ML040680369
| ML040680369 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 04/20/2004 |
| From: | Marshall M NRC/NRR/DLPM/LPD2 |
| To: | |
| Marshall M. NRR/DLPM 415-2734 | |
| References | |
| TAC MC0941 | |
| Download: ML040680369 (7) | |
Text
Enclosure 2 NRC Staff Initial Questions and TVA Responses
- 1.
Discuss whether any primary-to-secondary leakage existed in this unit prior to shutdown.
During Fuel cycle 12, Sequoyah (SQN) Unit 2 experienced a fuel defect beginning approximately 100 days into the cycle, which increased reactor coolant activity by about a factor of 1000 in iodine and xenon levels. The increased reactor coolant xenon activity increased the ability of the Condenser Vacuum Exhaust Radiation Monitors (CVE RMs) to detect primary-to-secondary leakage. This increased detection ability made possible the detection of a primary to secondary leak rate of 0.16 gallons per day (gpd) by the CVE RMs on December 5, 2002. The primary to secondary leak indication then dissipated and, following a forced outage in late December 2002, has decreased to less than detectable when using CVE RM indication. Steam Generator (SG) samples have been concentrated on resin impregnated filters and on cation columns. A small amount of activity has been noted in SGs 2 and 3. Further sampling using these methods indicated that a very small leak is probably in SG 3. Late in September of 2003, primary-to-secondary leakage spiked up to 0.25 gpd and decreased in October and has held steady at 0.14 gpd.
- 2.
Discuss the results of secondary side pressure tests.
Since primary-to-secondary leakage was so low, no pressure test is planned.
- 3.
For each SG, provide a description of areas examined, including the expansion criteria utilized and type of probe used in each area. Also, be prepared to discuss your inspection of the tube within the tubesheet, particularly the portion of the tube below the expansion/transition region.
Inspection Plan Full-length bobbin examination in all four SGs; 100 percent Hot leg top of tubesheet (TTS) rotating pancake coil (RPC) examination in all four SGs using a plus point probe +2/-8; 100 percent Rows 1-11; 100 percent, and Rows 12-20 U-bends using a +Point probe or Mitsubishi Heavy Industries (MHI) probe; 20 percent Free span dings greater than or equal to 2 volts (Cycle 11) from heat transfer system (HTS) to H07 using the +Pt probe; 100 percent Tube support plates (TSPs) H01 to C07 dented greater than or equal to 2 volts using the +Pt probe; 100 percent Antivibration bar (AVB) locations with dents with a +Pt probe; 100 percent All dents or dings in the U-bend region (any voltage) using the +Pt probe; 100 percent Expansion Criteria TTS +Point The top of TTS RPC examinations will be expanded to the cold leg (20 percent sample) if any SG is categorized as C-3 or if any tube with a TTS indication fails the Nuclear Energy Institute 97-06 performance criteria.
U-Bend MHI Probe If one or more circumferential crack-like indications are detected in Rows 9 through 11, then 100 percent of Rows 12 through 20 U-bends shall be examined with the MHI probe (or equivalent). If one or more circumferential crack-like indications are detected in Rows 12 through 20, then 100 percent of all U-bends shall be examined with the MHI probe (or equivalent). If axial crack-like indications are detected in Rows 3 through 8 U-bends, then 100 percent of Rows 12 through 17 shall be examined with the MHI probe (or equivalent). If axial crack-like indications are detected in Rows 9 through 10, then 100 percent of U-bends up to and including Row 25 shall be examined with the MHI probe (or equivalent).
Dented TSP Intersection +Point The hot leg dented intersections are considered the critical area and C07 is considered the buffer zone. If a primary water stress corrosion cracking (PWSCC) axial or circumferential at a dented TSP or outer-diameter stress corrosion cracking (ODSCC) circumferential at a dented TSP is identified at C07, then a expansion to 20 percent of the C06 intersection (dented greater than 2v) would then be required. The new buffer zone would be considered as C06.
If circumferential cracking is identified in the dent inspection in a lower-voltage dent (2-5 volts), then, 100 percent expansion to the greater than 1-volt dent population (critical area and buffer area to be defined) would be performed to bound the condition.
Refer to U1C10 operational assessment and U1C11 condition monitoring.
Straight Length Freespan Ding +Point If one or more crack-like indications are identified associated with free span dings from H06-H07, then 100 percent of greater than or equal to 2-volt dings (Cycle 11 voltage) between C07 and C06 will be inspected.
- 4.
Discuss any exceptions taken to industry guidelines.
Two exceptions with Rev. 6:
The MHI Intelligent Array Probe (IAP) utilizes a nominal 0.004 inch radial dent (360º) in lieu of the radial expansion for establishing phase. This deviation has no impact to the plant or its operation nor does it have any impact on the techniques capabilities. The methodology put into Revision 6 relied primarily on a particular vendor of the array probe and made no effort to accommodate variations. Either method provides for a reproducible method of establishing phase. The deviation was successfully utilized during the Peer Review process for the IAP at Electric Power Research Institute (EPRI) in Charlotte to qualify the probe we will be using for the detection of PWSCC in low row U-bends.
