ML19350C427
| ML19350C427 | |
| Person / Time | |
|---|---|
| Issue date: | 09/16/1981 |
| From: | Agrawal P, Gupta D, Singh S NRC, SARGENT & LUNDY, INC. |
| To: | |
| References | |
| NUDOCS 8104010515 | |
| Download: ML19350C427 (5) | |
Text
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t-SEISMIC STABILITY ANALYSES 9
OF EARTH-STRUCTURES FOR g#
A NEW MADRIO TYPE EVENT
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/'\\v roIe by D. C. Gupta P.
K. Agrawal, and S. Singh Paper to be submitted for possible presentiation at'the' Conference " Earthquakes and_ Earthquake Engineering:
The-Eastern U.S."'to be held September 14-16, 1981 at: Knoxville, Tennessee.
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U.S. Nuclear. Regulatory Commission, Bethesda, Md.
2.-
Sargent & Lundy, Chicago, Ill.
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As a part of'the facilities for a nuclear power plant situated e
in Illinois, a dam has been constructed across a creek to develop the cooling lake reservoir.
Within the reservoir, a submerged pond is created by building an earth dike across an existing natural stream channel..The submerged pond forms the ultimate heat sink for the emergency core cooling system of the power plant.
Since the ultimate heat sink is classified as a Sciemic Category I S tructure, its components, viz., the submerged dike and natural slopes forming its perimeter are also designated as Catecory I' structures.
Because of_the importance of these structures to the operation and safe shutdown of the power plant, a detailed seismic stability analysis was performed for1these structures.
Based on an ev51uation of the variables relevant to the stability o'f slopes, most critical section of the natural slope was selected for the seismic stabilityfanalyses.
The variables considered in the selection process are slope configuration, subsurface conditions,
- water elevation,- and effective soil parameters for soils forming the slope. :The-slope cross-section selected for the seismic stability analyses is 60 feet-high and-has a maximum slope of.3.5:1.
-The: submerged dike of the ultimate heat sink was' constructed using Wisconsinan Glacial Till.,The dikescross-section analyzed for seismic stability analyses has a height of 17 feet and side slopes of 5:1.
i' i-The' subsurface soil conditions at the locations of the natural slope Land,the' dike were_ investigated using a comprehensive boring program.
The: static and dynamic soil properties of the: materials forming i.
' these structures were determined by the use of appropriate: field and laboratory tests.
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.The plant design response spectra for the regional safe shutdown d
earthquake conform, at the ground surface, to the USNRC Regulatory Guide 1.60, and yield a maximum ground acceleration of 0.25 g at
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the foundation-elevation.
In addition, since the plant is located in the.mid-west region of the country, the effect of a distant, low frequency, long duration "New Madrid type earthquake" on the seismic
' response of Category I structures was also investigated.
For this event a " sustained" maximum acceleration of 0.10 g at the ground surface was specified.- It was estimated that such an earthquake might produce'scismic loads at the plant site for a 1 to 2 minute
' duration with energy input predominantly between 1 and 3 second
,9er iod s.
Five natural carthquake records with characteristics similar to those mentioned'above were used to simulate this postu-lated' event.
these records were scaled such that the third largest peak-of each attained the " sustained" maximum acceleration of 0.10 g.
The response spectra for these. records, after such scaling, were developed and compared'with the regional SSE spectrum.
It was'found that'the1 response' spectra of the " distant ~ earthquake" do not exceed the regional.SSE response spectra and therefore recional'SSE
'(Regulatory Guide 1.60) controls the design of power plant structural elements.-
For the' Category I carth structures, however, it was felt that their design may be, controlled:by the " distant earthcuake" because of larger number of_ cycles involved. LTherefore the stability analysis was performed for the. regional SSE as well as for the time histories representing.tho'" distant earthquake."
The_ analyses were' performed Page-2.
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,t using the dynamic finito element analyses approach which considers the strain-dependent material properties.
The procedure consists of the following steps:
a.
determination of the response of the dike / slope-foundation system to the compatible rock accelerations, including the evaluation of the induced shear stresses at various locations throughout the dike / slope and the foundation material; b.
representation of the irregular cycles of shear stresses induced in the dike / slope foundation system by an equivalent number of cycles of uniform shear stresses; c.
determination of.the static stresses existing in the dike /
j sl' ope-foundation system (prior to the rock accelerations);
d.
determination of the cyclic shear stresses required to cause strains greater;than 5 x:10-2:in the material for conditions representative of those existing in the dike / slope-foundation system'by means of appropriate cyclic load tests on repre-sentative specimens of the materials;.and
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evaluation of the. seismic stability of the earth structures, by comparing the shear stresses required to cause strain
-2 greater.than 5 x 10 with the equivalent shear stresses
- induced lur rock' accelerations.=
.The results, in terms of the factors of safety, obtained for various elements of the1 finite element models of the' dike and natural slope cross-sections for theftwo seismic events were compared.
It was
'found}that.the design of. earth. structures is also governed by the Page 3.
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regional SSE (USNRC Regulatory Guide 1.60 Spectra) and significantly higher factors of safety are obtained for the seismic loads corresponding to a "New Madrid type event."
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