ML19290C645

Updates & Confirms Conversations Re Unit 2 Excavation Authorized Under Amend 2 of Lwa.Moss Fill of Lean Concrete Will Be Required in Order to Comply W/Design Base Foundation Elevation.Analyses Will Be Submitted Upon Completion

ML19290C645
Person / Time
Site:	Black Fox
Issue date:	10/19/1979
From:	Ewing T PUBLIC SERVICE CO. OF OKLAHOMA
To:	Varga S Office of Nuclear Reactor Regulation
Shared Package
ML19262B963	List: ML19262B962 ML19290C640 ML19290C645
References
NUDOCS 8001220308
Download: ML19290C645 (7)

Text

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6212 DINS-018-4E4 PUBLIC SERVICE COMPANY OF OKLAHOMA '

A CENTRAL AND SOUTH WEST COMPANY > "f N f

l L s P O BOX 201/ TULSA OKLAHOMA 74102 > 1918. 5E3-3E11 Public Service Company of Oklahoma October 19, 1979 Black Fox Station Units 1 and 2 File 6212.125.3500.21L Excavation Seal Unit 2 U.S. URC Docket No. STN 50-556, 50-557 Mr. Steven A. Varga, Assistant Director Division of Project Management Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Washington, D. C. 20002

Dear Mr. Varga:

This letter updates and confirms several conversations concerning the excavation of Unit 2 which took place between representatives of PS0 and the technical staff of NRR and I&E Region IV during the latter part of July and mid-August of this year. During these discussions, we reported the preliminary results of the field testing activities which were completed in the foundation area for Unit 2 structures and the recomm'.indations of our Architect-Engineer regarding the nature of the subgrade meterial found in the Unit 2 excavation area.

Excavation for Unit 2 was authorized under Amendment 2 of the Limited Work Authorization on November 30, 1978. The init W work commenced in December, 197E, resumed in April, 1979 after the unusually reve..e winter forced shutdown and At that time, field tests reached the base foundation elevation in mid-June.

showed that a soft, siltstone layer underlaid a large portion of the excavation surface at depths ranging from two to seven feet. Our Architect-Engineer recomrended that the depth of the excavation in the Unit 2 foundation areas be extended to remove the soft siltstone layer. Subsection 2.5.4 of the BFS Pre-liminary Safety Analysis Report predicted such overexcavation below the base foundation elevation and stated that these areas would be backfilled with lean concrete.

As discussed previously with Dr. Cecil 0. Thomas, Licensing Project Manager of NRR and William A. Crossman, Chief, Projects Section, I&E Region IV, the soft, siltstone layer material has now been removed from the excavation area, and a nominal six-inch concrete seal coat has been applied to protect the underlying rock structure from weathering.

In order to bring the entire excavation up to design base foundation elevation, a mass fill of lean concrete will be required. The Unit 2 structures (i.e. , the 1785 004

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U. S. Nuclear Regulatory Commission October 19, 1979 Mr. Steven Varga Page Two fuel building, the reactor building, and auxiliary building) would therefore be supported on fill concrete founded directly on the same competent siltstone as Unit.l. However, in accordance with our oral commitment to Mr. Thomas, we intendThese to provide the NRC Staff with certain analyses prior to taking this action.

analys.es involve an assessment of the strength of the concrete as a foundation material as compared with the natural rock structure and the effect of this relative thick concrete fill on the dynamic responses of the safety-related structures.

During cur previous discussions, we expected to supply these analyses within a 30 to 60 day time period for Staff review. Unfortunately due to manpower con-straints associated with reviewing the Staff's TMI requirements in preparation for our upcoming hearing, we have fallen behind schedule in providing you with this information.

We will advise you further when the engineering effort on the analyses work is completed.

Very truly yours, T. N. Ewing, Ma _ r Black Fox Station Nuclear Project TNE:VLC:jk cc: BFS Service List e

A 1785 005

BFS 3.8.5 Foundations and Concretc y wrts The foundation mats will be founded on competent subsurface materials which will provide adequate support at appropriate bearing level. As presented in Subsection 2.5.4, unsuitable material is re=oved and backfilled with a lean concrete mix of 2,000 psi minimu= specified compressive strength

~~

at 28 days. (Refer to Figure 2.5-50 for excavation detail.)

Since an inordinate amount of unsuitable material had to be re=oved, additional studies were perfor=ed to assess the strength of the lean con-crete as a foundation material as compared with a natural rock structure and the effect of this relatively thick concrete fill on the dyna =ic responses of the safety-related structures. As established by the NRC staff in Table 3.7.2-1 of the Stan brd Review Plan 3.7.2, the soil-structure interaction effect is negligible for a supporting medium with a minimum shaar wave velocity of 3,500 fps. The competent Savannah for=ation under the expected lean concrete backfill has a minimu= shear wave velocity of 3.500 fps (PSAR Subsection 2.5.2.4). The lean concrete backfill provides a 16 shear wave velocity which can be computed as follows:

E = 57,000 = 57,000 2,000 - 2.549 x 106 p,i conc. = 2.246 - 10 lb sec /in

" cone. = 0.17 s = 6.964 x 10' in/sec = 5,800 fps 2(1 Because the shear wave velocity computed above is greater than 3,500 fps, no soil-structure interaction vill result due to seis=ic events.

