ML060540483
| ML060540483 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 02/22/2006 |
| From: | Pace P Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| TAC MC8712 | |
| Download: ML060540483 (13) | |
Text
February 22, 2006 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Gentlemen:
In the Matter of )
Docket No. 50-327 Tennessee Valley Authority
)
SEQUOYAH NUCLEAR PLANT (SQN) - RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING UNIT 1 STEAM GENERATOR (SG)
REPORTS FROM CYCLE 13 REFUELING OUTAGE (TAC NO. MC8712)
Reference:
NRC letter to TVA dated January 13, 2006, Sequoyah Nuclear Plant, Unit 1 - Request for Additional Information Regarding the Cycle 13 Steam Generator Tube Inservice Inspections (TAC No. MC8712)
The enclosure to this letter provides additional information requested by the referenced letter. The additional information includes diagrams that help clarify the design features of the SQN Unit 1 replacement SGs.
Please direct questions concerning this issue to me at (423) 843-7170 or J. D. Smith at (423) 843-6672.
Sincerely, Original signed by James D. Smith for:
P. L. Pace Manager, Site Licensing and Industry Affairs Enclosure cc: See page 2
U.S. Nuclear Regulatory Commission Page 2 February 22, 2006 cc (Enclosure):
Mr. Douglas V. Pickett, Senior Project Manager U.S. Nuclear Regulatory Commission Mail Stop O8G-9a One White Flint North 11555 Rockville Pike Rockville, Maryland 20852-2739 NRC Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy-Daisy, Tennessee 37379-3624 Regional Administrator U.S. Nuclear Regulatory Commission Region II 61 Forsythe St., SW, Suite 23T85 Atlanta, Georgia 30303-3415
E1-1 ENCLOSURE TENNESSEE VALLEY AUTHORITY (TVA)
SEQUOYAH NUCLEAR PLANT (SQN)
UNIT 1 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING THE CYCLE 13 STEAM GENERATOR TUBE INSERVICE INSPECTIONS NRC Question 1 Your report indicated that 100-percent of the dents and dings greater than or equal to 2-volts between the top of the hot leg tubesheet and the top of the cold leg tubesheet were examined with a +Point' probe. Discuss the results of these
+Point' probe inspections, including the number of dents/dings in each SG. In addition, discuss if there were any new dents/dings or any "anomalous" dent signals.
TVA Response All greater than or equal to 2 volt dents were examined during the Sequoyah (SQN) end of cycle 13 (EOC-13) inservice inspection (ISI) using Plus Point probes and no degradation was identified.
The number of dents identified per SG during the SQN EOC-13 ISI is compared to the number identified during the SQN preservice inspection (PSI) below.
Number of Dents Comparison 11 12 82 25 13 10 66 15 0
10 20 30 40 50 60 70 80 90 SG-1 SG-2 SG-3 SG-4 Dents >= 2v S1C12 PSI S1C13
E1-2 The total number of dents identified during the EOC-13 ISI was 104 as compared to 130 identified during the SQN PSI.
The differences in the reduction in numbers can be attributed to the following:
- 1. The screening criterion for identifying dents is 2 volts.
It is expected that with operation of the unit an oxide layer forms causing measured voltages to decrease slightly. Some of the baseline dents that were called near 2 volts are measured less than 2 volts during the ISI, thus not meeting the screening criterion. This could also be attributed to eddy current uncertainties.
- 2.
During the PSI, the rules for the computerized data system (CDS) for identifying dents were discovered to be too conservative inside the tubesheet. On the first two SGs (#3 and #4) when dents were examined with a plus point coil, some of the signals were actually slight geometry variations in the expanded region of the tube.
For the next 2 SGs and for the ISI (#1 and #2), the rules were modified. Thus fewer dents were identified.
Six new DNT indications were identified during the SQN EOC-13 ISI using the greater than or equal to 2 volt screening criterion and are tabulated as follows showing the comparison to the SQN PSI calls:
RSG Row Col Location Cyc13 ISI PSI 1
8 22 VS3+0.72 2.09v 1.94v 1
16 100 DS1+2.30 4.90v (1) 1 16 122 DS1+2.32 4.05v (1) 1 43 7
VS2+1.28 2.00v 1.93v 3
43 43 C02-1.60 2.41v 1.89v 3
43 49 C02-1.60 2.13v 1.98v Note (1):
A look back to PSI shows a geometry anomaly at the bend transition for these two tubes that was interpreted during the ISI (EOC-13) as a dent. The same small signal was recognized during the PSI as an occasional consequence of the tube bending process and not identified as a dent.
This signal is still present and unchanged.
The discussion above shows there were no new indications or anomalous signals identified during the EOC-13 ISI following operation after the baseline PSI.
