Analysis of ACR Nuclear Island Seismic SSI: Challenges ... · Analysis of ACR® Nuclear Island Seismic SSI: Challenges & Experiences N. Allotey, R. Gonzalez, A. Saudy & M. Elgohary

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Analysis of ACR® Nuclear

Island Seismic SSI:

Challenges & Experiences

Analysis of ACR® Nuclear

Island Seismic SSI:

Challenges & Experiences

N. Allotey, R. Gonzalez, A. Saudy & M. Elgohary

OECD SSI Workshop

Ottawa, Canada, October 6-8, 2010

UNRESTRICTED / ILLIMITÉ 2

Presentation Outline

• Introduction

• Design Basis Parameters

• Analysis Overview

• Issues & Challenges

• Summary


Introduction

• ACR® & EC6TM standard designs are based on existing

CANDU design

• Light water cooled, heavy water moderated pressure-

tube reactors

• Standard designs meet recent Canadian and IAEA

regulatory safety standards & customer requirements

• ACR seismic soil-structure-interaction analyses are

complete

• Two-unit plant include NSP & BOP

• Nuclear Island consists of Reactor Building & Reactor

Auxiliary Building founded on a common basemat


Introduction

RB

NI

RAB


Introduction

• Reactor Building – houses safety-related systems such as the reactor, steam

generators, fuelling machines, and heat transport systems

– prestressed concrete containment shell & reinforced concrete and steel internal structure

• Reactor Auxiliary Building – houses safety-related electrical and mechanical systems, new

and spent fuel storage, and associated fuel-handling facilities

– a single reinforced concrete structure around reactor building

• Basemat – Common to RB & RAB

– Stepped with two parts: square part & rectangular part

– Rectangular part is lower than square part


Introduction


Introduction

• ACR seismic soil-structure-interaction analyses

– CSA N289.3-10

– ASCE 4-98

– NRC SRP 3.7.1

• This paper gives a brief overview of ACR SSI analyses

plus encountered challenges & gained experiences

• This paper presents a point of view on the state-of-the-

art SSI requirements

• This paper does not aim to examine theoretical basis

of different clauses in various codes and standards


Design Basis Parameters

• Envelope seismicity of potential North American sites

• Design Basis Ground Response Spectra

–3 different design GRS: 2 CSA-based + 1 ENA-based

• Design Basis Ground Time Histories

–Single set of 3 compatible time histories for each design GRS

0

0.2

0.4

0.6

0.8

1

1.2

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al

Accele

rati

on

(g

)

CSA-Rock

CSA-Soil

ENA

x = 5%


Design Basis Parameters

• Envelope soil conditions of potential sites

• Design Basis Soil Profiles

–7 design basis soil condition: 6 layered soil profiles + elastic

half-space for hard rock condition

Shear wave velocity (m/sec) Profile Depth to bedrock

(m) Top Bottom Half-space

HR - - - 2500

A1 14 533 637 1500

B1 42 533 845 1500

B2 42 457 724 1500

B3 42 305 506 1500

C1 70 205 370 1500

D1 114 152 300 1500


Analysis Overview

• ACR Seismic SSI analyses

• Overview

–Model Development

–Analysis Procedure

–Sample Results

• Software:

–ACS SASSI

Analysis Overview

• Model Development –3D finite “coarse” element model of the nuclear island

– replaced an earlier lumped mass (stick) model

– Basemat: surface foundation under square part and partially embedded under rectangle part

– No. of nodes is in excess of 11000

– Developed in ANSYS and then converted to ACS-SASSI

–Adequate for modeling wave propagation of frequencies in excess of 50 Hz characterizing seismicity of ENA rock sites



