ML071420119
| ML071420119 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 05/21/2007 |
| From: | Coutu T Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, NRC/NRR/ADRO |
| References | |
| BW070040 | |
| Download: ML071420119 (11) | |
Text
Exelon GeneTation Company, LLC www.exeloncoTp.com Exe~n Nuci ear Braidwood Station
~ ~ S~t~S.
~ ~O~C7 May 21, 2007 BW070040 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Braidwood Station, Unit 1 Facility Operating License No. NPF-72 NRC Docket No. STN 50-456
Subject:
Response to Request for Additional information Regarding the Braidwood Station, Unit 1 Spring 2006 Steam Generator Inspection
Reference:
Letter from K. J. Poison (Exelon Generation Company) to U. S. NRC, Braidwood Station, Unit 1 Twelfth Refueling Outage Steam Generator inservice Inspection Summary Report, dated July 25, 2006 (ADAMS Ascension No.
ML06206066000)
Based on the review of the referenced submittal, the NRC determined that additional information was required in order to complete their evaluation of the Braidwood Station, Unit 1 Spring 2006 Steam Generator inservice Inspection Summary Report.
The attachment to this letter provides the Exelon Generation Company response to these NRC questions.
Should you have any questions concerning this letter, please contact Mr. Dale Amb!er, Regulatory Assurance Manager, at (815) 417-2800.
Respectfully, Thomas Coutu Site Vice President Braidwood Station
Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information Question 1:
In Section 5.1 of the report, you indicated that only one tube was identified with a dent/ding greater than or equal to 2.0-volts in the hot-leg region. Was this dent/ding signalnew? If so, please discuss the causal mechanism for this dent/ding signal. In addition, if there were other dents/dings examined during the refueling outage (RFO) 12 inspections, please discuss whether these dent/ding signals changed. If changes did occur, what is the causal mechanism and were the changes expected?
Response to Question 1:
As discussed in the Braidwood Station Unit 1 Spring 2006 Refueling Outage (RFO) 12 Steam Generator (SG) Inservice Inspection Summary Report1, only one dent/ding was identified in the hot leg region during the 100% full-length eddy current inspection of all four SGs. The one dent/ding was located 10.46 inches above the hot leg top of tubesheet in SG 1A Row 8 Col 11. The dent/ding was sized at 2.58 volts based on bobbin inspection. The dent/ding was inspected with +PointTM and no degradation was identified. This dent/ding has been present since initial pre-service inspection of the SG and has not changed.
Question 2:
For all loose parts/possible loose part indications identified during RFQ 12, please discuss whether they were identified with bobbin coil, rotating probe, visual inspection, or a combination of these techniques (indicate the specific combination). In addition, please discuss the scope ofyour visual inspection on the secondary side ofthe SGs. If all loose parts/possible loose part indications were not identified by all techniques (except for locations not accessible to visual inspection), please discuss the basis for the scope ofyour examinations, The main issue is whether the bobbin coil is sufficient to screen tubes for loose parts near the top of the tubesheet (refer to Information Notice 2004-17, Loose Part Detection and Computerized Eddy Current Data Analysis in Steam Generators).
Response to Question 2:
Wear from Secondary Side Foreign Objects During the Braidwood Station Unit 1 RFO 12, secondary side visual inspection was performed in the top of tubesheet region of all four SGs. The visual inspection scope included 100% of bundle periphery including hot leg, cold leg, and open tube lane.
Additionally, in-bundle visual inspection was performed on a limited basis in order to determine post sludge lance cleanliness conditions and view accessible areas when eddy current identified locations of possible foreign objects.
1 Letter from K. J. Poison (Exelon Generation Company) to U. S. NRC, Braidwood Station, Unit 1 Twelfth Refueling Outage Steam Generator Inservice Inspection Summary Report, dated July 25, 2006 (ADAMS Ascension No. ML06206066000) 1
Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information A total of eight indications of secondary side foreign object wear in six tubes were identified during the Braidwood Station Unit 1 RFO 12 100% full-length bobbin eddy current inspection of all four SGs. The data was originally transmitted as part of the Braidwood Station Unit 1 RFO 12 SG Inservice Inspection Summary Report (the Summary Report). The table below summarizes each indication and percent through-wall (%TW), followed by a detailed discussion of each indication.
