Information Notice No. 85-10, Supplement 1: Posttensioned Containment Tendon Anchor Head Failure

                                                       SSINS No.: 6835 
                                                       IN 85-10, Supp. 1 

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 

                               March 8, 1985 

                                                TENDON ANCHOR HEAD FAILURE 


All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 


This information notice is provided to supplement Information Notice 85-10 
by advising addressees of (1) the failure of an additional anchor head (from
a different fabrication lot control number), (2) discovery of water within 
the greased tendon system, and (3) the preliminary determination of the 
cause of the failures as provided by Alabama Power Company for Farley Unit 
2. It is expected that recipients will review the information for 
applicability to their facilities and consider actions, if appropriate, to 
detect a similar problem at their facilities. Suggestions contained in this 
notice do not constitute NRC requirements; therefore, no specific action or 
written response is required. 

NRC is continuing to obtain and evaluate pertinent information. If specific 
actions are determined to be required by NRC, an additional notification 
will be made. 


Information Notice 85-10 describing conditions that were recently discovered
at the Farley Unit 2 facility after the anchor heads had been installed and 
sealed in grease for nearly 8 years. Additionally, the information notice 
described anchor head failures that occurred during construction at 
Bellefonte Units 1 and 2 and at Byron Units 1 and 2. 

Description of Circumstances: 

On February 26, 1985 while continuing the tendon inspection program that 
Alabama Power Company had underway, an additional field anchor head (the 
third vertical tendon anchor held) was found in a failed condition at the 
bottom of a vertical tendon. In the as-found condition the anchor head was 
broken, but it was still carrying an estimated 25-30% of the original load. 
While detensioning the anchor head, it separated into five pieces. This 
anchor head was from a fabrication lot different than the two failed anchor 
heads that had 


                                                       IN 85-10, Supp. 1 
                                                       March 8, 1985 
                                                       Page 2 of 3 

been reported earlier in Information Notice 85-10. The anchor head that was 
noted to be cracked in the earlier report has now been detensioned, the 
anchor head removed, and the anchor head-examined metallographically. 

As of March 1, 1985, the licensee had completed the visual inspection of 
many of the anchor heads by removing the grease caps, cleaning grease from 
the anchor head, and visually inspecting for cracks, moisture, or other 
evidence of distress. All 130 field anchor heads (bottom) of vertical 
tendons have been inspected; nearly one-half of those were reported being 
found with moisture. Approximately one-half of the horizontal (hoop) tendon 
anchor heads have been inspected; of those, less than 10% were reported 
being found with moisture. About one-fourth of the dome tendon anchor heads 
have been inspected; moisture was found on nearly 5%. As discussed below, 
moisture may be a significant contributor to the failure of the three anchor 
heads discovered broken or with major cracks. 

Water was found in the grease cap or on the anchor head in each case of the 
three failed vertical tendon field anchor heads. The quantity of water found
associated with these varied from a few ounces to 1/2 pint. The maximum 
amount of water reported, to date, by the licensee was 1-1/2 gallons, which 
was found in one grease cap when it was removed from a vertical tendon field
anchor head. 

The licensee obtained preliminary results from two laboratories, on the 
analysis of failed anchor head material, that indicated the failures have 
been caused by hydrogen-stress cracking (HSC). The conditions necessary for 
HSC to occur include a high-strength steel subjected to sustained tensile 
stresses and a source of atomic hydrogen. Testing, to date, reveals evidence
that a corrosion cell was established between steel and zinc in the presence
of the available water. The zinc source may have been particles from the 
inside of the galvanized tendon sheaths that were abraded during tendon 
installation and tensioning or from the inside of the galvanized grease 
caps, some of which showed evidence of surface etching which points to an 
active corrosion cell. The corrosion cell produced the atomic hydrogen that 
was then apparently adsorbed by the steel, resulting in cracks and their 
growth by hydrogen-stress cracking. 

The resulting cracking surfaces exhibited intergranular separation and the 
magnitude of the cracking could continue to grow as the corrosion cell 
continued to produce hydrogen until a critical crack size was reached. Rapid
section failure then would occur as a result of increased stresses (same 
load but on a reduced area). 

At the present time the licensee is proceeding to remove and inspect, by 
magnetic particle testing (MT), the vertical tendon field anchor heads 
(located at the bottom of the verticals in the tendon gallery) that have not
yet been replaced with new heads. Hoop and dome tendon anchors found with 
significant amounts of water will be detensioned and the heads removed for 
MT. If cracking is found, they will be replaced; if no cracking is found the
old head will be reused. As of March 1, 1985, there had been 28 field anchor
heads replaced. The licensee is now precoating the anchor head, installing 
wires, buttonheading, and regreasing the completed anchor head assembly 
prior to retensioning the tendon. 


                                                       IN 85-10, Supp. 1   
                                                       March 8, 1985 
                                                       Page 3 of 3 

The licensee also conducted load tests on 4 removed field anchor heads. One 
was found by MT to have 3 ligament cracks before load testing, and 2 of the 
4 had water reportedly found with them. The cracks typically occur between 
two holes (a ligament crack) and may-extend fractions of an inch into the 
anchor head. Each anchor head was able to carry a minimum load of 140% of 
the guaranteed ultimate tensile strength of the tendon without failure. 
Three of the field anchor heads exhibited additional cracks after the load 
tests. The anchor head with the 3 original ligament cracks was determined to 
have intergranular separation. 


Current plant technical specifications, where posttensioned concrete 
containments are used with greased tendons, typically state that during 
required surveillance periods the sheath filler material (grease) is to be 
checked to verify that it has not undergone a "change in physical 
appearance." The intent of such a statement is not just to ascertain that 
the filler material continues to meet the original material specifications, 
but also that the filler material is performing its original function, which 
is to preclude moisture from entering the tendon assembly. Therefore, the 
presence of moisture or free water during any surveillance activity should 
be considered evidence of an abnormality and require further action. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 

                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  R. E. Shewmaker, IE 
                    (301) 492-7432 

List of Recently Issued IE Information Notices 

Page Last Reviewed/Updated Friday, May 22, 2015