ML20237D649
| ML20237D649 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 08/13/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20237D648 | List: |
| References | |
| NUDOCS 9808270107 | |
| Download: ML20237D649 (8) | |
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NUCLEAR REGULATORY COMMISSION
\\,...../a WASHINGTON, D.C. 30000 4001 SAFETY EVALUATION BY THE OFFICE OF NUCl FAR REACTOR REGULATION RELATED TO AMENDMENT NO.137 TO FACILITY OPERATING LICENSE NO. DPR-42 AND AMENDMENT NO.128 TO FACILITY OPERATION LICENSE NO. DPR-60 NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNITS 1 AND 2 DOCKET NOS. 50-282 AND 50-306 i
1.0 INTRODUCTION
By letter dated September 24,1996, as supplemented October 17,1996, January 3,
- January 20, and November 10,1997, and January 9, June 8, and July 20,1998, the Northern States Power Company (NSP or the licensee) requested amendments to the Technical Specifications (TS) appended to Facility Operating License No. DPR-42 for the Prairie Island Nuclear Generating Plant, Unit 1, and Facility Operating License No. DPR-60 for the Prairie
. lsland Nuclear Generating Plant, Unit 2. The proposed amendments would revise TS 4.12,
" Steam Generator Tube Surveillance," to provide Prairie Island with an altemative for dispositioning steam generator tubes with degradation in the tubesheet region. The steam generators at Prairie island are Westinghouse Model 51 steam generators with mill annealed, Alloy 600 tubing. Prairie Island Units 1 and 2 steam generators were fabricated with a 2.75-inch partial depth roll expansion of the ends of the tubes at the bottom of the tubesheet.
The licensee's submittals dated January 3, January 20, and November 10,1997, and January 9, June 8, and July 20,1998, provided additional clarifying information within the scope of the original Federal Register notice and did riot affect the staff's initial no significant hazards consideration determination.
l With these amendments steam generator tubes with degradation below the EF* distance in the l
tubesheet region would not roquire sleeving or plugging but could be repaired by installation of l
l additional roll expansion to meet the EF* criteria. The proposed changes are intended to l
l provide adequate assurance of steam generator tube integrity because the presence of the l
tubesheet in conjunction with the hardroll process ensures structural and leakage integrity of l
the tubes. The presence of the tubesheet provides constraint to the tubes by minimizing the I
amount of deformation a tube can undergo beyond its expanded outside diameter. The
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proximity of the tube and tubesheet, due to the hardroll expansion, limits the amount of primary-to-secondary leakage. The EF* criteria provide a similar level of protection for tube degradation in the tubesheet region as that afforded by Regulatory Guide (RG) 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," for degradation located outside the tubesheet region.
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/ The licensee supported its request with proprietary and nonproprietary versions of Westinghouse Report WCAP-14225, Revision 1, entitled "F* and Elevated F* Tube Plugging Criteria for Tubes with Degradation in the Tubesheet Region of the Prairie Island, Units 1 and 2 Steam Generators." In support of its repair methodology, the licensee also supplied the results of its test program to verify the adequacy of the reroll process for returning tubes to service.
This program is documented in ABB Combustion Engineering Repod CEN-620-P, Revision 05-P, " Series 44 & 51 Design Steam Generator Tube Repair Using a Tube Re-Rolling Technique (Proprietary ar.d nonproprietary versions available)."
2.0 BACKGROUND
General Design Criterion (GDC) 14 of Appendix A to 10 CFR Part 50 requires that the reactor coolant pressure boundary be designed, fabricated, erected, and tested so as to have an extremely low probability of abnormal leakage, of rapidly propagating failure, and of gross rupture. RG 1.121 provides guidance on an acceptable method for establishing the limiting safe conditions of tube degradation of steam generator tubing. Technical Specification 4.12,
" Steam Generator Tube Surveillance," assures the continued integrity of the steam generator tubes that are a part of the primary coolant pressure boundary. If a tube is found to be defective, the existing TS requires that the tube be removed from service by plugging, or repaired by sleeving or by utilizing F* in the lower region of the tubesheet.
