ML040680360
| ML040680360 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 04/20/2004 |
| From: | Marshall M NRC/NRR/DLPM/LPD2 |
| To: | |
| Marshall M, NRR/DLPM/301- 415-2734 | |
| References | |
| TAC MC0941 | |
| Download: ML040680360 (6) | |
Text
SUMMARY
OF CONFERENCE CALLS WITH TENNESSEE VALLEY AUTHORITY REGARDING CURRENT STEAM GENERATOR INSPECTION RESULTS AT SEQUOYAH, UNIT 2 November 24, 2003, Conference Call On November 24, 2003, the U. S. Nuclear Regulatory Commission (NRC) staff participated in a conference call with Tennessee Valley Authority (TVA) to discuss the ongoing steam generator (SG) inspection activities at Sequoyah Nuclear Plant (SQN), Unit 2.
At the time of the call, the licensee had acquired and analyzed approximately 80 percent of the SG inspection data. All of the scheduled top-of-tube sheet (TTS) and bobbin detection inspections were completed. The licensee was inspecting dents with the +Point' probe and performing +Point' inspections of bobbin indications. In addition, there were no TTS +Point' examinations reaching the C-3 categorization or failing the Nuclear Energy Institute (NEI) 97-06, Steam Generator Program Guidelines, performance criteria.
The inspection plan and expansion plan for the U-bends resulted from the Westinghouse Owners Group (WOG) report recommendations as a result of recent findings of circumferential cracking in the U-bends during the Diablo Canyon Power Plant (DCPP), Unit 2 11th Refueling Outage (2R11) SG inspection. The WOG recommendations are summarized as follows:
A.
Circumferential Primary Water Stress Corrosion Cracking (PWSCC) 100 percent +Point' (or equivalent) inspection of Rows 3 through 10, at the first inspection following spring 2003. It is assumed that Rows 1 and 2 have already been inspected at 100 percent.
Future outage scope is based on the first inspection results, consistent with the Electric Power Research Institute pressurized-water reactor SG Examination Guideline, Rev. 6.
B.
Axial PWSCC Inspection is +Point' or equivalent based. The application of the inspection is sample based and considers accumulated effective full-power year, T-hot, previous observation of axial PWSCC at tubesheet expansions, and cumulative observations from all potentially susceptible plants.
Inspection can range from 20 percent sample of Rows 13 through 16, to a 50 percent sample of Rows 13 through 16 with a 20 percent sample of Rows 12, 17, and 18.
Bobbin techniques can also be used as a defense in depth approach for detection of axial PWSCC.
The licensee had just started inspecting the U-bend area with the Mitsubishi Heavy Industries (MHI) Intelligent Array Probe and the +Point' probe. The licensee stated that the MHI probe has comparable sensitivity to U-bend cracks as the +Point' probe, and has a faster inspection speed. However, the MHI probe could not be used for the smaller radii U-bends found in Rows 1 through 5. One axial PWSCC indication in Row 1 has been found to date. No sizing information was available.
The licensee used the MHI probe with some difficulty in Rows 6 through 10. Therefore, the licensee obtained nine MHI probes from Japan which have a smaller diameter and are more flexible. The licensee noted that thee MHI probes did not have a certificate of conformance as required by Revision 6 to the industry guidelines, since these guidelines are not applied in Japan. However, the probes had undergone performance verification with calibration standards maintained on-site. MHI indications will be retested with the +Point' probe for verification.
The licensee provided the following summary of outer-diameter stress corrosion cracking (ODSCC) detected by the bobbin probe in the tube support plate (TSP) in accordance with Generic Letter 95-05 alternate repair criteria:
SG No. of Bobbin Indications Comments 1
245 17 indications greater than 1.0 V Largest indication 1.88 V No indications meeting criteria for plugging 2
237 22 indications greater than 1.0 V Largest indication 1.81 V 3
305 51 indications greater than 1.0 V Largest indication 9.76 V Next largest indication 2.49 V 4
730 65 indications greater than 1.0 V Largest indication 3.55 V Next largest indication 2.42 V The licensee stated that it still meets its condition monitoring performance criteria with the 9.76 V bobbin indication (5.8 V +Point' indication). The indication is measured at 0.66 inch in length, with an average depth of 58 percent and a maximum depth of 90 percent using the phase sizing of the +Point' probe. However, the +Point' voltage response indicates that the crack was likely 100 percent through-wall since the +Point' phase sizing tends to underpredict the depth of through-wall and near through-wall cracks. The indication appears to be a single microcrack and occurred in the same SG as the leakage detected during the outage. A look-back analysis of the past outage data revealed that this indication was measured at 1.98 V with the bobbin probe; however, no +Point' data was available.
