ML20195B469

Summary of 880524 Meeting W/Util in Rockville,Md to Discuss Proposed Change to Tech Spec 3.6.1.6, Containment Structural Integrity. Related Info Encl

ML20195B469
Person / Time
Site:	Summer
Issue date:	06/06/1988
From:	Hayes J Office of Nuclear Reactor Regulation
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 8806210425
Download: ML20195B469 (43)

Text

_ . __

JUN 0 61988 Docket No. 50 395 DISTRIBUTION LICENSEE: South Carolina Electric & Gas Company See attached sheet FACILITY: V. C. Summer Nuclear Station

SUMMARY

OF MEETING WITH SOUTH CAROLlHA ELECTRIC & GAS COMPANY I General On May 24, 1988, the NRC staff met with the South Carolina Electric & Gas '

Coapany (SCE&G) and their consultants to discuss the proposed change to l Technical Specification (TS) 3.6.1.6, "Containment Structural Integrity," for the V. C. Sumer Station. The meeting was held at Headquarters in Rockville, Ma ryland. A list of those persons who attended the meeting is included as  !

Enclosure 1. '

Discussion i

The purpose of the meeting was to discuss the SCE&G response of March 17, 1988  ;

to the NRC's request for additional information. The staff provided to SCE&G '

the information in Enclosure 2 via telecopy for the purpose of focusing the discussion for the meeting. Enclosure 3 was provided by SCE&G for the purpose of demonstrating, based upon the three surveillances of the containment tendons  ;

conducted, the projected point in time where the tendon force would be below the present Technical Specification value and the point in time if TS 3.6.1.6 was changed. Enclosure 4 provided a sumary of the events involving this proposed Technical Specification change. Enclosure 5 is SCE&G's response -

to the information presented in Enclosure 2. As a result of this meeting, SCE&G will commit to provide, either in plant procedures or the Technical Specifications, a report on those occasions where the tendon average force data would indicate that the tendon forces would drop below their TS limit-prior to the conduct of the next tendon surveillance. The approach to be  ;

taken will be discussed with the Project Director for Project Directorate 11 1 and the Technical Specification Branch prior to discussing with SCE&G. l The lead technical reviewer and his Branch Chief appeared to have all of their ,

technical issues addressed. i b\  !

John J. Hayes, Jr., Project Manager Project Directorate 11-1 l Division of Reactor Projects 1/11 i

Enclosures:

As stated cw closures: 62ggas $$o 5 0FC :PM:PD21:DRPR:D:PD' PR :  :  :  :  :

NAME : J :ch ; f n am :  :  :  :  :

..__/ _

____.:_f,[/88 DATE  : ']if/  ; h7[/88 :  :  :  :  :

0FFIClal RECORD COPY

~ .- . - . _ _ . _

c asa

1. [og UNITED STATES

[ p, NUCLEAR REGULATORY COMMISSION

j WASHINGTON, D. C. 20555

\ , , , , , #' JUN061NB Docket No. 50-395 LICENSEE: South Carolina Electric & Gas Company FACILITY: V. C. Sunner Nuclear Station

SUMMARY

OF MEETING WITH SOUTH CAROLINA ELECTRIC & GAS COMPANY General On May 24, 1988, the NRC staff met with the South Carolina Electric & Gas Company (SCE&G) and their consultants to discuss the proposed change to Technical Specification (TS) 3.6.1.6, "Containment Strectural Integrity," for the V. C. Summer Station. The meeting was held at Headquarters in Rockville, Ma ryland. A list of those persons who attended the meeting is included as Enclosure 1.

Discussion The purpose of the meeting was to discuss the SCE&G response of March 17, 1988 to the NRC's request for adJitional information. The staff provided to SCE&G the information in Enclosure 2 via telecopy for the purpose of focusing the discussion for the meeting. Enclosure 3 was provided by SCE&G for the purpose of demonstrating, based upon the three surveillances of the containment tendons conducted, the prnjected point in time where the tencon force would be below the present Technical Specification value and the point in time if TS 3.6.1.6 was changed. Enclosure 4 provided a summary of the events involving this .

proposed Technical Specification change. Enclosure 5 is SCE&G's response to the information presented in Enclosure 2. As a result of this meeting, SCE&G will commit to provide, either in plant procedures or the Technical Specifications, a report on those occasions where the tendan average force data would indicate that the tendon forces would drop below their TS limit i prior to the conduct of the next tendon surveillance. The approach to be l taken will be discussed with the Project Director for Project Directorate 11-1 and the Technical Specification Branch prior to discussing with SCE&G.

The lead technical reviewer and his Branch Chief appeared to have all of their technical issues addressed. 1 l

9 Joh J. Hayes, Jr. , roj ct Manager Project Directorate 11-1 Division of Reactor Projects 1/11

Enclosures:

As stated cc w/ enclosures:

See next page 4

. .-_ - ~ , - , . . . - - - - _ _ - - -

.y

• 4 N

Mr.:D. A. Nauman /

South' Carolina Electric & Gas Company Virgil C. Summer Nuclear Station cc: 3 Mr. William A.'= Williams,.Jr.

