ML090960558
| ML090960558 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 04/03/2009 |
| From: | Skaggs M Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| TAC MD9709 | |
| Download: ML090960558 (12) | |
Text
U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Gentlemen:
In the Matter of
)
Docket No. 50-390 Tennessee Valley Authority (TVA)
)
WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 - RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION RE: STEAM GENERATOR TUBE INSPECTION REPORT FOR CYCLE 8 (TAC NO. MD9709)
The purpose of this letter is to respond to NRCs request for additional information (RAI) dated March 9, 2009 (ADAMS No. ML090640936), concerning the subject steam generator tube inspection report. The subject report was originally submitted to the NRC by letters dated June 24 and September 11, 2008.
There are no regulatory commitments associated with this submittal. If you have any questions concerning this matter, please call Emmett Camp at (423) 843-8214.
Sincerely, Mike Skaggs Site Vice President
Enclosure:
cc: See Page 2 April 3, 2009 Original Signed by Mike Skaggs
U.S. Nuclear Regulatory Commission Page 2 Enclosure cc:
NRC Resident Inspector Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381 U.S. Nuclear Regulatory Commission Mr. John G. Lamb, Senior Project Manager Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation MS 0-8H1A Washington, DC 20555-0001 U.S. Nuclear Regulatory Commission Region II Sam Nunn Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303 April 3, 2009
E-1 of 8 ENCLOSURE REQUEST FOR ADDITIONAL INFORMATION (RAI)
WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 STEAM GENERATOR TUBE INSPECTION REPORT FOR CYCLE 8 By letters dated June 24, 2008 (ADAMS No. ML081840101) and September 11, 2008 (ADAMS No. ML082600068), TVA submitted information pertaining to the cycle 8 steam generator (SG) tube inspections for WBN Unit 1. The NRC issued a request for additional information (RAI) dated March 9, 2009 (ADAMS No. ML090640936). TVAs response to this RAI is given below.
Question 1 Please provide the cumulative effective full power months that the SGs had operated at the time of the WBN Unit 1 2008 refueling outage.
TVA Reponse The WBN Unit 1 Replacement Steam Generators (RSGs) had operated for one cycle at the time of the WBN Unit 1 Cycle 8 Refueling Outage (RFO). This cycle consisted of 432.5 Effective Full Power Days (EFPD), which equates to approximately 14.21 Effective Full Power Months (EFPM).
Question 2 Please provide a general description of WBN Unit 1 replacement steam generators. Please include in this response the following information regarding the design of your replacement steam generators: (i) the tubesheet thickness (with and without clad); (ii) the method used to expand the tubes into the tubesheet and the extent of expansion; (iii) the extent to which any tubes were stress relieved following bending; (iv) the radius of the smallest radius tubes; (v) the tube support plate thickness, material of construction, and hole configuration (including the flow distribution baffle, if any); (vi) the anti-vibration bar cross section (e.g., rectangular), material of construction and the depth of penetration of the anti-vibration bars; (vii) the tube pitch (e.g., 1.0 triangular); and (viii) the tube and steam generator fabricator. In addition, please provide a tubesheet map and a sketch of the stream generator showing the tube support naming convention.
TVA Response I.
A.
General
Description:
The WBN Unit 1 RSG is of the same basic design as the original steam generator (OSG), which is a vertical shell and continuous bend U-tube recirculating heat exchanger with an integral preheater. High temperature reactor coolant from the reactor vessel enters the SG through the primary inlet nozzle and flows inside the tubes (primary side) where it is cooled. The reactor coolant leaves the SG through the primary outlet nozzle. The primary head is divided into inlet and outlet chambers with a vertical partition plate extending from the head to the tubesheet. Feedwater enters the preheater region
E-2 of 8 (secondary side) through the main feedwater nozzle located near the tubesheet on the cold leg side of the tube bundle. This region is separated from the lower hot leg region by a divider plate in the tube lane.
The RSG preheater design differs significantly from the OSG design, which was a split-flow arrangement with multiple pass cross-flow. The preheater for the RSG is an axial counter-flow design with no baffles. Feedwater exiting the distribution box at the tubesheet flows beneath a distribution plate and upward as it is heated by forced convection to near saturation conditions at the top of the preheater. At this elevation, the heated feedwater mixes with cold leg downcomer water and secondary fluid from the hot leg side of the tube bundle.
Steam is generated on the outside of the tubes and flows upward while continually increasing in quality. Moisture separators mounted on a deck plate above the top of the tube bundle shroud separate the steam from the two-phase mixture. Separated water returns to the downcomer and steam flows upward through the secondary dryers and out the steam nozzle at the top of the vessel as main steam to the turbine.
B.
