ML20069M632
| ML20069M632 | |
| Person / Time | |
|---|---|
| Site: | Crystal River  |
| Issue date: | 04/19/1994 |
| From: | Beard P FLORIDA POWER CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML20069M635 | List: |
| References | |
| RTR-REGGD-01.121, RTR-REGGD-1.121 3F0494-09, 3F494-9, NUDOCS 9406220068 | |
| Download: ML20069M632 (10) | |
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y Florida Power CORPORATION U N ES l l April 19, 1994 l 3F0494-09 ' U. S. Nuclear Regulatory Commission Attn: Document Control Desk , Washington, D.C. 20555 l l
Subject:
Refuel 9 Inspection Plan for Once Through Steam Generators j
References:
- 1. FPC to NRC, letter 3F0392-02, dated March 30, 1992
- 2. FPC to NRC, letter 3F0792-02, dated July 10, 1992
- 3. FPC to NRC, letter 3F0793-02, dated July 29, 1993
- 4. NRC to FPC, letter 3N1293-21, dated Decem1cr 14, 1993 l
- 5. FPC to NRC, letter 3F0394-02, dated March 4, 1994
Dear Sir:
Florida Power Corporation (FPC) is hereby providing in Attachment 1 our Refuel 9 inspection plan for the Once Through Steam Generators (OTSGs). This inspection , plan constitutes an important milestone in FPC's continuing effort to maintain l OTSG integrity. One of the objectives of the plan is to achieve resolution and l to disposition low signal-to-noise (S/N) indications present in the Crystal River l Unit 3 (CR-3) OTSGs. The Inspection Plan and S/N Disposition Strategy adopted by FPC are the result of extensive interaction between FPC and the NRC staff. FPC is providing in Attachment 1, a background to sunmarize that interaction, a summary of the ongoing Refuel 9 plans, a proposed 3/N disposition strategy, an evaluation of the proposed S/N disposition criteria and a safety significance evaluation of that criteria. Also provided in Attachment 2 to this letter are several documents that serve as the technical basis for the overall disposition strategy of S/Ns. CRYSTAL RIVER ENEHGY COMPLEX e 15700 W. Power Line Street . Crystal River . Flonda 344284708 . (J04) 7956486 A Fkvida Progress Company 9406220068 940419 PDR C ADOCK 05000302 '
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M. *. . i.; 3F0494-09 Page 2 FPC will under separate cover withdraw Technical Specification Change Request ] Number (TSCRN) 198. It is both FPC and NRC conclusion that this amendment , request is inappropriate until such time when more information is available to support a more permanent inspection and repair criteria. Meanwhile, for Refuel 9 . only, FPC will disposition S/Ns as proposed in the S/N disposition strategy l described in Attachment 1. If the results of the inspections are substantially j different than expected or if any changes to the plan are deemed appropriate, FPC l will notify and gain the concurrence of the NRC staff. In particular, we will l either follow the expansion of the motorized rotating pancake coil (MRPC) sample as described in Attachment 1 or get NRC's concurrence. In addition to the normal l reporting requirements, FPC plans to provide the NRC with the results of the inspection and destructive examinations of the pulled tubes no later than Spring 1995. FPC believes that the supplemental inspection and disposition strategy applied to S/N indications, is consistent with the overall philosophy, intent and direction being taken by the NRC and industry on Degradation Specific Management Programs for steam generators. FPC is working with the B&WOG to collect the appropriate data that will allow a more permanent inspection method and final repair criteria. We have demonstrated our commitment and leadership in this area. We fully expect that the final criteria that will evolve over the next several months will substantiate and go beyond the current S/N disposition criteria. If subsequent destructive examination results are not positive, the i
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data collected in accordance with this inspection will be effective in determining the extent of any problems and in planning any supplemental i inspections or corrective actions. Sincerely', D -{buf P. M. Beard Jr. Senior Vice President Nuclear Operations PMB/LVC Attachments xc: Regional Administrator, Region II Senior Resident Inspector NRR Project Manager
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3F0494-09 Attachment 1 Page 1 INSPECTION PLAN AND S/N DISPOSITION STRATEGY BACKGROUND The analysis of eddy current data for the CR-3 April 1990 refueling outage (as well as previous inspections) revealed a significant number of low amplitude signals (signal-to-noise (S/N) ratios less than 5:1). These S/N indications were present in both OTSGs. Reference 1 provided FPC's plan for dealing with the S/Ns during the April 1992 refueling outage. The plan consisted of completing the eddy current inspection of all tubes which had not been previously inspected with bobbin coil, performing inspection of a limited number of tubes utilizing a motorized rotating pancake coil probe (MRPC) and performing tube pulls to investigate