Information Notice No. 85-10, Supplement 1: Posttensioned Containment Tendon Anchor Head Failure
SSINS No.: 6835
IN 85-10, Supp. 1
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 8, 1985
Information Notice No. 85-10, SUPPLEMENT 1: POSTTENSIONED CONTAINMENT
TENDON ANCHOR HEAD FAILURE
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
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.
Background:
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
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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.
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IN 85-10, Supp. 1
March 8, 1985
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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.
Discussion:
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
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