Technical Assistant - Nuclear Operations Santee Cooper P.O.: Box 764 (Mail Code 153)

Columbia, South Carolina 29218 J . ' B . Kno'tt s , J r. , E sq .

Bishop,: Cook, Purcell and Reynolds 1400 L Street, N.W.

Washington, D. C. 20005-3502 Resident inspector / Summer NPS -

c/o U.S. Nuclear Regulatory Coamission Route 1, Box 64 Jenkinsville, South Carolina 29065 Regional Administrator, Region II U.S. Nuclear Regulatory Commission, 101 Marietta Street, N.W., Suite 3100 I Atlanta, Georgia 30323 Chairman, Fairfield County Council P.O. Box 293 Winnsboro, South Carolina 29180 ,

Attorney General Box 11549 Columbia, South Carolina 29211 l Mr. Heyward G. Shealy, Chief Bureau of Radiological Health- l South Carolina Department of Health and Environmental Control 2600 Bull Street Columbia, South Carolina 29201 South Carolina-Electric & Gas Company Mr. A. R. Koon, Jr. , Manager Nuclear Licensing Virgil C. Summer Nuclear Station P. O. Box 88 Jenkinsville, South Carolina 29065 l

1

_ ,m_ . - - - ,, , . - ,,._ , _ , --_-- , , - - - _ . - .

ENCLOSURE 1 ATTENDANCE MAY 25, 1988 MEETING SCE&G NRC Gilbert /Comonwealth Associates W. Higgins J. Hayes, Jr. D. Krause J. Todd C. Tan J. Herr G. Bagchi C. Chen J. Fulton (Private Consultant) f

w. , _ . . . _.__. _- __. . -. _ _ _ _. . . _ _ . ___

• ENCLOSURE 2 I

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ENCLOSURE 4 i

SUMMARY

OF EVENTS ,.

PROPOSED TECH SPEC CHANGE

1. Third Tendon Surveillance - November December 1985 e Individual tendons below their respective 95% Base predicted values.  !

e Average of normalized hoop tendon forces below Tech Spec limit:  ;

1175 kips versus 1181 kips. Vertical and dome tendon group average forces we.re above their respective Tech Spec limits: Verticals at 1236 kips versus 1195 kips and domes at 1187 kips versus 1115 kips.

i e Vcrti al tendons projected to go below 1195 kips Tech Spec limit in second I half of 1989.

2. Thirty day special report required by Tech Spec filed with NRC (G Al Report No. 2610).

• Eve' .ation ofindividual tendon forces: effects of stress relaxation and effects of prestress loss analysis methodology.

e Evaluation of analyses previously performed to establish Tech Spec values for minimum required average tendon forces for all three tendon groups. [

e For hoop tendons, a more accerate representation of the hoop tendon forces based on a model of the containment wall indicated that the Tech  ;

Spec limit 1181 kips for minimum required average tendon force could be i I

reduced to 1078 kips and provide the 750 kips per ft. minimum required I design hoop prestress. i e Consideration of increase in tendon force due to tendon elongation under i containment expansion allowed minimum required average tendon forces to be reduced from current Tech Spec limits: Verticals from 1195 kips tc 1160 kips, domes from 1115 kips to 1065 kips, and hoops from II78 kips to 1000 kips.

e Recommended: (1) Use final wire stress relaxation test results to determine revised Base values; (2) Use non-interaction prestress loss calculation

, (; -

rpethod; and (3) Rcvise Tech Spec limits for minimum required average tendon force.

E

3. Proposed Tech Speuhange to NRC - September 1986 e Specify Tendon Surveillance Program for remainder of plant operation after Third Surveillance.

• Reduce required minimum average prestress force levels.

e Eliminate tables from Tech Spec that related to the first three surveillances.

4. NRC Questions (RFI) First Round - June 1987 SCE&G Reply to RFI First Round - August 1987 5.

I <

6.' NRC' Questions (RFI)Second Round (Informal Transmittal of Questions)-

, December 1987

7. NRC Questions (RFI) Second Round (Formal Transmittal of Questions)- March 1988 i A

8. SCE&G Reply to RFI Second Round - April 1988 )

9. NRC Questions Third Round (Informal)- May 1988 t

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i j '

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ENCLOSURE 5 l

6 l l

RESPONSES TO NRC INFORMAL INFORMATION REQUEST DATED MAY 19,1988 QUESTION 1:

1 The staff has reservations on the use of tendon elongation under accident l pressure as a means to reduce the required minimum average tendon force for the following reasons:

i QUESTION 1a:

On the basis of the analysis of the tendon installation and sumeillance data, the linear relationship between tendon force and tendon elongation does not always exist, for instance for tendon no. V68:

l P3 of 16 1602k

-424 1178k + 10.5" = 112.2k/in 1461k I 1

-424 1037k + 9.5" = 109.2k/in j P4 of 16 1562 424 1159 + 9.16" = 126.5k/in 1

1324 l

-424 900 + 8.47" = 106.4k/in Such a condition is more pronounced in hoop and dome tendons due to the presence of friction. l RESPONSE TO QUESTION 1a:

The tendon selected as an example, tendon V68,is curved over a portion of its I

length as it deflects around the personal access opening. The fact that the apparent stiffness of the tendon V68 varies during stressing is due to several

effects. At original stressing of V68, the tendon was initially stressed up to 1602k monotonically. Then the force was reduced, shims instaHed, and the tendon was seated. As a final operation, the tendon was lifted off and a lock-off force of 1461k was recorded. When the tendon force was reduced from 1602k to 1461k, the friction forces were reversed along the tendon over a certain distance in the curvature region. Because of this effect and possibly measurement inaccuracies, the apparent stiffnes is slightly different (109.2k/in.) from that due to monotonical stressing of the tendon (112.2k/in.).