The industry experience has shown that defects have developed between the mid-thickness of the tubesheet and the top of the tubesheet. The staff believes that tubes having degradation in i
the tubesheet region may remain in service provided the degradation is below a specified I
distance from the bottom of the roll transition, and the undegraded portion of the tube in the tubesheet can maintain adequate structural and leakage integrity under loading from normal operations, anticipated operational occurrences, and postulated accident conditions. RG 1.121 recommends that the margin of safety against tube rupture (or pullout) under normal operating conditions should not be less than a factor of three at any tube location where defects have been detected. For postulated accidents, RG 1.121 recommends that the margin of safety against failure be consistent with the margin of safety determined by the stress limits in NB-3225 of Section lil of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME). Structural loads imposed on the tube-to-tubesheet joint primarily result from the differential pressure between the primary and secondary sides of the tubes. The peak postulated loading occurs during a main steam line break due to a lowering of
- the secondary side pressure. However, normal operating loads, cyclic loading from transients (e.g., startup/ shutdown), and potential thermal expansion loads can also be significant. The l
analysis supporting the EF* criteria should address all loading conditions necessary to maintain adequate integrity of the tube-to-tubesheet joint.
The elastic preload between the tube and tubesheet not only prevents pullout of the tube from
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the tubesheet, but also provides a leak tight barrier minimizing the potential for
-j primary-to-tecondary coolant leakage. With sufficient length of hardroll, the tube-to-tubesheet joint will limit leakage under normal and faulted conditions. If a through-wall crack is present in the joint, primary coolant may leak to the secondary side. Under the proposed EF" criteria, I
licensees must demonstrate that leakage integrity of the joint is maintained under all analyzed conditions.-
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' 3.0 EVALUATION 3.1 EF* Encaaement Distance Determination The application of the EF* criteria applies to those tubes where an additional roll expansion is performed above the mid-plane of the tubesheet. The EF* criteria are based on determining the length of hardroll engagement necessary to resist tube pullout forces during normal operation, test, upset, and faulted conditions. The necessary engagement length (as discussed below, this measurement is 1.62 inches) applicable to the Prairie Island steam generators was determined based on a semi-empirical method of quantifying the axialload bearing capability of the rolled joint, resulting from the radial contact preload pressure and the associated friction between the tube and the tubesheet. A test program was conducted by Westinghouse to quantify the degree of interference fit between the tube and the tubesheet provided by the hardrolling operation.. The calculation of the value of the EF* digtance is based on determining the length of hardroll necessary to offset the applied loads (i.e., rolling process preload, thermal expansion preload, pressure preload tubesheet bow loss, and the axial pullout forces) during the maximum normal operating conditions or faulted conditions, whichever provides the largest value. The applied loads are balanced by the load-carrying ability of the hardrolled tube as calculated using an empirically derived coefficient of friction at the tube-to-tubesheet interface.
The EF* criteria will be implemented by forming an additional roll expansion joint up to 2 inches below the top of the tubesheet. The EF* distance when applied below 2 inches from the top of the tubesheet, considering the loads discussed above including the effects of tubesheet bowing above the neutral bending axis of the tubesheet, has been determined to be 1.62 inches (excluding NDE [ nondestructive examination) uncertainty) for the Prairie island steam generators.
Limitation of Primarv-To-Secondarv Leakaoe Under the proposed repair criterion, tubes containing throughwall flaws may be allowed to remain in service provided the indications are located below a specified length of tube-to-tubesheet hardroll (i.e., EF* distance). The presence of a hardroll above any tube cracking provides significant resistance to primary-to-secondary (or secondary-to-primary)
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leakage through such flaws. Operating experience with the F* repair criteria approved for i
Prairie Island Unit 2 indicates that rerolled tubing has a higher potential for leakage than j
originally determined in leak rate qualification testing of simulated tubesheet hardroll expansions. In the 1997 Prairie Islcnd Unit 2 refueling outage, secondary side hydrostatic pressure testing identified several F* tubes as the possible source of primary-to-secondary j
leakage measured prior to plant shutdown. The licensee quantified the maximum leak rate from the F* tubes through in-situ pressure testing. Based on the results of the inspection findings and the in-situ pressure testing, the licensee determined a bounding leak rate for a l
single steam generator assuming all tubes had been repaired by rerolling. The calculated leak rate is lower than the site allowable leak rate limit of 1 gallon per minute.
