Figure 9.5.2 Conceptual figure for the response spectra of horizontal earthquake ground motion in seismic basement (A-H show control points determined by magnitude and equivalent hypocentral distance)

Figure 9.5.3 Response spectra of the horizontal earthquake ground motion with no specific source locations for each of the S wave velocities

response spectra thus obtained are enveloped by the “Design Response Spectra”, “Design Response Spectra” may represent them.

3. Elastically design earthquake ground motion Sd (edegmSd)

The EDEGM Sd is defined from the engineering standpoint to ensure, with higher confidence, maintaining seismic safety functions of the facilities under the DBEGM Ss.

It is obtained by multiplying the DBEGM Ss by the coefficient a. Hie coefficient is usually decided in an engineering judgment considering the ratio of the input earthquake load for the safety functional limits against that for the elastic limit. The value of a is 0.5 to 1.0 and established for each site independently.

2. Seismic Assessment

The assessment of seismic capability of a nuclear installation is performed utilizing the earthquake ground motion obtained in Sec. 9.5.1 and first applying it to analyze the deformation and stress in the ground and to evaluate the ground seismic stability. Then using an analytical model integrated with the ground and the building or structure as a unit and applying the ground earthquake motion to determine the deformation and stress in the building, the seismic assessment of the building is done. The building floor response spectra which are derived from the building response analysis are used as input motion to the equipment and piping system, and the seismic assessment is also performed with the analysis of the deformation and stresses in the equipment and piping system.

An outline of the seismic assessment process is shown schematically in Figure 9.5.4.

The principle of seismic assessment for the ground, buildings and structures, equipment and piping is discussed below.

(1) Seismic assessment of the ground

The assessment of seismic capability of the ground must follow the “Guideline for Safety Review of Geological Features and Foundation for Nuclear Power Plants” published by the Committee on Examination of Nuclear Reactor Safety, and it is done following the process shown concretely in

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(Modeling of

Outside shields

Fuel handling building

Auxiliary building

B

sxxwwwWWWWww -

Modeling of the ground

©Deformation and stress

In the equipments

(1) Seismic assessment of the ground

fl I.

©Deformation and stress —HA*, in the ground ' (1)1

"Steam generator

Inside, concrete

ft

©Earthquake ground motion

Figure 9.5.4 Schematic of the seismic assessment process

(3)Scismic assessment of the equipment and piping system

©Ground motion of __ , .. ,

buildings ©Ground motion of

(Seismic response) equipmen s (Seismic response)

the “Aseismic Design Technology Guidelines for Nuclear Power Stations” published by Japan Electric Association as summarized below:

1. Geological investigation

For the purpose of understanding the geological structures, borings, a test tunnel and other geological investigations on the bedrock of the reactor building and the ground slope in its vicinity, are conducted.

2. Investigation and testing of the ground

For the purpose of determining the physical properties in the stability analysis of the ground, testing of the rock and bedrock are conducted.

3. Stability assessment of the reactor building foundation and neighboring slopes

Based on the investigations carried out, the ground is classified into the surface layer, bedrock, weak stratum, etc.; then using the finite element analytical model shown in Figure 9.5.5, the ground displacement, stress and others induced by the DBEGM Ss are evaluated to assess the ground stability. It should be noted that the effect of tsunamis must be addressed with the design wave height obtained from numerical simulation considering the historical records of tsunamis around the site.

(2) Seismic assessment of the buildings and structures

The seismic assessment of buildings and

structures must follow the Seismic Design Examination Guide published by the Nuclear Safety Commission. In reality, they must be designed to be safe according to the dynamic seismic force or the static seismic force corresponding to their importance as shown in Table 9.5.2. The dynamic seismic forces are applicable to class S facilities, and obtained by the horizontal and vertical dynamic analysis against the EDEGM Sd and DBEGM Ss.

In the dynamic seismic analysis, a model considering the interaction between the foundation rock /soil and the building is used. As shown in Figure 9.5.6, the embedded Sway Rocking Model, where the lateral and rotational springs simulate the ground under the building basemat and lateral springs simulate the ground beside the building, and the Lattice Model (multi-mass in-line ground model), where multiple mass points simulate the ground, etc. are applied.

The static seismic forces are applied to the different classes of structures as follows: For class S facilities, the story shear coefficient (by multiplying the weight, the shear force loaded on that story is calculated) 3.0Ci is applied, and for class B facilities, 1.5Ci and for class C , l.OCi. For the class S facilities, the vertical static seismic force is applied simultaneously in the most conservative direction also.

The story shear coefficient of l.OCi corresponds to the earthquake force as defined in the Building

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Figure 9.5.5 Schematic stability analysis model for the ground under the reactor building foundation

Standard Law and it is equivalent to the earthquake force applicable to general building construction.

For example, because the reactor building has the auxiliary safety function of class S and it also contains other class S facilities, it is generally designed according to the following process. First, Allowable Stress Design is done with the dynamic seismic force determined from the results of a dynamic response analysis (elastic seismic response analysis) using the EDEGM Sd or the static seismic force obtained from the story shear coefficient 3.0Ci, whichever is larger. Then, a dynamic response analysis (elasto-plastic earthquake response analysis) of the reactor building is performed with the DBEGM Ss and it is confirmed if the deformation in each story is within the limit*¹. It is evident that some local plastic deformation of the building is allowed under the DBEGM Ss.

The flow of the seismic assessment for the buildings and structures in class S is shown in Figure. 9.5.7.

In the elasto-plastic earthquake response analysis, an analysis utilizing the restoring force characteristics (the relation between deformation and applied force) of the building based on the proving test data is performed.

In the seismic response analysis, a building is often simulated with a lumped mass model where the masses of the building are concentrated for each floor, and the analyzed earthquake response of each floor (response acceleration time history or floor

Figure 9.5.6 Schematic dynamic response analysis model for the ground and building interaction

⁽*¹¹ The shear strain intensity in the main wall (shear wall) remains less than 2 x lO ’radian.

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