ML20216D737
| ML20216D737 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 04/10/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20216D733 | List: |
| References | |
| NUDOCS 9804150417 | |
| Download: ML20216D737 (7) | |
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. UNITED STATES i
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NUCLEAR REGULATORY COMMISSION 1
,t WASHINGTON, D.C. 30006 4001 l-l r...../
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO390TO l
. FACILITY OPERATING LICENSE NO. DPR-51 ENTERGY OPERATIONS. INC.
ARKANSAS NUCLEAR ONE. UNIT NO.1 DOCKET NO. 50-313 l
1.0 INTRODUCTION
l By letter dated February 9,1998, Entergy Operations, Inc. (the licensee) submitted a request for changes to the Arkansas Nuclear One, Unit No.1 (ANO-1) Technical Specification (TS). The requested changes would allow the use of the repair roll technology (reroll) for the upper tubesheet region of the ANO-1 steam generators. The reroll technology is an attemative to sleeving or plugging steam generator tubes found during inservice inspections to have defects that exceed the stated repair criteria in the upper tubesheet region of a steam generator tube.
The proposed amendment would allow new rolljoints to be installed t'elow indications of defects in the upper tubesheet region of the steam generators. The proposed use of reroll technology is based on a qualification program, documented in Framatome (formerly B.ibcock and Wilcox)
Topical Report, BAW-10232P (proprietary), "OTSG Repair Roll Qus%c9 tion Report (Including Hydraulic Expansion Evaluation)," Revision 00, which was incluc'ed in the licensee's submittal.
j-A non-proprietary version of the supporting topical report was suc,itted on February 26,1998,
2.0 BACKGROUND
Arkansas Nuclear One, Unit 1 has two model 177FA once-through steam generators manufactured by Babcock and Wilcox. The tubes were fabricated from mill annealed Alloy 600 i
material and were restrained by the roll expansion joints in the upper and lower tubesheets. The original tube-to-tubesheet rolls were expanded by a hardroll process and are approximately 1-2 inches in axial length extended into the upper tubesheet from the tube end. The upper tubesheet is about 24 inches thick.
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 j
extremely low probability of abnormal leakage, of rapidly propagating failure, and of gross
. rupture. A significant portion of the reactor coolant pressure boundary is maintained by steam generator tubes which have experienced various levels of degradation. Regulatory Guide (RG) l-1.121 provides guidance on an acceptable method for establishing the limiting safe conditions of tube degradation. In addition, the plant TS require periodic inspections of steam generator tubes. TS also require those tubes with defects in excess of the repair limits (e.g.,40 percent through-wall) be repaired or removed from service.
'9804150417 980410 PDR ADOCK 05000313 P
js 2-The joint between the tube and tubesheet is an interference 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 tube-to-tubesheet roll joint provides sufficient strength to maintain adequate structural and pressure boundary integrity.
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The industry experience has shown that defects have developed in the tube-to-tubesheet roll joints er a result of various degradation processes. In general, tubes with degraded rolljoints are either plugged or repaired by sleeving. However, the NRC has accepted attemate 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 loadings from normal operation, anticipated operational occurrence, 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 ill of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME). Normal structural loads imposed on the tube-to-tubesheet roll primarily are derived from the differential pressure between the primary and secondary sides of the tubes. Loadings from a postulated main steam line break event can be significant. In addition, cyclic loading from transients (e.g., startup/ shutdown) should also be 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 a 1-inch rolllength for the newjoints would carry all structuralloads and minimize potentialleakage. The qualification program consisted of (1) preparing the mockup, (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 eight steam generator 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 roll joints, the licensee applied loads to the sample tubes to simulate or exceed normal, thermal and pressure cycling transient, and postulated accident conditions. In accordance with RG 1.121, the test pressure applied to the sample tubes exceeded 3 times normal operating pressure and 1.43 times mala steam line break pressure. To obtain conservative leakage results, the sample tubes were severed 360 degrees through the i
tube wallin the rolljoints.
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. 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 thermalloads may cause
3 the tubesheet to bow in one direction or the other which can cause 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, the licenses specified an exclusion zone in the tubesheet where the reroll joint would not be installed.