For this outage, the licensee has implemented a plan to +Point' all bobbin indications greater than 1.0 V to detect indications which may result in large voltage growth rates similar to those seen in the DCPP 2R11 SG inspection. The licensee plans to perform preventive plugging if there is a potential for a large voltage growth rate by the next outage as a consequence of the cracking growing entirely through-wall over a significant length (e.g., cracks that run from edge to edge or abnormal +Point'/bobbin voltage response ratio).
With respect to anti-vibration bar (AVB) wear and cold-leg (CL) thinning, the applicant provided the following summary of indications:
SG AVB Wear CL Thinning 1
11 19 2
44 50 3
16 29 4
17 27 Of the AVB wear indications detected, one tube will be plugged (i.e., a tube with a 41 percent wear indication). Of the CL thinning indications detected, eight tubes will be plugged. The largest CL thinning indication was 46 percent maximum in depth. All CL thinning indications are measured with +Point' to ensure that the indication is wear and not cracking. These indications have been mapped and are found at the periphery of the tube bundle.
A foreign object search and retrieval and sludge lancing of the SGs is scheduled near the end of the SG inspection. At the time of the conference call, one SG had completed this phase of the inspection with no significant pieces of foreign material found and two sludge rocks identified.
November 26, 2003, Conference Call On November 26, 2003, the NRC staff participated in a conference call with TVA to discuss the ongoing SG inspection activities at SQN, Unit 2. The licensee had acquired approximately 96 percent of the SG inspection data and was continuing to analyze the data.
The purpose of this call was to address the following questions:
Describe the types of flaws contained in the Examination Technique Specification Sheet 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 SQN, Unit 2.
Provide any other data that may provide insight on the ability of the MHI probe to detect axial and circumferential crack cracks in the U-bends. Provide any comparative data that may exist between the +Point' and MHI probe concerning the detection of U-bend cracks.
Discuss the quality of the data being obtained in the SQN 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 from 0.3 V to 2 V.
For ODSCC at the tube support plates, describe your criteria, if any, for implementing preventive plugging of tubes exhibiting bobbin voltages between 1.0 V and 2.0 V.
You state that the ODSCC findings satisfied performance criteria for condition monitoring. From a structural standpoint, does this include satisfying both probability of burst for the population of indications/SG and 1.4 times main streamline break pressure for the most limiting (9.8 V) indication?
With respect to the first question, the licensee described its assessment to qualify the use of the MHI probe. The qualification data set for this probe consisted of electro-discharge machined notches. This same data set was used to qualify both bobbin and +Point' probes as well. However, the staff referenced Regulatory Information Summary 00-22, Issues Stemming from NRC Staff Review of Recent Difficulties Experienced in Maintaining Steam Generator Tube Integrity, which discusses the need to update the qualification data set to include more representative cracks. While the MHI probe is considered qualified for use in the U-bend area of the tubes, the qualification data set is not representative of the types of flaws that may be found in this area.
In response to the second question, the licensee stated that site-specific performance demonstration training used data from the DCPP 2R11 SG inspection and the indications observed with the MHI probe are similar to those observed with the +Point' probe. The licensee indicated that its confidence in the MHI probe to detect the cracks similar to those found during the DCPP 2R11 SG inspection is based on its comparable sensitivity to the U-bend cracks as the +Point'. The staff requested that the comparative data between the
+Point' and MHI probes be made available to the NRC inspector who was on-site. In addition, the licensee provided direct comparisons of the +Point' and MHI c-scans for several SQN Unit 2 U-bend signals to illustrate the comparable sensitivity to U-bend circumferential cracks.