Primary Side:
The primary side steam generator pressure boundaries are the lower hemispherical head of the vessel, the tubesheet, and the tubing. The primary head divider plate partitions the head into an inlet and outlet plenum. Reactor coolant enters the inlet plenum through the primary inlet nozzle, flows through the U-tubes to the outlet plenum, and exits through the primary outlet nozzle.
- 1.
Tube and Tubesheet:
The U-tube bundle consists of 5,128 tubes whose average heated length is 67.54 feet with 0.75 inch outer diameter (OD) x 0.043 inch average wall thickness. These nickel-chromium-iron (Ni-Cr-Fe) thermally treated Alloy 690 tubes are hydraulically expanded into the tubesheet and welded to the Ni-Cr-Fe alloy clad primary face of the tubesheet. The tubesheet base metal is 22 inches thick and clad with 0.25 inch minimum thick Ni-Cr-Fe alloy material.
- 2.
Primary Nozzles:
One primary inlet nozzle 31.03 inches nominal inner diameter (ID) and one primary outlet nozzle 31.03 inches nominal ID are located in the primary head. Both inlet and outlet nozzles are clad with 0.2 inch minimum thick Ni-Cr-Fe cladding and have a welded ring for mounting nozzle dams for use during refueling or other maintenance operations.
- 3.
Primary Manways:
Two 16-inch nominal ID primary manways are located in the primary head, one on the inlet side and one on the outlet side. A stainless steel gasket insert plate and spiral wound gasket are held in place by the manway cover which is secured with sixteen studs and nuts to the manway flange.
E-3 of 8 C.
Secondary Side:
- 1.
Connections and Nozzles:
The secondary side steam generator boundaries are the top head, upper shell, conical transition section, lower shell, and the secondary side of the tubesheet and tubing. Connections are provided in the shells and upper head for feedwater, auxiliary feedwater, steam outlet, recirculation water, pressure tap, hot and cold leg blowdown, draining, and water level instrumentation. The upper shell is provided with two 16-inch nominal ID gasketed manways. The manway cover seats externally and is held in place by 20 studs, nuts, and washers. The lower shell is provided with two 6-inch nominal ID and two 8-inch nominal ID gasketed handholes.
Two inspection ports are provided at the tubesheet elevation and one is provided at a mid-bundle location. The inspection port, recirculation nozzle, and handhole covers seat externally and are held in place by eight studs, nuts, and washers. The top head has an integrally forged steam outlet nozzle, 32-inch nominal OD. The steam outlet nozzle includes seven alloy 690 flow-limiting venturi inserts.
- 2.
Tube Supports and Shroud:
An assembly of tube supports is located throughout the secondary side below the separator support plate. These supports provide stability to the tubes by limiting movement due to water/steam flow-induced vibration.
Surrounding the tubes and supports is a shroud that creates a circulation path for secondary water to come up through the tube bundle and a return path down between the shroud and outer shell, ending at the tubesheet.
- 3.
Steam Separation Equipment:
The steam separation equipment is located in the upper shell and top head section (steam drum). Separation equipment consists of cylindrical shaped steam-water separators provided with spinner blades in their lower sections to direct the steam-water mixture to the perforated mid-section of the separators. Steam dryers are located above the separators and are designed with high capacity vanes to remove moisture from the steam-water mixture exiting from the separators. Drains are provided to return extracted moisture from the dryer to the separator support plate where it is mixed with separator water and then recirculated.
- 4.
Vessel Supports:
Each SG is supported vertically on four main support pads. These pads, located 90° apart, are integrally forged with the primary head. Six holes are machined into each pad to securely fasten the steam generator to the field support system. The steam generator support pads are machined with a 0.38 inch allowance for shimming the unit to achieve vertical alignment during installation at the site.
E-4 of 8
- 5.
Loose Part Monitoring Sensors:
Contact surfaces are provided for mounting loose part monitoring sensors on the primary head. Contact surfaces are also provided for mounting loose part monitoring sensors on the lower shell.
II.
Specific Elements of Question 2:
A.
Tubesheet Thickness:
The tubesheet has a base metal nominal thickness of 22 inches plus a 0.25 inch minimum clad thickness (i.e., total thickness of 22.25 inches minimum).
B.
Tube-to-Tubesheet Joint:
The RSG tubes are fusion welded to the primary face of the tubesheet and hydraulically expanded to maximize mechanical strength and to minimize the tube-to-tubesheet crevice.
A single-pass fusion weld seals the tube to the tubesheet. The smooth weld profile results in negligible tube diameter reduction. The tube-to-tubesheet weld was designed, analyzed, welded, and examined in accordance with ASME B&PV Code,Section III, criteria.
