ML20196K632
| ML20196K632 | |
| Person / Time | |
|---|---|
| Site: | Crystal River  |
| Issue date: | 06/28/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20196K630 | List: |
| References | |
| NUDOCS 9907090230 | |
| Download: ML20196K632 (4) | |
Text
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'I' NUCLEAR REGULATORY COMMISSION n'
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WASHINGTON,.A C. 20555
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.180 TO FACILITY OPERATING LICENSE NO. DPR-72 REPAIR ROLL CRITERIA FOR STEAM GENERATOR TUBING FLORIDA POWER CORPORATION CRYSTAL RIVER UNIT 3 DOCKET NO. 50-302
1.0 INTRODUCTION
By letter dated August 31,1998, as revised on March 18,1999, Florida Power Corporation (the licensee), submitted a request to amend the technical specifications (TSs) for Crystal River Unit 3 (CR-3). The purpose of the amendment is to implemant altamate repair criteria for steam generator (SG) tubes that have degraded rolljoints inside the upper tubesheet. The alternate repair criteria would allow new roll joints to be installed below the degraded roll joints in the upper tubesheet. The alternate repair criteria were developed based on the results of a qualification program, documented in Framatome topical report, BAW-2303P, Revision 3, "OTSG Repair Roll Qualification Report," which was included in the submittal dated August 31, 1998. Subsequent to the initial application for this amendment, the licensee and its vendor revised the limiting accident for the CR-3 once-through steam generators (OTSGs) that was considered in BAW-2303P. On March 18,1999, the licensee oubmitted an additional topical report, BAW-2342P, Revision 1, "OTSG Repair Roll Qualification Report - Addendum A," to appropriately modify the previous analyses and proposed changes to the TSs to support the amendment. The information contained within this submittal superseded that provided by the August 31,1998, submittal. The March 18,1999, submittal also included a double repair roll methodology which was not included in the original submittal. As a result, a revised proposed no significant hazards consideration determination was noticed in the Federal Reaister.
CR-3 has two model 177FA OTSGs that were manufactured by Babcock and Wilcox. The OTSG tubes were fabricated from Alloy 600 material and are secured by roll expansion joints
- in the upper and lower tubesheets. The tubesheet is approximately 24-inches thick, and a seal weld at the pnmary face of each tubesheet prevents primary-to-secondary leakage around the hardroll expansions.
2.0 BACKGROUND
General Design Criterion (GDC) 14 of Appendix A to 10 CFR Part 50 requires that the reactor coolant pressure boundary (RCPB) be designed, fabricated, erected, and tested so as to have an extremely low probability of abnormalleakage, of rapidly propagating failure, and of gross
- rupture. A significant portion of the RCPB is maintained by SG tubes which have experienced vanous levels of degradation. U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," provides l
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q guidance on an acceptable method for establishing the limiting safe conditions of tube j
. degradation. In addition, the plant TSs require periodic inspections of SG tubes and require that those tubes with defects in excess of the repair limits (e.g.,40 percent through-wall) be repaired or removed from service.
The joint between the tube and tubesheet is an interference fit constructed by roll expanding I
the tube into the bore of the tubesheet, followed by a seal weld at the primary face of the tubesheet. The original tube-to-tubesheet roll joint provides sufficient strength to maintain adequate structural and pressure boundary integrity.
- Industry experience has shown that defects have developed in the tube-to-tubesheet roll joints s's a result of various degradation processes. In general, tubes with degraded rolljoints are either removed from service or repaired. The NRC has accepted altemate repair criteria allowing repaired tubes with degraded roll joints to remain in service provided that the repaired tubes can maintain adequate structural and leakage integrity under loadings from normal
. operation, anticipated operational occurrences, and postulated accident conditions.
RG 1.121 recommends that the margin of safety against tube rupture under normal operating conditions should be equal to or greater than three at any tube location where defects have i
been detected. For postulated accidents, RG 1.121 recMmends that the margin of safety
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.' against tube rupture be consistent with the margin of safety determined by the stress limits specified in NS-3225 of Section ill of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers. Structuralloads imposed on the tube-to-tubesheet roll under normal operating conditions result primarily from the differential pressure between the primary and secondary sides of the tubes. Loadings from a postulated small break loss of coolant accident are significant and are limiting for OTSG tubing. Cyclic loading from transients (e.g.,
startup/ shutdown) were also considered in the qualification of the rolljoints.
3.0 EVALUATION 3.1 Qualification Proaram On the basis of the qualification program, the licensee established that either a single or double roll repair will carry all structural loads and minimize potential leakage. The need to use a double roll depends on the location of the tube within the tube bundle. The qualification program consisted of (1) preparing a mockup to simulate tubesheet conditions, (2) establishing tube loads for the qualification tests, and (3) performing verification tests and analyses.
The mockup consisted of a perforated metal block inserted with SG tubes that simulates the tube-to-tubesheet configuration in the field. The tubes were expanded into the mockup tubesheet using an expanding tool that had the same critical dimensions as the tool used in the field.
