torque Dry Sludge In Crevice The samples were prepared by performing a rerolljoint in a steam generator tube sample.
The tube samples were prepared from.875" x.050" Inconel 600 tubing and the tubesheet blocks were fabrinted from two inch carbon steel bar stock. The tubes were rolled at one end in order to simulate the original hard roll. The two samples with hard sludge had a layer of magnetite slurry applied to the tube outside diameter and block inside diameter prior to hard rolling. These two samples were then placed in an oven at a temperature of
-650'F for eight to ten hours.
The test sampics were then placed in a test fixture in order to perform the rerolling operation. The Repair 2000 system was used to perform the proper process steps to the samples.
The lield reroll procedure was utilized to assure that the proper joint configuration was produced. A roll expander with a 1.25" effective roll length, identical to that used in field applications, was used to make the rerolljoints.
5.0 Iest Results The test samples had the original rolled joint machined away and fixtures were welded to the tube and the block in order to allow the pulling of the tube from the tubesheet block.
.... The table below summarizes the test results of this program.
FA PULL TEST DATA Sampic No.
Ilole Dia.
% Wall Reduction Max. Load 1
.887 110 in lbs 165.1 in-lbs 5.06 3262 lbs 2
.887 1li)in-lbs 165.3 in-lbs 5.80 3358 lbs 3
.893 70 in lbs 127.4 in-lbs 4.02 3166lbs 8
.893 60 in-ibs 120.4 in-lbs 3.65 3117lbs 5
.890 90 in-lbs 145.5 in lbs 4.06 3470 lbs 6
.890 80 in lbs 143.7 in-l'bs No Data' 6624lbs
' Fitting Failure i
USNRC November 10,1997 Page 12 of 13 All the test samples were pulled to failure. The mode of failure was joint slippage, except for Sample 6, which had a failure of the threaded fitting at the point where it was welded to the tube.
For the a.iditional reroll test prgram described as Phase 2 tests in Revision 04-P of CEN 629 P, the samples were pulled to failure, it was decided that pulling the samples represented a more conservative approach than the push test. The rerolljoints made from Prairie Island tubing failed at a minimum load of greater than 3200 pounds. For the Prairie Island tubing, the maximum failure load was greater than 5400 pounds. For all of the additional testing described in Revision 04-P to CEN 620-P, which included actual plant tubing and additional pull tests for both EF* and F* lengths of addit!onal roll expansion, the minimum pull out load was greater than 3100 pounds.
Qgg.ption 2,d:
The ABBICE report dated Decernber 1996, stated that rerolling tubes into hard sludge is considered off dealgn, and the decision to return to service tubes rerolled into hard packed crevices will be based on operational concerns.
Provide the basis for returning to service tubes that have been rerolled into hard sludge.
Response
Pull out loads for EF* rerolls installed in hard dry sludge packed crevices have been performed. Pull out with baked on dry sludge occurred at greater than 6000 pounds.
This pull out load exceeds the pull out force of 2886 pounds which would result from three times the normal operating di"erential pressure. Based on these tests, there is still sufficient restraining force in a tube with hard packed crevices. It is proposed that leakage could occur in crevices which have a partial packing of the crevice due to channeling through the sludge in the crevice even though in all appearances the EF*
reroll is acceptable. Based on the in situ pressure tusts at Prairie Island, this leakage will still be acceptable. The profilometry acceptance criteria established at Prairie Island should prevent leaving rerolls in service with significant hard sludge present.
CONCLUSIONS NSP has evaluated the leakage identified with two F* reroll repairs in the Prairie Island Unit 2 steam generators and has concluded that:
.1 Strucbral and leakage integrity is maintained during normal and accident conditions,
- 2. Even though there is an increase in the total maximum predicted leal;3ge from 0.08 GPM in the original EF* license amendment request to the revised 0.85 GPM maximum value, the total leakage under accident conditions is bounded by the maximum
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USNRC November 10,1997 Page 13 of 13 I
allowablo leakage which insures offsite and control room dose limits are not exceeded.
The conclusions of the safety evaluation and significant hazard evaluation in the September 24,1996 EF* License Amendment Request remains valid, j
- 3. The NRC Safety Evaluation included with the May 15,1995 F* License Amendment supports the promise that tiw F* repair criterion is a leak limiting criterion,
- 4. Prairie Island can operate safely utilizing the EF* reroll repair process, and
- 5. Improvements made to the reroll process have minimized the expected leakags from F*
reroll joints and are expected to minimize leakage from EF* reroll joints.
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USNRC November 10,1997 l
Page 13 cf 13 i
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REFERENCES i
' ABB Combustion Engineering report CEN S20 P Revision 00, March 1995," Series 44 l
& 51 Design Steam Generator Tube Repair Using a Tube Re-rolling Technique" l
8 DESIGN BASES DOCUMENT," ACCIDENT ANALYSIS", DBD-TOP-01, Rev.1, l
I Sections 3.4.4 and 5.2.1
- FSAR Section 14.2.5, " Rupture of a Steam Pipe", page 14.2-34d, amendment 33,4 9-73 l
' Prairie Island Modification 95L486 Addendum 1:
- Add A Hardroll Over The Original l
i Roll Transition Zone To The Additional Roll Expansion Repair Process for Steam Generator Tubes and Add Additional Acceptance Criteria for F* Rerolls" j
' ABB Combustion Engineering Report 97-TR FSW-009 Revision 00, " Test Report on the Pull Testing of Reroll Joints in Steam Generator Tubes", dated 2-15 97 I
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