Information Notice No. 85-10: Posttensioned Containment Tendon Anchor Head Failure
SSINS No.: 6835
IN 85-10
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
February 6, 1985
Information Notice No. 85-10: 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 alert recipients of current
information relating to a potentially significant problem regarding recent
failures of 170-wire posttensioned containment tendon anchor heads at Unit 2
of the Farley Nuclear Station. 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. However,
suggestions contained in this information 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, an additional notification will be
made.
Description of Circumstances:
Farley Unit 2
On January 28, 1985, while conducting a preintegrated leak rate test
walkdown of the exterior of the containment structure at the Farley Unit 2
facility, an alert utility worker noted grease leakage and a deformed
vertical tendon anchor grease cap on the top of the containment ring beam.
When the grease cap on the same tendon was inspected in the tendon access
gallery, it also revealed a deformed grease cap. Removal of the grease cap
showed that the field anchor head had broken into seven pieces. The
posttensioning force (approximately 1.5 x 10-6 pounds) also had been
released and numerous broken wires from the 170-wire tendon were found.
On the basis of this finding, the utility removed some additional tendon
anchor grease caps. Of the first eight anchor heads uncovered for
inspection, one was found to be cracked. Inspection was curtailed until the
cracked anchor head can be detensioned. The tendon associated with this
anchor head is still transmitting posttensioning force to the containment.
The utility determined from their records that the broken anchor head and
the cracked anchor head have the same fabrication lot control number.
8502060136
.
IN 85-10
February 6, 1985
Page 2 of 3
Additionally, the utility has sent some of the pieces of the broken anchor
head to two laboratories for a series of tests for failure analysis,
including tests on metallurgical, mechanical, chemical and other physical
properties. Testing of the corrosion inhibitor grease is under way.
Currently, the utility has personnel from the architect-engineer and the
material supplier on-site in the continuing investigation. Neither the NRC
nor the licensee has yet fully integrated the information regarding the
results of previous tendon surveillance activities at the Farley site into
this information notice. Oral information from the licensee indicates the
tendons at Farley Unit 2 were posttensioned in early 1977. The unit has been
operational since May 1981. The specific tendon whose anchor head failed and
the one found with a cracked anchor head were not included in the sample of
tendons that were subjected to surveillance activities since the plant began
operation. Thus, there is no definitive information currently available on
the time of occurrence of the breakup of the one anchor head or the crack
formation in the other anchor head.
While no specific conclusions have been reached at this time regarding the
cause of the failures, the NRC believes that based on the conversations with
the supplier, INRYCO, that all material from the same fabrication lot
control number as the failed heads was utilized exclusively at Farley Unit
2.
The previous history of anchor head failures before the event at Farley Unit
2, in nuclear applications, has been confined to occurrences during the
construction phase (during or shortly after posttensioning). It is during
this time that the tendon system, including the anchor head, undergoes the
maximum loading force.
As background information,, previous 170-wire tendon anchor head failures
during construction at other facilities are briefly summarized below.
Bellefonte Units 1 and 2
During 1975 and 1976 a series of eight rock anchor heads, supplied by INRYCO
for the containments at Bellefonte Units 1 and 2, failed during construction
installation. In the phased construction process these 170-wire assemblies
were sealed for long periods in a highly alkaline water environment. These
anchor heads were to be coupled to the posttensioned containment vertical
tendons to serve as a direct tie between the containment and the rock
foundation material. In these instances the anchor head also broke into
several pieces. The licensee's investigations completed on these failures
cited several possible contributors. These included: (1) high anchor head
stress as a result of a 1.4-inch-diameter hole in the head for grout
passage, (2) inclusions in the steel found oriented parallel to the final
failure plane, (3) bending of shims and anchor plate, and (4) unknown
environmental conditions which facilitated stress corrosion cracking. The
NRC had an independent study made that concluded possible stress corrosion
cracking as the initiator.
The resolution of the problem resulted in the removal of all the anchor
heads and replacement with new anchor heads made from a vacuum degassed
(cleaner)
.
IN 85-10
February 6, 1985
Page 3 of 3
steel with the center grout hole eliminated and the anchor head coated for
temporary environmental protection. The NRC is aware of no further failures
at Bellefonte after this corrective action.
Byron Units 1 and 2
In November of 1979 two 170-wire anchor heads on horizontal tendons were
reported to have failed during construction of the Byron containments. One
failure occurred one day after stressing and seating the tendon and the
other occurred 13 days after stressing and seating. By the end of January
1980, two additional anchor heads had been reported as having failed. The
supplier of the anchor heads was INRYCO. Investigations were made by INRYCO
on the material from seven separate fabrication lots. It was found that the
basic steel material used in several of the batches had been manufactured by
a process that utilized vanadium grain refinement causing an incompatibility
with the postfabrication heat treatment. This resulted in a different steel
chemistry that would have required a higher temperature for proper heat
treatment. As a result of this conclusion all anchor heads that had received
improper heat treatment for the basic steel chemistry were removed and
replaced. The NRC is not aware of any failures at Byron since the corrective
action.
Discussion
Because the integrity of the posttensioned concrete containment structure is
based on a highly redundant system of numerous tendon elements (several
hundred), the failure of one such element in a family of tendons does not
jeopardize containment structural capability. It does, however, necessitate
a determination that a mechanism or systematic problem has not arisen under
service conditions when one such failure in a tendon is revealed. Specific
tendon geometry, tendon size, containment design details, and location of
individual tendons with lost or lowered strength properties would dictate
the critical number of tendons that could be lost before containment
integrity is jeopardized.
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. Shewmaker, IE
(301) 492-7432
Attachment: List of Recently Issued IE Infomation Notices
.
Page Last Reviewed/Updated Tuesday, March 09, 2021