E1-3 NRC Question 2 Please indicate if the 11 wear indications in the area of the 2nd and 4th vertical straps were inspected with bobbin and
+Point' probes. If the wear indications were not inspected with a +Point' probe, please discuss how you concluded that the indications were wear. In addition, the staff noted that several of the wear indications were located in the same general vicinity. Please provide any insights on the pattern of the wear indications.
TVA Response The eleven wear indications in the area of the 2nd and 4th vertical straps were inspected with both bobbin and plus point probes. Wear was confirmed with plus point.
Westinghouse analyzed these wear indications by developing WCAP-16407-P to describe the likely cause and examine the probability of future indications. The results are summarized below.
Cause:
The most likely cause of tube wear was identified as local clearance variations. Vertical strip arrays VS2 and VS4 have no lock-bars so tube to support clearance has the potential for small localized deviations from optimal clearance. For example, when the vertical strips of VS2 and VS4 were welded to the middle arch plates, the strips might have had local deviations in spacing from each other or might have deviated from a vertical orientation due to weld distortion. Either case would result in less than optimal tube to support clearances. It is feasible that a few vertical strips could have slightly larger tube clearances. These larger clearances would result in increased tube vibration amplitudes. While the SQN U1 RSGs upper bundle support (UBS) design provides excellent support against flow induced vibration (FIV), the high circulation ratio, which is beneficial in avoiding tube corrosion phenomena, results in increased fluid forces in the UBS region. In some local regions enlarged tube to support clearances in combination with increased fluid forces might have lead to the mild tube wear indications.
Future Wear Prediction:
Comprehensive wear progression trending data for tube wear at vertical strips was available from operating experience of Reference SGs at a CE plant. The geometry of the UBS is
E1-4 similar between the Reference plant and Sequoyah in the local region of interest. Trending demonstrates indication stabilization (i.e., rate slows down as tube to support bearing area increases) and suggests volumetrically linear wear progression. Using SQN U1 RSG Tube Wear Curves of WCAP-16407-P it can be projected that a tube which incurred a 17%
wall penetration in the first cycle, would not reach the ASME Section XI (40%) plugging criteria until sometime during the ninth cycle of operation.
NRC Question 3 Your report provided information about flaws in specific tubes and at specific locations within a tube. In order for the U.S. Nuclear Regulatory Commission staff to better understand where your indications are being detected and for future reference, please provide the following information regarding the design of your replacement SGs: a tubesheet map, a tube support plate diagram, the heat transfer surface area, the tubesheet thickness (with and without the clad),
the lattice grid thickness, the tube support plate thickness, and a description of the U-bend support system, including the thickness and the tubing supported by the various supports.
In addition, discuss whether you have a flow distribution baffle to distribute flow at the top of the tubesheet, including the thickness.
TVA Response Steam Generator Design:
There is no flow distribution baffle in the SQN U1 Replacement SG design.
Sequoyah Unit 1 is a Westinghouse PWR with 4 steam generators. These are replacements for the previous Westinghouse Model 51 steam generators. Each steam generator contains 4983 U-bend tubes made of thermally treated Inconel 690. The generators are designated as Model 57AG. This comes from approximately 57,000 square feet of surface area (57) and an Advanced Grid design for the tube supports, hence 57AG. The tubes are 0.750 in diameter with a nominal wall thickness of 0.043. All support structures are 409SS.
Various descriptions and drawings follow.
E1-5 Sequoyah Unit 1 Tubesheet The tube sheet, as shown above, is a triangular pitch design with tubes designated by row and column. There are 103 rows and 125 columns in each leg of the four steam generators.
The tubes are expanded full length of the tube sheet using a hydraulic expansion process. There are 13 blank spaces in each tubesheet for stay rods. The locations of these blanks are as follows:
Stay rod tube numbers 79-39 79-63 79-87 47-27 47-51 47-75 47-99 15-15 15-33 15-51 15-75 15-93 15-111 Location of Stay Rods The Advanced Tube Support Grid (ATSG) structures are 409 stainless steel which is a ferritic stainless. Tubes will be even numbered row - even numbered column or odd numbered row
- odd numbered column. Support structure measurements are provided in the following table while figures following illustrate a cross sectional view of the steam generator support structures and specific support nomenclature.
Additional tables list all dimensional and design specifics relative to the support structures within the Sequoyah steam generators and give the u-bend support structures through
E1-6 which the various tubes pass. In addition, lists of tube dimensions by row with respect tostraight and curved sections are provided. Finally, figures show drawings of the supports and Ventilated Support Bars (VSB). The tubes through and including row 16 were heat treated the entire length.