At the first surveillance, V68 was selected to be detensioned and then retensioned after a test wire was removed. The detensioning of the tendon also caused the friction forces to be redistributed and the apparent stiffness of the tendon increased.

Considering all straight vertical tendons that were involved in the first three  !

surveillances (13 total), and based on the elongations that were measured at l

original stressing, an average effective Young's modulus, E, of 29.6 x 106 psi l was calculated. This value compares with 29.0 x 106 psi used for E in the i prestress increase calculations. Consequently, the measured elongations substantiate that the value of E used to determine the increase in prestressing force is conservative.

The above effects which cause differences in the apparent tendon stiffness are l associated with end stressing of the tendon. These effects are not nearly as significant in the determination of increase in tendon force due to expansion I of the containment for the following reason. Even though tendons are unbonded, there is a high degree of strain compatibility between the tendons and concrete. This condition does not exist when the tendon is stressed by applying a force ~at each end. For example, the radial expansion of the containment for the accident pressure load case produces essentially a uniform pressure load on the hoop tendons. There is compatibility in the radial displacements of the containment wall and hoop tendons. For all practical purposes, the strains in the tendon are uniform over most of its length and are equal to the hoop strain in the concrete, which is essentially uniform over the containment circumference.

For the dome, the prestress compressive stress levels in the concrete vary depending on the particular tendon. This was taken into account in the determination of the increase in dome prestress (refer to Calculations 1.18.2.3 previously submitted).

QUESTION 1b:

The computation of elastic shortening, shrinkage, creep and relaxation is at best an approximation. Any variation may cancel out the gain in tendon elongation, also the increase in tendon force may result in increase in creep.

RESPONSE TO QUESTION 1b:

Calculations of elastic shortening, shrinkage, creep, and stress relaxation are approximations based on theory and test results. Variation in tendon force losses due to these parameters can result in lower average forces in tendon groups than predicted by calculations. However, to establish the prestress levelin the containment, these calculations are not relied upon. R- Ser tendon surveillances are performed to measure the tendon forces on a sample basis.1.ift off measurements of tendon force provide assurance that the required minimum prestress levels are available and also establish whether force losses are occurring in a predictable manner, thus assuring that required minimum prestress levels will be available in the period between surveillances.

The first three surveillances for the V. C. Summer containment indicate that losses greater than predicted have occurred. However, the straight line semi-log paper plot of average measured force for each tendon group versus time for the first three surveillances indicates force losses are occurring in a predictable manner. These measured forces eliminate the approximations in the original calculations to predict losses since actual measured losses from j three surveillances are available.

Predicted creep losses will not be influenced by the increased prestress force in the shell which occurs for internal presure loads. Creep losses occur slowly over time - months and years. The postulated accident pressure loads occur for a period of hours - at most a few days. Any increase in creep will be I

1

insignificantly small during the relatively short duration accident pressure loading.

QUESTION 1c:

For hoop tendons, the buttresses are under compression which will reduce the tendon elongation.

RESPONSE TO QUESTION 1c:

The effect on tendon force of buttress shortening due to compression resulCng from hoop tendon prestress force under service conditions is accounted for by taking actuallift-off measurements of tendon fc.rce during survell;ances. Any elastic shortening loss is reflected in the measured tendon force.

The effect of elastic shortening across the face of the buttress due to the increase in tendon force under accident pressure is negligible. The STRUDL analysis in calculation 1:18.2.1 submitted previously in response to NRC questions indicates a net shortening between the buttress faces of 0.0578 ,

inches for a loading corresponding to 0.7 GUTS force (1402 kips) in the hoop ,

teadons.

The increase in average hoop tendon force occurring under the 1.5 times accident pressure loading is 78 kips. Assuming, conservatively, the full 78 kips increase is reflected at the anchor heads, the elastic shortening of the buttress due to 78 kips can be calculated by direct ratio of the STRUDL analysis results for 1402 kips force.

