ML20217R340
| ML20217R340 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 08/28/1997 |
| From: | Cruse C BALTIMORE GAS & ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9709050072 | |
| Download: ML20217R340 (4) | |
Text
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Italtimore Oas and Electric Company Vice President Cah ert Cliffs Nuclear Power Plant Nuclear Energy 1650 Calven Cliffs Parkw ay Lusby, Maryland 20657 410 495-4455 August 28,1997 U. S. Nuclear Regulatory Commission Washington, DC 20555 1
ATTENTION:
Document Control Desk
SUBJECT:
Calvert Cliffs Nuclear Power Plant Unit No.1; Docket No. 50 317 Containment Tendon Surveillance Day Reoort
REFERENCES:
(a)
Calvert Cliffs Nuclear Power Plant, Unit 1, Technical Specifications, Appendix "A" to License No. DPR-53, Amendment No. 220 l
(b)
Calvert Cliffs Nuclear Power Plant, Units 1 and 2, Updated Safety l
Analysis Report, Revision 20 i
Baltimore Gas and Electric Company, holder of Operating License No. DPR-53, submits this report pursuant to the requirements of the Calvert Cliffs Nuclear Power Plant Unit i Technical Specification 4.6.1.6.4 (Reference a).
Technical Specification 4.6.1.6.4 requires that abnormal degradation of the containment structure detected during tendon surveillance shall be reported to the Commission within 30 days of the test. The 20-year tendon surveillance for Unit 1 post-tensioning containment structure, required by Technical Specification 4.6.1.6, has been completed. This report contains all abnormal degradations detected during this surveillance. The first reportable abnormal degradation of the surveillance was detected on July 29,1997.
BACKGROUND The Calvert Cliffs Nuclear Power Plant post-tensioning system consists of:
- 1. Three groups of 68 dome tendons oriented at 60 degrees to each other for a total of 204 tendons anchored at the vertical face of the dome ring girder.
- 2. Two-hundred-four vertical tendons anchored at the top surface of the ring girder and at the bottom of the base slab.
3.
Six groups of 78 hoop tendons, each enclosing 120 degrees of arc, for a total of 468 hoop tendons anchored at the 6 vertical buttresses.
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Document Control Desk
. August 28,1997 Page 2 Each tendon consists of 90 high-strength steel wires 1/4-inch in diameter with stressing washers and buttonheads on each end of the wires. The details of the containment structure are shown on Figure 51
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of Reference (b).
l I
INSPECTION PROCEDilllE The 20 year tendon surveillance program for the containment post-tensioning system consists of the following operations:
Itchnical Sgtcificallop 4.6.1.6.1 Determining that for a representative sample of at least nine tendons (three dome, three vertical, a.
and three hoop), each tendon has a normalized lift-off force equaling or exceeding its lower limit expected range for the time of the test. If the normalized lifl-off force of any one tendon in a group lies between the lower limit expected range and the lower bound individual, an adjacent tendon on each side shall be checked for lift-off force. If both of these tendons are found acceptable, the surveillance program may proceed considering the single deficiency as unique and acceptable, if either of the adjacent tendons is found unacceptable, it shall be considered as evidence of possible abnormal degradation of the containment structure. In addition, more than one unacceptable tendon out of those selected for surveillance (from all three tendon groups) shall l
be considered as evidence of possible abnormal degradation of the containment structure, l
if the normalized lift-off force of any single tendon lies below the lower bound individual, the occurrence should be considered as evidence of possible abnormal degradation of the containment structure, in addition, determining that the average of the normalized lift-off forces for each sample population (hoop, vertical, dome) is equal to or greater than the required average prestress level; 536 kips for hoop tendons,622 kips for vertical tendons, and 555 kips for dome tendons. If the average is below the required average prestress force, it shall be considered as evidence of possible abnormal degradation of the containment structure.
- b. Removing one wire from each of a dome, vertical, and hooo tendon checked for lift off force, and determining over the entire length of the wire:
- 1. The extent of corrosion, cracks, or other damage. The presence of abnormal corrosion, cracks, or other damage shall be considered evidence of possible abnormal degradation of the containment structure.
2.
A minimum tensile strength value of 240 ksi (guaranteed ultimate strength of the tendon material) for at least three wire samples (one from each end and one at mid-length) cut from each removed wire. Failure of any one of the wire samples to meet the minimum tensile strength test is evidence of possible abnormal degradation of the containment structure.
Perform a chemical analysis to detect changes in the chemical properties of the sheath filler c.
grease. Any unusual changes in physical appearance or chemical properties that could adversely affect the ability of the filler grease to adhere to the t:ndon wires or otherwise inhibit corrosion shall be reported.
u
Docum:nt Control Desk
. August 28,1997 Page 3 InhnicalSpeelnention 4.6.1.64 End Anchorages and Adjacent Concrete Surfaces. The structural integrity of the end anchorages and adjacent concrete surfaces shall be demonstrated by determining through inspection of a representative sample of tendons (reference Technical SpeciGcation 4.6.1.6.1) that no apparent changes have occurred in the visual appearance of the end anchorages or their adjacent concrete exterior surfaces.
