TXX-6859, Forwards Response to NRC Requesting Addl Info Re Util Results Rept for Rev 3 to Issue Specific Action Plan Ii.C, Maint of Air Gab Between Concrete Structures
| ML20236H462 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 10/30/1987 |
| From: | Counsil W TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC) |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| TXX-6859, NUDOCS 8711040144 | |
| Download: ML20236H462 (4) | |
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MM Log # TXX-6859 ;
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File # 10068 1UELECTRIC.
October 30, 1987 j William o. Counsil l Emuone Vke Premkar -
U. S.. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555
SUBJECT:
COMANCHE PEAK' STEAM ELECTRIC STATION (CPSES)
DOCKET NOS. 50-445 AND 50-446 REQUEST FOR ADDITIONAL INFORMATION ON q ISAP RESULTS REPORT II.c Gentlemen:
Your letter dated September 16, 1987 requested additional.information concerning the TV Electric Results Report for ISAP II.c.
On October 21, 1987 we received a'n extension for submitting this response from your Mr. Jim Lyons until November 4,1987.
We hereby respond to the above item in the attachment to this letter.
Very truly yours, 7
W. G. Counsil JCH/grr Attachment c - Mr. R. D. Martin, Region IV ResidentInspectors,CPSES(3) ,
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l 8711040144 B71030 c l PDR- ADOCK 05000445 ]
A PDR-j t \ 4 400 North Olive Street LB 81 Dallas, Texas 75201 \ l 1 3
Attachment to TXX-6859
- October 30, 1987 Page 1 of 3
- NRC REQUEST FOR ADDITIONAL INFORMATION
!- ON ISAP RESULTS REPORT II.c ISAP II.c (Revision,3), Maintenance of Air Gap Between Concrete Structures:
Question 1 1 l
Paragraph 5.3 of ISAP II.c Results Report refers to adjustments made to concrete walls in order to provide the minimum required gap width. The Results Report states, in part, on page 18, "...some areas may require concrete to be " shaved" from the surface to allow building movements predicted by the calculations". This paragraph also refers to a third-party review of an, "... assessment of significance of these removal activities as they relate to design requirements such as structural integrity and rebar coverage".
The NRC inspector has witnessed gap width adjustment activities where grinding and chipping were used to widen the ga). The NRC inspector also witnessed final as-built inspections of gap widt1 and condition where exposed and subsequently coated rebar was present at the surface of the concrete within the gap being inspected. Some of this rebar was apparently exposed during gap widening and cleaning activities. If rebar is actually exposed within the gap, then additional rebar must have less than adequate concrete coverage.
Certainly some portion of the length of the rebar exposed will have less than adequate concrete coverage. In addition, the reduction in wall thickness could affect structural integrity.
Please provide the assessment of the significance of these removal activities including justification that the walls remain in compliance with ACI-Code and the FSAR. Also provide the third-party review of this assessment.
1 Response to Question 1: '
Instances of exposed rebar have been observed in gap cleaning activities. The l cleaning and widening activities have not, in themselves, been the cause of i exposing rebar. Rather, the removal of debris from concrete surfaces within !
the gaps have revealed instances where rebar is exposed. The " shaving" !
activities, as explained below, were of a very localized nature and involved j the removal of a small amount of material from the surface. It is the i practice at CPSES to document occurrences of exposed rebar on Nonconformance i Reports (NCRs) in accordance with the Project procedure NE0 3.05, " Reporting i and Control of Nonconformances". I In preparing this response, a documentation review was performed by the Project that identified 23 NCRs related to exposed rebar. These NCRs were generated in the time period from 1985, when gap investigations began, to the present. The 23 documented cases of exposed rebar within the seismic gap are not a result of shaving or an indication that other areas have ;
less-than-adequate rebar cover. These documented cases of exposed rebar were i the result of protruding rebar and/or misplaced rebar at the time of placement (15 cases), inadequate concrete consolidation (4 cases), underpouring of concrete (3 cases), and spalling of the concrete surface (1 case). Each NCR was dispositioned by the Engineer as either " Repair", " Rework", or "Use As is". In each case the exposed rebar was determined not to be detrimental to the structural integrity of the concrete structure and, therefore, was determined to be acceptable by the Engineer. All seismic gaps l
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Attachment to TXX-6859
'0ctober 30, 1987
. Page 2 of 3 Response to Question 1 (continued):
at CPSES are currently undergoing a 100% post-cleaning inspection of concrete surfaces, which include inspections for exposed reinforcement. Any additional findings of exposed rebar in the seismic gaps during the final gap inspections will be documented on NCRs and dispositioned by the Engineer as required by Project procedures.