Foreign Object Wear Summary A 102 u~ 61 TSH + SVI 59 Y Y 0.22 A 102 61 TSH+ SVI 88 Y Y 0.29 A 96 107 TSH+ VOL 17 N N 0.11 A 98 107 TSH+ VOL 16 N N 0.10 A 97 108 TSH+ VOL 16 N N 0.16 B 98 47 TSH VOL 23 N N 0.01 B 53 90 8H SVI 37 Y Y
+_0.47 B 53 90 8H SVI 56 Y Y
+ 0.79 SVI = Singie Volumetric Indication VOL = Volumetric Indication TSH =Top of Tubesheet hot Leg 8H 81h Lattice Grid Support on the Hot Leg Side In the 1 A SG, tube Row 102 Column 61, had two indications of secondary side foreign object wear slightly above the hot leg top of tubesheet intersection. The wear was originally identified during the full-length bobbin inspection of the tube and was classified as a Distorted Tubesheet Indication (DII). in accordance with the Exelon Steam Generator Eddy Current Program, all indications classified as I-Codes require further analysis using the +PointlM probe. The +PointTM probe classified the indication as two Single Volumetric Indications (SVI). The two indications were sized at 88% TW and 59% 1W via the ÷PointTMprobe.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information The +PointlM inspection scope was expanded to the surrounding 18 tubes in the hot leg top of tubesheet region (two tubes in all directions). No additional wear or evidence of secondary side foreign objects was identified. Structural and leakage integrity of these two indications was demonstrated through In-Situ pressure testing as described in Section 5.2.2 of the Summary Report. Subsequent secondary side visual inspection of this region identified that the object that caused the tube wear was still present and could not be retrieved. The object is best described as a piece of metallic gasket material approximately 1.75 inches in length (SG Object No. 1A005). Tube Row 102 Column 61 was plugged and stabilized in the region of the foreign object wear. The surrounding six tubes were also preventively plugged and stabilized in the region of the foreign object in order to preclude further tube damage.
In the 1A SG, three tubes were identified with relatively small amounts of tube wear based on +PointTM sizing, (i.e., Row 96 Col 107 (17% 1W), Row 98 Col 107 (16% TW) and Row 97 Col 108 (16% 1W)). These indications were not detectable during the 100% full-length bobbin inspection program. During the secondary side visual inspection program of the top of tubesheet region in the 1A SG, a piece of metallic gasket material approximately 1.25 inches long x 0.180 inch high x 0.050 inch thick (SG Object No. 1 A001) was identified at Row 101 103 and Column 109 110 on the hot leg.
The object was successfully retrieved from the 1A SG. Since the object was in a relatively high flow region of the SG, +PointTM inspection was performed on 23 tubes in the surrounding area. During this additional +PointTM inspection, the three tubes listed above were identified as having small amounts of wear. Based on this information, the
+PointTM inspection scope was expanded to the hot leg top of tubesheet region by an additional 15 tubes. No additional tube wear or evidence of foreign objects was identified. Based on secondary side visual inspection of the areas surrounding these tubes, it was confirmed that no foreign objects remained in the region. Since these indications were significantly below the Technical Specification (TS) plugging criteria of greater than or equal to 40% 1W and visual inspection confirmed that secondary side foreign objects were no longer in the region, these tubes were allowed to remain in service.
In the 1 B SG, tube Row 98 Column 47 had a 23% 1W wear indication as sized by the
+PointTM probe. This indication was not detectable during the 100% full-length bobbin inspection program. The indication was identified during the planned 20% hot leg top of tubesheet +PointlM inspection program as described in the Summary Report. Once identified, the three additional tubes were added to the already planned 20% hot leg top of tubesheet +PointTM inspection program in order to inspect two tubes in all directions.
No additional wear or evidence of foreign object(s) was identified. Based on secondary side visual inspection of the area surrounding this tubes it was confirmed that no foreign objects remained in the region. Since this indication was significantly below the IS plugging criteria of greater than or equal to 40% TW and visual inspection confirmed that secondary side foreign objects were no longer in the region, this tube was allowed to remain in service.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information In the 1 B SG, tube Row 53 Column 90 had two indications of secondary side foreign object wear at the 8th Lattice Grid in the hot leg side. These indications were originally identified during the full-length bobbin eddy current inspection program and classified as Distorted Support Indications (DSI). The followup +PointTM probe examination classified the indication as two Single Volumetric Indications (SVI). The two indications were sized at 56% 1W and 37% 1W using the +PointTM probe. The +PointTM inspection scope was expanded to the surrounding 18 tubes (two tubes in all directions) through the 8th Lattice Grid location 3 to +4 on the hot leg side. No additional wear or evidence of secondary side foreign objects was identified.
Based on the eddy current signal, the object that caused the wear in tube Row 53 Column 90 appears to remain lodged at the ~ Lattice Grid on the hot leg side.
Structural and leakage integrity of the two indications within this tube was demonstrated through In-Situ pressure testing as described in Section 5.2.2 of the Summary Report.
Secondary side visual inspection of this region was not possible due to its location at the 8th Lattice Grid. Tube Row 53 Column 90 was plugged and stabilized in the region of the foreign object wear. The surrounding nine tubes were also preventively plugged and stabilized in the region of the foreign object in order to preclude further tube damage.