REPORTABLE ABNORMAL DEG.lLADATIONS During the performance of Tcchnical Specification 4.6.1.6.1 Surveillance, conditions which may represent abnormal degradation of the containment structure have been found. Three tendons were found to have lift-off forces lower than the Technical Specification lower limit expected lin-oft range for the time of the test. In addition to these low lift-off values, several vertical tendons have been found to contain active corrosion, lioop Tendon 421171 exhibited a lift off force of 596 kips as opposed to its lower limit expected value of 608.3 kips. Per Technical Speci0 cation 4.6.1.6.1.a. adjacent Tenoons 421170 and 421172 were tested and were found to be acceptable with lin-off measurements above 608.3 kips. After 421170 and 421172 were found acceptable,421171 was retensioned to its required value.
Iloop Tendon 511114 also exhibited a lin off force of 549.4 kips as opposed to the lower limit expected value of 608.3 kips. Adjacent Tendons 511113 and 511115 were tested and found acceptable. Tendon 511114 was then detensioned, as planned, to allow removal of a sample wire and then restored to its required value.
Vertical Tendon 45V9 was discovered to have a lift-off value of 518K versus the lower limit expected value of 616.8 kips. This low lin-off force was due to the shim having uneven heights on the two sides.
The top most shim in the stack was 1/2-inch on one side and 3/8-inch on the other. The 1/2-inch side exhibited normal lin-off while the 3/8-inch side, as expected, was low. Averaging brought the overall value down. Adjacent Tendons 45V8 and 45V10 were then tested and found to have acceptable lift-off measurements. Ilowever, during the lift-off testing of 45V10, noises were heard that indicated wires may have broken. Visual examination of 45V10 revealed two wires had broken during the lift-off test, and a third wire broke aner the ram had been removed from the tendon Subsequent evaluation of the broken wires determined a ductile failure in an area of thinning caused by corrosion. The corrosion began three to four inches below the buttonhead and the breaks occurred Eve to seven inches below the bottom of the buttonheads. After 45V10 lift-off testing was completed,45V9 was restored to its required force.
Further visual examinations revealed Tendon 34VI1 contained five broken wires that showed severe corrosion in the same approximate area as the 45V10 wires. These wires had corroded over the fracture surfaces and we were, thus, unable to come to a Grm conclusion on their fracture mode. The wires were thinned by corrosion suggesting a ductile overload fracture. Tendons adjacent to 34VI1(i.e.,34V10 and 34V12) were tested and found acceotable. At that point,34Vil was tested for lift-off, found acceptable, and then detensioned. Two additional wires of Tendon 34Vil were found broken when the ram was l
removed. The breaks in all seven wires occurred four to seven inches below the bottom of the buttonheads. The corrosion areas began three to four inches below the bottom of the buttonheads. The fracture surfaces were examined and failure was determined to be ductile overload in an area where the effective cross-section was reduced by corrosion. The seven broken wires in 34Vil were retained for
Document Control Desk
, August 28,1997 Page 4 further analysis. Visual examination of the removed tendon wires did not show any signs of significant degradation except in the area of the breaks.
No signs of abnormal degradation of the end anchorage or adjacent concrete surface of the containment have been found in any of these areas. Some grease samples displayed various visual indications of water absorption. Ilowever, subsequent chemical analysis of the grease samples showed no detectable changes in the chemical properties.
CDRRECTIVE ACTIONS At the present time, we are evaluating compensatory actions to be taken until all unacceptable tendons are properly dispositioned. We have also expanded the scope of the lift off testing and visual inspections to 100 percent of the vertical 1:ndons to determine the extent and cause of the corrosion. These inspections are ongoing, and results will be reported separately as described below. Although we have identified tendons with mMerate to heavy corrosion, all of the tendons with this condition that have been lift-off tested to date have had lift off values above the required surveillance minimum. The surveillance load is above the design basis accident load per tendon. llence, the containment is operable.
Ongoing efforts incluae engineering evaluation to further analyze the lifbofTtest results addressed above and to complete the expanded scope of testing and inspection on the vertical tendons. Additionally, we continue to evaluate root cause(s), determine appropriate compensatory actions, and establish plans for long-tenn corrective actions. Our findings will be reported to the Commission by October 28,1997, within the 90 day period allowed by Reference (a) for engineering evaluation.
Should pu have questions regarding this matter, we will be pleased to discuss them with you.
Very truly yours,
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R. S. Fleishman, Esquire
- 11. J. Miller, NRC J. E. Silberg, Esquire Resident inspector, NRC Director, Project Directorate 1-1, NRC R.1. McLean, DNR A. W. Dromerick, NRC J.11. Walter, PSC k
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