ML20044E737
| ML20044E737 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 05/20/1993 |
| From: | Cross J PORTLAND GENERAL ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9305260031 | |
| Download: ML20044E737 (6) | |
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(,aws.ee Portland General Electric Company James E. Cross
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Vice President and Chief Nuclear Officer May 20, 1993 Trojan Nuclear Piant Docket 50-344 License NPF-1 U. S. Nuclear Regulatory Commission
.j Attn: Document Control Desk Washington, DC 20555
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Dear Sirs:
i Transmittal of Additional Steam Generator Information i
On November 9,1992, the Trojan Nuclear Phmt (TNP) was shut down as a result of a steam
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generator tube leak that exceeded the primary-to-secondary leakage limits of Trojan Technical Specifications. On November ?6,1992, Portland General Electric (PGE) submitted License Change Application (LCA) 227, proposing to defer the unscheduled steam generator inservice inspection required by Trojan Technical Specifications. A letter containing supplemental information regarding LCA 227 was submitted by PGE November 28,1992.' LCA 227 was later withdrawn by PGE.
Further steam generator tube inspections and analyses were performed in support of another technical specification amendment that was to be requested prior to restart of the Trojan plant.
The attachment to this letter provides a summary of the results of these inspections and analyses. This summary is being provided at the request of the Nuclear Regulatory Commission Staff, and is intended for information only. Na further action is planned by Ponland General Electric to investigate further or confirm the inspection results.
Sincerely, I
Attachment c:
Mr. John B. Martin Regional Administrator, Region V
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U. S. Nuclear Regulatory Commission 9305260031 930520 PDR ADOCK 05000344
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pyg 121 SW Salmon Street, Portland, OR 97204
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L-I 503/464-8897
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, Document Control Desk l
May 20, 1993
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t Page 2 Mr. David Stewart-Smith State of Oregon Department of Energy Mr. Kenneth Johnston NRC Resident Inspector Trojan Nuclear Plant i
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, Trojan Nuclear Plant Document Control Desk Docket 50-344 May 20, 1993 License NPF-1 Attachment Page1of4 i
Follow-Up Steam Generator Tube Inspection Activities and Results Following the November 9,1992 Tube Leak Additional Ilobbin Coil Inspection Program Discussion Subsequent to the closing of the steam generators at Trojan following the steam generator tube sleeve inspection, a decision was made to review the data obtained from the small bobbin coil probes (probes having an outside diameter ranging from 0.540 to 0.580 inches) to determine if significant defects could be identified from the data. This data was obtained during the course of eddy current inspection of sleeved intersections, with the small diameter probe traversing intersections not containing sleeves.
The objectives of the analysis of this data were first to resolve correlation questions concerning the appropriateness of employing the data obtained from the small bobbin coil in flaw characterization for TSP intersections not containing sleeves, and second, to confirm the flaw growth rates are not inconsistent with the safety analysis for the Cycle 14 Interim Repair Criteria.
During the course of the preliminary review of this data, questions arose regarding the potential effect oflateral movement and wobble of the eddy current probe as it traversed intersections not containing sleeves, due to the difference in diameter between the outside diameter of the small probe, and the inside diameter of the non-sleeved intersections. Due to these concerns, an additional inspection program was developed, using the 0.720-inch eddy current probe that is typically used in eddy current inspection of 7/8-inch steam generator tubes.
The scope of this additional inspection conducted in Steam Generator "C", using the 0.720-inch eddy current bobbin coil probe, included indications previously reported as PI (Potential Indications) in the 1991 inspection (241 tubes), marginal RPC indications reported as MI (Marginal Inservice Indications) in the 1991 inspection (29 indications),
and tubes first identified as PI during the review of the data obtained from eddy current inspection activities following the November 9, tube leak. A total of 402 indications were inspected using the bobbin coil probe in Steam Generator "C", of which 84 are OD indications, defined by an indication phase angle equal to, or greater than,35 degrees in both the 1991 and 1992 inspections.
Results The first stated objective of this additional inspection program was to resolve correlation questions concerning the appropriateness of employing the data obtained from the small bobbin coil in flaw characterization for TSP intersections not containing sleeves.
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, Trojan Nuclear Plant Document Control Desk Docket 50-344 May 20, 1993 License hTF-1 Attachment Page 2 of 4 comparison of the bobbin coil indication values obtained for the 0.540 to 0.580-inch and 0.720-inch probes was conducted, the results of which are shown below in Figure 1.
