ML20063L114

Forwards Evaluation of Reactor Containment Capacity in Response to Allegations Re Cad Weld Reliability.Vertical Tendons Have Sufficient Capacity to Resist Net Membrane Tensile Forces in Containment Wall

ML20063L114
Person / Time
Site:	Summer
Issue date:	09/03/1982
From:	Dixon O SOUTH CAROLINA ELECTRIC & GAS CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
NUDOCS 8209080637
Download: ML20063L114 (7)

Text

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SOUTH CAROLINA ELECTRIC & GAS COMPANY l

Post orFicE 7m i

COLuuslA. SOUTH CAROUNA 29218 i

-[

O. W. DixON, JR.

uve s1.""o'[.'nIr"[ous September 3, 1982 l

t t

.i Mr. Harold R..Denton, Director p

Office of Nuclear Reactor Regulation j

U.S. Nuclear Regulatory Commission'

[

Washington, D.C.l20555 t

Subject:

. Virgil.C. Summer Nuclear Station j

Docket No. 50/395

(

Operating License No. NPF-12 Cadweld Allegation l

Dear Mr. Denton:

i EIn response to NRC staff questions, South Carolina Electric i

and Gas Company (SCEEG) hereby provides a summary report

(

i concerning the structural significance of the vertical l

cadwelds in the reinforcing bar within the containment at the i

Virgil C. Summer Nuclear Station.

Because this is a matter i

before the V.

C. Summer Nuclear Station Atomic Safety and t

' Licensing Board, copies of this document will also be sent to the service list in that proceeding.

s If you have any quetions, please let us know.

I i

Very truly yours, t

t i

{

/

O. W.

ixon, Jr.

[

NEC: OWL'/f jc cc:

V.

C.

Summer M. N.

Browne G. H. Fischer G. J. Braddick H.

N. Cyrus J. L.

Skolds

[

T.

C. Nichols, Jr.

J. Braddick O. W. Dixon, Jr.

J. Fulton

['

M. B. Whitaker, Jr.

D. A. Nauman J.

P. O'Reilly Herbert Grossman i

H. T. Babb Frank F. Hooper i

D. A.

Nauman Gustave A. Linenberger C.

L. Ligon (NSRC)

Richard P. Wilson W. A.-Williams, Jr.

Steven C. Goldberg

l R

R. B. Clary NPCF O. S. Bradham File t

F A.

R. Koon l

[

.B. A. Bursey j

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O p

p.

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- EVALUATION OF REACTOR CONTAINMENT

^

CAPACITY RELATED TO ALLEGATIONS

- ON CADWELD RELIABILITY 21.0

' INTRODUCTION-Th's report describes the results of an evaluation of the ability of i

the V. C. Summer Reactor Building containment structure to resist the l

load combinations which control the design of the containment. The capacity of a typical containment usually-depends on the strength 2 properties and quantity of the prestressing. tendons and reinforcement 3

within the containment. The evaluation described herein for the-V..C.

Summer containment determines the effect on the capacity of the containment of neglecting the capacity of the vertical reinforcement, t

,This condition is evaluated as a worst case assumption which results t

from allegations that some of'the Cadwelds~used to splice the vertical reinforcement may not be reliable.

s The evaluation does not. determine the acceptance of the containment relative to the: acceptance criteria specified in either the ASME Contain-ment Code or the FSAR. This evaluation is performed as a realistic determination of the capability of the. containment to resist the con-trolling design loads,and it-is similar to the ultimate. pressure capa-city evaluations currently being performed.in the industry for the Hydrogen Detonation condition.

i 2.0 ORIGINAL DESIGN CRITERIA The original design criteria for the Reactor Building containment was established in 1972/1973, prior to availability of the ASME Section III, Division 2 Code (Containment Code). As a. result, the structure was i.

designed such that when tension, stresses-occur in the concrete due to membrane forces they are resisted by conventional reinforcement.

Pro-visions of the Containment Code. allow taking into account the additional-capacity of the prestressing tendons up to 90% of their yield strength.

~

3.0-CURRENT EVALUATION 3.1 TENDON CAPACITY The capacity of the vertical prestressing tendon system corresponding, to its yield strength was accounted for in the current evaluation. The f

minimum ~ tested yield strength (yield at 1% strain) of the actual pro-duction tendon wir'e supplied for V. C. Summer was used. The minimum tested yield strength corresponds to 90% of the guaranteed Ultimate-Tensile Strength (GUTS) of 240 ksi.