Objects Identified by Visual Inspection During the Braidwood Station Unit 1 RFO 12 there were a number of secondary side foreign objects that were newly identified by visual inspection that were not associated with tube wear. Table 1 lists each of these foreign objects, including additional actions taken and final disposition for each object. The objects were either removed from the secondary side of the SG, or if retrieval was not successful, the objects were either determined to be acceptable via engineering disposition or the surrounding tubes were preventatively plugged and stabilized.
Ability to Identify Secondary Side Foreign Objects! Wear In response to industry issues concerning the ability to identify SG secondary side foreign objects / wear, as discussed in NRC Information Notice 2004-17, Loose Part Detection and Computerized Eddy Current Data Analysis in Steam Generators, Exelon has taken the following actions:
- As identified in Operating Experience Report (OE1 8385) dated May 11, 2004, and summarized in NRC Information Notice 2004-17, a utility utilizing computerized data screening did not identify wear at the top of tubehseet region due to improper setup of the sort parameters. The improper setup dealt with an analysis gap in which the eddy current data for a section of the tube was not being analyzed. In response to OE1 8385, the Exelon SG Program procedures were revised in order to preclude similar events. The automated eddy current data sorts are reviewed to ensure there are no gaps over the required inspection lengths.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information This review is performed prior to the start of data collection and signed off by the Exeion Eddy Current Level Ill, Independent Qualified Data Analyst and Westinghouse Lead Level III.
As part of Exelons review of OE1 8385, Westinghouse performed a review of the automated eddy current data sorts used during the previous Braidwood Station Unit 1 Outage (RFO 11), and confirmed that analysis gaps as described in the OE did not exist during the inspection.
- As identified in NRC Information Notice 2004-1 7, a utility was unable to detect secondary side foreign object wear using bobbin eddy current techniques since the wear indication was masked by the residual signal from the top of tubesheet expansion transition. Subsequent review of the bobbin eddy current data determined that the signal could have been detected if a three-frequency mix had been used at the top of tubesheet transition. The three-frequency mix helps to minimize the residual interfering signal from the top of tubehseet transition. Prior to issuance of the NRC Information Notice, the Braidwood Station Unit 1 RFO 11 SG inspection included use of a three-frequency mix at the top of tubesheet transition in order to identify indications of secondary side foreign object wear. The use of the three-frequency mix was optional as contained in the Exelon SG Program procedures.
Based on the information contained in the NRC Information Notice, the Exelon SG Program procedures were revised to make use of the three-frequency mix a requirement. Therefore, the three-frequency mix was used during the Braidwood Station Unit 1 RFO 12 and will continue to be used in future outages.
- Realizing the potential for SG secondary side foreign objects to cause significant tube wear, the Braidwood Station Unit 1 RFO 12 inspection included detailed video inspection of the accessible regions of the top of tubesheet region. The highest flow areas at the top of tubehseet region in the Braidwood Station Unit 1 SGs are at the periphery of the tube bundle. Once inside the tube bundle the secondary side flow velocities significantly reduce; thus, having less potential to cause significant tube wear between inspections.
- During the Braidwood Station Unit 1 RFO 12, video inspection was performed from the bundle periphery looking towards the tube bundle in order to determine if foreign object were present that were not able to be detected via eddy current inspection. If foreign objects were visually identified in high flow regions, the surrounding tubes received an additional +PointTM inspection. The +PointTM inspection was performed in order to determine: 1) if wear was present that might not have been detectable using the bobbin eddy current techniques and 2) if other potential foreign objects were present that might not have been able to be seen via the video inspection. The results of these inspections are described in Table 1.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information
- As previously discussed, during the Braidwood Station Unit 1 RFO 12 inspection, when eddy current detected evidence of wear or signal from a secondary side foreign object, the +PointTM inspection program was expanded to include the surrounding tubes. If a foreign object was visually identified in a high flow region of the tube bundle that had the potential to cause tube wear, the tubes surrounding the object received +PointTM inspection. These additional inspections helped ensure that secondary side foreign object / foreign object wear would be identified.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information Table 1 Secondary Side Foreign Objects Newly Identified During Al Ri2 SG Location (Object Description Bas:don Additional Actions Disposition See discussion in text Object removed during Foreign Object Metallic Gasket concerning the three tubes with Search And Retrieval (FOSAR).
1A Material ITS HL law level wear in SG 1A (1 A001) 1.25 x 0.180 x Row 101 -103 Row 96 Cal 107(17% 1W)
Col 109-110 0.050 Row 98 Cal 107(16% 1W)
Row 97 Cal 108 (16%TW)
Metallic Gasket 23 tubes were +PointTM Object could not be successfully 1A Material TTS HL inspected in the region of the removed. Four tubes surrounding the (1A002) 0.75 x 0.180 x Row 112-114 object. No wear or evidence of object were preventatively stabilized 0.050 Col 88-89 additional foreign objects were and plugged.
identified.