Nine probes utilized during the inspection were expedited to site from Japan and from Issaquah, WA. These were previously manufactured probes that did not have a certificate of conformance as required by Rev. 6. The probes were, however, receipt-inspected onsite and each probe was connected to a test instrument and functionally tested.
- 5.
Provide a summary of the number of indications identified to-date of each degradation mode and steam generator tube location (e.g., TSP, top-of-tubesheet, etc.). Also provide information, such as voltages, and estimated depths and lengths of the most significant indications.
S/G Row Col Location Crack X of X
ID/OD Ax/Circ FLDA 360 Degree Degraded Area Length Max Depth Max Volts Burst from NDE indicated Inch De ODS 2
10 23 HTS+0.2 OD AXIAL 24.7 0.19 49 0.12 7861 2
14 67 HTS+0.2 OD AXIAL 26.5 0.12 48 0.11 7936 4
4 5
HTS-0.85 1OF OD AXIAL 33.3 0.16 57 0.23 7345 4
4 5
HTS-0.26 3OF OD AXIAL 22.0 0.16 33 0.15 8210 4
4 5
HTS-0.35 2OF OD AXIAL 16.4 0.21 47 0.26 8517 4
5 6
HTS-0.53 1OF OD AXIAL 38.1 0.09 61 0.13 7177 4
5 6
HTS-0.09 3OF OD AXIAL 47.6 0.08 93 0.09 6526 4
5 6
HTS-0.28 2OF OD AXIAL 57.6 0.09 81 0.11 5697 4
6 6
HTS-0.26 OD AXIAL 30.4 0.21 49 0.16 7412 4
7 6
HTS-0.16 OD AXIAL 25.6 0.48 49 0.61 7351 4
13 32 HTS-0.08 OD AXIAL 60.6 0.16 84 0.37 5117 4
20 33 HTS-0.02 OD AXIAL 15.8 0.22 61 0.15 8568 ODS 3
15 22 HTS+0.0 OD CIRC 0.00 30 1
0.13 4
8 53 HTS-0.09 OD CIRC 0.00 29 4
0.12 ODS 2
16 57 HTS+0.9 OD AXIAL 35.4 0.27 62 0.12 6859 PWS 1
15 33 HTS-8.67 ID AXIAL 66.6 0.69 98 2.94 2
11 60 HTS-6.74 ID AXIAL 27.0 0.47 47 0.59 2
19 72 HTS-0.79 ID AXIAL 38.2 0.17 68 0.35 6904 2
20 60 HTS-0.52 ID AXIAL 42.6 0.15 85 0.42 6692 2
35 40 HTS-7.25 2of3 ID AXIAL 61.5 0.77 98 2.39 2
35 40 HTS-6.75 1of3 ID AXIAL 27.5 0.10 41 0.33 2
35 40 HTS-7.96 3of3 ID AXIAL 25.6 0.09 41 0.49 3
21 30 HTS-9.48 ID AXIAL 65.1 0.21 99 0.72 3
35 31 HTS-9.42 ID AXIAL 38.5 0.31 86 1.91 4
6 46 HTS-0.68 ID AXIAL 28.6 0.10 61 0.51 7932 PWS 3
8 23 HTS-0.03 ID CIRC 17.8 1.35 27 39 0.50 9003 3
24 31 HTS-0.22 ID CIRC 34.7 4.04 41 65 0.71 8767 3
34 31 HTS-9.80 ID CIRC 44.3 5.56 45 96 1.35 8656 We will discuss ODSCC indications at supports (Generic Letter (GL) 95-05) on the phone call as well as AVB Wear and Cold Leg Thinning.
- 6.
Describe repair/plugging plans for the SG tubes that meet the repair/plugging criteria.
All crack-like indications will be plugged on detection except for ODSCC at TSPs, which will follow alternate repair criteria GL 95-05.
AVB wear and Cold Leg Thinning is plugged at 40 percent. All other wear is plugged on detection.
- 7.
Discuss the previous history of SG tube inspection results, including any "look backs" performed. Specifically for significant indications or indications where look backs are used in support of dispositioning (e.g., manufacturing burnish marks (MBM).
All new indications, including MBM, will be traced back to the first optical recording of that data (Cycle 5 - April 1992). Indications exhibiting change or not verifiable in the Cycle 5 data will be dispositioned by enhanced techniques and/or repair.
- 8.
Discuss, in general, new inspection findings (e.g., degradation mode or location of degradation new to this unit).
We have found no new degradation.
- 9.
Discuss your use or reliance on inspection probes (eddy current or ultrasonic) other than bobbin and typical rotating probes, if applicable.
MHI array probe is being used for the U-Bend exams in Rows where the probe will pass through the U-Bend. This probe was qualified through the EPRI Appendix H process for 7/8 tubing
- 10.
Describe in situ pressure test plans and results, if applicable and available, including tube selection criteria.
No indication, thus far, exceeds criteria for in situ testing.
- 11.
Describe tube pull plans and preliminary results, if applicable and available; include tube selection criteria.
No tube pull is required for this outage.