the nature of the S/N indications. The results of the April 1992 inspection and tube pulls were conveyed to the NRC via Reference 2 (15 day Technical Specification report) and Reference 3 respectively. Preliminary information had been provided to the Staff by telecon as soon as FPC learned that sections of tubes pulled from the first span of the "B" 0TSG revealed that the degradation mechanism present was intergranular attack (IGA) in the form of pits, referred to as pit-like IGA (November 1992). FPC met with the NRC staff on September 20, 1993 to discuss the detailed results of the tube pull project reported in Reference 3. The S/N signals in tubes removed from CR-3 OTSGs exhibited low volume IGA as the predominant damage mechanism. Questions were raised as to how to treat these low volume indications which appeared so different in morphology and safety significance from other defect mechanisms found during normal eddy current inspections. FPC submitted Reference 4 to request an interim license amendment to revise the current criteria contained in the CR-3 Technical Specifications for the inspection and repair of steam generator tubes. FPC met with the NRC staff on March 31, 1994, to discuss the amendment request and its basis. FPC's presentation had the objective of providing clarificction to any questions the NRC may have had about the TSCRN or the Regulatory 1.121 based evaluation which was provided by Reference 4. During a subsequent meeting with the NRC staff on April 12, 1994, it was concluded that FPC would withdraw TSCRN 198 and instead would follow the following inspection plan and S/N disposition strategy.
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3F0494-09 Attachment 1 Page 2 REFUEL 9 OTSG PLANS FPC's OTSG inspection plan for Refuel 9 is as follows: Inspect with bobbin coil approximately 23% of the total tubes of each OTSG. This sample inspection consists of a number of tubes randomly selected and all inservice tubes with previous recorded indications, including S/Ns. Perform a supplemental MRPC probe sample inspection of 20% of the total known S/N population. This aspect of the inspection has been discussed with the NRC staff. Specific details of this sample inspection are addressed in the S/N disposition strategy section contained herein. Perform preventive sleeving of 164 tubes in the lane region of each 0TSG. Additionally, FPC will attempt to pull four tube samples with indications at the 7th and 9th tube support plates and distorted tube sheet signals at the lower tube support face. Chemical and metallurgic analyses will be performed on these tubes. The intent of this second tube pull project is to obtain additional data for a future degradation specific repair criteria.
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3F0494-09 Attachment 1 Page 3 S/N DISPOSITION STRATEGY FPC strategy for the disposition of S/Ns consists of repairing all S/N indications greater than 2V and indications exceeding the dimensional limits of axial size greater than 0.25" or a circumferential size greater than 0.60". S/Ns which are dispositioned as wear or manufacturer's burnish marks will be dealt with separately. Their disposition is fully described in Section 12 of Attachment 2. The strategy also provides for a supplemental sample inspection consisting of a non-random 20% sample .of the total population of low signal-to-noise indications of CR-3 OTSGs. This' supplemental inspection will be conducted by MRPC. It will include S/N indications with voltages less than 2V which have axial sizes greater than 0.15" or circumferential sizes greater than 0.30". The balance of the initial sample will consist of the remaining indications of the population which exhibit the highest voltages. Tubes with S/N indications known to exceed the voltage and dimensional repair criteria prior to the selection of the 20% MRPC sample will be repaired but not counted towards bobbin or MRPC expansions. MRPC sample expansions will be performed if the failure rate of the indications in the initial MRPC sample exceeds 5% of the total number of S/N indications with voltages below the arithmetic median of that sample. Failure rate is defined as the percentage of indications within the sample exceeding the dimensional limit of axial size greater than 0.25" or a circumferential size greater than 0.60". The configuration of the MRPC sample expansions will depend upon the voltage range where failures are identified. Several examples illustrating the~1ogic that will be followed for the selection of the expanded samples (MRPC or bobbin) are given in section 12.7 of Attachment 2. l _ _ -
