ML20098D628
| ML20098D628 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 05/21/1992 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 92-343, TAC-M82916, TAC-M82917, NUDOCS 9205290198 | |
| Download: ML20098D628 (5) | |
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4 ViltGINIA NI.ECTIfl0 AND l'OWER COM]%NY H iciinoxn, Vntorsir une6 May 21,1992 U. S. Nuclear Regule. tory Commission Serial No.92-343 Attn.: Document Control Desk NL&PffAH: R2 Washington, D.C. 20555 Docket Nos.: 50 338 50-339 License Nos. ; F F-4 NPF-7 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 RESPONEE TO REQUEST FOR ADDITIONAL INFORMATION SETTLEMENT MONITORING PROGRAM FOR CLASS 1 STRUCTURES TAC NOS. M82916 AND M82917 On October 2,1989, Oginia Electric and Power Company submitted a request for a lic, <? 'mendment (Serial Number 89-682) to revise our Settlement Monitoring Pro
'.. for Class 1 Structures. By letter dated May 8,1992, you requested additional c
infor,aation in four specific areas of concern.
Attached is the information you requested.
We conclude from the nature of your concerns that the main impetus of your request is reconfirmation that the margin of safety for the safety relatad piping will not be reduced. As stated in the license amendment request dated October 2,1989, the margin of safety as defined in the Technical Specification is preserved by ensuring that settlement of safety related structures and piping is within the allowed limits of the Technical Spec;fication. These limits are established to ensure that stresses on the rafety related piping remains within the code allowable stress limits. The improved i
survey methods reduce random errors and improve the accuracy of the surveys thus reducing the survey uncertainty to five percent or less. The reduced uncartainty factor assures that the potential pipe stresses resulting from the proposed Technical Specification Allowaale Limits wi:1 remain within the code allowable limits. Therefo*e, the margin of safety for settlement of the safety related piping will not be reduced.
If you have any further questions, please contact us.
Very truly yours, 7
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p W. L. Stewart Senior Vice President - Nuclear Attachment l
9205290198 920521 b;()
PDR ADOCK 0500o338 p
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Docket Nos.: 50 338&S0-339 Smial No.92-343 Page 2 of 2 pc: U.S. Nuclear Regulatory Commission Region ll 101 Marietta Street, N.W.
Suite 2900 Atlanta, Georgia 30323 Mr. M. S. Lesser NRC Senior Resident Ir.spector North Anna Power Station l
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Docket Nos.: 50 338&50-339 Serial No.92-343 Attachment Page 1 of 3 13'dSPONSE TO REQUEST FOR ADDIT;ONAL INFORMATION SETTLEMENT MONITORING PROGR AM FOR CLASS 1 STRUCTURES Concern 1 Vertical, "translational" differential settlements which are considered as boundary cenditions would impose a significant inf!uence on the buried pipe stresses, if anchor rotations are restrained. This fact should be considered in the parametric study of the pipe stress calculation.
Response 1 We agree with the staff position that the differential settlements which are considered as boundary conditions would impose significant influence on the buried pipe stresses,if anchor rotations are restrained, in order to account for that, the anchor rotations were restrained in the supporting stress calculation, Stone and Webster Engineering Calculation #12050-NP(B)-094-X9, Rev.1 (Reference 1).
It is recognized that the higher the value of the rotational stiffness, the higher the stress due to imposed differential settlement A conservative value of rotational anchor stiffness was used in the analysis to obtain a conservative estimate of pipe stress. Answer 2 in the attachraent of our letter dated September 29, 1989, Peference 2) provided the basis for determination of anchor stiffn: ss and the reference stiffness calculation of our consultant, Stone & Webster Engineering l
Corporation. The result of that calculation showed that the rotational stiffness of the 9 n-lb/iadian. It is realized that a rotational anchor stiffness i
anchor was 6.8 X 10 lower than the calculated value will yield a lower estimate of stress. Therefore, to predict a conservative estimate of stress in the pipe due to postulated differential 10 n-lb/ radian settlement, a conservative rotational anchor stiffnees value of 1.0 X 10 i
l was used in the analyses. This magnitude of stiffness represents an increase of l
about 50% over the calculated value.