Based on the magnitude and thoroughness of the investigations / corrective I actions conducted for ISAP II.c and the small number of findings (23 NCRs), a I reasonable conclusion can be drawn that there does not exist a significant problem related to the exposure of rebar at CPSES. The following will address the more general subject of the adequacy of concrete coverage of rebar.
There is no load being carried by the top of secondary walls; therefore, exposed rebar at the top of the secondary walls has no significant effect on the structural capacity of the walls. l All walls at eismic building-to-building gaps have been designed as exterior walls, having a concrete cover for reinforcement equal to or in excess of ACI code requirements. Specifically, ACI 318-71, Section 7.14, " Concrete Protection for Reinforcement", has as the most restrictive case a minimum coverage requirement of 2 inches. Seismic gap surfaces are not exposed to earth or weather by virtue of their being protected by such means as metal covers, flashing, boot seals, and water stops. This being the case, it is appropriate that the ACI 318-71, Section 7.14, requirements for surfaces not exposed to weather or in contact with the ground be applicable. Accordingly, the minimum coverage requirement becomes 3/4 inches for bars smaller than #14 size rebars, and 1-1/2 inches for #14 size rebar or larger.
Review of the initial as-built gap condition (prior to beginning corrective measures) and the subcontractors shaving records indicate that the need for shaving of concrete surfaces was primarily due to local form relaxation. The form relaxation (bulging) results in a seismic gap reduction and potentially adds to the coverage thickness of that already afforded by the conservative application of ACI requirements. Concrete shaving in the gaps has, to date, required the removal of a maximum of 3/8" from any one wall.
A worst case scenario would consider the fact that with the minimum cover provided by the design at CPSES (2"), reduced by the maximum 3/8" shaved from the wall surface, would provide a resulting coverage of 1-5/8," which still exceeds the allowable established for interior surfaces per ACI 318-71.
Therefore, the coverage existing in the gaps at CPSES is in excess of the ACI minimum requirements.
The above establishes the adequacy of concrete coverage for rebar in areas affected by the corrective actions in effect for ISAP II.c. It is worth noting that the total naye.itude of the wall area affected by the shaving operation was less than 1300 sq. ft. of wall area out of more than 100,000 sq.
ft, of gap surface area. Only a few square feet of area have been shaved at most locations.
, , Attachment to TXX-6859
. October 30, 1987 l , Page 3 of.3 i
Response to Question 1 (continued):
The structural adequacy of walls as a result of ISAP II.c corrective actions l is not in question. This is easily seen by comparing the maximum depth of shaving (3/8") to the nominal wall thickness (2' -6") at the point of maximum removal. The shaving depth corresponding to 1.25% of the total wall thickness occurs at very localized areas. Accordingly, the structural impact associated with concrete shaving is considered to be negligible.
The third party has been made aware of the questions raised by your staff and !
concurs with the response provided herein. The third party plans to document their review of this matter (and others related to ISAP II.c discussed below) at the point in time that is appropriate considering the status of the ongoing corrective actions. Third party has not finalized its evaluation since gap cleaning activities are continuing.
Question 2:
Numerous activities including: 1) removal of debris or rotofoam, 2) analysis of the final as-built condition, 3) inspection and documentation of the final as-built condition, 4) analysis of seal design, 5) calculations to support l decision to leave design items in the gap, etc. are left open in the ISAP II.c Results Report (see paragraph 7.0). How will these activities be monitored by the third party and finally closed?
Response to Question 2 Commitments made in ISAP II.c Results Report by the third party, as well as those imposed on the Project by the third party, have been captured and collected in a computerized commitment tracking system. Actions required of the third party are overview activities and, as such, will be accomplished in accordance with Revision 4 of the CPRT Program Plan, Appendix H. T1ese activities will be directed by the Senior Review Team. Many of the activities specific to ISAP II.c are pending completion of the corrective actions required for issue closure.
The Project will document closure of this issue by a revision to Specific Technical Issue Report CPRT-S-010, " Building Separation Gaps", at the conclusion of the corrective actions. This summary report will be made available to the Staff for review at that time.
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