The licensee will implement changes in the rerolled tube acceptance criteria from the process used under the F* repair criterion. These changes should improve the resistance to leakage of l
repaired tubing to that afforded by the installation procedures utilized in previous applications of l
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the F* repair criterion. The licensee has also demonstrated by analysis that the postulated leak rate from all tubes repaired by rerolling is within acceptable limits. The staff concludes that the proposed rerolling repair process will provide adequate leakage integrity to those tubes retumed to service using the EF* repair criterion.
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l Tube Intearity Under Postulated LOCA Conditions The licensee must also demonstrate tube integrity under secondary-to primary differential pressure loading during a postulated loss-of-coolant accident (LOCA). A review of tube collapse strength characteristics indicates that the constraint provided to the tube by the
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tubesheet gives a significant margin between tube collapse strength and the limiting secondary-to-primary differential pressure condition, even in the presence of circumferential or axialindications.
Insoection of EF* Tubes
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The EF* criteria are based on determining the length of hardroll engagement necessary to
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resist tube pullout forces and/or leakage within the tubesheet area. The necessary engagement distance was determined to be 1.62 inches. An eddy-current examination is performed to determine the acceptability of the reroll. This examination, as noted below, verifies the presence of the reroll as well as the location. Since the rerolling tool used results in a 2-inch roll in previously unexpanded tubing, this examination verifies the existence of the necessary engagement. Accordingly, addressing eddy-current measurement uncertainty is unnecessary. Rerolled joints will be subject to a bobbin coil profilometry examination to verify the adequacy of the installed joint. Data from the profilometry inspection will be used to confirm that the roller forces were sufficient to fully expand tubes into the tubesheet bore consistent with the repair requirements for EF* tubes.
The proposed TS require that all tubes that have had the EF* criteria applied will be inspected in the EF* regions of the roll expanded regions. The region of these tubes below the EF*
i regions may be excluded from the plugging criteria of the TS. The requirement to inspect all I
' EF* tubes during each TS inspection is acceptable to the staff.
1 Renair Methodoloav in support of the proposed EF* amendment, the licensee submitted ABB Combustion j
Engineering Report CEN-620-P, Revision 05-P, " Series 44 and 51 Design Steam Generator 1
Tube Repair Using a Re-Rolling Technique." This report details the analyses and testing completed to verify the structural and leakage integrity of the repair hardroll introduced in the rerolling process. To retum steam generator tubes to service with confirmed degradation below the EF* distance in the tubesheet region, the licensee will implement a field rerolling process that expands tubes into the tubesheet over a 2-inch length located up to within 2 inches of the tubesheet secondary face. Field-installed hardrolls will be inspected to verify that (1) the new expanded tube region is free from defects over the EF* distance, (2) the hardroll is in the proper location, and (3) the tube was adequately expanded into the tubesheet.
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s 5-One factor that could affect the integrity of the repair is the presence of sludge between the i
outer tube wall and the tubesheet bore. In the qualification test program, the vendor modeled and evaluated the adverse effects of crevice sludge on the tube-to-tubesheet joint. Tubes were hardrolled into a tubesheet collar for testing with both wet and dry conditions. Several different tests were conducted to verify acceptability of EF* tubes. These tests included static and cyclic j
loading in addition to leak resistance testing. Based on the results of the test program, the
'j licensee concluded that the field-installed hardrolled joints would have adequate structural integrity. The leakage was also shown to be well within acceptable limits.
Recent operating experience at Prairie Island Unit 2 has indicated that field-installed hardrolled joints for tubes repaired under the F* criterion have lower resistance to leakage than demonstrated in the qualification testing for the rerolling process. To address this issue, the licensee implemented several changes in the F* repair procedure that are similcrly applied to the EF* repair process that will minimize the potential for returning tubes to service by rerolling with unacceptably low resistance to leakage. As described in CEN-620-P, Revision 05-P, the licensee will perform bobbin coil profilometry to ensure a minimum expansion level was achieved in the rerolling process. The revised repair procedure will also add an additional roll expansion over the original roll transition zone. A third change includes the performance of a i
secondary side hydrostatic test to verify the leakage integrity of newly installed rerolls. These measures provide assurance that tubes returned to service via the EF* repair criterion will have adequate leakage integrity during normal operating and postulated accident conditions.