Elastic Shortening :- 78 kips /1402 kips x 0.0578 inches = 0.0032 inches The tendon force corresonding to the 0.0032 inch shortening is: ,

Change in tendon force due to elastic shortening across buttress =

4AE/L = (0.0032" x 8.345 sq. in. x 29 x 10U + (12"/ft. x 269 ft.) =

0.24 kips or 240 pounds Therefore, the effect of the elastic shortening across the face of the buttress l for the additional 78 kip force per hoop tendon corresponds to less than 1 kip. j 4

QUESTION 1d:

Depending on the ambient temperature at the time of construction,the tendon force may increase or decrease resulting in the variation of tendon forces. ,

i RESPONSE TO QUESTION id: l l

it is agreed that the temperature of the tendon at any pointin time can be different from that which occurred at original installation and stressing. This is expected to be the case during normal operating condition with the l containment at an elevated steady state operating temperature. In this case, the tendon temperature is higher than that at original stressing. If the containment were not similarly affected by the elevated temperature, the tendon would experience an elastic thermal force decrease. However,it is generally accepted that the average temperature across the containment wall and dome is approximately equal to the tendon temperature. Because of this  !

and due to the fact that the tendons and concrete have similar linear I coefficients of thermal expansion (6.5 versus 5.5 micro inches per inch per

• F.), l the thermal growth of the containment concrete and the tendon are l practically the same, and there is no significant decrease in tendon force due to the elevated steady state operating temperature condition within the containment. For example, based on a 90' F. vertical tendon temperature, a 5 kip decrease in tendon force could result,which represents 0.4% of the minimum required force in the vertical tendons.

QUESTION 1

SUMMARY

l On the basis of the above considerations,it is felt that the margin of safety l l

resulting from tendon elongation should not be used to reduce minimum I required average tendon force.

RESPONSE TO QUESTION 1

SUMMARY

REGARDING MARGIN OF SAFETY Accounting forincreased prestress levels under postulated accident pressure conditions to the extent proposed still provdes significant design margins as indicated in the following discussion.

5-

From one major standpoint, consideration of the tendons in the design of the V. C.

Summer containment is conservative by the current standards reflected in the ASME B&PV Concrete Containment Code. In the current Code, under the factored load combinations, tendens are allowed to be considered acting together with the reinforcement for load resisting purposes. This is reflected in Article CC-3423 where I the axial tensile capacity of the tendon is limited to 0.90 fpy,in which fpy is the yield strength of the tendon. In effect, the tendons may be considered as non-bonded reinforcement. Consequently, for each factored load combination, after the 1

effective prestress and dead load compression stresses arc overcome, the tendons together with the reinforcement are considered to resist the net tensile membrane force and accompanying moment. For V. C. Summer, these code provisions were not used; tendon forces beyond the effective prestress compressive stresses that they produce in the containment are not utilized. Significant reinforcement is provided to resist the net internal forces and moments.

For vertical tendons, for example, under the current Code the tendon; at 0.9 fpy have a permissible tensile capacity of 1442k based on the minimum specified yield strength of the tendon wire material. This is significantly greater (by 282k) than the minimum required average tendon force of 1160k that is proposed. The 282k difference is equivalent to over 5 in.2 of Grade 60 reinforcement. This advantage was not utilized for the V. C. Summer containment design to determine the required reinforcement. Therefore, again using the vertical tendons as an example, accounting for tendon elongations and the accompanying 35k increase in tendon force beyond its effective prestress level corresponding to 1160k tendon force does not significantly impact the margin provided under the current Code where a 282k increase in tendon force would be allowed.

QUESTION 2:

In your January 17,1986 submittal, it was indicated that stress relaxtion tests were underway at Fritz Laboratories. Provide the details and results of the tests.

6-

RESPONSE TO QUESTION 2:

Wire stress relaxation is not related to the determination of the minimum required prestress level. Wire stress relaxation is used as part of the input data to determine predicted forces in the individual tendons versus time.

As part of the investigation to determine why greater than anticipated losses were occurring in the tendon, wire stress relaxation tests were performed on tendon wire samples.

The testing is complete. The result; indicate that the 81/2 percent relaxation used in the original calculations to predict losses is low. Test results indicate between 12 and 13 percent relaxation losses over 40 years.

Testing was performed on wire samples in 68* F.,90* F., and 104* F.

environments. The 90* F. tests are representative of the tendon enironment in the containment. Force loss measurements were recorded at various times out to about 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. The results were plotted on log-log sheets and projected to the 40 year end of plant life.

Four samples at 68* F. projected to an average relaxation of 9.6% at 40 years.

Three samples at 90* F. projected to an average of 12.8% at 0 years.

Two samples at 104* F. projected to an aerage of 17% at 40 years.

As a check of the 90* F. results, linear interpolation of the 68* F. and 104* F.

results to 90* F. indicates 14% relaxation at 90* F. compared to the average of 12.8% for the 90* F. tests.

QUESTION 3:

Provide hand calculations which approximate the internal pressure values that together with the applied prestress in forces will result in zero hoop and axial l membrane stresses.

7-

e ,

RESPONSE TO QUESTION 3:

The following approximate calculations indicate the value of internal pressure that combined with the effective design prestress will result in zero hoop and axial membrane stresses.

AXIAL Design Effective Prestress = 335 k/ft.

Internal Pressure: No = PR/2 = 335 k/ft.

P = 2 x 335k/ft./R = 2 x 335k/ftJ63 ft. = 10.63 ksf = 73.8 psi P = 73.8 psi = 1,29 Pa, Pa = S7 psig (DBA)

HOOP Design Effective Prestress = 750 k/ft.

