oecd seminar prague 2011 - nuclear energy agency · •asce 4-98 seismic analysis of safety –...

Nuclear Research Institute Rez plc

OECD Seminar Prague 2011

Seismic Engineering Knowledge Transfer Seminar

21 – 25 November 2011

Nuclear Research Institute Rez, Czech Republic

Part A – Seismic design of civil engineering

structures


Monday 21 Nov. updated afternoon programme

Seismic Design and Evaluation of the NPP Civil Engineering Structures

• Applicable Stadards

• Seismic Classification of civil structures

• Seismic Input

• Basic Rules for Seismic Design

• Soil – Structure interaction

(Jan Maly, NRI Rez)

• Practical examples of civil structure seismic analyses

(Daniel Makovička, Klokner Institute, Czech Technical University)

Computational Model and Finite Elements

• Rules for FEM Model Development

• Methods of Dynamic Analysis

• Generatin of the Floor Response Spectra

• Structural Acceptance Criteria

(Martin Lukavec (M.L.Engineering & Consulting)


Basic codes and standards

• NS-G-3.3 Evaluation of Seismic Hazard for Nuclear Power Plants (2002 - 2010)

• SSG-9 Seismic Hazards in Site Evaluation for Nuclear Installations (2010)

• NS-G-3.6 Geotechnical Aspects of Site Evaluation and Foundations for Nuclear Power Plants (2004)

• NS –G-1.6 Seismic Design and Qualification for Nuclear Power Plants (2003)

• NS-G-2.13 Evaluation of Seismic Safety for Existing Nuclear Installations (2009)

• SRS No.28 Seismic Evaluation of Existing Nuclear Power Plants (2003)

• IAEA-TECDOC-1333 Earthquake Experience and Seismic Qualification by Indirect Methods in Nuclear Installations (2003)


Basic codes and standards

• ASCE 4-98 Seismic Analysis of Safety – Related

Nuclear Structures and Commentary

• ASCE 43-05 Seismic Design Criteria for

Structures, Systems, and Components in Nuclear

facilities

• European Utility Requirements for LWR Nuclear

Power, volume 2 Chapter 4 – Design Basis, 2.4.6

Seismic Design, Appendix A – Method of Seismic

Analysis


Evaluation of ground motion Hazard

• Determination of the ground motion hazard for a plant have to be based on the geological, geophysical and seismological characteristics of the region

• Typically two levels of ground motion hazard are evaluated for each plant

• SL-2 corresponds directly to safety requirements and represents the maximum level for design purposes (year occurence probability is usually 10 E-04) The minimum peak ground acceleration is 0.1 g.

• SL-1 corresponds to to a less severe, more likely earthquake (year occurence probability is usually 10 E-02)


Seismic input data

Design ground motions are typically specified in terms of several parameters: •Peak ground acceleration (PGA), or peak ground velocity •Response spectra •Effective duration of the seismic motion •Three components set of accelerations time histories In general, two orthogonal horizontal components and one vertical component shall be considered.(components of the motion shall be statistically independent. Correlation coefficient should not exceed 0.3) Seismic input motions shall be appropriate for the geological and seismological environment


Response Spectra

• Site specific horizontal response spectra (site dependent spectra) incorporate specific consideration of the tectonic environment and subsurface conditions

• Site independent horizontal response spectra (Typical examples are Newmark spectra in NUREG/CR-0098, or standard spectra NRC R.G. 1.60, EUR standard spectra)

• Vertical response spectra can be obtained by scaling the corresponding ordinates of the horizontal component by two-thirds throughout the entire frequency range but only for far field earthquakes (rules for near-field earthquakes can be found in ASCE 4-98)


Site response spectra calculated from natural accelerograms

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 5 10 15 20 25 30 35

Frequency [Hz]

Accele

rati

on

[m

/s 2 ]

Natural Hor Natural Ver


standard broadband NUREG/CR-0098 spectra


Response Spectra


EUR – design basis ground motion spectra

(horizontal, 5% damping)


Time Histories

• One or more natural (recorded) or synthetic ground motion time histories may be used.

• Zero-period acceleration (ZPA) shall equal or exceed the design ground acceleration

• Compatibility of the time histories and response spectra have to be checked. No one point of the mean spectrum (calculated from time history) shall be more than 10% below the design spectrum. Also power spectral density (PSD) functions shall be generated for acceleration time histories to determine the distribution of power in the motion as a function of frequency.


Power Spectral Density Functions

• PSD functions shall be generated for acceleration time histories used in seismic response characteristics to determine the distribution of power in the motion as a function of frequency.

The average value of the PSD shall have adequate power at all important frequency ranges.

Specification for minimum PSD requirements can be found in NUREG-0800, chapter 3.7.1


Rules for Developing synthetic time histories


Synthetic time histories

• Sufficiently long duration and small time increment

• Nyquist frequency of at least 50Hz (N=1/2Δt)

• Spectral acceleration at 5% damping

• Synthetic time histories may be used for linear seismic

analysis only

• Modified recorded accelerogram can be produced by

scaling Fourier amplitudes such that the resulting response

spectrum envelopes the target response spectrum.


Synthetic accelerograms generated for natural site response

spectra

-1.5

-1

-0.5

0

0.5

1

1.5

0 5 10 15 20 25

t [s]

a[m

/s2]

Synthetic accelerogram hor .th


Comparison of NUREG and Calculated Backward Response Spectra


NS-G-3.6 Geotechnical aspects…

Site categorization

• Type 1: vs > 1100 m/s

• Type 2: 1100 m/s > vs > 300 m/s

• Type 3: 300 m/s > vs

Vs = best estimate shear wave velocity below

the foundation level


NS-G-3.6 Geotechnical Aspects

• Input ground motion is usually provided at surface

level. Deconvolution of the input motion to the

foundation level should be carried out for sites

Type 2 and 3

• Site specific response spectra should be

determined in case of Type 3 site

• Soil-structure interaction (SSI) analysis should be

performed for sites of Type 2 and 3. Fixed base

support may be assumed for Type 1 sites.


