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Information Notice No. 89-33: Potential Failure of Westinghouse Steam Generator Tube Mechanical Plugs
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 March 23, 1989 Information Notice No. 89-33: POTENTIAL FAILURE OF WESTINGHOUSE STEAM GENERATOR TUBE MECHANICAL PLUGS Addressees: All holders of operating licenses or construction permits for pressurized-water reactors (PWRs). Purpose: This information notice is being provided to alert addressees to the potential for failure of Westinghouse steam generator tube mechanical plugs. Such failures could lead to a large primary-to-secondary leak and possible damage to adjacent tubes. It is expected that recipients will review the information for applica-bility to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice do not con-stitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: On February 25, 1989, North Anna Unit 1 experienced a large primary-to-secondary leak of about 70 gallons per minute (gpm) following a reactor trip from 76 percent power. Unit response to the trip was normal. No safety injection (manual or automatic) was initiated or required during the event, and post-trip cooldown and depressurization were conducted in a stable manner. Primary-to-secondary leakage in the affected steam generator (S/G) had been negligible (< 1.0 gallon per day) with no increasing trend before the reactor trip. The licensee's investigation performed after plant shutdown showed the leak was associated with a broken plug located in the hot leg of tube R3C60. This tube had been plugged in 1985 because a 55 percent indication had been found at the first support plate location. The broken plug was a "mechanical" plug supplied by Westinghouse. The top portion of the plug was completely severed from the body of the plug, which was expanded against the tube and tubesheet. The severed portion of the plug was apparently propelled up the length of the tube by the primary system pressure to a location just above the U-bend transition where it impacted the outer curvature of the tube approximately 4 inches above the seventh support plate. 8903230075 . IN 89-33 March 23, 1989 Page 2 of 2 Further investigation with a modified Welsh-Allyn video probe showed that the impact of the broken plug piece had punctured the tube over an area approxi- mately 2� inches long and 3/4 inches wide. The broken plug piece subsequently impacted and dented the adjacent tube R4C60 directly above. Discussion: Westinghouse informed several licensees on January 17, 1989 that a few utilities had observed dripping or wetness around tube ends plugged with Westinghouse mechanical plugs. When some of these plugs were removed and examined, inter-granular cracks were found. Westinghouse reported that these cracks appeared to be associated with minimal intergranular carbide precipitation, which, in turn, may be the result of a low mill annealing temperature. Preliminary information obtained informally from Westinghouse indicates that plugs from two heats (numbers 3513 and 3962) may exhibit this susceptible microstructure. The broken plug at North Anna Unit 1 was fabricated from heat number 3962. The licensee for North Anna Unit 1 is currently determining where other plugs from the susceptible heats may be installed at Units 1 and 2 and what remedial measures need to be taken. During a recent inspection of steam generator tube plugs at Millstone Unit 2 (a Combustion Engineering-designed reactor), in response to the North Anna event, the licensee removed four similar Westinghouse mechanical plugs. Current indi-cations are that these plugs were from the suspected susceptible heats. One of the plugs severed during the removal operation as a result of a circumferential crack. The NRC staff is currently investigating the potential generic implications of these plug failures. No specific action or written response is required by this information notice. If you have any questions about this matter, please contact the technical contact listed below or the Regional Administrator of the appropriate regional office. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contact: E. Murphy, NRR (301) 492-0945 Attachment: List of Recently Issued NRC Information Notices . Attachment IN 89-33 March 23, 1989 Page 1 of 1 LIST OF RECENTLY ISSUED NRC INFORMATION NOTICES _____________________________________________________________________________ Information Date of Notice No._____Subject_______________________Issuance_______Issued to________ 89-32 Surveillance Testing 3/23/89 All holders of OLs of Low-Temperature or CPs for PWRs. Overpressure-Protection Systems 89-31 Swelling and Cracking 3/22/89 All holders of OLs of Hafnium Control Rods or CPs for PWRs with Hafnium control rods. 89-30 High Temperature 3/15/89 All holders of OLs Environments at or CPs for nuclear Nuclear Power Plants power reactors. 89-29 Potential Failure of 3/15/89 All holders of OLs ASEA Brown Boveri or CPs for nuclear Circuit Breakers power reactors. During Seismic Event 89-28 Weight and Center of 3/14/89 All holders of OLs Gravity Discrepancies or CPs for nuclear for Copes-Vulcan power reactors. Air-Operated Valves 89-27 Limitations on the Use 3/8/89 All holders of OLs of Waste Forms and High or CPs for nuclear Integrity Containers for power reactors, the Disposal of Low-Level fuel cycle Radioactive Waste licenses and certain by-product materials licenses. 89-26 Instrument Air Supply to 3/7/89 All holders of OLs Safety-Related Equipment or CPs for nuclear power reactors. 89-25 Unauthorized Transfer of 3/7/89 All U.S. NRC Ownership or Control of source, byproduct, Licensed Activities and special nuclear material licensees. ______________________________________________________________________________ OL = Operating License CP = Construction Permit ..
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