Sequoyah Support Structure Measurements Location Inches from TE Center to Center Tube End 0.0 Top of Tubesheet 25.25 25.25 Center #1 TSP 45.25 20.0 Center #2 TSP 89.22 43.97 Center #3 TSP 133.19 43.97 Center #4 TSP 177.16 43.97 Center #5 TSP 221.13 43.97 Center #6 TSP 265.10 43.97 Center #7 TSP 309.07 43.97 Model 57AG
E1-7 SEQUOYAH 1 DESIGN SPECIFICS Tube-sheet Data:
Hole diameter - 0.758 Pitch 1.0625 Thickness 25.25 (includes 0.25 clad)
Expansion process -
Hydraulic (full expansion)
Tube Support Data:
Hole geometry Lattice Grid Lattice dimensions -
0.765 minimum tube clearance Width x thickness -
2.00 & 1.00 x 0.145 (thick portion)
Material -
409 SS Ventilated Support Bar Data:
Lock Bar Shape -
rectangular Width x thickness 0.5 x 0.09 Straps Shape rectangular Width x thickness 2.0 x 0.16 Material -
409 SS Tube Data:
OD 0.750 ID 0.664 Wall thickness:
Nominal -
0.043 Number of tubes/SG -
4983 U-bend radius:
Maximum 57.375 Minimum 3.188
E1-8 Rows Structures Through Which the Tubes Pass 1-36 DS1 VS3 DS2 37-75 DS1 VS2 VS3 VS4 DS2 76-103 DS1 VS1 VS2 VS3 VS4 VS5 DS2 U-Bend Support Intersections ATSG Lattice
E1-9 ATSG Top View ATSG
E1-10 U-Bend Support Strap Ventilated Support Bar (VSB)
Bend Support Structures Ventilated Support Bars (VSB)
E1-11 Tube Dimensions by Row 103 rows x 125 colum ns Tubes R ow "A"
"B" "C "
"D "
(D eveloped Length)
R ow "A"
"B" "C "
"D" (D eveloped Length) 1 61 6.37500 10.013830 322.26000 319.07250 648.15883 53 53 61.62500 96.800324 367.76000 336.94750 770.69532 2
62 7.43750 11.682798 323.13500 319.41625 650.51530 54 52 62.68750 98.469295 368.63500 337.29125 773.05179 3
63 8.50000 13.351769 324.01000 319.76000 652.87177 55 53 63.75000 100.138266 369.51000 337.63500 775.40827 4
62 9.56250 15.020740 324.88500 320.10375 655.22824 56 52 64.81250 101.807237 370.38500 337.97875 777.76474 5
63 10.62500 16.689711 325.76000 320.44750 657.58471 57 51 65.87500 103.476208 371.26000 338.32250 780.12121 6
62 11.68750 18.358682 326.63500 320.79125 659.94118 58 52 66.93750 105.145179 372.13500 338.66625 782.47768 7
63 12.75000 20.027653 327.51000 321.13500 662.29765 59 51 68.00000 106.814150 373.01000 339.01000 784.83415 8
62 13.81250 21.696624 328.38500 321.47875 664.65412 60 50 69.06250 108.483121 373.88500 339.35375 787.19062 9
63 14.87500 23.365595 329.26000 321.82250 667.01060 61 49 70.12500 110.152092 374.76000 339.69750 789.54709 10 62 15.93750 25.034566 330.13500 322.16625 669.36707 62 50 71.18750 111.821064 375.63500 340.04125 791.90356 11 63 17.00000 26.703538 331.01000 322.51000 671.72354 63 49 72.25000 113.490035 376.51000 340.38500 794.26003 12 62 18.06250 28.372509 331.88500 322.85375 674.08001 64 48 73.31250 115.159006 377.38500 340.72875 796.61651 13 61 19.12500 30.041480 332.76000 323.19750 676.43648 65 47 74.37500 116.827977 378.26000 341.07250 798.97298 14 62 20.18750 31.710451 333.63500 323.54125 678.79295 66 48 75.43750 118.496948 379.13500 341.41625 801.32945 15 55 21.25000 33.379422 334.51000 323.88500 681.14942 67 47 76.50000 120.165919 380.01000 341.76000 803.68592 16 62 22.31250 35.048393 335.38500 324.22875 683.50589 68 46 77.56250 121.834890 380.88500 342.10375 806.04239 17 61 23.37500 36.717364 336.26000 324.57250 685.86236 69 45 78.62500 123.503861 381.76000 342.44750 808.39886 18 62 24.43750 38.386335 337.13500 324.91625 688.21884 70 46 79.68750 125.172832 382.63500 342.79125 810.75533 19 61 25.50000 40.055306 338.01000 325.26000 690.57531 71 45 80.75000 126.841803 383.51000 343.13500 813.11180 20 62 26.56250 41.724277 338.88500 325.60375 692.93178 72 44 81.81250 