3F0494-09 l Attachment 1 Page 4 EVALUATION OF THE S/N DISPOSITION CRITERIA The basis for the disposition strategy of S/Ns at CR-3 OTSGs is discussed more fully in Attachment 2. The current revision of the Regulatory Guide 1.121 evaluation provided in that attachment reflects FPC's disposition of NRC questions and comments on the original revision submitted as part of the basis for TSCRN 198 (Reference 4). The evaluation also demonstrates the adequacy of the strategy adopted for dealing with S/Ns. FPC considers the essential eler.ents of this evaluation to be: Structural Adequacy OTSG tubing with pit-like IGA or similar flaws retain adequate structural capability. Section 5 of Attachment 2 discusses the results of a structural analysis performed by MPR and Associates Inc. The MPR structural analysis calculated the maximum allowable tube wall degradation for various types of damage mechanisms of the CR-3 OTSG tubing. The proposed supplemental inspection and disposition criteria provides substantial margin by demonstrating that the tubes will not catastrophically fail at pressures suggested by RG 1.121. These pressures are very conservative. Further, the structural analysis was done assuming 100% through wall (TW) defects. The size of defects analyzed far exceeds those found in the CR-3 generators. Growth of Tube flaws at CR-3 Neither the data or our understanding of the degradation mechanisms present indicate any potential for growth. Such IGA is limited by the amount of sulfur (or similar chemicals) available. Section 6 of Attachment 2 documents a review performed by the EPRI NDE Center to assess growth of IGA indications observed in the first span of the "B" OTSG and a comparison of the last three eddy current inspections completed by B&W. The objective of this comparison was to assess growth of tube flaws at CR-3. EPRI concluded that the IGA patches had not arown since they were detected. B&W also found no significant growth for indications present at the free span or the support plates. Nevertheless, the proposed inspection and disposition criteria include an allowance for growth between inspections. Accuracy of Detectability and Sizing for CR-3 pit-like IGA Bobbin coil methodology was relatively effective at detecting the IGA defects. The probability of detection increased with size, as would be expected, and is effective at detecting those that may warrant further examination. Section 7 of Attachment 2, provides a discussion of EPRI's findings regarding sizing accuracy. A linear regression and comparison of eddy current indications and actual metallurgical test results demonstrated that the phase angle method of sizing could not accurately size the small volume indications found at CR-3. A 25% correlation coefficient and 27% RMS error are indicative of the inadequacy in applying phase angle techniques for these indications.
3F0494-09 Attachment 1 Page 5 Conservatism of Signal Amplitude Screen and Dimensional Limits The supplemental inspection and disposition criteria are conservative in appropriate respects. The 2V upper boundary for the sample of the supplemental inspection is also the limit beyond which tubes will be repaired. This 2V limit will assure tubes with indications that have the potential to be structurally significant will be removed from service. MRPC has been shown by experience, and would be expected, to conservatively estimate the size (axial and circumferential extent) of such indications. Section 7.5 of Attachment 2 discusses this more fully. Section 8 of Attachment 2 provides the basis for the 2V repair limit and for the dimensional disposition criteria of indications with axial extent less than or equal to 0.25 inches and circumferential extent less than or equal to 120 degrees as well as for the repair criteria that will apply beyond those limits. The previously noted EPRI NDE Center correlation would predict 100% TW at approximately 3.4V. (Such a flaw would be structurally acceptable based on the MPR analysis.) Outline of Analyst Guidelines Section 10 of Attachment 2 discusses in detail the analyst guidelines to be used during Refuel 9. The guidelines were revised to reflect feedback from the NRC during their review of Reference 4 and subsequent discussions. The revised guidelines contain sections on noisy data and the use of MRPC clip plots to assess the axial and circumferential size of an indication. Margin Against Tube Rupture. Section 5 of Attachment 2 summarizes the extensive investigation and burst testing performed by FPC as part of the Refuel 8 Tube Pull Project and burst testing performed on OTSG tubing with simulated flaws that envelope all known OTSG damage mechanisms. Leakage Considerations FPC does not believe that any of the defects left in service are of such a magnitude to have a significant potential for leakage under Main Steam Line Break (MSLB) loading conditions. The likelihood of a MSLB which fully depressurizes the OTSG is very small. Section 9 of Attachmet 2 discusses primary-to-secondary leakage rates under normal operating pressure or worse-case MSLB conditions. Recently completed calculations by MPR indicate that approximately 10% of the original tube wall needs to remain intact to prevent leakage under such conditions. Two independent statistical evaluations indicate that there is a high likelihood that the defects are well within this value.