3.2 Evaluation of Prooosed TS Chanaes The licensee proposed the following changes to the TS to implement the EF* repair criteria.
- 1. TS 4.12.B.3 " Steam Generator Tube Samole Selection and insoection" The existing TS is modified to state the following:
In addition to the sample required in Specification 4.12.B.2.a through c, all tubes which have had the F* or EF* criteria applied will be inspected in the F* and EF* regions of the roll expanded regions. The region of these tubes below the F* and EF* regions may be excluded from the requirements of 4.12.B.2.a.
- 2. TS 4.12.D.11f)
The definition of Repair Limit is modified to state that the definition does not apply to l
the portion of the tube in the tubesheet below the F* or EF* distance provided the tube i
is not degraded (i.e., no indications of cracks) within the F* or EF* distance for F* or i
EF* tubes.
- 3. TS 4.12.D.11i)
The definition of F* Distance is modified to clarify that the F* distance applies to roll expanded regions below the midplane of the tubesheet.
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u 4. Prooosed New TS 4.12.D.111)
The following new TS is added to provide the definition of EF* distance.
l EF* distance is the distance from the bottom of the upper hardroll transition toward the bottom of the tubesheet that has been conservatively determined to be 1.62 inches (not including eddy current uncertainty). EF* distance applies to roll expanded regions l
I when the top of the additional roll expansion is 2.0 inches or greater down from the top I
of the tubesheet.
- 5. Prooosed New TS 4.12.D.11m) l The following new TS is added to provide the definition of EF* tube.
EF* tube is a tube with degradation, below the EF* distance, equal to or greater than 40%, and not degraded (i.e., no indications of cracking) within the EF* distance.
- 6. IS 4.12.D.2 "Acceotance Criteria" The existing TS is modified to state the following:
The steam generator shall be determined OPERABLE after completing the I
corresponding actions (plug or repair by sleeving all tubes exceeding the repair limit and all tubes containing through-wall cracks or classify as F* or EF* tubes) required by Table TS.4.12-1.
- 7. TS Table 4.12-1 " Steam Generator Tube insoection" The existing TS Table is modified to state the following:
The words " plug or sleeve" in the action required columns for the three sample inspections are replaced with the word " repair."
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- 8. TS 4.12.E.4 "Recorts" The existing TS is modified to state the following:
1 The results of inspections performed under Specification 4.12.B for all tubes that have defects below the F* or EF" distance, and were not plugged, shall be reported to the l
Commission within 15 days following the inspection. The report shall include:
- a. Identification of F* and EF* tubes, and 1
- b. Location and extent of degradation f
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- 9. Table TS 4.12-1 " Steam Generator Tube insoection" The words " plug or sleeve" are changed to " repair" to acknowledge the variety of repair criteria.
- 10. Proposed Revision to Bases Section The Bases for TS Section 4.12 have been revised to address changes to the TS.
The staff has reviewed the TS changes and finds that they incorporate the EF* criteria consistent with the safety evaluation as discussed above and will provide adequate assurance of steam generator tube integrity. Therefore, the proposed changes are acceptable.
3.3 Conclusion Based on its review of the licensee's proposal, the staff has determined that the proposed changes will provide adequate assurance of steam generator tube integrity because (1) the presence of the tubesheet in conjunction with the hardroll process significantly reduces the potential for tube failure and/or leakage from tubes retumed to service through the application of EF* repair criteria, (2) the presence of the tubesheet provides for constraint to the tube by.
minimizing the amount of deformation a tube can undergo beyond its expanded outside diameter, and (3) the proximity of the tube and tubesheet, due to the hardroll expansion, limits the amount of primary-to-secondary leakage. Therefore, the staff concludes that tubes can be i
left in service with eddy-current indications of pluggable magnitude that are below the EF*
l distance, provided the tube is not degraded within the EF* distance.
4.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Minnesota State official was notified of the proposed issuance of the amendments. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
The amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NRC staff har, determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no l
significant hazards consideration and there has been no public comment on such finding (61 FR 64388). Accordingly, the amendments meet the eligibility criteria for categorical l
exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b), no environmental 3
impact statement or environmental assessment need be prepared in connection with the l
issuance of the amendments.
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6.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: P. Rush I
Date:
August 713,1998 4