ML18153A838
| ML18153A838 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 06/08/1995 |
| From: | Ohanlon J VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 95-036F, 95-36F, NUDOCS 9506130151 | |
| Download: ML18153A838 (4) | |
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VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 June 8, 1995 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D. C. 20555 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNITS 1 AND 2 Serial No.
NL&P/CGL Docket Nos.
License Nos.
95-036F RO' 50-280 50-281 DPR-32 DPR-37 CORE UPRATE
- MATERIALS & CHEMICAL ENGINEERING BRANCH FOLLOW-UP QUESTIONS TO PREVIOUS REQUEST FOR ADDITIONAL INFORMATION The Surry Core Uprate Technical Specification change request was submitted for NRC review by an August 30, 1994 letter (Serial No.94-509).
Subsequently, responses to an NRC request for additional information from the Materials & Chemical Engineering Branch (EMCB) of NRR were provided by letters dated March 23, 1995 (Serial No. 95-036A) and April 13, 1995 (Serial No. 95-0368).
Further EMCB questions were discussed during May 17, 1995, May 30, 1995, and June 6, 1995 conference calls between the NRC, Virginia Power, and Westinghouse. The two follow-up questions specifically discussed address steam generator tube plugging margin and tube U-bend inspection scope and frequency. The following paragraphs provide additional information regarding these two items.
Steam Generator Tube Plugging Margin Our April 13, 1995 submittal (Serial No. 95-0368) established Section XI, Paragraph IWB-3521.1 of the ASME Code as the basis for the 40% plugging margin used at Surry Units 1 and 2. It was further established that the uprating conditions will have little or no effect on the established 40% limit.
As discussed during the May 17, 1995 conference call, an evaluation of minimum acceptable tube wall thickness demonstrates the conservatism of the 40% limit.
Using the formula for thickness evaluation provided in Paragraph NB-3324.1 of Section Ill of the ASME Code, a minimum required wall thickness of 0.020 inch is obtained for the 0.875 X 0.050 inch tubing of Surry 1 and 2 steam generators. This is based on the allowable limit of 1/3 of ultimate strength and normal operation primary to secondary pressure differential of 1465 psi.
Providing an additional 0.01 O inch for eddy current test uncertainty and potential growth of degradation results in a limit for plugging of 0.020 inch or 40% of the nominal tube wall thickness of 0.050 inch.
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Steam Generator Tube U-bend Inspection Scope and Freguency Our March 23, 1995 submittal (Serial No. 95-036A) indicated that, although core uprate generates circumstances that result in an increased potential for vibration, these circumstances do not contribute to any significant decrease in long term bundle integrity for the Model 51 F steam generators. Our submittal also indicated that evaluations for Model 51 steam generators, including the Model 51 F, show that the additional tubes subject to wear at the anti-vibration bar (AVB) intersections as a result of equivalent core upratings constitute less than 0.3% of the total tube count over the life of the generator.
During the May 17, 1995 conference call, the NRC referred to another plant's Safety Evaluation (SE) issued for core uprate or rerate. This SE indicated that, because the analysis determined that there was a slight increase in the potential for tube wear, that licensee planned to inspect all (Model F) steam generator tubes in Rows 25 or greater during the three refueling outages following the rerate. The SE also indicated that a significant fraction of these inspections would be completed during the first or second refueling outage following the rerate. The NRC requested we perform similar steam generator tube inspections at Surry.
As discussed during a follow-up conference call with the NRC on May 30, 1995, we have assessed this request and have determined that requiring similar inspections at Surry is not appropriate. The basis for this determination is presented in the following paragraphs.
Our NSSS vendor, Westinghouse, has reviewed U-bend wear evaluations performed for various steam generator models. These evaluations were done to assess the effect of power increases and/or steam pressure reductions on U-bend wear at the AVBs.
The steam pressure reductions were a result of assumed tube plugging or primary temperature reductions. The attached table shows the results of these evaluations in terms of the increase in percentage and number of tubes subject to plugging as a result of the operating condition changes. Evaluations were performed for steam generator Models F, 51, and D3.
With respect to U-bend/AVB considerations, the Surry steam generators (Model 51 F) are most like the Model 51 steam generators, having the same tube diameter and a two-bar AVB configuration. The Model F generators are significantly different, having a smaller tube size and a three-bar AVB configuration. Although the Surry steam generator design incorporates some Model F features (e.g., stainless steel support plates with broached holes, Alloy 600 thermally treated tubes, and hydraulically expanded tube to tube sheet joints), these features are not impacted by nor do they impact the U-bend/AVB wear considerations.
As presented in the attachment, the Model 51 steam generator evaluation demonstrates a potential increase in plugging of only one tube per steam generator for uprated conditions, compared to 5 tubes for the Model F uprating. Furthermore, the Model 51 uprating evaluation bounds the Surry uprating in terms of both power increase (4.5% versus 5%) and steam pressure decrease (due to tube plugging of 7%
versus 20%). These evaluation results are consistent with operating experience, which has shown that the Model 51 steam generators are less susceptible to U-bend wear than the Model F steam generators.
The tube plugging history for the Surry steam generators is also consistent with these evaluation results. Since the Surry steam generator replacements occurred in 1980, 7 tubes (5 in Unit 1 and 2 in Unit 2) have been plugged due to minor AVB wear. [Note that our March 23, 1995 submittal indicated 8 tubes. Upon further review of the data, one of the plugged Unit 1 tubes was a free span indication near an AVB, rather than at an AVB contact point.] These 7 tubes were preventively plugged prior to reaching the Technical Specification through wall plugging limit.
Based on the evaluation results and operating/plugging history, the Surry core uprating is expected to have no significant effect on U-bend wear. In view of these considerations, the existing Technical Specification inspection requirements are adequate for Surry at the uprated conditions without additional inspection commitments (like those cited above for the Model F steam generators). As previously noted in our March 23, 1995 letter and as required by Surry Technical Specification 4.19.F, we will continue to report steam generator plugging performed, along with a description of the type of indications seen.
As discussed during a subsequent follow-up conference call with the NRC on June 6, 1995, we will notify the NRC (in the annual plugging report required by Technical Specification 4.19.F) of any significant increase in wear-related indications seen during the three refueling outages following implementation of core uprate at both Surry Units 1 and 2.
If you have questions or require additional information, please contact us.
Very truly yours, Rf ~~,or James P. O'Hanlon Senior Vice President - Nuclear Attachment - Increased Tube Plugging as a Result of Operating Condition Changes cc:
U. S. Nuclear Regulatory Commission Region II 101 Marietta Street, N. W.
Suite 2900 Atlanta, Georgia 30323 Mr. M. W. Branch NRC Senior Resident Inspector Surry Power Station Commissioner Department of Radiological Health Room 104A 1500 East Main Street Richmond, Virginia 23219
ATTACHMENT INCREASED TUBE PLUGGING AS A RESULT OF OPERATING CONDITION CHANGES S/G Model, Operational Change Model F, Uprating Model F, Uprating and T hot Reduction Model 51, Uprating and 20% Tube Plugging Model D3, 18% Tube Plugging Surry - Model 51 F, Uprating and 7%
Tube Plugging Uprating 4.5 4.5 5.0 4.5 Steam Pressure Decrease (psi) 47 85 110 105 None*
Increase in Plugging
% of Number Total per S/G 0.08 5
0.15 8
0.03 1
None None For the uprating and assumed 7% plugging, design steam pressure was maintained at Surry as a result of the large margin in the original design.
- The Model 51 evaluation results above are applicable for the Surry Model 51 F steam generators.