ML20196J612
| ML20196J612 | |
| Person / Time | |
|---|---|
| Site: | Crystal River  |
| Issue date: | 06/28/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20196J605 | List: |
| References | |
| NUDOCS 9907070369 | |
| Download: ML20196J612 (4) | |
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UNITED STATES S
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WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTpR REGULATION RELATED TO AMENDMENT NO.179TO FACILITY OPERATING LIC 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 C River Unit 3 (CR-3). The purpose of the amendment is to implement alternate repair c steam generator (SG) tubes that have degraded rolljoints inside the upper tubesheet. The in the upper tubesheet. The alternate repair criteria were qualifbation program, documented in Framatome topical report, BAW-2303P, Revisio "OTSG Repair Roll Qualification Report," which was included in the submittal dated 1998. %Dsequent to the initial application for this amendment, the licensee and its vendo rev5ed the limiting accident for the CR-3 once-through steam generators (OTSGs) that considered in BAW-2303P. On March 18,1999, the licensee submitted an additional report, BAW-2342P, Revision 1, "OTSG Repair Roll Qualification Report - Addendum A appropriately modify the previous analyses and proposed changes to the TSs to support t August 31,1998, submittal. The Marchamendment. The information contained w 18,1999, submittal also included a double repair nll no significant hazards consideration determiaauon was notic CR-3 has two model 177FA OTSGs that were manufactured by Babcock and Wilcox. The OTSG tubes were fabr'.ted from Alloy 600 material and are secured by roll expansion in the upper and lower tubesheets. The tubesheet is approximately 24-inches thick and weld at the primary face of each tubesheet prevents primary-to-secondary leakage aroun hardroll expansions.
2.0 BACKGROUND
General Design Criterion (GDC) 14 of Appendix A to 10 CFR Part 50 requires that the re coolant pressure boundary (RCPB) be designed, fabricated, erected, and tested so as to h an extremely low probability of abnormalleakage, of rapidly propagating failure, and of gros rupture. A significant portion of the RCPB is maintained by SG tubes which have experien various levels of cegradation. U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," prov 4
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. l guidance on an acceptable method for establishing the limiting safe conditions of tubn 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 intc 'arence fit constructed by roll expanding 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 rolljoint provides sufficient strength to maintain adequate structural and pressure boundary integrity.
Industry experieace has shown that defects have developed in the tube-to-tubesheet roll joints as a result of various degradation processes. In general, tubes with degraded rolljo'ints are either removed from service or repaired. The NRC has accepted alternate repair criteria allowing repaired tubes with degraded rolljoints to remain in service provided that the repaired tubes can maintain adequate structural and leakage integrity under loaaings 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 been detected. For postulated accidents, RG 1.121 recommends that the margin of safety against tube rupture be consistent with the margin of safety determined by the stress limits specified in NB-3225 of Section 111 of the Boiler and Pressure Vessel Code of the American Society of Mechanicai 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. Leadings 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 roll joints.
3.0 EVALUATION 3.1 Qualification Proaram On the basis of the qualc.:ation 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 e. ;:anded into tho mockup tubesheet using an exranding tool that had the same critical dimensions as the tool ust.d 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 operatir,g 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.
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. In the qualification program, the licensee also considered 'he impact of tubesheet bowing on the rolljoints because the tubesheet bore diameter can chan2e during certain opersting conditions. The combined effects of primary-to-secondary pressure differential and thermal loads may cause the tubesheet to bow in one direction 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 tae 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 the reroll joint would not be installed.
3.2 Structural and Leakaoe Inteority Based on the results of the qualification testing,- the licensee determined roll lengths sufficient 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 tJbesheet under the worst possible combination of loadings. No motion of the tubes relative to the simulated tubesheet was observed during the thermal and fstigue cycling tests. With regard ts leakage integrhy, the qualification tests showed that if each of the tubes in a SG were rerolled in the upper tubesheet and contained a 100 percent through-wall flaw at the upper edge of the reroll, the total expected leakage from all flaws would be weil below the operational leak rate limit in the CR-3 TS.
3.3 Field installation and Inspection The licensee proposed to repair tubes by the installation of either one or two hardroll joints l
(reroll) in the tubes that have degradation in or near the original roll region. The repaired roll is f
typically installed 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 l
controlled during the rerolling and is recalibrated after installation of a certain number of rerolls 1
to ensure the minimum torque is maintained to produce proper fit.
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After the installation, the licer Tee will inspect all rerolls using addy current techniques to ensure proper diametral exp.pdon and positioning of the reroll repairjoint. In addition, the inspechons will verify that the reroll regions are hee of degradation. Any reroll not satisfying i
the acceptance criteria will be either plugged or repaired with a method other than rerolling.
For future inservice inspechons, the licensee will inspect all rerolled tubes during steam generator inspechon 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 REPAIR TECHNIQUE i
.The licensee proposed to implement an attemate 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 reports BAW-2303F, Revision 3, and BAW-2342P, Revision 1.
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, The staff has determined that (1) the licensee's altemate 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 leakage tests were specified and applied in accordance with RG 1.121, and (3) the proposed changes to plant TS satisfied all regulatoy requirements applicable ta SG tube integrity.
On the basis of submitted information, the staff concludes that the proposed reroll repair for degraded rolljoints 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 SJ. ATE 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 notification of issuance of license amendments.
6.0 ENVIRONMENTAL CONSIDERATION
S The amendment changas requirements with respect to installation or use of a facility cor".pnt located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments inyolve 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 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
Based on its review of the licensee's proposal, the staff has determined that the proposed changes to the 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 staff concludes that (1) there is reasonable assurance that the health and safety of the pubhc will not be endangered by operation in the proposed manner, (2) such activities will be conducted irt ceinpiisnce wi;.h tre Commission't. 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 Contnbutor: P. Rush, DE/EMCB Date: June 2, 1999