3.2 Structural and Leakane Intearity Based on the results of the qualification testing, the licensee determined a roll length of 1-inch is necessary to ensure adequate margins of structural and leakage integrity. - With regard to the structural integrity, the licensee demonstrated by their ultimate load testing (testing to simulate accident conditions) that the tube with the new roll would not be pulled out from the upper
. tubesheet under the worst possible combination of loadings. Also, no motion of the tubes relative to the simulated tubesheet were observed during the thermal and fatigue cycling tests.
With regard to the leakage integrity, the qualification tests showed that if each of the tubes (about 15,500 tubes) in a steam generator was rerolled in the upper tubesheet and had a 100 percent through-wall flaw in the reroll, the total leakage from all flaws would be minimal. As a defense-in-depth measure, the licensee proposed to implement a primary-to secondary leakage limit of 150 gallons per day per steam generator in the plant TS. The 150 gallons per day requirement is more limiting and conservative than the current TS limit of 500 gallons per day and is consistent with the staff's position regarding altemate tube repair criteria.
The licensee's proposed altemate repair criteria would change the licensing basis of the operating license because the primary pressure boundary would be modified and indications above the reroll location would be allowed to remain in service. For ANO-1, the staff believes that the tube integrity under accident conditions as a result of the license amendment will not be decreased. The rerolled joints essentially restore the tube-to-tubesheet joints to their original-design condition.
- 3.3 Field instointion and Inspection The licensee proposed to apply a single repair method to install one roh ; reroll) in the tubes that have degradation in the upper tubesheet region. The repair rollis typically installed using a manipulator and a tool head, monitored by a control system that tracks tha ; osition and monitors
' the torque of the roll expander. The roll expander is 1-inch long but the actt.al roll will have a 1/4 inch taper on each end. The torque is automatically controlled during the rerolling and is recalibrated after installation of certain number of rerolls to ensure the minimum torque is maintained to produce proper fit. An optional hydraulic expansion may be utilized prior to
- rerolling the tube to minimize the potential for denting between the original roll and the repair roll.
After the installation, the licensee will inspect all rerolls using eddy current techniques to ensure proper diametral expansion and that the reroll regions (including hydraulic expansion transition region) 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
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-4 licensee will inspect all rerolled tubes during steam generator inspection activities. If degradation were to be found in the reroll region, the affected tube will be plugged or repaired by means other than rerolling because only one raroll per tube is allowed by the proposed amendment. The licensee did not propose a method for multiple rerolls on the same tube in this amendment.
3.4 Proposed Technical Specification Changes The significant changes to TS sections ao as follows:
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- TS 3.1.6.3.b if the leakage through the tubes of any one steam generator equals or-exceeds 150 gallons per day (0.104 gpm), a reactor shutdown shall be initiated within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and the reactor shall be in cold shutdown condition within the next 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
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The proposed leakage limit of 150 gallons per day for one steam generator is more limiting than the previous limit of 500 gallons per day and is consistent with the staff's position regarding attemate tube repair criteria; therefore, it is acceptable.
i TS 4.18.3.b.
All tubes which have been repaired using the reroll process will have the new roll area inspected during the inservice inspection.
This surveillance requirement clarifies the licensee's intent that all reroll regions of the repaired tubes will be inspected. The staff finds this requirement acceptable because it provides a -
comprehensive monitoring of potential degradation in the rerolled regions of the repaired tubes.
. An associated editorial change renumbering previous TS 4.18.3.b. to TS 4.18.3.c. is also acceptable.
Notes (3) Where special inspections are performed pursuant to 4.18.3.b, defective or degraded tube indications found in the new roll area as a result of the inspection and any indications found in the originally rolled region of the rerolled tubes, are not included in the determination for the inspection results category of a general steam generatorinspection.
l This requirement is not a relaxation from the current TS. The indications found in the new rolls need not be included in determining the inspection Results Category because the licensee i
proposed to inspect all reroll regions of repaired tubes and to repc't to the NRC the results of the i
' inspection.- The staff believes that these two actions, inspection and reporting, provide
.. adequate monitoring of the rerolls in the repaired tubes.
TS 4.18.4.c.1.