Seismic categorization of civil structures

New IAEA guide NS-G-1.6 introduces four seismic categories:

• Seismic category 1 – covers all buildings and structures important to safety and should be designed to withstand the consequencies of SL-2 earthquakes

• Seismic category 2 – buildings that may have spatial interactions due to collapse, falling,… or other interactions such as fire, flooding,…(SL-2 level is required, but lower safety margins)

• Seismic category 3 – items that could pose a radiological hazard (not related to the reactor). SL-2 level required, but safety margins according to radiological consequences.

• Seismic category 4 – items designed in accordance with practice for non-nuclear structures


The seismic category 1 includes firstly:

items whose failure could directly or indirectly cause accidental

conditions (seismic adequacy required up to SL2 as minimum),

items required for safe shutting down the reactor, monitoring its

critical parameters, maintaining the reactor in a shutdown

condition and removing residual heat over a long period (minimum

three days, seismic adequacy required up to SL2 as minimum),

items that are required to prevent radioactive releases or to

maintain releases below limits established for design accidental

conditions (seismic adequacy required up to SL2 as minimum).


For second group of structures their seismic resistance up to

SL-1 (OBE) is required.

They form a part of building structures in which handling

with fresh fuel assemblies before their loading into reactor is

done,

they form a part of building structures in which handling

with low and medium radioactive liquid media is done, even

when it is proven that the possible leaks of these matters into

ambient environment at failure due to a seismic event will

not cause exceeding of limit doses defined as limits for the

given locality.


Seismic categorization of civil structures – VVER 1000 – Temelin NPP

Building No. Description 1) Required SeismicResistance

800/01-06 Reactor building SL-2803/01, 03 Reactor building ventilation stack SL-2807/01, 02 Air pressure vessels foundations SL-2586/01-03 Cooling pools with spraying systems SL-2586/4 Switchboards for technical water systems SL-2588 Ducts for technical service water systems piping SL-2594/01 Water treatment for technical service water

systemsSL-2

442/01-03 Diesel generator, pumping and compressorstations

SL-2

445/01-03 Diesel oil handling SL-2350 Cable ducts SL-2352/02 SMS sensors building SL-2801/01 Auxiliary building (fresh fuel assemblies storage) SL-1801/01 Auxiliary building (wardrobes and laboratories) SL-1801/03 Auxiliary building (RA media treatment station) SL-1803/02 Auxiliary building ventilation stack SL-1


Rules for modeling of structures

• The model shall represent the actual locations of masses

and centers of rigidity,accounting for the torsional effects

• Three dimensional analytical model shall be used for the

seismic response analysis

• The selection of the type of finite elements and

discretization parameters shall consider the size, shape and

number of nodal points

• Structural mass shall be lumped so that the total mass as

well as the center of gravity is preserved


Rules for modeling of structures

• The selection of the type of finite elements and

discretization parameters shall consider the size, shape and

number of nodal points. A detail model that represents the

structural configuration shall be used for direct

determination of stresses. The model shall include gross

discontinuities such as large openings etc.

• The number of dynamic degrees of freedom (and number

of lumped masses) shall be selected so that all significant

vibration modes of the structure can be evaluated


Seismic Response Analysis

• One – step analysis, All seismic responses in a structural system are determined in a single analysis.

• Multistep method, In the first step the overall seismic response (displacement, acceleration, inertial forces) is determined. The response obtained in the first step is than used as input to models for the subsequent analyses of the various portions of the structure or as input to seismic analyses of equipment and subsystems.


Dynamic coupling criteria

• Coupled analysis of a primary structure and

secondary system shall be performed when effects

of interaction are significant.

• Coupling is not required if the total mass of the

secondary system is 1% or less of the mass of the

supporting primary structure or if a coupled

analysis will not increase the response of the

primary system over that of a decoupled analysis

by more than 10%


Decoupling criteria for secondary systems


VVER 1000 - Reactor building - calculation model, general view


VVER 1000 - Reactor building - vertical section of the containment and

in-built structures


VVER 1000 - Reactor building - layout at the level +16,50


VVER 440 - Reactor building - calculation model, general view


Acceptable FEM analytical methods

• Time history method (via modal decomposition or direct

numerical integration in case of nonlinear analysis)

• Response spectrum method (number of modes included in

the analysis have to be checked in order to avoid „missing

mass effect“. Total modal mass considered in the response

have to be at least 90% of the total system mass)

• Complex frequency-response method (response time

history or transfer function method can be used)

• Equivalent static method (simplified method recommended

for simple secondary structures)


Seismic Qualification of civil structures

• Qualification by analysis is prevailing method for buildings and civil structures

• FEM analytical models are usually used for calculation of earthquake response.

• Consistent system of codes and standards have to be used for structural capacity assessment

• Structural joints should be designed to provide high ductility and capability to accommodate large displacement

• Required level of safety margins and acceptance criteria selection depend on seismic categorization


Seismic Qualification of civil structures

Uncertainities in the mechanical properties should be taken into account through parametric studies.

A range of variation in soil properties is suggested in the IAEA

NS-G 3.6

• Variation of the shear modulus between

G x (1+cv) and G/(1+ cv)

• The minimum value of the coefficient of variation

cv = 0.5

oecd seminar prague 2011 - nuclear energy agency · •asce 4-98 seismic analysis of safety –...

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