ML20215C588
| ML20215C588 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 06/04/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20215C509 | List: |
| References | |
| NUDOCS 8706180177 | |
| Download: ML20215C588 (8) | |
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- v y ,I WASHINGTON, D. C. 20555 i R.....l SAFETY. EVALUATION BY THE- 0FFICE OF NUCLEAR REACTOR REGULATION
. RELATING TO SEISMIC BEARING CAPACITY OF THE i 1
- REACTOR SUPPORT STRUCTURE AND VAPOR CONTAINER FOUNDATIONS YANKEE ATOMIC ELECTRIC COMPANY YANKEE NUCLEAR POWER STATION DOCKET NO.50-029 1.0' INTRODUCTION The Yankee Nuclear Power Station at Rowe, Massachusetts was not originally designed for seismic loads. As a result of NRC's Systematic Evaluation Program, the licensee, Yankee Atomic Electric Company (YAEC),; has perfonned seismic analyses and is upgrading.the plant to sustain earthquake effects.
These analyses indicated that the foundations of the. Reactor Support Structure
- (RSS).and the vapor container structure (VCS) will apply transient loads to the supporting soil .that are more than double the long-term loads anticipated in j
the original plant design. The capability of the supporting soils to resist
, the increased foundation loads is the focus of this safety evaluation.
I 2.0 DISCUSSION
.The Yankee Atomic Electric Company investigated the bearing capacity of the soils beneath the foundations by three methods and concluded that they could safely support a load of at least 20 KSF (kips per square foot). The basis for this value is presented in Reference 1. By the first " empirical" method, a standard penetration test N value of 100 wa assumed for the supporting soil and resulted in sufficient bearing capacity, From " pub'ished tables," a dense glacial till material is expected to have adequate bearing capacity. Thirdly, using a " theoretical method" and an assumed angle of internal friction (9) value of 40 degrees for the soil, an adequate capacity value is obtained.
Initial staff review of Reference 1 did not lead to a confirmation of the
. assumptions and methodology utilized to conclude that the foundation supports were adequate for earthquake effects. Questions were sent to the licensee on June 6, 1986 (Reference 2). The licensee provided the information in Reference 3 through 6; the staff then performed an independent assessment using the methodology suggested in Reference 7.
The licensee had performed analyses of the performance of the VCS under various loading conditions in Reference 8. For the loss-of-coolant accident (LOCA), the staff's independent assessment confirmed that the bearing capacity beneath the critical footing was about 20 KSF, while the expected soil loading was about 10.4 KSF. Therefore, the foundations would perform as expected during a LOCA event.
8706180177 B70604 PDR ADOCK 05000029 P PDR
2 However, for the seismic event, the staff assessment concluded that footing uplift would begin due to the overturning moment. In response to this concern, the licensee performed a reanalysis of the VCS (Reference 9) which is evaluated below.
3.0 REACTOR SUPPORT STRUCTURE 3.1 Foundation Description The RSS is supported by six concrete columns attached symmetrically to a concrete ring foundation and by two symmetric concrete columns resting on a concrete mat foundation interior to the ring. The ring has an inner radius of 30.25 ft.; it is 13 ft. wide and 7 ft. deep. The base of the rinc h 10.25 f t, below the ground surf ace. The mat is 43 ft. square, 8.5 ft. osep and its base is also 10.25 ft. below the ground surface. The foundations were sized for a static soil bearing pressure of about 8.0 KSF.
3.2 Loads on Rino and Mat Foundations The transient loads imposed on the ring foundation and the mat foundation during the earthquake (NRC site-specific spectra at 0.199) are given in Table F-2 of Reference 4. The resultant load (EV upward) on the mat is about 13,400K has an eccentricity of 4.0 ft., and acts at an angle of 11 decrees from the vertical. The resultant load on the ring is about 24,000K, has an eccentricity of 23.6 ft. and acts at an angle of 11 degrees from the vertical.
The peak soil pressure under the edge of the 43 ft, wide mat is about 15.1 KSF and the peak soil pressure under the outer edge of the 13 ft, wide' ting is about 19.3 KSF. Negligible transient soil pressures exist on the opposite edges of the mat and ring.