Internal Pressure: NO = PR P = 750k/ft./63 ft. = 11.90 ksf = 82.6 psi P = 82.6 psi = 1.45 Pa, Pa = 57 psig (DBA)

The above calculations are approximate in the sense that biaxial effects and dead load compressive stresses are not included. ,

These calculations demonstrate that the summer containment is designed as a "partially prestressed" structure. That is, the prestress level is not intended to maintain compression at zero membrane stress state under the controlling factored load combinations. For these load combinations, the resulting  :

internal net tensile forces are resisted by significant amounts of reinforcement in the containment. In the hoop direction of the wall, #14 @ 12" spacing (outside face) and # 11 @ 12" spacing (inside face) are generally provided. In )

the vertical direction of the wall, over the lower half, #18 @ 12" spacing l (outside face) and #14 @ 12" spacing (inside face) are generally provided. In i the upper half of the wall, #14 @ 12" spacing (outside face) and #14 @ 12' spacing (inside face) are generally provided. In the dome, the meridional and hoop reinforcement varies and #11 an'., #14 bars are provided.

4 e

DISTRidVT10N FOR MEETitiG

SUMMARY

DATED:

JllN 0 6 1988 Facility: V. C. Surnmer Nuclear Station Docket File llRC Participarits liRC PDR Local POR J. Hayes PDll-1 Reading C. Tan E. Adensam G. Bagchi P. Anderson J. Hayes OGC E. Jordan (Mt4BB 3302)

J. Partlow (9AE)

ACRS(10) cc: Licensee / Applicant Service List

i t

DISTRIBUTION FOR MEETING

SUMMARY

DATED:

JUN 96Ithi ,

Facility: V. C. Summer Nuclear Station 40cckef?Eilhl NRC Participants

• NRC POR Local PDR J. Hayes PDil-1 Reading C. Tan E. Adensam G. Bagchi P. Anderson J. Hayes ,

OGC E. Jordan (MNBB 3302)

J. Partlow (9A2)

ACRS (10) cc: Licensee / Applicant Service List ,

t l

I

O JUN 0 61998 Docket No. 50-395 DISTRIBUTION LICENSEE: South Carolina Electric & Gas Company See attached sheet  ;

FACILITY: V. C. Sumer Nuclear Station

SUMMARY

OF MEETING WITH SOUTH CAROLINA ELECTRIC & GAS COMPANY

. General On May 24, 1988, the NRC staff met with the South Carolina Electric & Gas Ccmpany (SCE&G) and their consultants to discuss the proposed change to Technical Specification (TS) 3.6.1.6, "Containment Structural Integrity," for the V. C. Sumer Station. The meeting was held at Headquarters in Rockville, Ma ryla nd. A list of those persons who attended the meeting is it.cluded as Enclosure 1.

Discussion The purpose of the meeting was to discuss the SCE&G response of March 17, 1988 to the NRC's request for additional information. The staff provided to SCE&G the information in Enclosure 2 via telecopy for the purpose of focusing the discussion for the meeting. Enclosure 3 was provided by SCE&G for the purpose of demonstrating, based upon the three surveillances of the containment tendons conducted, the projected point in time where the tendon force would be below the present Technical Specification value and the point in time if TS 3.6.1.6 was changed. Enclosure 4 provided a sumary of the events involving this proposed Technical Specification change. Enclosure 5 is SCE&G's response to the information presented in Enclosure 2. As a result of this meeting, SCE&G will comit to provide, either in plant procedures or the Technical Specifications, a report on those occasions where the tendon average force data would indicate that the tendon forces would drop below their 15 limit prior to the conduct of the next tendon surveillance. The approach to be taken will be discussed with the Project Director for Project Directorate 11-1 and the Technical Specification Branch prior to discussing with SCE&G.

The lead technical reviewer and his Branch Chief appeared to have all of their technical issues addressed.

h\

John J. Hayes, Jr., Project Manager Project Directorate 11-1 Division of Reactor Projects I/II

Enclosures:

As stated cc w/ enclosures:

See next page OFC :PM:PD21:DRPR:D:PD R:  :  :  :  :

NAME : J

. . :ch

. . .: ,@b.E a  :  :  :  :  :

DATE :

[/88 , h7[/88  :  :  :  :  :

OFFICIAL RECORD COPY

0 Mr. D. A. Nauman South Carolina Electric & Gas Company Virgil C. Sumer Nuclear Station cc:

Mr. William A. Williams, Jr.

Technical Assistant - Nuclear Operations Santee Cooper P.O. Box 764 (Mail Code 153)

Columbia, South Carolina 29218 J. B. Knotts, Jr., Esq.

Bishop, Cook, Purcell and Reynolds 1400 L Street, N.W.