The RSG tubes are hydraulically expanded through essentially the entire thickness of the tubesheet. A nominal crevice depth of 0.071 inches is achieved at the secondary face of the tubesheet. The length of the expansion mandrel was determined by the thickness of the tubesheet. The procedure included fine tuning of mandrel length, if necessary, to account for local variations of tubesheet thickness. The hydraulic seals of the expansion mandrel are of elastomeric material and are designed so that no metal parts are impressed upon the inside surface of the tube when the hydraulic pressure was applied. The position of the seal at the secondary face of the tubesheet was controlled to ensure that expansion of the tube was as close as possible to the secondary face of the tubesheet without going past the face.
After expansion, the inside profile of each tube was measured by an eddy current method through the entire expanded area of the tubesheet. The measurements indicated both the position and condition of the tube expansion and became a baseline for subsequent inservice inspections (ISI).
To ensure that all tube-to-tubesheet joining operations could be satisfactorily performed, a mockup sample was constructed. It simulated the full tubesheet thickness and used materials identical to those utilized in the SG. All processes, procedures, and inspections approved for use in manufacturing the tube to tubesheet joint were performed. The sample was then examined by sectioning to verify that manufacturing operations were correctly performed and that results were satisfactory.
Tests on hydraulically expanded joints made with Alloy 690 tubes, in triangular-pitched holes, have shown that residual stresses exist in the transition region
E-5 of 8 between the expanded and unexpanded tube. The hydraulic expansion process has been designed and qualified to minimize residual stresses while maintaining joint integrity.
C.
Stress Relieved Following Bending:
Rows 1 through 38 have been heat treat stress relieved for the full length following bending.
D.
Smallest Tube Radius:
The minimum tube bend radius is 3.19 inches nominal and maximum bend radius is 58.44 inches nominal.
E.
Tube Support Plate:
WBN Unit 1 SGs do not have tube support plates. Rather, they have Advanced Tube Support Grids (ATSGs) made of type 409 stainless steel specially machined 1 inch and 2 inch tall bars. Design features of the ATSGs include:
Low flow resistance compared to the OSG Minimum tube-to-support contact Superior vibration restraint and fretting resistance Low tendency to accumulate deposits compared to a broached or drilled plate Elimination of denting potential due to selection of stainless steel The ATSGs provide support for the straight portion of the tubes. Tie rods provide spacing, and the periphery of the ATSGs is fixed to the shroud. In the preheater area, the ATSGs are made in two halves with the secondary divider plate between them.
The ATSG is in operation in the Sequoyah Nuclear Plant (SQN) Unit 1 RSG design, and it is an improved version of the Eggcrate design typically employed in such designs as the Palo Verde Units 1, 2, and 3 and the recent series of plants in operation or under construction in South Korea. The ATSG was developed specifically for the SQN Unit 1 RSG where the tube size and pitch selected for the SQN Unit 1 RSGs design afforded the opportunity to optimize the Eggcrate design to minimize the line contact between tubes and supports. This configuration has been tested in a prototypical arrangement of tubes and support spacing to verify acceptable vibration characteristics. The ATSG design was adopted for the Watts Bar Unit 1 RSG. Design and fabrication details are identical to the SQN Unit 1 RSG since the tube size and pitch are the same.
The arrangement of the heat transfer surface includes an axial flow preheater section adjacent to the cold feedwater entrance. The arrangement of the axial
E-6 of 8 flow preheater including the feedwater box is based on similar designs applied in operating plants of the original Palo Verde series, Korean plants, and the Palo Verde 2 RSG. Use of the axial flow preheater for the RSG maintains the same external interface with feedwater piping as the OSG design and provides for efficient use of heat transfer surface. Incoming feedwater is distributed circumferentially in the feedwater box where it turns downward to pass flow openings in the foot of the feedwater box. The fluid enters the space between the shell and periphery of the tube bundle and then enters the tube bundle in cross-flow, where the restriction of the 1 inch thick flow distribution plate with 0.775 inch drilled holes on a 1.06 inch triangular pitch pattern helps develop a uniform upward and axial flow along the tubes in the preheater section. The flow distribution plate is made of type 405 stainless steel.
F.
Anti-Vibration Bar:
The Upper Bundle Support (UBS) System is designed to support the U-bends against harmful wear and vibration, to minimize the potential for sludge deposition, and to promote circulation. The design configuration is a refinement of the design employed for the SQN Unit 1 RSG currently in operation.
The UBS features diagonal, vertical, and branch bars which provide support to the U-bends against flow induced vibration (FIV), seismic loads, transportation loads, and handling and assembly loads during fabrication. These assemblies are fabricated from perforated 2 inch tall strips made of type 409 stainless steel.