3.3 Foundation Stability The resultant transient load on the mat or ring foundation will not cause slippage between the bottom of the foundation and the supporting soil because the inclination of the resultant force is only 11 degrees. Other combinations of the vertical earthquake load (downward rather than upward) would not change this conclusion. The resultant transient load will not cause overturning of the mat or ring foundation because the eccentricity values are tolerable compared to the foundation dimensions and the soil is not expected to fail in i bea rinc. .
l 3.4 Properties of Soil Foundation Information concerning the soil characteristics beneath the RSS foundations is l
I available in Appendix A of Reference 3. Logs of borings S-1, S-3, and S-5 l between 1011.25 ft. and about 995 ft. provide field datr most pertinent to 4 bearing capacity of the critical ring foundation. (The. ring foundation would have the lowest bearing capacity because it is only 13 ft. wide).
The borings logs show that the RSS foundation is supported by a 3 or 4 ft, thick )
layer of well-graded brown sand, medium compact to compact, which overlies the :
lodgement till composed primarily of well-graded, dense (compact) )
olive-colored silty sand. The standard penetration test N values range from 1 15 to 62 in the upper brown sand. N values recorded for the till are high,
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n 3
probably due to the presence of gravel and cobbles in the till. One sample drive, SS-3 in boring S-1 appears to be uninfluenced by gravel and cobbles; the N value for this sample was 88.
Soil strength parameters can be estimated from Figure 7 (page 7.1-149) of
- Reference 7. From this figure, the upper brown sands (SW) are likely to have an angle of internal friction greater than 35 degrees; the lower silty sands (SM) are also likely to have an angle of internal friction greater than 35 degrees.
3.5 Bearing Capacity of Ring Foundation The ultimate bearing capacity (failure load) for the critical ring foundation can be estimated using Figure 3a on page 7.2-133 of Reference 7. Linear interpolation of Figure 3a for the depth of the foundation, the angle of internal friction of the soil and the inclination of the resultant load on the ring foundation was used to estimate the bearing capacity. It was assumed that the water table was at the ground surface and the submerged unit weight of the soil was 60 pcf.
The estimated ultimate bearing capacity for the ring foundation is 30 KSF.
4.0 VAPOR CONTAINMENT STRUCTURE / FOUNDATION 4.1 Description The VCS is a 175 ft. diameter spherical steel shell that surrounds the reactor and ancillary components. The bottom of the sphere is 24 ft. above ground and is supported at its equator by 16 steel columns symmetrically spaced around the circumference and tied to an appropriate steel framework. The bottom of each column is bolted to a 5.33 ft. diameter reinforced concrete pedestal that extends to, and is supported by, an integral 10.5 ft. square footing. The botton of the footings were placed 6.0 to 14.3 ft. below the ground surface on soils judged capable of safely supporting a static bearing pressure of 8,000 pounds per square foot (8 XSF). Additional details of the VCS and its foundation are contained in References 5, 6 and 8.
4.2 Properties of the Soil Foundation 1
Soil characteristics beneath the VCS footings are as discussed above for the RSS.
Soil modulus values were calculated from field seismic tests, as outlined in Reference 3. These small-strain modulus values were reduced by a factor of 5 to account for the large strain imposed on the soil by the transient footing movements (Reference 9).
4.3 Foundation Model for Seismic Analysis j 1
The analysis of the VCS presented in Reference 8 assumed that the footing 0 foundations could move vertically and rotate in the rocking mode, and that these motions were resisted by a linear elastic soil. The reanalysis of the footing foundations presented in Reference 9 assumed that the footings could j move vertically and horizontally and rotate in the rocking mode; these !
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4 motions were resisted by a soil with a much lower modulus that could not develop tensile stresses..