To determine the strength of the rolljoints, the licensee applied loads to the sample tubes to simulate or exceed normal, thermal and pressure cycling transients, and postulated accident conditions.. in accordance with RG 1.121, the test pressure applied to the sample tubes exceeded three times normal operating pressure and 1.43 times peak accident pressures. To obtain conservative leakage results, the sample tubes were severed 360 degrees through the tube wallin the rolljoints.
3-In the qualification program, the licensee also considered the impact of tubesheet bowing on the roll joints because the tubesheet bore diameter can change during certain operating conditions. The combined effects of primary-to-secondary pressure differential and thermal loads may cause the tubesheet to bow in one d!rection or the other which can lead the tubesheet bore to dilate or shrink. When the tubesheet bore is dilated, the contact stress between the roll joint and the tubesheet would decrease and, thereby, reduce the pullout resistance of the rolljoint. Considering the bowing effect, topical reports BAW-2303, Revision 3, and BAW-2342P, Revision 1,'specified an exclusion zone in the tubesheet where 1
the reroll joint would not be installed.
3.2 Structural and Leakaae Integaty Based on the results of the qualification testing, the licensee determined roll lengths sufficient l
to ensure adequate margins of structural and leakage integrity. With regard to structural integrity, the licensee demonstrated through ultimate load testing (testing to simulate accident conditions) that the tube with a new hardroll expansion would not be pulled out from the upper j
tubesheet under the worst possible combination of loadings. No motion of the tubes relative to j
the simulated tubesheet was observed during the thermal and fatigue cycling tests. With regard to leakage integrity, the qualification tests showed that if each of the tubes in a SG were rerolled in the upper tubeshoei and contained a 100 percent through-wall flaw at the upper edge of the feroll, the total expected leakage from all flaws would be well below t'le i
operational leak rate limit in the CR-3 TS.
3.3 Field installathn and inspection The licensee proposed to repair tubes by the installation of either one or two hardroli joints (reroll) in the tubes that have degradation in or near the original roll region. The repaired roll is typically instalied using a manipulator and a tool head, monitored by a control system that tracks the position and monitors the torque of the roll expander. The torque is automatically controlled during the rerciling and is recalibrated after installation of a certain number of rerolls 1
to ensure the minimum torque is maintained to produce proper fit.
4 After the installation, the licensee will inspect all rerolls using eddy current techniques to ensure proper diametral expansion and positioning of the reroll repair joint. In addition, the inspections will verify that the reroll regions are free of degradation. Any reroll not satisfying
. the acceptance criteria will be either plugged or repaired with a method other than rerolling.
- For future inservice inspections, the licensee will inspect all rerolled tubes dunng steam generator inspeupon activities. If degradation is identified in the reroll region, the affected tube
. will be plugged or repaired by means other than rerolling. Only one reroll repair per tube is allowed by the proposed amendment.
4.0 EVALUATION OF REROLL RFPAIR TECHNIQUE The licensee proposed to implement an altemate repair method using a hardroll expansion process to repair tubes having indications of tube. degradation in the original roll regions of the upper tubesheet. The technical basis for the proposed reroll method is documented in topical repotts BAW-2303P, Revision 3, and BAW-2342P, Revision 1.
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, The staff has determined that (1) the licensee's attemate repair criteria were established on the basis of the qualification tests that used specimens simulating the actual tube-to-tubesheet
. Joint configuration of the SGs, (2) the loads for structural and laakage tests were specified and applied in accordance with RG 1.121, and (3) the proposed changes to plant TS satisfied all regulatory requirements applicable to SG tube integrity.
On the basis of submitted information, the staff concludes that the proposed reroll repair for
. degraded roll joints in SGs at CR-3 is acceptable because the licensee has demonstrated through an acceptable qualification program that the reroll satisfies GDC 14 of Appendix A to 10 CFR Part 50 and RG 1.121.
5.0 STATE CONSULTATION
Based upon a letter dated March 8,1991, from Mary E. Clark of the State of Florida, Department of Health and Rehabilitative Services, to Deborah A. Miller, Licensing Assistant, U.S. NRC, the State of Florida does not desire notifi.ation of issuance of license amendments.
6.0 'ETRONMENTAL CONSIDERATIONS The amendment changes requirements 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 has 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 j
Commission has previously issued a proposed finding that this amendment involves no significant hazards consideration for both the August 31,1998, submittal (63 FR 56249) and the March 18,1999, revised submittal (64 FR 19557), and there has been no public comment
' on either finding. Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
7.0 CONCLUSION
I Based on its review of the licensee's proposal, the staff has determined that the proposed changes to tlw CR-3 TSs to allow the installation of reroll repairjoints in OTSG tubing in the upper tubesheet using the process described in B&W reports BAW-2303P, Revision 3, and BAW 2342P, Revision 1, will continue to provide adequate assurance of SG tube integrity.
The suff concludes that (1) theie 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 amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor P. Rush, DE/EMCB DatefJune 28,1999