The diagonal bars are 35° 45 from horizontal. Inner, middle, and outer vertical bars are integral to the diagonal bars. Branch bars are integral to the outer vertical bars and are configured 35° 45 from horizontal. These bars have slotted holes. As the tube bundle is assembled, these assemblies are positioned and interlocked by 15 lock bars made of type 409 stainless steel at the center vertical bar. The top of the inner, middle, and outer vertical bars are captured above the tube bundle by five arch plates made of A-36 carbon steel. These arch plates are supported by the shroud. The ends of the diagonal and branch bars are also linked together. The ends of the assembly, where the center vertical and diagonal bars intersect, are supported and spaced by a slotted tee attached to the top-most ATSG. In effect, there is no u-bend section of any tube without support.
At the completion of tube bundle assembly, the UBS is integral with the U-bends of the tube bundle and generally moves with the tube bundle during heatup and cooldown. The vertical deadweight of UBS assembly is nominally transferred to the bundle through the center vertical grid. The out-of-plane support of the U-bends is provided by the arch plate structure and I-beam structure external to the bundle. Additionally, there are lateral load supports on the arch plates that provide support during assembly operations (vessel rotation), shipping, and seismic events.
G.
Tube Pitch:
The tube bundle has a 1.06 inch triangular pitch.
E-7 of 8 H.
Tube and Steam Generator Fabricator:
The WBN Unit 1 RSG design has many unique design features. Westinghouse designed the RSGs. Doosan Heavy Industry and Construction (South Korea) was the primary manufacturer. Valinox (France) manufactured the tubing. CFI (France) supplied the primary head forgings, cone forgings, and some shell forgings. Kyung Sung Manufacturing (South Korea) supplied the tube support grids, UBS parts, and primary moisture separators. Peerless Manufacturing (USA) manufactured the steam dryers.
III.
Additional Requested Information:
The tubesheet map and SG sketch for the naming convention are provided as Attachments 1 and 2, respectively.
Question 3 Regarding the scope of WBN Unit 1 inspections, discuss whether any secondary side inspections (including foreign object search and retrieval inspections) were performed. If so, discuss the results. If any loose parts or degraded conditions were identified, please discuss the actions taken to ensure acceptable SG performance until the next scheduled inspection.
TVA Response Sludge Lancing was performed during the Unit 1 Cycle 8 RFO and subsequent inspections found the top-of-tubesheet to be acceptably clean of loose sludge. The following amounts were removed: SG 1 - 8.5 lbs, SG 2 - 5.75 lbs, SG 3 - 7.25 lbs, SG 4 - 8.0 lbs.
Foreign Object Search And Retrieval (FOSAR) was completed for all four SGs prior to SG closure and all identified foreign objects were retrieved. No wear from foreign material was identified during the U1C8 inspection either by eddy current or FOSAR. Possible Loose Parts (PLPs) identified by eddy current were provided to FOSAR and were investigated. No PLPs resulted in FOSAR identifying foreign material. The order of sludge lance and FOSAR was SG 3-2-1-4.
Notable results of the FOSAR were as follows:
No foreign objects were identified in SG 3.
In SG 2, the sludge lance vendor lost a black delron protective piece and initially it was not identified by FOSAR. The initial review of the FOSAR tape occurred prior to sludge lance identifying the missing delron piece. However, a second review of the FOSAR tape after sludge lance identified the missing piece of delron resulting in it being retrieved (Problem Evaluation Report (PER) 139205).
In SG 1, one foreign object was identified and retrieved (PER 139331).
Prior to the U1C8 outage, the SG 4 loose parts monitoring system identified a noise possibly indicating a foreign object. The SG 4 feedwater flow distribution box was opened to provide access for FOSAR to do a visual search for any foreign material. A quantity of foreign material was identified in SG 4. FOSAR discovered a couple of foreign objects on the hot leg top-of-tubesheet and numerous (approximately 15 - 20) foreign objects on the cold leg top-of-tubesheet. Additionally, a quantity of foreign objects (approximately 50) was identified in the SG 4 feedwater flow distribution box (PER 139399). FOSAR utilized
E-8 of 8 a vacuum and was able to remove all of the foreign objects. The foreign objects were pieces of flexitalic gasket material, weld slag, wire, and other unidentifiable objects. None of the objects were such that a source could be conclusively determined.
The identified foreign objects were removed from the SGs.
No upper bundle inspections were performed during the U1C8 inspection. The wrapper in the WBN Unit 1 SGs are of a very robust design. No interference was identified by the sludge lance personnel. Therefore, no additional inspections were performed related to wrapper drop or wrapper cracking.
A1-1 of 1 ATTACHMENT 1 WATTS BAR NUCLEAR PLANT REPLACEMENT STEAM GENERATOR TUBESHEET MAP
A2-1 of 1 ATTACHMENT 2 WATTS BAR NUCLEAR PLANT STEAM GENERATOR TUBE SUPPORT NAMING CONVENTION