4.4 'Results of Yankee Reanalysis of VCS ;
4.4.1 Loads'and Moments
' As described in Reference 9, the reanalysis of the VCS involved changes to the structural and foundation model, changes in the soil modulus and inter-action assumptions, and a change in analysis method from modal to time-history.-
These changes resulted in a significant reduction in the computed moments' !
transmitted by the footings to the supporting soil. A comparison of the ;
critical vertical footing loads and maximum moments from Reference 9 (1987) J and Reference 8 (1984) follows:
Footing No. Axial Load (K) Overturning Moment (in-K)
(1984) (1987) (1984) (1987) 1 146 172 23,435 8,195 5 141 172 17,203 5,883 '
9 155 159 17,316 6,156 ;
13 185 204 26,282 9,091 l 4.4.? Ultimate Bearing Capacity of Footings '
One of the attachments to Reference 9, a report by Geotechnical Engineers, Incorporated (GEI)datedMarch 27, 1987, provides an analysis of the ultimate bearing capacity of shallow footings subjected to the forces and moments resulting from the reanalysis of the VCS undergoing NRC seismic effects (an earthouake with a peak ground acceleration of 0.19g). A footing is classified as " shallow" if the ' depth of the footing below the ground surface is no greater than its minimum width.
As shown on Table A.1 of the GEI report, the maximum axial load, horizontal i shearing forces and rocking moment are sustained between 10 and 12 seconds after the onset of the earthquake. The calculated rotation of the footings, as shown on page 17 of Reference 9, varies from about 4 to 6 mils. These rotations were considered sufficient to reduce the bearing pressure on the bottom of the footing to zero over a significant fraction of the footing surface such that the footing " effective width" is reduced; Table B.1, page 2 of 3, of the GEI report lists the effective footing width, B', for each footing. The actual footing width is 10.5 ft., but the effective width varies from 10.45 to 1.01 ft. q To account for the possible influence of the reduced footing widths on the ultimate bearing capacity predictions, GEI assumed the footings were " deep foundations" and used conventional analysis methods to calculate their capacity; these results are presented in Reference 10. Also, because most seismic stability analyses of foundations and earth structures are based on the undrained strength of saturated soils,'a " deep foundation" analysis of the ultimate bearing capacity of the VCS footings was conducted using strength values extrapolated from laboratory tests on samples of lodgement till. The capacity results are given in Reference 10. Use of undrained strength values is justified because instantaneous peak seismic soil stresses occur before the pore water in the saturated soil has time to move with respect to the soil grain' skeleton. j 1
i 5
, 4 4.5 Staff Evaluation The following paragraphs describe the basis for the staff evaluation of References 9 and 10 and the justification for staff conclusions; this section addresses the bearing capacity of the 16 individual footings supporting the VCS during an NRC site-specific spectra earthquake.
4.5.1 Loads on Footings !
Table 1 of Reference 9 gives the loads imposed on each of the 16 footings supporting the VCS. The loads include dead load, temperature change loads (negligible) and NRC earthquake loads. The resultant axial force, horizontal forces and rocking moments on the base of the footing are also given in Table
- 1. The location of each footing is shown on References 2 and 3.
The VCS is essentially symmetric, but the peak seismic loads on the footings are not. An examination of Table 1 of Reference 9 shows that the largest axial forces due to the earthquake occur on footings number 7 and 15 and the smallest axial forces occur on footings number 3 and 11. Similarly, the largest horizontal loads occur on footings 3 and 11 and the smallest on 7 and
- 15. These observations suggest that the model of the VCS vibrates in a predominately east-west direction about an axis in the north-south direction; the direction of vibration response is probably due to the assumed direction of the earthquake motion. Because actual earthquake motions could be oriented in any direction, the maximum VCS footing load could occur on any of the 16 footings. Therefore, any footing should be capable of supporting the maximum instantaneous forces and moments caused by the earthquake. According to Reference 9, Table 1, footing number 15 on the east side of the VCS sustains the largest axial forces due to the earthquake and, according to page 1 of Table B.1, produces the maximum soil bearing pressure of 15.8 KSF. The period for one cycle of footing loads is about 1.25 seconds, so peak loads would be on the footing for only a fraction of a second during each cycle.