Analysis Overview


Analysis Overview

• Analysis Procedure

–Control motion defined at underside of square part of

basemat as outcrop motion

–Cut-off frequency is set at 50 Hz for harder rock profiles and

reduces to 25 Hz for softer soil profiles

–Coherent motion assumed for combinations of different

design soil profiles & design ground response spectra

– Incoherent motion considered for hard rock condition only

–Abrahamson’s hard rock coherency model used to account

for wave incoherency effects

Design Ground Response Spectra Design Soil Profile

CSA-based Rock HR, A1, B1, B2

CSA-based Soil B3, C1, D1

ENA-based HR, A1, B1, B2


Analysis Overview

• Sample Results –Selected primary seismic responses at key locations

–Coherent motion analyses

– At lower elevations, B3-Soil CSA-based combination governed the acceleration response

– At higher elevations, B2-Rock CSA-based & HR-Rock CSA-based combinations governed the acceleration response

– In the high frequency range, HR-ENA combination governed the ISRS response

– Incoherent motion analysis

– About 20% reduction in RB ISRS peaks at high frequencies

– Reduction generally greater in vertical ISRS

– Corresponding RAB ISRS for RAB were, however, mixed


Issues & Challenges

• SSI analyses of ACR nuclear island using ACS SASSI

resulted in issues, challenges, lessons, & experiences

of interest to technical community

–Stick model vs. refined model

–Model size, run-times & exportability

–Effect of foundation fixity

– ISRS development: Is SRSS that important?

–The unending damping debate

–Wave incoherency effects: other matters

–Limits of SASSI-based frequency domain methods

– Interaction nodes

–Transfer function acceptability


Issues & Challenges

• Stick Model vs. Refined Model

–Use of finite element model becoming more trendy

– to address HF contents per regulatory requirements

– due to increase computing powers

–Advantages of refined models

– more precise structural response

– no need to verify model adequacy

– ability to capture in-plane and out-of-plane flexibilities

– no need to introduce “lollipops”

–However:

– Large volume of data is produced

– Need to communicate results in a controlled, seamless,

effective and auditable manner


Issues & Challenges

x = 5%

0

1

2

3

4

5

6

7

8

9

0.1 1 10 100

Frequency (Hz)

Spect

ral A

ccele

ratio

n (

g)

Mid-section

Right Corner

Left Corner


Issues & Challenges

• Model Size, Run-Times & Exportability

– Model capability to transmit all frequencies of interest

– Need to ensure sufficient model discretization for effective

wave propagation; especially, in softer soil profiles

– Smaller element size needed to model embedment region, due

to soft soil profile, leading to larger overall model size

– A balancing act needed in standard design between model

refinement vs. model size

– Node numbering of large models & run-time optimization

– Run-times for one frequency point (32-bit PC)

– Without node optimization: 8.5 hours (outrageous)

– With node optimization: 55 minutes


Issues & Challenges

• Model Size, Run-Times & Exportability (cont.)

–Exporting SSI results for use in stress analyses

–Which response to export?

– SSI peak acceleration responses as equivalent static loads

– Very conservative designs

– SSI peak or transient element forces combined with other loads

– Use of same finite element model for SSI & stress analyses

– Significant cost-savings

– Due to size of the stress analysis model (27,000+ nodes),

SSI peak floor accelerations were applied

–ACS SASSI-ANSYS Integration module

– A tool that would export SSI responses at critical time steps

from a coarser model to a more refined stress analysis model


Issues & Challenges

• Effect of Foundation Fixity –Need for inclusion of hard rock profiles in standard designs

– Peak of vertical ISRS at top of CS for hard rock case considerably larger than that for rest of soil profiles

– Attributed to foundation fixity resulting in increased structural deformations due to larger mass participation

x = 5%

0

1

2

3

4

5

6

7

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tion

(g

) HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S


Issues & Challenges

• ISRS development: Is SRSS that important?

– Nuclear codes require ISRS to be computed as SRSS of

ISRSs due to the individual components of ground motion

– Question: how, under coherent input motion do cross-

directional ISRSs influence the computed ISRS?

– ISRSs at top of CS, CS springline, and two top RAB corners

– Hard rock profile

– With & without SRSS combination


Issues & Challenges

• ISRS development: Is SRSS that important? (cont.)