- 12.
Discuss the assessment of tube integrity for the previous operating cycle (i.e., condition monitoring).
Each indication is sized using appropriate sizing techniques, and a burst pressure is calculated. If the burst pressure is below performance criteria, the indication will be in situ pressure tested. Each indication is also evaluated for leakage using voltage screening.
- 13.
Provide the schedule for SG-related activities during the remainder of the current outage.
Eddy current will be completed on November 26, 2003.
- 14.
Discuss the following regarding loose parts:
<> what inspections are performed to detect loose parts Bobbin coil exam identifies potential loose parts. Foreign object search and retrieval is performed on all four SGs. Look down several columns, down the tube lane, both sides of the annulus.
<> a description of any loose parts detected and their location within the SG No significant parts have been identified thus far.
<> if the loose parts were removed from the SG
<> indications of tube damage associated with the loose parts No indications of loose parts wear thus far
<> The source or nature of the loose parts, if known
- 15.
If SGs contain thermally treated tubing (Alloy 600 or 690), discuss actions taken (if any) based on Seabrooks findings documented in NRC Information Notice (IN) 2002-21).
N/A NRC Staff Additional Questions and TVA Responses
- 1.
Describe the types of flaws contained in the Examination Technique Specification Sheet (ETSS) qualification data set for U-bends. Discuss the degree to which this data set is representative of cracks which may potentially exist in the U-bends at Sequoyah Unit 2.
The MHI array coil is an EPRI-qualified technique with the ETSS number 23514. The technique was qualified on the same Row-1 U-bend data set that was used to qualify the plus point and pancake coils. These samples consist of inside diameter axial and circumferential electro discharge machining (EDM) notches placed at various locations with respect to the bend tangent. With regard to how representative the notches are of indications which may be present at Sequoyah, there are the following observations:
A) In general, ID EDM notches will yield eddy current test signal amplitudes that are greater than those from PWSCC for a given depth.
B) The Row 1 U-bend interference present in the samples from over-bending and mandrel bulges are considerably more adverse than the bend geometry present in the tubes being examined at Sequoyah.
The minimum U-bend that is being examined with the array probe is Row 5.
- 2.
Provide any other data that may provide insight on the ability of the MHI probe to detect axial and circumferential cracks in the U-bends. Provide any comparative data that may exist between the plus point and MHI probe concerning the detection of U-bend cracks.
The MHI array coil was used at Diablo Canyon in February 2003 to examine low Row U-bends that were identified as containing circumferential indications with the plus point coil. The same indications present on the plus point coil data were present in the MHI coil data.
- 3.
Discuss the quality of the data being obtained in the Sequoyah U-bends with the + Point and MHI probes. How does data quality compare between the two probes? Discuss whether data quality poses a concern with respect to reliable detection of circumferential cracks in the U-bends which produce + Point responses of from 0.3 to 2 volts (as seen at Salem).
For the plus point examinations of Rows 1 and 2, noise measurements are recorded at the apex of each U-bend by analysts who are specifically dedicated to data quality. Any peak to peak or vertical maximum voltage which exceeds (by 10 percent) of the average values from the qualification data set are identified for further review by the resolution team. The resolution team may use circumferential averaging filters, review of prior cycle data, and/or request retests. In general, the data quality from the U-bend exams is commensurate with previous examinations at Sequoyah. For U-bend examinations with plus point or the MHI probe above Row 2 the data quality analysts review tubes on a sampling basis using qualitative judgment.
We have no specific information regarding what was reported at Salem several weeks ago. However, the circumferential indications from Diablo Canyon were clearly visible and we have no reason to suspect that similar flaws, if present at Sequoyah would be masked. In general, the MHI data in the U-bends appears to be good quality without much interfering noise.
- 4.
For the ODSCC at the tube support plates, describe your criteria, if any, for implementing preventive plugging of tubes exhibiting bobbin voltages between 1 and 2 volts.
During the inspection of support plates utilizing the bobbin probe our testing will be performed in accordance with our alternate repair criteria. TVA has elected to +Point examine distorted support indication (DSI) indications of 1 volt or greater. Once these support plate locations are +Point tested, various parameters will be evaluated. The parameters being reviewed will be the length of the indication, the numbers and types of cracks (e.g., several microcracks versus one macrocrack), the voltage ratio of the
+Point to bobbin, the elevation of the support plate, and the growth of the indication from outage to outage. Combinations of the aforementioned parameters shall be considered when evaluating the disposition of these indications. No specific criteria as to preventive plugging of this category of tubes have been determined at this point.
- 5.
It was stated that the ODSCC findings satisfied performance criteria for condition monitor. From a structural standpoint, does this include satisfying both probability of burst for the population of indications/SG and 1.4 x main streamline break pressure for the most limiting indication?
Yes, a preliminary calculation was performed based on the population of indications as of November 24, 2003, prior to the phone call. At that time with the data available, the population met condition monitoring burst and leakage performance criteria. An updated set of data was evaluated and this set also met condition monitoring burst and leakage performance criteria. We are in the process of evaluating growth so that we can do a preliminary operational assessment calculation.