Washington, D. C. 20005-3502 Resident Inspector /Sumer #1 c/o U.S. Nuclear Regulatory Comission Route 1, Box 64 Jenkinsville, South Carolina 29065 Regional Administrator, Region II U.S. Nuclear Regulatory Comission, 101 Marietta Street, N.W., Suite 3100 Atlanta, Georgia 30323 Chairman, Fairfield County Council P.O. Box 293 Winnsboro, South Carolina 29180 Attorney General Box 11549 Columbia, South Carolina 29211 l

Mr. Heyward G. Shealy, Chief l Bureau of Radiological Health i South Carolina Department of Health and Environmental Control 2600 Bull Street Columbia, South Carolina 29201 1

South Carolina Electric & Gas Company l Mr. A. R. Koon, Jr., Manager Nuclear Licensing Virgil C. Sumer Nuclear Station -

P. O. Box 88 Jenkinsville, South Carolina 29065

ENCLOSURE 1 ATTENDANCE MAY 25, 1988 MEETING SCE&G NRC Gilbert /Connonwealth Associates W. Higgins J. Hayes, Jr. D. Krause J..Todd C. Tan J. Herr G. Bagchi C. Chen J. Fulton (Private Consultant) y%* .

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. ENCLOSURE 4

SUMMARY

OF EVENTS PROPOSED TECH SPEC CHANGE

1. Third Tendon Surveillance - November December 1985 e Individual tendons below their respective 95% Base predicted values.

• Average of normalized hoop tendon forces belowTech Speclimit:

1175 kips versus 1181 kips. Vertical and dome tendon group average forces were above their respective Tech Spec limits: Verticals at 1236 kips versus 1195 kips and domes at 1187 kips versus 1115 kips.

e Vertical tendons projected to go below 1195 kips Tech Spec limit in second half of 1989.

2. Thirty day special report required by Tech Spec filed with NRC (gal Report No. 2610),

o Evaluation of individual tendon forces: effects of stress relaxation and effects of prestress loss analysis methodology.

• Evaluation of analyses previously performed to establish Tech Spec values j for minimum required average tendon forces for all three tendon groups.

e For hoop tendons, a more accurate representation of the hoop tendon forces based on a model of the containment wallindicated that the Tech Spec limit 1181 kips for minimum required average tendon force could be reduced to 1078 kips and provide the 750 kips per ft. minimum required design hoop prestress.

l e Consideration of increase in tendon force due to tendon elongation under containment expansion allowed minimum required average tendon forces to be reduced from current Tech Spec limits: Verticals from 1195 kips to i 1160 kips, domes from 1115 kips to 1065 kips, and hoops from 1I78 kips to 1000 kips.

• Recommended: (1) Use final wire stress relaxation test results to determine revised Base values; (2) Use non interaction prestress loss calculation

method; and (3) Revise Tech Spec limits for minimum required average tendon force.

3. Proposed Tech Spec change to NRC - September 1986

• Specify Tendon Surveillance Program for remainder of plant operation after Third Surveillance.

e Reduce required minimum average prestress force levels.

e Eliminate tables from Tech Spec that related to the first three surveillances.

4. NRC Questions (RFI) First Round - June 1987

5. SCE&G Reply to RFI First Round - August 1987

6. NRC Questions (RFI) Second Round (Informal Transmittal of Questions) -

December 1987

7. NRC Questions (RFI) Second Round (Formal Transmittal of Questions)- March 1988

8. SCE&G Reply to RFI Second Round - April 1988

9. NRC Questions Third Round (Informal)- May 1988 i

.- .- a

ENCLOSURE 5 4

RESPONSES TO NRC INFORMAL INFORMATION REQUEST DATED M AY 19,1988 l QUESTION 1:

The staff has reservations on the use of tendon elongation under accident pressure as a means to reduce the required minimum average tendon force for the following reasons:

QUESTION 1a:

On the basis of the analysis of the tendon installation and surveillance data, the linear relationship between tendon force and tendon elongation does not i always exist, for instance for tendon no. V68:

P3 of 16 1602k j

-424 l 1178k + 10.5" = 112.2k/in 1461k

-424 1037k + 9.5" = 109.2k/in P4 of 16 1562

-424 1159 + 9.16" = 126.5k/in 1324

-424 ,

900 + 8.47" = 106.4k/in Such a condition is more pronounced in hoop and dome tendons due to the presence of friction. .

RESPONSE TO QUESTION 1a:

The tendon selected as an example, tendon V68,is curved over a portion of its length as it deflects around the personal access opening. The fact that the apparent stiffness of the tendon V68 varies during stressing is due to several 1

effects. At original stressing of V68, the tendon was initially stressed up to 1602k monotonically. Then the force was reduced, shims installed, and the tendon was seated. As a final operation, the tendon was lifted off and a lock-off force of 1461k was recorded. When the tendon force was reduced from 1602k to 1461k, the friction forces were reversed along the tendon over a certain distance in the curvature region. Because of this effect and possibly measurement inaccuracies, the apparent stiffnes is slightly different (109.2k/in.) from that due to monotonical stressing of the tendon (112.2k/in.).

At the first surveillance, V68 was selected to be detensioned and then retensioned after a test wire was removed. The detensioning of the tendon also caused the friction forces to be redistributed and the apparent stiffness of the tendon increased.

Considering all straight vertical tendons that were involved in the first three surveillances (13 total), and based on the elongations that were measured at origi.wl stressing, an average effective Young's modulus, E, of 29.6 x 106 psi ,

was calculated. This value compares with 29.0 x 106 psi used for E in the prestress increase calculations. Consequently, the measured elongations substantiate that the value of E used to determine the increase in prestressing force is conservattve.