–With & Without SRSS: H1

–With & Without SRSS: H2

0

2

4

6

8

10

12

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tion

(g

)

CS-Top

CS-Spring line

RAB-DA Corner

RAB-BA Corner

0

2

4

6

8

10

12

0.1 1 10 100

Frequency (Hz)

Spect

ral A

ccele

ratio

n (

g)

CS-Top

CS-Spring line

RAB-DA Corner

RAB-BA Corner

0

2

4

6

8

10

12

0.1 1 10 100Frequency (Hz)

Spect

ral A

ccele

ratio

n (

g)

CS-Top

CS-Spring line

RAB-DA Corner

RAB-BA Corner

0

2

4

6

8

10

12

0.1 1 10 100

Frequency (Hz)

Spect

ral A

ccele

ratio

n (

g)

CS-Top

CS-Spring line

RAB-DA Corner

RAB-BA Corner


Issues & Challenges

• ISRS development: Is SRSS that important? (cont.)

– With & Without SRSS: V

– Significant effect of SRSS combination in case of vertical ISRS

– Horizontal responses critically contribute to vertical response in case of

CS springline & RAB

– Locations with high eccentricity from centre of rigidity

– SRSS combination has no effect on vertical response at top of CS

– Location at containment’s axis of symmetry; i.e. centre of rigidity

0

1

2

3

4

5

6

7

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tio

n (

g)

CS-Top

CS-Spring line

RAB-DA Corner

RAB-BA Corner

0

1

2

3

4

5

6

7

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tio

n (

g)

CS-Top

CS-Spring line

RAB-DA Corner

RAB-BA Corner


Issues & Challenges

• The unending damping debate –Nuclear codes recommend material damping ratios for linear

analyses of structures

– Recognize damping is linked to stress level and recommend different damping ratios for different design level earthquakes

– RG 1.61 recommends 7% & 4% damping for reinforced concrete structures at SSE and OBE levels; respectively

–Codes recommend soil/rock material damping be estimated from free-field analyses, i.e. SHAKE-like programs

– For hard soil/rock conditions, material damping can be quite low

– For HR to B2 profiles, material damping to be less than 3%

– Issue: Would damping of a structure exceed that of its supporting soil/rock medium during any design earthquake?

– Damping of structures is stated by codes while damping of foundation medium is estimated by analytical simulations


Issues & Challenges

• Wave incoherency effects: other matters

–Reduction in ISRS, especially for frequency > 10Hz, when

wave incoherency of ground motion is accounted for

– However, rotational & torsional responses may increase in

some cases and adequate SSI models should be used

– Both reductions & increases observed in ACR SSI analysis

– Increase at RAB side with embedment

–Two issues that need industry guidance

– Stochastic characteristic of incoherent ground motion:

– Issue: Should not it be a requirement that multiple sets of time-

history sets be used, rather than a single set?

– Besides its effect on ISRS, wave incoherency affects would

influence acceleration, and displacement responses too

– Issue: Should wave incoherency be used in stress evaluations?

Issues & Challenges

• Limits of SASSI-based frequency domain methods –Sub-structure frequency method used in SASSI-based codes

is based on linear visco-elastic assumption

– Very attractive: once transfer functions are generated; responses can be easily obtained for different loading functions

– Nonlinearity?: iterative equivalent-linear solutions

– Primary nonlinearity due to wave propagation is captured by with strain-compatible modulus and damping ratio curves

–However, secondary nonlinearity effects including wave-trapping in weak backfills

– Additional structural elements to model near-field soil

– Very challenging & its effect has been proven harmfully unpredictable

– Issue: Would not time-domain approaches be a better alternative?