Metallic Gasket Object was in periphery trough Object removed during FOSAR.
1A 1~1~SCL area away from the tube bundle.
Material 0.50 x Periphery (1A004) 0.180 x 0.050 Trough Area No evidence of tube wear in area.
Metallic Gasket See discussion in text Object could not be successfully TTS HL 1A Material concerning wear in SG 1A Tube removed. Tube with wear plugged and (1A005) 1.75 x 0.180 x Row 102 Row 102 Col 61 stabilized and surrounding six tubes Col 60-62 0.050 preventatively plugged and stabilized.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information Slag Metal ITS HL Object was in periphery trough Object removed during FOSAR.
1B 0.250 x 0.125 x Periphery area away from the tube bundle.
(1BOO1) No evidence of tube wear in 0.064 Trough Area area.
Metallic Gasket 19 tubes +PointTM inspected in Object removed during FOSAR.
ITS HL 18 Material the region of the object. No Row 111 (18002) 1.00 x 0.180 X wear or evidence of additional Cal 46-47 0.050 foreign objects were identified.
Object was in periphery trough Object removed during FOSAR.
1B Wire ITS HL Periphery area away from the tube bundle.
(18003) 0.50 x 0,010 Trough Area No evidence of tube wear in area.
Metallic Gasket 16 tubes +PointTM inspected in Object removed during FOSAR.
ITS HL lB Material the region of the object. No (18004) 0.150 x 0.180 x Row 17 wear or evidence of additional Col 140 0.050 foreign objects were identified.
Object was in periphery trough Object removed during FOSAR.
1B Wire ITS HL area away from the tube bundle.
(1B005) 0.25 x 0.010 Periphery No evidence of tube wear in Trough Area area.
18 tubes +PointTM inspected in Object could not be successfully the region of the object. No removed. Surrounding tubes allowed lB Wire TTS HL wear or evidence of additional to remain in service based on Row 87 foreign objects were identified. engineering evaluation that determined (18010) 1.25 x 0.031 Cal 50-52 if potential wear were to occur over next cycle of operation it will be substantially below tube integrity limits.
Metallic Gasket TTS HL 23 tubes +PointlM inspected in Object could not be successfully 18 Material Row 103 the region of the object. No removed. Seven tubes surrounding (1 BOll) 2.00 x 0.180 x wear or evidence of additional the object were preventatively 0.050 Col 50-52 foreign objects were identified, stabilized and plugged.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information 22 tubes +PointTM inspected in Object could not be successfully the region of the object. No removed. Object is in a relatively low wear or evidence of additional flow region of the SG. Surrounding Metallic Gasket 1C Material ITS HL foreign objects were identified. tubes allowed to remain in service Row 37 based on engineering evaluation that (1C002) 1.00 x 0.180 X Cal 63 determined if potential wear were to 0.050 occur over next cycle of operation it will be substantially below tube integrity limits.
28 tubes +PointTM inspected in Object could not be successfully the region of the object. No removed. Object is in a relatively low wear or evidence of additional flow region of the SG. Surrounding Metallic Gasket TTS HL 1C Material foreign objects were identified. tubes allowed to remain in service (1COO3) 1.00 x 0.180 X Row 58 based on engineering evaluation that Col 63 determined if potential wear were to 0.050 occur over next cycle of operation it will be substantially below tube integrity limits.
Metallic Gasket 12 tubes +PointTM inspected in Object removed during FOSAR.
1C Material ITS CL the region of the object. No (1C004) 0.250 x 0.180 x Row 106 wear or evidence of additional Cal 41 0.050 foreign objects were identified.
Metallic Gasket ITS HL 16 tubes +PointTM inspected in Object removed during FOSAR.
1C Material the region of the object. No Row 112 (1C005) 0.250 x 0.180 X Col 46 wear or evidence of additional 0.050 foreign objects were identified.
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Attachment A Braidwood Station Unit 1 Spring 2006 Refueling Outage Steam Generator Inspection Summary Report Response to Request for Additional Information ITS CL Object was in periphery trough Object removed during FOSAR.
1D Wire area away from the tube bundle.
(1DOO1) 0.250 x 0.010 Periphery No evidence of tube wear in Trough Area area.
Machining ITS CL Object was in periphery trough Object removed during FOSAR.
1D Remnant area away from the tube bundle.
(1DOO2) 0.100 x 0.060 x Periphery No evidence of tube wear in Trough Area 0.010 area.
Machining ITS CL Object was in periphery trough Object removed during FOSAR.
1D Remnant 0.100 x Periphery area away from the tube bundle.
(1 DOO3) No evidence of tube wear in 0.060 x 0.030 Trough Area area.
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