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 +PointŽ probe. Discuss the results of these
+PointŽ 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 90SG-1SG-2SG-3SG-4Dents >= 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
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 2 nd and 4 th vertical straps were inspected with bobbin and
+PointŽ probes. If the wear indications were not inspected
with a +PointŽ 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 RSG's 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 SG's 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 VS3VS4 DS2 76-103 DS1VS1VS2 VS3VS4 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 columnsTubesRow#"A""B""C""D"(Developed Length)Row#"A""B""C""D"(Developed Length)1616.3750010.013830322.26000319.07250648.15883535361.6250096.800324367.76000336.94750770.69532 2627.4375011.682798323.13500319.41625650.51530545262.6875098.469295368.63500337.29125773.051793638.5000013.351769324.01000319.76000652.87177555363.75000100.138266369.51000337.63500775.408274629.5625015.020740324.88500320.10375655.22824565264.81250101.807237370.38500337.97875777.76474 56310.6250016.689711325.76000320.44750657.58471575165.87500103.476208371.26000338.32250780.12121 66211.6875018.358682326.63500320.79125659.94118585266.93750105.145179372.13500338.66625782.47768 76312.7500020.027653327.51000321.13500662.29765595168.00000106.814150373.01000339.01000784.83415 86213.8125021.696624328.38500321.47875664.65412605069.06250108.483121373.88500339.35375787.19062 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