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3F0494-09 Attachment 1 Page 6 SAFETY SIGNIFICANCE EVALUATION Florida Power Corporation has reviewed the safety significance of the S/N disposition criteria. FPC has concluded that it does not present a potential adverse effect in plant safety. In support of this conclusion the following analysis is provided:
- 1. The S/N disposition criteria will not significantly increase the probability or consequences of an accident previously evaluated. The relevant accidents are excessive leakage or steam generator tube rupture (as a consequence of MSLB or otherwise).
RG 1.121 establishes a standard method for demonstrating structural integrity under worse-than-DBE conditions. The existing TS is based on this RG. The S/N disposition strategy continues to rely on this guidance. TW sizing utilizing existing techniques may well ; low defects greater than the current limit to remain in service since t'.ese techniques do not accurately size the pit-like IGA degradation. Therefore, the probability of a Steam Generator Tube Rupture (SGTR) is not increased and may well be decreased by effective implementation of this S/N disposition strategy. The probability of OTSG tube leakage during normal operation or accident conditions is not adversely affected by this S/N disposition strategy. Current leakage limits are retained. Pit-like degradation which might leak during worse-case MJLB conditions have not been observed in the CR-3 OTSGs. If they were to be detected the criteria is at least as effective in determining which should be removed from service as are the existing TS limits. The supplemental inspection and S/N disposition strategy are an integral part of an overall effort to better understand these and similar phenomena in OTSGs.
- 2. The S/N disposition strategy will not create the possibility of a new or different kind of accident from any accident previously evaluated. The key 'new or different' accidents addressed in this and similar proposals is the potential for MSLB-induced multiple SGTR or excessive primary-to-secondary leakage during such events. While these events are addressed in CR-3 Emergency Operating Procedures (EOPs), they are beyond those licensed for the facility.
However, as noted above, the likelihood of MSLB induced multiple SGTR is reduced by more effective screening and plugging criteria. The probability of detection and identification of tubes which should be removed from service is maintained or improved by the supplemental inspection and S/N disposition strategy. The likelihood of adverse effects from plugging sound tubes is reduced. The operation of the OTSG or related structures, systems or components is otherwise unaffected.
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. 3F0494-09 Attachment 1 Page 7 SAFETY SIGNIFICANCE EVALUATION (continued)
- 3. The supplemental inspection and S/N disposition criteria will not involve a significant reduction to any margin of safety.
The margins of safety defined in RG 1.121-(primarily the required pressure j used in the structural analysis) are retained. The likelihood of i detecting defects is unchanged since bobbin coil methods will continue to be the primary means on initial detection. The likelihood of leakage remains acceptably small. The practice of inspecting all previous S/N's adds to the margin of safety by elimination of reliance on random _ sampling for these tubes. The overall B&W0G and FPC programs (tube pulls, sleeving, enhanced S/N evaluations) has and will continue to add to the existing margin of safety. In particular, the fact that -tubes removed from the generator have been and are continuing to be destructively examined (burst tested and metallurgically) adds to the margin of safety, l i
l Florida Power CORPORATION l Docket so-aar June 7, 1994 3F0694-ll U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555
Subject:
Request for Non-Withholding of Information from Public Disclosure , l
Reference:
FPC to NRC letter, 3F0494-09, dated April 19, 1994
Dear Sir:
Florida Power Corporation (FPC) provided the Refuel 9 Inspection Plan for the Once Through Steam Generators in the reference letter. Attachment 2 to that letter contained information that had been marked as containing information l proprietary / copyrighted to FPC and/or MPR and Associates. FPC is hereby stating that such information need agi be considered proprietary / copyrighted to FPC or MPR and Associates and the information could be made available for inspection and copying in the NRC Public Document Room. Sincerely, G. L. oldt, Vice President Nuclear Production GLB/LVC:ff xc: Regional Administrator, Region II Senior Resident Inspector NRR Project Manager cmsTAL nrVER ENmGY COMPLEX 15700 W. Poww Une Sannet . CW Fh . Ronda 34436706 . po4) 7966486 A DwMa Prognise Cwnpany YO b6
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