Summary

• Seismic SSI analyses of ACR nuclear island have

recently been completed

• Standard design basis parameters

• Overview of SSI analysis model development, analysis

procedures and sample results

• Discussion of learnt lessons, encountered challenges

& gained experiences

• Identified few issues that, authors believe, remain

open-ended & require clearer industry guidance



Issues & Challenges

• Interaction Nodes

–Located on soil-structure interface; i.e. shared between

structure & foundation medium

–Need for guidance on uniform distribution of interaction nodes

over interaction surface

– A highly non-uniform distribution of interaction nodes could

introduce unrealistic torsional effects

– Lead to inconsistent SSI results

– After a peer-review, a more uniform distribution of interaction

nodes was adopted instead

– Inconsistencies were rectified


Issues & Challenges

• Transfer Function Acceptability –SSI analysis evaluates transfer function of input motion to

desired location at discrete frequency points

– Interpolation algorithm to develop continuous transfer function

– Accurate interpolated transfer function, over the desired frequency range, leads to accurate SSI results

– Act of balancing: very few (poor TF) vs. too many (expensive) frequency points

–Logically, refined models requires far more frequency points then lumped mass stick models

– 200 frequency pts. Refined model/coherent motion

– 300 frequency pts. Refined model/incoherent motion

– 50 frequency pts. Lumped mass model/coherent motion


Issues & Challenges

• Transfer Function Acceptability (cont.) –Assessing adequacy of interpolated transfer functions is

important and labour-intensive

– Useful tools are available, such as viewing TF & ITF, and frequency search

– Still, a developed in-house macro worksheet was used to reduce considerably time spent on checking and cross-checking

– Particularly, in SSI analyses with incoherent motion, with many interpolation and smoothing parameters

– A Restart option that uses stored impedances from initial run, as input for subsequent runs

– Without this feature, SSI analyses with incoherent motion would not have been possible


Analysis Overview

• Sample Results (cont.)

* Letter code relates to locations in the analysis model # Combinations of design soil profile and design ground response spectra;

e.g., “B3-S” refers to B3 profile & CSA-based Soil motion

Floor acceleration (g) Location Symbol

*

H1 H2 V

Basemat: square part a 0.64 B3-S# 0.52 B3-S 0.58 B3-S

Basemat: rectangular part b 0.50 B3-S 0.47 B3-S 0.40 B3-S CS top c 2.53 B2-R 2.36 B2-R 0.98 HR-E IS top d 1.6 HR-R 1.6 B2-R 0.84 B2-R RAB square part top e 1.55 HR-R 0.84 B2-R 0.57 HR-E RAB rectangular part top f 0.98 HR-R 1.15 HR-R 0.76 HR-R

Maximum peak of 5% damped ISRS (g) Location Symbol

*

H1 H2 V

Basemat: square part a 2.53 B3-S 2.45 B3-S 2.36 B3-S Basemat rectangular part b 1.74 B3-S 1.74 B3-S 1.44 B2-R CS top c 14.85 B2-R 12.45 B2-R 5.86 HR-R IS top d 8.71 B2-R 8.76 B2-R 3.01 B2-R RAB square part top e 8.0 HR-R 3.78 B2-R 2.17 B2-R RAB rectangular part top f 4.54 HR-R 5.72 HR-R 3.12 HR-R


Analysis Overview


–Top of CS: H1, H2, and V

0

2

4

6

8

10

12

14

16

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tion

(g

) HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S

0

2

4

6

8

10

12

14

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tion

(g

) HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S

0

1

2

3

4

5

6

7

0.1 1 10 100

Frequency (Hz)

Sp

ectr

al A

cce

lera

tion

(g

) HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S


Analysis Overview


–Top of IS: H1, H2, and V

0

1

2

3

4

5

6

7

8

9

10

0.1 1 10 100

Frequency (Hz)

Spectr

al A

ccele

ratio

n (

g)

HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S

0

1

2

3

4

5

6

7

8

9

10

0.1 1 10 100

Frequency (Hz)

Spectr

al A

ccele

ratio

n (

g)

HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S

0

0.5

1

1.5

2

2.5

3

3.5

0.1 1 10 100

Frequency (Hz)

Spectr

al A

ccele

ratio

n (

g)

HR-E

HR-R

A1-E

A1-R

B1-E

B1-R

B2-E

B2-R

B3-S

C1-S

D1-S

Documents

Analysis of ACR Nuclear Island Seismic SSI: Challenges ... · Analysis of ACR® Nuclear Island Seismic SSI: Challenges & Experiences N. Allotey, R. Gonzalez, A. Saudy & M. Elgohary