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Information Notice No. 91-67: Problems with the Reliable Detection of Intergranular Attack (IGA) of Steam Generator Tubing
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 October 21, 1991 NRC INFORMATION NOTICE 91-67: PROBLEMS WITH THE RELIABLE DETECTION OF INTERGRANULAR ATTACK (IGA) OF STEAM GENERATOR TUBING Addressees All holders of operating licenses or construction permits for pressurized-water reactors (PWRs). Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform licensees of recent problems experienced at the Trojan Nuclear Plant concerning the reliable detection of general intergranular attack (IGA) of the steam generator (SG) tubes at the tube-to-support plate (TSP) intersections. This information notice complements NRC Information Notice 90-49, "Stress Corrosion Cracking in PWR Steam Generator Tubes," which discussed, in part, problems in detecting intergranular stress corrosion cracks (IGSCC) in steam generator tubes at TSP intersections. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances The Trojan Nuclear Plant is a 4-loop pressurized water reactor (PWR) designed by Westinghouse Electric Corporation with Model 51 steam generators. During a refueling outage in the summer of 1991, the Portland General Electric Company (the licensee) conducted an inservice inspection of the steam generator tubes. The licensee used an eddy current test bobbin probe to inspect all tubes in each steam generator from the hot leg tube end to at least the top support plate on the cold leg side. A 20% sample of tubes in one steam generator was inspected with a bobbin probe over the full tube length. The licensee per- formed additional inspections with a more sensitive motorized pancake coil (MRPC) probe to confirm and better characterize indications found with the bobbin probe. The MRPC probe was also used by the licensee to inspect all tubes at the expansion transition region at the top of the tubesheet on the hot leg side and all row 2 tubes in the u-bend region. (All row 1 tubes were previously plugged). As a result of these inspections, the licensee identified 297 tubes with axial indications at the TSP intersections, 26 tubes with axial or circumferential indications at the expansion transitions, and ten tubes with axial or circumferential indications in the row 2 u-bends. 9110160088 . IN 91-67 October 21, 1991 Page 2 of 3 The licensee removed a tube exhibiting an axial indication at the first (i.e., lowermost) TSP location for laboratory examinations. These examinations revealed circumferential bands of IGA at the first, second, and third TSP locations. The IGA morphology is a relatively uniform attack of all grain boundaries over a portion of the tubing surface. Stress is not a significant contributor to the morphology of IGA. The licensee found the IGA at the first TSP to range to a maximum depth of 92 percent through-wall and to extend 270 degrees around the tube circumference. The maximum depth of IGA at the second and third TSPs was found to be 40 percent and 42 percent through-wall, respectively. However, no eddy current test indications had been called in the field at the second and third TSPs prior to tube removal. The pulled tube examinations prompted the licensee to reassess the field bobbin probe data and to determine that indications were, in fact, present at these locations. The voltage amplitudes of these indications were below the thresh- old criteria of the plant data analysis guidelines. These guidelines stated that the amplitude be at least 1.5 volts before a signal would be designated a "possible indication" (PI) and the tube subjected to a confirmatory MRPC inspection. Accordingly, the licensee revised the guidelines for data analysis to ensure that any flaw-like signal, regardless of amplitude, would be desig- nated as a PI. Further, only one rather than two data channels having a flaw-like signal response would be sufficient for reporting a signal as a PI. An added refinement to the data analysis guidelines, based on the examinations of the removed tube specimen, was that the residual TSP signal response (from the 400 kHz and 500/100kHz differential mix channels) to volumetric IGA need not exhibit a rapid transition across the support plate centerline to be reported as a PI. The licensee reassessed all of the bobbin probe field data and identified an additional 2500 PIs at the TSPs using the revised guidelines for data analysis. The licensee performed MRPC inspections and confirmed approximately 34 percent of these 2500 PIs. The licensee evaluated the data from the MRPC at a rela- tively high gain level to better detect flaws. Using the MRPC, the licensee inspected 200 other TSP intersections without PIs to validate the effectiveness of the bobbin probe as the screening method for IGA. However, the licensee found eight of these intersections (without PIs) to contain indications. For this reason, the licensee concluded that it should rely exclusively on the MRPC probe for detecting the degradation at the support plates. Accordingly, the licensee performed MRPC inspections of all TSP intersections up through the fifth support plate on the hot leg side. These MRPC inspections confirmed indications in approximately 10 percent of the tubes not exhibiting PIs with the bobbin probe. Sample inspections with the MRPC were also conducted at the sixth and seventh support plates and confirmed these support elevations to be relatively free of indications. The licensee has identified a total of about 2160 tubes with indications during the current outage to date. Each of these tubes will be plugged or sleeved. Discussion Licensees are finding increasing numbers of small amplitude, axial and circum- ferential indications at TSP intersections in steam generators constructed by Westinghouse and Combustion Engineering. Licensees have examined affected . IN 91-67 October 21, 1991 Page 3 of 3 tubing removed from North Anna Unit 1, Farley Units 1 and 2, McGuire Unit 1, and Catawba Unit 1 and have found degradation characterized mainly as IGSCC, sometimes associated with limited amounts of IGA. However, the Trojan findings show that crack-like indications may also indicate IGA in which cracks have opened between the grains, thus dominating the eddy current response of the region affected by IGA. The threshold at which IGA can be detected reliably with MRPC and bobbin probes is not well understood. The licensee for Trojan has taken the conservative position that MRPC indications at the TSPs may indicate IGA flaws greater than the 40 percent plugging limit in the Technical Specifications, even where the bobbin probe does not show a PI. The licensee plans to remove additional tube specimens before restarting from the current outage. Examination of these specimens is expected to provide additional data on the Trojan TSP degradation mechanism and the associated eddy current signal responses and to validate the effectiveness of the inspection methodology and data analysis criteria at Trojan to detect degradation greater than the 40 percent plugging limit. NRC Information Notice 90-49 discussed potential non-conservatisms associated with the use of bobbin probe voltage amplitude criteria for identifying IGSCC at TSP locations in excess of the plugging limit in the Technical Specifica- tions. Although the licensee for Trojan ultimately elected not to rely on the bobbin probe to detect IGA flaws at the TSPs, the Trojan experience with the bobbin probe further underscores the potential non-conservatisms associated with the use of voltage amplitude criteria for evaluating indications at the TSPs in the absence of corroborating data from removed tube specimens. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contact: E. Murphy, NRR (301) 492-0710 Attachment: List of Recently Issued NRC Information Notices .
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