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Information Notice No. 91-80: Failure of Anchor Head Threads on Post-Tensioning System During Surveillance Inspection
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 December 11, 1991 NRC INFORMATION NOTICE 91-80: FAILURE OF ANCHOR HEAD THREADS ON POST-TENSIONING SYSTEM DURING SURVEILLANCE INSPECTION Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to problems associated with failure of internal threads on post-tensioned containment tendon anchor heads during a tendon surveillance. 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 On August 1, 1991, while the Duke Power Company (the licensee) was conducting the post-tensioning system surveillance inspections at Oconee Nuclear Station, Unit 1, as required by the technical specifications, the internal threads on one of the tendon anchor heads failed while the tendon was being loaded (stressed) with a hydraulic jack (stressing ram). The stressing load was applied to the tendon by the pull bar on the stressing ram which couples to the internal threads on the anchor head. The pull bar was engaged to three-fourths of the threads on the anchor head. When the threads on the anchor head failed, the stressing ram disengaged from the anchor head, swung free from the tendon, and posed a safety hazard to personnel performing the tendon surveillance. Fortunately no injuries occurred. The licensee has suspended further tendon surveillance operations until it reviews the results of an investigation into the cause of the anchor head thread failure. The licensee visually examined the remaining threads on the damaged anchor head and found that the threads had yielded before the current tendon surveillance inspection. Duke Power also visually examined anchor heads on other tendons and determined that the threads were also deformed, having been subjected to stresses exceeding the material yield point. The licensee performed a stress analysis of the threads on the anchor heads under application of maximum loads applied during tendon stressing operations. This analysis disclosed that the actual load applied to the threads during stressing operations exceeded the anchor head material shear yield stress, which explains the observed 9112050161 . IN 91-80 December 11, 1991 Page 2 of 2 deformations of the threads on the anchor heads. Duke Power Company is continuing to investigate this problem and is evaluating its effect on future tendon surveillance inspections. Discussion Because the integrity of the post-tensioned concrete containment structure is based on a highly redundant system of several hundred elements, the failure of one such element in a family of tendons would not significantly jeopardize containment structural capability. However, the failure of the threads as occurred at Oconee could adversely affect occupational safety. 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: Joseph J. Lenahan, RII (404) 331-4190 Attachment: List of Recently Issued NRC Information Notices .
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