The above effects which cause differences in the apparent tendon stiffness are associated with end stressing of the tendon. These effects are not nearly as significant in the determination of increase in tendon force due to expansion of the containment for the following reason. Even though tendons are unbonded, there is a high degree of strain compatibility between the tendons l and concrete. This condition does not exist when the tendon is stressed by j applying a force at each end. For example, the radial expansion of the containment for the accident pressure load case produces essentially a uniform pressure load on the h7op tendons. There is compatibility in the radial displacements of the containment wall and hoop tend'ons. For all practical purposes, the strains in the tendon are uniform over most of its length and are equal to the hoop strain in the concrete, which is essentially uniform over the containment circumference, i

2-

i For the dome, the prestress compressive stress levels in the concrete vary depending on the particular tendon. This was taken into account iri the determination of the increase in dome prestress (refer to Calculations 1.18.2.3 previously submitted).

QUESTION 1h:

t The computation of elastic shortening, shrinkage, creep and relaxation is at best an approximation. Any variation may cancel out the gain in tendon elongation, also the increase in tendon force may result in increase in creep.

RESPONSE TO QUESTION 1b:

Calculations of elastic shortening, shrinkage, creep, and stress relaxation are l approximations based on theory and test results. Variation in tendon force losses due to these parameters can result in lower average forces in tendon groups than predicted by calculations. However, to establish the prestress levelin the containment, these calculations are not relied upon. Rather l tendon surveillances are performed to measure the tendon forces on a sample basis. Lift off measurements of tendon for:e provide assurance that the 1 required minimum prestress levels are available and also establish whether force losses are occurring in a predictable manner, thus assuring that required minimum prestress levels will be available in the period between surveillances.

, Tha first three surveillances for the V. C. Summer containment indicate that i losses greater than predicted have occurred. However, the straight line semi-log paper plot of average measured force for each tendon group versus time

, for the first three surveillances indicates force losses are occurring in a 1 predictable manner. These measured forces eliminate the approximations in the original calculations to predict losses since actual measured losses from three surveillances are available.

Predicted creep losses will not be influenced by the increased prestress force in a

the shell which occurs for ir.ternal presure loads. Creep losses occur slowly over time - months and years. The postulated accident pressure loads occur for a period of hours - at most a few days. Any increase in creep will be j

4 3-

s insignificantly small during the relatively short duration accident pressure loading.

QUESTION 1c:

For hoop tencons, the buttresses are under compression which will reduce the tendon elongation.

RESPONSE TO QUESTION ic:

The'er'fect on tendon force of buttress shortening due to compression resulting from hoc,p tendon prestress force under service conditions is ace.ounted for bytaking actuallift off measurements of tepdon force during surveillances. Any elastic shortening loss is reflected in thq measured tendon force.

, , 'i The effect of elastic shortening across the face of the buttress due to the increase in tendon force under accident pressura is negligible. The STRUDL analysis in calculation 1:18.2.1 submitted previously in response to NRC questions indicates a net shortening between the buttress faces of 0.0578 ,

inches for a loading corresponding to 0.7 GUTS force (1402 kips)in the hoop i tencons. ,

T The increase in average hoop tendon force occurring under the 1.5 times ,

accident pressure loading is 78 kips. Assuming, conservatively, the full 78 kips l increase is reflected at the anchor heads, the elastic shortening of the buttrest l

due to 78 kips can be calculated by direct ratio of the STRUDL analysis results for 1402 kips force.

1 Elastic Shortening = 78 kir.s/1402 kips x 0.0578 inches = 0.0032 inches The tendon force corresonding to the 0.0032 inch shortening is:  ;

Change in tendon force due to elastic shortening across buttress u AAE/L = (0.0032" x 8.345 sq. in. x 29 x 103) + (12"/ft. x 269 ft.) =

0.24 kips or 240 pounds Therefore, the effect of the elastic shortening across the fece of the buttress

<' or the additional 78 kip force per hoop tendon corresponds to less than 1 kip.

l 4

4 QUESTION 1d:

Depending on the ambient temperature at the time of construction, the tendon force may increase or decrease resulting in the variation of tendon forces.

RESPONSE TO QUESTION 1d:

It is agreed that the temperature of the tendon at any point in time can be different from that which occurred at originalinstallation and stressing. This is expected to be the case during normal operating condition with the containment at an elevated steady state operating temperature. In this case, the tendon temperature is higher than that at original stressing. 'f the contair, ment were not similarly affected by the elevated temperature, the tendon would experience an elastic thermal force decrease. However,it is generally accepted that the average temperature across the containment wall and dome is approximately equal to the tendon temperature. Because of this and due to the fact that the tendons and concrete have similar linear coefficients of thermal expansion (6.5 versus 5.5 micro inches per inch per

• F.),

the thermal growth of the containment concrete and toe tendon are practically the same, and there is no significant decrease in tendon force due to the elevated steady state operating temperature condition within the containment. For example, based on a 90' F. vertical tendon temperature, a 5 kip decrease in tendon force could result,which represents 0.4% of the minir .sm required force in the vertical tendons.