128.510774 384.38500 343.47875 815.46827 21 61 27.62500 43.393249 339.76000 325.94750 695.28825 73 43 82.87500 130.179746 385.26000 343.82250 817.82475 22 60 28.68750 45.062220 340.63500 326.29125 697.64472 74 42 83.93750 131.848717 386.13500 344.16625 820.18122 23 61 29.75000 46.731191 341.51000 326.63500 700.00119 75 43 85.00000 133.517688 387.01000 344.51000 822.53769 24 60 30.81250 48.400162 342.38500 326.97875 702.35766 76 42 86.06250 135.186659 387.88500 344.85375 824.89416 25 61 31.87500 50.069133 343.26000 327.32250 704.71413 77 41 87.12500 136.855630 388.76000 345.19750 827.25063 26 60 32.93750 51.738104 344.13500 327.66625 707.07060 78 40 88.18750 138.524601 389.63500 345.54125 829.60710 27 59 34.00000 53.407075 345.01000 328.01000 709.42708 79 36 89.25000 140.193572 390.51000 345.88500 831.96357 28 60 35.06250 55.076046 345.88500 328.35375 711.78355 80 38 90.31250 141.862543 391.38500 346.22875 834.32004 29 59 36.12500 56.745017 346.76000 328.69750 714.14002 81 39 91.37500 143.531514 392.26000 346.57250 836.67651 30 60 37.18750 58.413988 347.63500 329.04125 716.49649 82 38 92.43750 145.200485 393.13500 346.91625 839.03299 31 59 38.25000 60.082959 348.51000 329.38500 718.85296 83 37 93.50000 146.869457 394.01000 347.26000 841.38946 32 58 39.31250 61.751931 349.38500 329.72875 721.20943 84 36 94.56250 148.538428 394.88500 347.60375 843.74593 33 59 40.37500 63.420902 350.26000 330.07250 723.56590 85 35 95.62500 150.207399 395.76000 347.94750 846.10240 34 58 41.43750 65.089873 351.13500 330.41625 725.92237 86 34 96.68750 151.876370 396.63500 348.29125 848.45887 35 59 42.50000 66.758844 352.01000 330.76000 728.27884 87 33 97.75000 153.545341 397.51000 348.63500 850.81534 36 58 43.56250 68.427815 352.88500 331.10375 730.63531 88 32 98.81250 155.214312 398.38500 348.97875 853.17181 37 57 44.62500 70.096786 353.76000 331.44750 732.99179 89 31 99.87500 156.883283 399.26000 349.32250 855.52828 38 58 45.68750 71.765757 354.63500 331.79125 735.34826 90 30 100.93750 158.552254 400.13500 349.66625 857.88475 39 57 46.75000 73.434728 355.51000 332.13500 737.70473 91 29 102.00000 160.221225 401.01000 350.01000 860.24123 40 58 47.81250 75.103699 356.38500 332.47875 740.06120 92 28 103.06250 161.890196 401.88500 350.35375 862.59770 41 57 48.87500 76.772670 357.26000 332.82250 742.41767 93 27 104.12500 163.559168 402.76000 350.69750 864.95417 42 56 49.93750 78.441642 358.13500 333.16625 744.77414 94 26 105.18750 165.228139 403.63500 351.04125 867.31064 43 57 51.00000 80.110613 359.01000 333.51000 747.13061 95 25 106.25000 166.897110 404.51000 351.38500 869.66711 44 56 52.06250 81.779584 359.88500 333.85375 749.48708 96 22 107.31250 168.566081 405.38500 351.72875 872.02358 45 55 53.12500 83.448555 360.76000 334.19750 751.84355 97 21 108.37500 170.235052 406.26000 352.07250 874.38005 46 56 54.18750 85.117526 361.63500 334.54125 754.20003 98 20 109.43750 171.904023 407.13500 352.41625 876.73652 47 51 55.25000 86.786497 362.51000 334.88500 756.55650 99 17 110.50000 173.572994 408.01000 352.76000 879.09299 48 54 56.31250 88.455468 363.38500 335.22875 758.91297 100 16 111.56250 175.241965 408.88500 353.10375 881.44947 49 55 57.37500 90.124439 364.26000 335.57250 761.26944 101 13 112.62500 176.910936 409.76000 353.44750 883.80594 50 54 58.43750 91.793410 365.13500 335.91625 763.62591 102 10 113.68750 178.579907 410.63500 353.79125 886.16241 51 53 59.50000 93.462381 366.01000 336.26000 765.98238 103 7
114.75000 180.248878 411.51000 354.13500 888.51888 52 54 60.56250 95.131353 366.88500 336.60375 768.33885