This will account for any perceived uncertainty in the parameters.
The same conservative value of rotational anchor stiffness was uniformly used in calculrtion. tarting from the initial Technical Specification basis and proposed basis to the parametric study documented in Refarence 2. Results of the parametric l
study presented as Answer 4 in Reference 2 wluch utilized the conservative (a 50%
i larger than the calculated) rotational anchor stiffness demonstrated no significant sensitivity of critical pipe stress level to a reasonable bounding increase or decrease in other parameters.
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Docket Nos.: 50 338&50-330 Serial No.92-343 Attachment Page 2 of 3 9.pacern 2 Currently there is little margin existing te absorb any additional pipe stress.
Therefore, complete pipe stress calet. ations incorporating the above mentioned paiametric study and the proposed lew Technical Specification permissible difierential settlements should be submitted for staff r6*,:ew. The submittal should include a justification of pipe configuration used for the analysis.
Response 2 The settlement induced stress it. the pipe for both the current and proposed differential settlement limit is still within the allowable limit of the applicable code.
Therefore, the. proposed change to the differential settlement limit does not represent any significant reduction in the margin of safety.
The complete pipe stress calculation is documented in Reference 1. The results of the parametric study to document the influence of soil parameters, stiffness of soil springs, soil spring spacings in the model and the anchor stiffness was submitted in-Reference 2.
The influence of anchor rotational stiffness is addressed in Response 1 (above).
The mathematical model representing the configuration of the pipe !s documented in Reference 1.
A complete physical layout and technical basis of other input parameters for the analyses were submitted in Reference 2.
Concern 3
-Without assurance that the stresses of the buried pipes have been properly.
calculated, and in light ci the potential significant reduction of the margin of safety as a result of the precedmg differential settlements, the staff questions the validity of the licensee's 10 CFR 50.92(c) evaluation.
Response 3 l-F.esults of the analyses documented in Reference 1 and subsequently submitted in Reference 2 provide adequate technical basis to conclude that the stresses in the buried pipes will not e.<ceed the code allows.M9 value for the proposed limiting value of differential settlement. It is our p;cidon that because the stresses have been conservatively calculated and the expected stresses will remain within code allowable limits our 10 CFR 50.92(c) evaluation remains valid and no potential significant reduuion in any margin of safety exists.
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Docket Nos.: 50 338&50-339 SerL11 No.92-343 Attachment Page 3 of 3 Concern 4 The acceptance of the above stress calculation is contingent upon the staff acceptance of the survey measurements of the survey points including but not limited to survey points 113 and 117, ir connection to total and differential settlements.
Response 4 We agree with the staff concern about the acceptance of the survsy measurements, because the level of stress is related to the magnitude of the differential settlement.
Our submitta: dated October 2,1989, (Reference 3) fully detailed the survey methodology which is applicable only for the revised allowable differential settlement limits for survey points 113R and 117. Your letter dated March 31,1992, (Refe_rence 4) states, "The NRC staff also finds acceptable the licensee's proposed method of direct measurement of the future change in the elevation of points 117 and 113R in order to determine the additional differential settlement of these points in the future." The remaining points will be surveyed by standard loop survey methods utilizing improved measurement devices and several shortar loops in place of one long loop. These improvements will enhance the accuracy of the surveys and reduce the additive random errors attendant in past measurements.
References:
- 1. Stone and Webster E,wineering Calculation #12050-NP(B)-094-X9, Rev.1
- 2. VEPCO letter to NRC datet: September 29,1989
- 3. VEPCO ietter to NRC dated October 2,1989
- 4. NRC letter to VEPCO dated March 31,1992 l
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