QUESTION 1

SUMMARY

On the basis of the above considerations,it is felt that the margin of safety resulting from tendon elongation should not be used to reduce minimum required average tendon force.

RESPONSE TO QUESTION 1

SUMMARY

'EGARDING MARGIN OF SAFETY Accounting for increased prestress levels under postulated accident pressure conditions to the extent proposed still provdes significant design margins as indicated in the following discussion.

t

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From one major standpoint, consideration of the tendons in the design of the V. C.

Summer containment is conservative by the cuirent standards reflected in the ASME B&PV Concrete Containment Code. In the current Code, under the factored load combinations, tendons are allowed tqbe considered acting together with the reinforcement for load resisting purr 7ses. This is reflected in Article CC-3423 where the axial tensile capacity of the tenddn is limited to 0.90 fpy,in which fpy is the yield strength of the tendon, in eff'ect, the tendons may be considered a~s non-bonded reinforcement. Consequently, for each factored load combination, after the' effective piestress and dead load comprosion stresses are overcome, the tendons together with the reinforcement arrpensidered to resist the net tensile membrane force and accompanying moment. For V. C. Summer, thece code provisions were not used; tendon forces beyond the effective prestress compressive stresses that they produce in the containment are not utilized. Significant reinforcement is provided to resist the net internal forces and moments.

For vertical tendons, for example, under the current Code the tendons at 0.9 fpy have a permissible tensile capacity of 1442k based on the minimum soecified yield strength of the tendon wire material. This is significantly greater (by 282k) than the minimum required average tendon force of 1160k that is proposed. The 282k difference is equivalent to over 5 in.2 of Grade 60 reinforcement. This advantage was not utilized for the V. C. Summer containment design to determine the required reinforcement. Therefore, again using the vertical tendons as an example, accounting for tendon elongations and the accompanying 35k increase in tendon force beyond its effective prestress level corresponding to 1160k tendon force does not significantly impact the margin provided under the current Code where a 282k increase in tendon force would be allowed.

QUESTION 2:

In your January 17,1986 submittal,it was indicated that stress relaxtion tests were underway at Fritz Laboratories . Provide the details and results of the tests.

i C

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l 6-

RESPONSE TO QUESTION 2:

Wire stress relaxation is not related to the determination of the minimum required prestress level. Wire stress relaxation is used as part of the input data to determine predicted forces in the individual tendons versus time.

As part of the investigation to determine why greater than anticipated losses were occurring in the tendon, wire stress relaxation tests were performed on tendon wire samples.

The testing is complete. The results indicate that the 81/2 percent relaxation used in the original calculations to predict losses is low. Test results indicate between 12 and 13 percent relaxation losser,over 40 years.

Testing was performed on wire samples in 58' F.,90' F., and 104' F. j environments. The 90* F. tests are representative of the tendon enironment in the containment. Force loss measurements were recorded at various times out to about 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. The results were plotted on log-log sheets and projected to the 40 year end of plant life.

Four saotples at 68* F. projected to an averau celaxation of 9.6% at 40 years.

Three samples at 90 F. projected to an average of 12.8% at 40 years.

Two samples at 104* F. projected to an aerage of 17% at 40 years.

As a check of the 90' F. results, linear interpolation of the 68* F. and 104* F.

results to 90* F. indicates 14% relaxation at 90' F. compared to the average of 12.8% for the 90' F. tests.  ;

1 QUESTION 3:

Provide hand calculations which approximate the internal pressure values that together with the applied prestress in forces ~will result in zero hoop and axial membrane stresses.

7

RESPONSE TO QUESTION 3:

The following approximate calculations indicate the value ofinternal pressure that combined with the effective design prestress will result in zero hoop and axial membrane stresses.

AXIAL Design Effective Prestress = 335 k/ft.

Internal Pressure: No = PR/2 = 335 k/ft.

P = 2 x 335k/ft./R = 2 x 335k/ftJ63 ft. = 10.63 ksf = 73.8 psi P = 73.8 psi n 1.29 Pa, Pa = 57 psig (DB A)

HOOP Design Effective Prestress = 750 k/ft.

Internal Pressure: NO = PR P = 750k/ft./63 ft. = 11.90 ksf = 82.6 psi P = 82.6 psi -

' 45 Pa, Fo = 57 psig (DBA)

The above calculations are approximate in the sense that biaxial effects and dead load compressive stresses are not included.

These calculations demonstrate that the summer containment is designed as a "partially prestressed" structure. That is, the prestress level is not intended to maintain compression at zero membrane stress state under the controlling factored load combinations. For these load combinations, the resulting internal net tensile forces are resisted by significant amounts of reinforcement in the containment. In the hoop direction of the wall, #14 @ 12" spacing (outside face) and #11 @ 12" spacing (inside face) are generally provided. In the vertical direction of the wall, over the lower half, #18 @ 12" spacing (outside face) and #14 @ 12" spacing (inside face) are generally provided. In the upper half of the wall, #14 @ 12" spacing (outside face) and #14 @ 12" spacing (inside face) are generally provided. In the dome, the meridional and hoop reinforcement varies and #11 and #14 bars are provided.

8-