With respect to the third question, the licensee indicated that dedicated analysts review the data for quality. The licensee also stated that the data quality was such that indications capable of producing +Point' responses within the range of 0.3 V and 2.0 V should be reliably detectable.
Results of the U-bend inspections are summarized below:
axial indications in Row 4 of SGs No. 2 and No. 4 resulted in an expansion relative to the initial inspection scope because the WOG report states that the hoop stresses in Rows 3 through 8 are similar to hoop stresses in Rows 13 through 17. Instead of sampling 20 percent of Rows 12 through 20, as recommended in the latest revision of the WOG guidelines, the licensee completed 100 percent inspection of Rows 12 through 17 and 20 percent of Rows 18 through 20.
circumferential indications similar to that found at DCPP were detected in Row 7 of SG No. 4. This tube was also tested with +Point' and the resident inspector will review the results.
all U-bend indications will be in situ tested.
The licensee indicated, in response to Question 5, that the population of indications met condition monitoring burst and leakage performance criteria.
December 1, 2003, Conference Call On December 1, 2003, the NRC staff participated in a conference call with TVA to provide an update on the SG inspection activities at SQN, Unit 2. The licensee had completed its SG inspection and discussed the final results of its analyses.
The licensee stated that there were ODSCC indications obtained through the bobbin inspection in SG No. 4 that meet the C3 reporting criteria. The summary of ODSCC indications at the TSP is summarized in the table below.
SG No. of Indications Largest Indications Detected by Bobbin:
1 247 1.88 V 2
252 1.81 V 3
307 9.76 V (next largest is 2.49 V) 4 739 3.55 V The condition monitoring calculations performed by the licensee indicate that the limiting SG is SG No. 3; however, the operational assessment results indicate that the limiting SG is SG No. 4. Due to the large voltage indication in SG No. 3, the probability of burst was determined to be 3.8 x 10 -3 and a postulated leak rate of 1.69 for the next end-of-cycle. The growth rate distribution for indications found is not voltage-based.
The licensee reiterated that bobbin indications 1.0 V and greater are reexamined with the
+Point' probe. From the +Point' data, the indication is sized and the profiles are checked to ensure that the indication is not a long, single, macrocrack or multiple cracks capable of linking.
In addition, the licensee indicated that it preventively plugged 10 indications to preclude the rapid voltage growth found at DCPP, Unit 2 and SQN, Unit 2 prior to the next inspection.
The following table summarizes the number of indications greater than 1.0 V.
SG No. of Indications 1
14 2
21 3
29 4
65 The licensee stated that 62 tubes were plugged in all four SGs and that no indications exceeded the performance criteria. The following summarizes the plugged indications:
One AVB wear indication which exceeded the 40 percent plugging limit. The AVB wear indication was 41percent.
Three CL thinning indications greater than 40 percent were plugged. The largest CL thinning indication was 47 percent and is an update of the information previously provided.
Sixteen ODSCC indications at the TSP which were plugged. None of these indications were significant with respect to the performance criteria.
Two TTS circumferential OD cracks Nine axial PWSCC in the tubesheet (TS)
Four tubes plugged for circumferential cracks within the TS Ten tubes plugged for ID cracks in the U-bend area The inspections at the U-bend portions of the SG tubes were completed as follows:
Rows 1 through 5 with the +Point' probe; 100 percent Rows 6 through 17 with the MHI probe; 100 percent Rows 18 through 20 with the MHI probe; 20 percent The results of these inspections yielded four axial indications detected by the +Point' probe.
Two indications were found in Row 1. Two indications were found in Row 4.
Circumferential cracks in the U-bends were found in SGs No. 2 and No. 4, similar to those found at DCPP, Unit 2 and Salem. Ten tubes had several circumferential crack indications.
These indications were detected using the MHI probe and verified through the +Point' probe.
In situ testing was performed on all U-bend indications and no leakage was detected.
For the circumferential indications, the largest indication was 1.52 V (by +Point') and the smallest was 0.35 V. The arclength of all indications was below 50 (i.e., most indications were around 20 and one indication was at 45).