Figure 1 0.540/0.560/0.580 to 0.720-Inch Probe Voltage Comparison 5.00 -
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0.00 0.00 1.00 2.00 3.00 4.00 5.00 0.720 Bobbin Coil Voltage In Figure 1 above, bobbin coil voltage readings were plotted using values obtained from a 0.720-inch probe as x-axis coordinates, and values obtained using 0.540/0.560/0.580-inch probes as y-axis components (for the same TSP intersections). The darkened line on the figure has a slope of 1 (y=x), and represents theoretical indications in which equal bobbin coil values were obtained for both the smaller and larger size bobbin coils. Points lying above the line indicate that for the same TSP intersection, the larger probe indicated a lower voltage reading than did the smaller probe size. Looking at the data shown above, the smaller probe size typically indicated a higher bobbin coil voltage than did the larger size for the intersections inspected with both sizes of probes at Trojan in the "C" Steam Generator. Therefore, it was concluded that the data obtained through use of the smaller (0.540/0.560/0.580-inch) probes were not acceptable for flaw characterization at TSP intersection not containing sleeves. The data obtained using standard 0.720-inch probes would be used.
The second stated objective of the additional inspection program in Steam Generator "C" was to confirm the flaw growth rates are not inconsistent with the safety analysis for the Cycle 14 Interim R.cpair Criteria. Westinghouse Electric Corporation conducted an evaluation of the eddy current voltage growth rates for Cycle 14 operation prior to the tube leak, and compared the results with the prior analysis ofgrowth rates as documented in WCAP-13129, " Trojan Nuclear Plant SG Tube Repair Criteria for Indications at Tube Support Plates."
, Trojan Nuclear Plant Document Control Desk Docket 50-344 May 20, 1993 l
License NPF-1 Attachment Page 3 of4 l
Due to the permanent cessation of operations at the Tro sn Plant, this evaluation of the j
Cycle 14 voltage growth rates was never finalized. However, preliminary results indicate that the voltage growth from 1991 to 1992 is in agreement with the voltage growth distribution for all OD phase angle indication from 1990-91 as documented in WCAP-13129. In addition, the projected end of cycle (EOC) indications, for those indications left in service in 1991 based on the interim repair criterion of 1.0 volt for marginal RPC indications, are consistent with the PGE safety analysis for Cycle 14 and WCAP-13129.
This is based upon the growth rates found for 1991-92 being consistent with those in WCAP-13129.
t Additional Inspection / Cellular Effect on Tube Burst Strength
Background
Analysis of the bobbin coil data obtained from the inspection conducted in Steam Generator "C" indicated that eleven tubes had indications that exhibited the potential for some circumferential cracking involvement. In an effort to obtain more data regarding these indications, PGE conducted a further examination of these intersections using RPC techniques. The subsequent RPC data obtained indicated that of the eleven intersections examined, indications existed in four tubes that appeared to be indicative of cellular SCC (stress corrosion cracking), in which short, tight, axial cracks can overlap in " patches" covering varying amouets of the tube circumference. Experience with this cellular SCC mechanism at other utilities has shown that this cellular arrangement of axial SCC microcracks has had little effect on the structural integrity of the tube. The existence of a circumferential cracking mechanism not cttributable to cellular SCC is not bounded by the Cycle 14 Interim Tube Repair Criteria, and would necessitate repair based upon the 40 percent tube repair criterion. PGE elected to conduct further testing on two of these intersections, the objective of which was to confirm that in fact the cracking mechanism being observed was cellular SCC, and to assess the acceptability of these tubes for continued operation for the remainder of Cycle 14. The remaining seven tubes examined by RPC exhibited RPC traces that reflected axial cracking typical of the cracking experienced by other tubes at Trojan, and these tubes did not warrant further examination.
UTEramination andResults Of the four tubes described above that had indications that demonstrated possible cellular SCC involvement, two tubes (R32C75 and R19C66) were selected for ultrasonic testing (UT). These tubes were selected based upon two criteria. First, the intersections to be examined in these tubes were physically located below tube sleeves, facilitating easier access to the intersections by the UT probe than would intersections above tube sleeves.
Second, the indications in these tubes were more pronounced, or judged to bound, the
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,,' Trojan Nuclear Plant Document Control Desk i
Docket 50-344 May 20 1993 i
License NPF-1 Attachment Page 4 of 4 i
indications in the other tubes with regard for the potential for circumferential cracking f
involvement.
1 The results of the UT examination demonstrated that the cracking present at the intersections examined were multiple axially oriented short tight cracks that did overlap, indicative of cellular SCC. Westinghouse preliminarily concluded that tubes could have i
t been left in service for the remainder of Cycle 14, at which time they would have been evaluated for continued service or repair.
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