4.5.2 Seismic Bearing Capacity of Footings i Because the maximum soil bearing pressure could occur on any of the 16 footings supporting the VCS, an estimate of the minimum bearing capacity available on any of the 16 footings was needed. The minimum capacity can then be compared to the maximum pressures to judge the expected performance of the footing foundation under earthquake loads. '
. Reference 10 presents the bearing capacity formulation for deep foundations in paragraph 1 of the attached GEI report. This eouation indicates that the bearing capacity is directly proportional to the effective overburden pressure, q, and that the frictional component would not change for the same loading conditions and footing geometry. The weakest footings, F-10 and F-11, are at a depth of 6.0 ft. below the ground surface, but footing F-15 is 10.25 ft.
below the ground surface. Using table D of Reference 10, the expected capacity of footing F-10 or F-11 with F-15 loads applied may be approximated by:
6 q (ultimate) = 0.122 + (0.625/0.890) (17.256) = 12.2 KSF j If the angle of internal friction of the drained soil is 40 degrees instead of 35 degrees, using Table B.2 from Reference 9 gives:
q (ultimate) = (0.122/0.204) (0.777) + (0.625/0.890) (33.552) = 24.0 KSF i
Reference 10, Table E, lists the bearing capacity values for the VCS footings using undrained soil strength. The bearing capacity does not depend on the footing depth within the range of depths that exist for the VCS footings.
A capacity of about 22 KSF would be available for F-15 loads on footings F-10 or F-11.
5.0 CONCLUSION
S The staff finds that the transient seismic load imposed on the RSS ring foundation is about 17.5 KSF while the foundation ultimate bearing capacity is in excess of 20 KSF. The foundation soils beneath the RSS have adequate strength to support the RSS foundation loads when subjected to the NRC site-specific spectra (anchored at 0.199). .
Based on the reanalysis of the VCS provided by the licensee in References 9 and 10, the staff notes that the maximum bearing pressure of any footing on the supporting soil is 15.8 KSF due to the NRC earthquake (0.199) motions. These same references indicate that the minimum static bearing capacity of any footirl would exceed 12.2 KSF using conservative assumptions of soil strength, and would likely be in the range of 20 KSF using undrained soil strength values. Because the loads on the footings are cyclic and peak values are ,
sustained for less than half a second, the footings will perform adequately 4 during the earthquake. We conclude that the foundation soils beneath the VCS footings have adequate strength, and the footings are adequately sized and placed, to support the foundation loads imposed by the NRC earthquake (0.1991 '
6.0 ACKNOWLEDGEMFNT i Principal Contributor: L. Heller l
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i
7.0 REFERENCES
- 1. Letter from G. Papanic (YAEC) to J. Clifford (NRC), dated May 5, 1986,
Subject:
- Soil Mechanics Investigation. 3
)
- 2. Letter fron E. McKenna (NRC) to G. Papanic (YAEC), dated June 6,1986.
- 3. Report by Weston Geophysical Corporation dated January 19, 1979, !
entitled: Geology and Seismology, Yankee Rowe Nuclear Power Plant '
(prepared for Yankee Atomic Electric Company).
- 4. Report by CYGNA Energy Services dated March, 1983, Revison 3 entitled:
Reactor Support Structure Yankee Nuclear Power Station Structural Analysis Report (for Yankee Atomic Electric Company), transmitted by letter dated April 22, 1983 from J. Kay (YAEC) to D. Crutchfield (NRC).
- 5. Drawing Number 9699-FC-59A entitled: FDN. DETAILS-VAPOR CONTAINER - S1 by Stone and Webster Engineering Corporation.
- 6. Drawing Number 9699-FC-59B entitled: FDN. DETAILS-VAPOR CONTAINER - S2 by Stone and Webster Engineering Corporation.
- 7. Design Manuals 7.1, Soil Mechanics, and 7.2, Foundations and Earth Structures, Department of the Navy, Naval Facilities Engineering Command, May, 1982.
- 8. Report by CYGNA Energy Services dated April,1984, Rev. 3, entitled:
Vapor Container Structure, transmitted by letter dated May 8,1984 from J. Kay (YAEC) to D. Crutchfield (NRC).
- 9. Letter from G. Papanic (YAEC) to E. McKenna (NRC) dated April 2,1987
Subject:
Ultimete Bearing Capacity.
'10. - Letter from G. Papanic (YAEC) to E. McKenna (NRC) dated May 6,1987.
Subject:
Ultimate Bearing Capacity.
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