Load Combinations EThe load combinations which control the design of the containment are the' following:

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Page 2 aof. 6'

,- gj-Abnormal D + F'+ To + 1.5TPo+Ta

~

Abr.ormal/ Severe' Environmental D'+ F + To + 1.25 P '+ T + 1.25E a

a Abnormal / Extreme Environmental D.+ F + To + Pa + E' + Ta where:

]

D = Dead Load F = Prestress Force (0.9 GUTS) i' j

Pa = Design Accident Pressure (57 psi)

)

E -= Operating Basis Earthquake (OBE) (0.lg, 2% Damping)

E' = Design Basis Earthquake (DBE) (0.15g, 2% Damping)

To = Operating Temperature (Taken as Zero)

Ta = Accident Temperature (Taken as Zero)

Secondary stresses due to discontinuities and thermal effects are not considered when evaluating the capacity of the containment. The secon-dary stresses have little or no influence on overall containment capacity. The operating and' accident temperature loads are not in-cluded in the evaluation because the thermal forces due to these loads are self-equilibriating over the cross section of the contain-ment wall ~and, therefore, do not affect the capacity of the containment.

Results i

l The vertical membrane forces that are produced by'the load combinations-identified above were obtained at two elevations in the containment wall.

The locations selected-are El. 420 ft. and El. 480 ft. which bound the region in which the Cadwelds are proported to be suspect. The net-tensile forces predicted to occur are indicated in Table 1 along with the yield capacity of the vertical tendons. The capacity of the i

vertical reinforcement is neglected.

l The net tensile forces shown in the table include the direct membrane forces produced by Dead Load and Design Accident Pressure. For the OBE and-DBE, the forces include the direct membrane force due to overturning moment and also the indirect tangential shear forces. For purposes of comparison with tendon capacity the tangential shear. forces are con-'

sidered the same as a direct membrane force because the tendon is assumed to resist the shear through a shear friction mechanism with a friction-coefficient of 1.0.

The results in Table 1 indicate that the tendons alone are capable.of providing sufficient membrane tensile capacity to resist the controlling

' loading conditions, and vertical reinforcement is not required.

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g 3.2 REINFORCEMENT REQUIREMENTS FOR TANGENTIAL SHEAR The results of the capacity evaluation in Table 1 indicate that the tendons alone have sufficient capacity to resist the combined membrane In this and tangential shear forces occurring in the containment wall.

evaluation the tendons are assumed to be ef fective in resisting the tangential shear through a shear friction mechanism with a friction coefficient of 1.0.

in effect, Shear friction as a method of resisting tangential shear was, specified as part of the design criteria for the V. C. Summer containment and was consistent with industry practice at the time [1].

However, in this criteria, reinforcement rather than tendons is required to provide the tan-the " clamping action" which permits shear friction to resist gential shear forces. Therefore, consistent with this criteria, a capacity evaluation was performed in which it is assumed that the tendons are effective in resisting only the membrane tension forces and that only the reinforcement is effective in resisting the tangen-tial shear forces. The evaluation was performed at all typical Cad-weld splice elevations between El. 420 ft. and El. 480 ft.

of the containment wall. The results are indicated in Table 2 only for the bounding elevations; however, it was determined that the results at El. 480 ft. control the reinforcement evaluation.

Membrane Tension As the results in Table 2 indicate, the tendon capacity exceeds the required membrane tension capacity for all load combinations.

Tangential Shear The largest tangential shear forces occur for the Abnormal / Extreme Environmental condition. The values are indicated in the table as 113 kips /ft. at El. 420' and 102 kips /ft. at El. 480'.

The area of vertical reinforcement generally provided near El. 420' is 6.25 in.2/ft.

(El. 423-4') and near El. 480' is 3.81 in.2/ft. (El. 482').

Based on the shear values of 113 kips /ft. and 102 kips /ft. and a permissible reinforcement stress of 54 ksi, the reinforcement area required is 2.09 in.2/ft. at El. 420' and 1.89 in.2/ft. at El. 480'.

The current Containment Code and the Building Code for Reinforced Con-crete, ACI 318-77, require that mechanical connections such as Cadwelds be used to splice the #14 and #18 reinforcement in the containment.

These requirements are intended to apply to all structural members, most of which are subjected to membrane and/o'r flexure forces. An example of this condition is the vertical membrane tension forces in the contain-ment wall and the vertical reinforcement. However, as discussed above,

[1] " Criteria for Reinforced Concrete Nuclear Power Containment Structures",

Reported by ACI Committee 349, Title No. 69-2, ACI Journal, January, 1972.

Pag: 4 of 6 n.

.']

thise tension forces are able to be resisted by the vertical tendons.

Therefore, the vertical reinforcement only has to be developed suffi-ciently to resist the tangential shear forces.

In a realistic assessment of this condition the reinforcement should be able to resist the tangential shear forces even if the Cadwelds are assumed to be ineffective. The reinforcement has a development (embedment) capacity c

due to the stagger of the Cadwelds and resulting overlap of the bars.

In this mechanism the tangential shear forces produce tension forces in the vertical reinforcement.

These tension forces are limited by the develop-ment capacity of the reinforcement above and below the shear plane.