The computed value of the shear wave velocity will be verified by actual tests in the field to ensure that it is not lo~er than 3,500 fps.

To investigate the effect of the mass fill of lean concrete on the structural behavior under other dynamic loads, the cafety/ relief valve (S/RV) discharge loads were chosen as representative of the dynamic loads which act, on the Reactor Building structures. The other dyna =1c loads, besidas seismic, are those associated with the loss-of-coolant accident (LOCA). It was decided to select the SRV loads because they produce greater structural responses and soil-structure interaction effects.

3.8-55 785 006 lo-

BFS The S/RV load case selected for the study was the all (19) valve case, random actuation. This case produces the largest structural response in both the vertical and the horizontal (rocking) directions. For this study, an axisymmetric finite element model of the Reactor Building and support soil was analyzed. See Figure 3.8-49 for a plot of the model used. The

• analysis was performed using program S73 (Ghosh-Wilson program ASESD2 described in Appendix 3A). The dynamic load is input in the fcrm of a Fourier Series to describe the load variation around the circumference.

Based on a large nu=ber of computer analyses of the S/RV loads, it was evident that the structural response, including particularly the rocking motion, was dominated by the first Fourier har=enic, i.e., the ter= Ay cos 0, where Ay represents the magnitude of the load and cos e represents the circumferential distribution of the load. Since the first har=enic represented the dominant structural response, it was not necessary to extend the analyses to include the full range of the Fourier harmonics.

Three computer analyses were performed to investigate the effects of 16 fill concrete on structural response. The first ana.'7 sis assumed nor=al bedrock with a mini =um fill of lean concrete, the second analysis assumed the mass fill concrete of 9.25 feet thick under the mat, expected for Black Fox, and the third analysis had a layer 29.25 feet thick. The natural soil properties in the model, as represented by the shear wave velocity V,, for all runs were as follows:

Elevation 539 to 510 V, = 3,500 fps Elevation 510 to 480 V,= 4,000 fps Below Elevation 480 V,= 4,400 fps The calculaticas were performed replacing the layer of soil im=ediately under the Reactor Building foundation with lean concrete (V, = 5,800 fps) 150 feet in diameter and 9.25 feet and 29.25 feet thick. All deviations in the resulting structural responses lie within 5 per cent of the response compared with only natural material, i.e., minimu= fill concrete under the building. Since this is within the calculational range of accuracy, it is concluded that the use of lean concrete backfill results in an insignificant effect on the structural response.

3.8-55a 1785 007 16-

EFS To simplify computer modeling, the shear wave velocity of the natural material, i.e. , 3,500 fps, vill be used for any future dynamic analyses of the 16 Raactor Building because of the similarity in structural response using fill concrete or natural material.

3.8.5.1 Reactor Building Foundation

,J.8.5.1.1 Description of the Foundation. The reactor building foundation is a circular reinforced concrete mat 138 feet in diameter and H feet lB thick. It supports the shield beilding, the steel containment vessel, the RPV pedestal, the dryvell, the weirvall and other internal structures. The 3

mat is structurally independent of other foundations.

16 Within the perimeter of the containment structure the foundation mat 0 is lined with steel plate. The liner is a part of the containment syste=, @'

refer to 3.8.2.1. The embedded skirts of the dryvell and the RPV pedestal b pass through the liner plate, as indicated on Figures 3.8-4a and 3.8-4b.

However, continuity of the steel membrane is maintained.

7 The liner is covered by about 10 feet of concrete in the area lB between the RPV pedestal and the weir vall, and 3 feet of fill inside the pedestal. There is no fill on the liner in the suppression pool.

3.8.5.1.2 Applicable Codes, Standards and Specifications. Sa=e as 3.8.3.1.2,

) except that Reference (3), ASME code Subsection NE, acclies to the liner plate suppression pool floor. B 3.8.5.1.3 1.oads and I,oading combinations. Same as 3.8.3.1.3, except tha+

reference to 500 cycles of temperature variation under 3.8.3.1.3.2 is omitted.

In addition to the load combinations referenced above, the combina- 3 tions utilized as minimum design criteria against sliding and overturning n due to earthqusher. vinds, and tornadoes, and against flotation due to g floods, are listed below. 5

a. D + H + Feqc

b. D+B+W

c. D+H >2 Fegs .

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