LIC-97-0036, Special Rept:On 970321,excessive Grease Voids Were Found in Helical Wall Tendons on Containment Tendon Prestressing Sys.Insp Has Been Conducted

Special Rept:On 970321,excessive Grease Voids Were Found in Helical Wall Tendons on Containment Tendon Prestressing Sys.Insp Has Been Conducted

ML20137G688
Person / Time
Site:	Fort Calhoun
Issue date:	03/21/1997
From:	Gambhir S OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LIC-97-0036, LIC-97-36, NUDOCS 9704010470
Download: ML20137G688 (5)

Text

Omaha Public PowerDistrict 444 South 16th Street Mall Omaha NE68102-2247 March 21, 1997 LIC-97-0036 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station P1-137 Washington DC 20555

References:

1. Docket No. 50-285

2. NRC Regulatory Guide 1.35. " Inservice Inspection of Prestressed Concrete Containments," Revision 3 dated July 1990

3. American Society of Mechanical Engineers. Boiler and Pressure Vessel Code.Section XI Sub Article IWL. 1992 Edition, 1992 Addenda

4. Letter from OPPD (W. G. Gates) to NRC (Document Control Desk),

dated February 11, 1992. LIC-92-0485

5. Letter from OPPD (W. G. Gates) to NRC (Document Control Desk).

dated May 11, 1992 LIC-92-1685

SUBJECT:

Special Report on the Containment Tendon Prestressing System. Excessive Grease Voids found in Helical Wall Tendons Omaha Public Power District (OPPD), holder of Operating License DPR-40, submits this special report pursuant to the requirements of the Fort Calhoun Station (FCS) Ur.it No.1 Technical Specification (TS) Surveillance Requirement 3.5(7), "Prestressing System."

The FCS TS Surveillance Requirement Acceptance Criteria 3.5(7)f.(v)(f) states that, the difference between the amount of grease injected into a tendon to replace the I

amount which was removed during inspection shall not exceed 5% of the net tendon  !

sheath (duct) volume when injected at the original installation pressure. TS 3.5(7)(g) requires an immediate investigation to determine the causes and extent of any non-conformance with the acceptance criteria and it requires the results to be f reported to the Commission within 90 days via a special report in accordance with TS 4 21 5.9.3.

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b. S. Nuclear Regulatory Commission LIC-97-0036

' j Page 2 The twenty-forth year Containment Building Prestressing System Surveillance l l including helical wall tendons, was completed by Precision Surveillance Corporation '

(PSC) on January 12, 1997. Additional testing was also conducted to complete the investigation started during the 1991/1992 Prestressing Surveillance. Grease was added to 26 helical wall tendons. Calculations were made to determine the grease void measurement on each tendon tested. Grease void measurements on 18 of 26 2

tendons were greater than the 5% net. duct void listed as reportable by Technical Specification 3.5(7)f.(v)(f). Results of grease void measurements are presented in Attachment 1.

, While Reference 2, which is the basis for TS 3.5(7)f.(v)(f) requires reporting of 1 measured grease voids in excess of 5% net duct volume, industry experience shows  !

voids greater than 5% can be expected due to installation practices.  !

The NRC has approved the applicable portion of the ASME Code (Reference 3) which l recognizes 10% grease void as the reporting limit. 11 of 18 reportable tendons had grease voids >5% but <10% and are acceptable on this basis. )

i Voids greater than 10% have been noted in industry applications. Research conducted 4 by Inryco in 1984 and 1985 found 3 original installation factors which contribute to i grease voids. These installation factors are:

1

1. A 6% void could result from shrinkage of grease which is injected at  !

temperatures between 120*F and 220*F.

2. A void of 7% net duct volume could develop due to inadequate tendon bundle soak time. The area between the close set wires requires dwell time to allow the grease to flow to the inside of the wire bundle.

3. Air entrapment can result in a 2% void.

This could result in a worst case condition of 15% net void, even if installation conditions were ideal. Tendons H-2121 H-4009, and H-3105 indicated voids between 10% and 15% when tested in 1991/1992. Results are reported in Attachment 2. When tested 5 years later, grease voids were significantly lower. This indicates the voids were most likely caused by initial grease filling practices. The four tendons l which recently measured grease void results between 10% and 15% are considered acceptable based on FCS experience that indicates the voids are due to initial filling practices. However, these tendons will be checked at the next surveillance interval to confirm grease volume.

The three tendons with grease volume voids of greater than 15% are indicated as leaking or potentially leaking. Tendons H-1087 and H-1078 have been tested for grease voids in both 1991/1992 and 1996/1997 and had comparable results in each test.

O

'U. S. huclear Regulatory Commission LIC-97-0036 Page 3 Inspection of exterior concrete surfaces in the vicinity does not reveal leakage directly traceable to these tendons. Both tendons travel through the intersection of concrete pours CW-10 and CW-9A. This area has repour vents installed, which were initially empty. The accessible vent piping is observed to be filling with a large quantity of tendon grease. Previous testing conducted at FCS has proven that a layer of grease, adequate to prevent corrosion remains on the tendon even when voids 250% have been noted.

Tendon H-2153 is slightly above the 15% void increment. Inspectiori of the concrete containment surfaces in the vicinity of this tendon run does not reveal grease leakage although this tendon also traverses the concrete pour CW-10, CW-9A interface. Void volume and visual inspection of these three tendons will be conducted at the next test performance.

Based on a comprehensive inspection for grease leakage, lack of corrosion in wire and anchorage locations and continued satisfactory tension measurements of the Containment Prestressing Tendons, OPPD is confident in the ability of the containment structure to continue to perform its safety related function.

If you have any questions, please contact me.

Sincerely, i

S. K. Gambhir Division Manager -

Engineering & Operations Support SKG/mah Attachments c: Winston & Strawn (w/o Attachments)

E. W. Merschoff. NRC Regional Administrator, Region IV (w/o Attachments)

L. R. Wharton, NRC Project Manager (w/o Attachments)

W. C. Walker, NRC Senior Resident Inspector (w/o Attachments)

Document Control Desk (w/o Attachments)

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