Development strength can be achieved due to the physical confinement of the vertical reinforcement. The concrete clear cover on the vertical bars is 5 inches. Additional confinement is provided by the horizontal reinforcement on the outside face of the wall and by the liner on the inside face.

The minimum developed vertical reinforcement was determined to be 4.50 in.2/ft. for El. 420' and 2.45 in.2/ft. for El. 480'.

These values are noted as VERTICAL REINFORCEMENT AVAILABLE in Table 2.

As indi-cated in the table, the available reinforcement exceeds that required to resist tangential shear.

Another mechanism of resisting tangential shear acts in conjunction with shear friction, but it is conservative to consider the two independently.

This mechanism is the direct shear action (dowel action) of the rein-forcement crossing the shear plane.

Here the shear forces are equili-briated primarily by shear stresses in the bars, and the tension forces in the bars are considerably less than that calculated using the shear friction approach. Consequently, the requirement for tension resis-tance of the reinforcement, in the form of Cadwelds or development capacity, is much less than that calculated above. The results of testing performed over the years have confirmed that dowel action is a significant mechanism by which shear forces are transferred; however, the codes for reinforced concrete construction have not yet reflected this in their design provisions.

4.0 CONCLUSION

The vertical tendons have sufficient capacity to resist the net membrane tensile forces in the containment wall as well as the tangential shear forces without the need for vertical reinforcement.

However, a conservative assumptien can be made that the tendons do not have any effectiveness in resisting the tangential shear.

If this assumption is made, then vertical reinforcement is required to resist the tangential shear forces.

In a realistic evaluation of this condition, it is calculated that the tangential shear forces can be resisted by the vertical rein-forcement without the need for Cadwelds. The method for this resistance is the development capacity of the vertical bars which exists because the embedment lengths of these bars are generally well staggered and confined within the containment. wall. Dowel action is another mechanism available for the vertical reinforcement to resist the tangential shear forces without the need for Cadwelds.

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REQUIRED CAPACITY TENDON CAPACITY TENDON NET TENSILE FORCE CAPACITY EXCEEDS LOAD ELEV.

FOR LOAD COMBIN.*

F = (0.9 GUTS)

REQUIRED CAPACITY COMBINATION STN #

(FT)

-(KIPS /FT)

(KIPS /FT)

YES/NO,,

5 420 281 504 Yes s

Abnormal

[0]

D + 1.5 Pa 4

9 480 317 504 Yes

[0]

0 m

Yes Abnormal /

]

Severe Environmental D + 1.25 Pa + 1.25 E 9

4R0 441 504 Yes

[85]

5 420 492 504 Yes Abnormal /

[113]

Extreme' Environmental D + Pa + E' 9

480 413 504 Yes

[102]

ST N.# 5 f

$ L,40$'

(

[EL.41?_'

• Membrane + Tangential Shear 1

i

[] = Tangential Shear TABLE 1 CAPACITY COMPARISON IN VERTICAL DIRECTION OF WALL -

w Q UIRED VERSUS PROVIDED BY TENDONS

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,---9 m

---

,1-

• J MEMBRANE TENSION TANGENTIAL SHEAR (T.S.)

M TENDON VERTICAL VERTICAL

• 3EINFORCEMENT CAPACITY REINFORCEMENT REINFORCEMENT AVAILABLE REQUIRED TENDON EXCEEDS CALC.

REQUIRED AVAILABLE EXCEEDS LOAD ELEV.

CAPACITY CAPACITY REQUIRED SHEAR FOR T.S.

FOR T.S.

REQUIRED 2

2 COMBINATION STN #

(FT)

(KIPS /FT)

(KIPS /FT)

YES/NO (KIPS /FT)

~

(IN /FT)

(IN /FT)

YES/NO 5

420 281 504 Yes 0

0 4.50 Yes Abnornal D + 1.5 Pa l

9 480 317 504 Yes i

0 0

2.45 Yes 1

5 420 407 504 Yes 94 1.74 4.50 Yes Abnormal /

Severe Environmental l

9-480 356 504 Ye=

1 85 1.57 2.45 Yes D + 1.25 Pa + 1.25 E j

+

Abnormal /-

5 420 379.

504 Yes 113 2.09 4.50 Yes Extreme Environmental 9

480 311 504 Yes 102 1.89 2.45 Yes D + Pa + E, j'

t

• Vertical reinforcement available for tangential shear is. equal to the area of vertical reinforcement provided and reduced by the ratio of the Embedment Length (E.L.) to the Development Length (D.L.) for bars with E.L. less than 1

D.L.

All Cadwelds are assumed to be ineffective.

o TABLE 2

$m CAPACITY COMPARISON IN VERTICAL DIRECTION OF WALL -

VERTICAL REINFORCEMENT AVAILABLE FOR TANGENTIAL SHEAR i

_