If the leaking tube has been repaired by the reroll process and is leaking in the new roll area, all of the tubes in the steam generator that have been repaired by the reroll process will have the new roll area inspected. If the results of this inspection fallinto the C-3 category, all of the tubes with rerolled areas in the other steam generator will also be similarly inspectet1. This inspection will be in lieu of the first sample inspection specified in Table 4.18-2.
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-. i This requirement is a part of unscheduled inspections for the cases when primary to secondary leakage exceeds the leakage limits of TS 3.1.6.3.b (e.g.,150 gallons per day). This specification -
verifies the condition of the non-leaking steam generatorif the degradation in the leaking steam generator reaches C-3 category. This requirement is consistent with the staff position and, therefore, is acceptable.
TS 4.18.5.a.7.
Plugging Limit means the imperfection depth at or beyond which the tube -
shall be restored to serviceability by the installation of a sleeve, rerolled, or removed from service because it may become unserviceable prior to the next inspection; it is equal to 40% of the nominal tube wall thickness.
The reroll repair process will only be used to repair tubes with defects in the upper tubesheet area. The reroll repair process will be performed only once per steam generator tube using a 1 inch reroll length.- The new roll area must be free of detectable degradation in order for the repair to be considered '
acceptable. The reroll repair process is described in the topical report, BAW-10232P, Revision 00.
This proposed TS 4.18.5.a.7. modification defines the applicable usage of the reroll repair process. The reroll repair is limited to the upper tubesheet area and can only be applied once to a single tube.. The specification also provides a clear definition of the length of the reroll and l
- acceptance criteria of rerolled tubes. The staff finds the definition in this specification acceptable.
I TS 4.18.5.a g.
Tube Inspection means an inspection of the steam generator tube from the i
point of entry completely to the point of an exit. For tubes that have been repaired by the reroll process within the upper tubesheet, that portion of the tube above the new roll can be excluded from future periodic inspection requirements because it is no longer part of the pressure boundary once the j
repair rollis installed.
This requirement clarifies the scope of tube inspection. Since the reroll regions are the new pressure boundary, the original roll areas need not be inspected The staff finds this specification acceptable.
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i TS 4.18.6.d.
(report to the NRC staff)... Number of tubes repaired by rerolling and number of indications detected in the new roll area of the repaired tubes.
- This requirement is a part of TS 4.18.6 that specifies that results of the steam generator tube inservice inspection be reported to the NRC within 45 days following completion of the inspection. The requirement is a logical extension of the existing TS reporting on degraded
- tubes and would provide the staff with a status of conditions of the reroll regions and is, therefore, acceptable.
. In addition to the above changes, the licensee proposed to revise Bases 4.18 to elaborate on the
. reroll process and revise Table 4.18-2 to include the use of the reroll f schnology as a tube
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repair attemative. The licensde proposed to delete references to allowing tubes to remain in service with intergranular attack indications with depths greater than 40% since the NRC had accepted this provision for only one operating cycle (Cycle 14) of operation. These changes are acceptable in that they merely constitute administrative changes required to implement the TS e
change discussed above.
i 3.5 Summary The licensee proposed to implement an altemate repair method using reroll to repair tubes having indications in the upper tubesheet region of the steam generators. The technical basis for the proposed reroll method was documented in Topical Report, BAW-10232P, Revision 00,.
'Jonuary 1998.
The staff has determined that (1) the licensee's attemate repair criteria using reroll were established on the basis of the qualification tests that used specimens simulating the actual tube-to-tubesheet joint configuration of the steam generators, (2) the loads for structural and leakage
. tests were specified and appl:ed in accordance with RG 1.121, and (3) the proposed changes to plant TS satisfied the regulatory requirements and technical basis.
On the basis of submitted information, the staff concludes that the proposed reroll repair for indications in the upper tubesheet regions of the steam generators at ANO-1 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.
4.0 STATE CONSULTATION
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in accordance with the Commission's regulations, the' Arkansas State official was notified of the.
proposed issuance of the amendment. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the amendment involves no significant j
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 1
amendment involves no significant hazards consideration, and there has been no public i
comment on such finding (63 FR 9268). 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.
J 6.0 fd2NQMJSION 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
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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: A. Kiem, NRR/EMCB Date:I April 10, 1998 i
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