United States Nuclear Regulatory Commission - Protecting People and the Environment

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 

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                                                       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 


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