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Information Notice No. 93-79: Core Shroud Cracking at Beltline Region Welds in Boiling-Water Reactors
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 30, 1993 NRC INFORMATION NOTICE 93-79: CORE SHROUD CRACKING AT BELTLINE REGION WELDS IN BOILING-WATER REACTORS Addressees All holders of operating licenses or construction permits for boiling-water reactors (BWRs). Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees that cracks have been observed in the weld regions of the core support shroud in boiling water reactors. 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 During the current refueling outage at Brunswick Unit 1 (a BWR-4 reactor), in-vessel visual inspection revealed cracks at weld regions of the core support shroud. The shroud is a stainless steel cylinder that serves to direct the flow of water inside the reactor vessel. The shroud is completely contained inside the 15.2 centimeter [6 inch] thick reactor vessel. The structural integrity of the reactor vessel is not impacted by the cracks in the shroud. Carolina Power and Light Company (CP&L), the licensee for Brunswick, found both circumferential and axial cracks in the shroud. The circumferential cracks were located in the inside shroud surface in the heat-affected zone (HAZ) of weld H-3 and extended 360 degrees around the circumference of the shroud (see Figures 1 and 2). Weld H-3 is a horizontal weld which fuses the top guide support ring to the lower shroud. The first axial crack discovered was located on the outside shroud surface of weld H-4 in the lower shroud. CP&L performed additional visual testing (VT) and ultrasonic testing (UT) of the shroud and removed boat samples to evaluate the length and size of the cracks. Discussion In 1990, crack indications were reported at core shroud welds located in the beltline region of an overseas reactor (BWR4). This reactor had completed approximately 190 months of power operation before the cracks were discovered. As a result of this discovery, General Electric (GE) issued Rapid Information Communication Services Information Letter (RICSIL) 054, "Core Support Shroud 9309290050. IN 93-79 September 30, 1993 Page 2 of 3 Crack Indications," on October 3, 1990, to all owners of GE BWRs. The RICSIL summarized the cracking found in the overseas reactor and recommended that at the next refueling outage plants with high-carbon-type 304 stainless steel shrouds perform a visual examination of the accessible areas of the seam welds and associated HAZ on the inside and outside surfaces of the shroud. Since early July, CP&L has performed VT inspections of the Unit 1 inside and outside shroud surface in the vicinity of welds. These inspections were performed in accordance with GE RICSIL 054 and discovered cracks in the weld regions. CP&L determined that in order to perform an adequate VT it was necessary to remove the outer blade guides, pre-clean inspection areas, and obtain an improved resolution of "1-millimeter wire" (in lieu of the Code-prescribed resolution). Camera and lighting positions were also found to be crucial in performing adequate VTs. Also, CP&L has worked with GE to develop more sophisticated UT equipment to identify how deeply into the shroud metal the crack extends. Additional VT inspections revealed more axial cracks at the inside surface of weld H-4 as well as cracks at welds H-1, H-2, and H-5 of the shroud. One of the additional cracks, a circumferential crack at weld H-5, appears to be approximately 76.2 centimeters [30 inches] in length. The majority of the cracks are located in the HAZ of the welds, although one crack was discovered in the central region of shroud plate P-6. The crack in P-6, however, may be associated with a possible weld repair of a surface defect in the plate after its fabrication at the mill. The results from the boat samples indicated intergranular stress-corrosion cracking (IGSCC) as the mechanism. Preliminary results suggest that the crack in the HAZ of weld H-3 may be 3.8 centimeters [1.5 inches] or more in depth. The location of this crack is shown in Figure 2. As a result of the shroud cracks being discovered on Unit 1, CP&L re-examined the results of the inspection performed during the 1991 refueling outage of Unit 2. The re-examination revealed three minor crack indications in the HAZ of weld H-2. The licensee concluded that the cracks do not pose a concern to normal operation of the reactor. CP&L plans to repair the Brunswick Unit 1 core shroud before the plant is brought back into service. CP&L intends to restore the strength of the shroud by adding stiffening braces around the top portion of the shroud. However, the licensee will continue to examine and evaluate the cracks in the core shroud. General Electric issued Revision 1 to RICSIL 054 on July 21, 1993, to update the information on the core support shroud cracks and to provide revised interim recommendations to perform visual examination of accessible areas of the shroud at all GE BWRs during the next scheduled outage. The NRC has been informed by GE that they are evaluating the Brunswick results and will provide . IN 93-79 September 30, 1993 Page 3 of 3 updated information to owners of GE BWRs. The NRC staff is evaluating the implications of the shroud cracks for reactor core configuration and emergency core cooling system performance under accident conditions at operating plants and will consider the need for additional generic communications. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. /s/'d by BKGrimes Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: R. A. Hermann, NRR J. Medoff, NRR (301) 504-2768 (301) 504-2715 P. Byron, Region II T. Greene, NRR (919) 457-9531 (301) 504-1175 Attachments: 1. Figure 1: Weld and Plate Locations in the Beltline Region of the Brunswick Unit 1 Core Shroud 2. Figure 2: Details of Weld Locations H-2 and H-3 in the Brunswick Unit 1 Core Shroud 3. List of Recently Issued NRC Information Notices .
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