New Nuclear Engineering and Technology · 2016. 12. 20. · Original Article Potentiality of Using Vertical and Three-Dimensional Isolation Systems in Nuclear Structures Zhiguang

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Original Article

Potentiality of Using Vertical and Three-Dimensional Isolation Systems in NuclearStructures

Zhiguang Zhou a,*, Jenna Wong b, and Stephen Mahin c

a Research Institute of Structural Engineering and Disaster Reduction, Tongji University, 1239 Siping Road, Shanghai

200092, PR Chinab Lawrence Berkeley National Laboratories, 1 Cyclotron Road, MS74R316C, Berkeley, CA 94720, USAc Department of Civil and Environmental Engineering, 777 Davis Hall, University of California, Berkeley, CA 94720,

USA

a r t i c l e i n f o

Article history:

Received 26 September 2015

Received in revised form

30 January 2016

Accepted 19 March 2016

Available online 5 April 2016

Keywords:

3D Isolation Systems

Isolator

Nuclear Power Plant

Response Spectrum

Rocking Effect

Vertical Isolation

* Corresponding author.E-mail address: [email protected] (Z.

http://dx.doi.org/10.1016/j.net.2016.03.0051738-5733/Copyright © 2016, Published by Elthe CC BY-NC-ND license (http://creativecom

a b s t r a c t

Although the horizontal component of an earthquake response can be significantly

reduced through the use of conventional seismic isolators, the vertical component of

excitation is still transmitted directly into the structure. Records from instrumented

structures, and some recent tests and analyses have actually seen increases in vertical

responses in base isolated structures under the combined effects of horizontal and

vertical ground motions. This issue becomes a great concern to facilities such as a

Nuclear Power Plants (NPP), with specialized equipment and machinery that is not only

expensive, but critical to safe operation. As such, there is considerable interest

worldwide in vertical and three-dimensional (3D) isolation systems. This paper ex-

amines several vertical and 3D isolation systems that have been proposed and their

potential application to modern nuclear facilities. In particular, a series of case study

analyses of a modern NPP model are performed to examine the benefits and challenges

associated with 3D isolation compared with horizontal isolation. It was found that

compared with the general horizontal isolators, isolators that have vertical frequencies

of no more than 3 Hz can effectively reduce the vertical in-structure responses for the

studied NPP model. Among the studied cases, the case that has a vertical isolation

frequency of 3 Hz is the one that can keep the horizontal period of the isolators as the

first period while having the most flexible vertical isolator properties. When the ver-

tical frequency of isolators reduces to 1 Hz, the rocking effect is obvious and rocking

restraining devices are necessary.

Copyright © 2016, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society. This

is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

Zhou).

sevier Korea LLC on behamons.org/licenses/by-nc

lf of Korean Nuclear Society. This is an open access article under-nd/4.0/).

http://creativecommons.org/licenses/by-nc-nd/4.0/


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Fig. 1 e Sketch of thick-rubber-layer bearing after ref [14].


1. Introduction

Conventional isolation systems are generally intended to

reduce seismic demands due to the horizontal components of

ground shaking. However, they do not prevent vertical seismic

forces from being transmitted directly into the structure.

Under certain circumstances, isolation systems can amplify

or add to the vertical vibrations similar to that experienced in

a fixed-base structure. For instance, elastomeric bearings can

have flexibilities by themselves or in combination with the

flexibility of the structural system shift the effective vertical

frequency of the isolated system into an amplified range of the

vertical pseudo-acceleration spectrum. As such, the vertical

response could beworse than that experienced by a fixed-base

structure. Sliding bearings are typically stiff in the vertical

direction, so the vertical response may be similar to a fixed-

base structure. However, if vertical excitations become large,

uplift can occur unless tension-capable bearings are used.

While moderate uplift may be acceptable in some applica-

tions, the uplift and reseating behavior of the bearing system

may result in impact loads that produce additional vertical

vibrations in the structure.

Various attempts have been made to provide enhanced

protection against the vertical component of response by: (1)

using a complete three-dimensional (3D) seismic isolation

solution; and (2) adding localized vertical isolation systems

to individual parts of a horizontally isolated structure

[1e11].

Early efforts to develop 3D isolation systems focused on

modifying the design parameters of laminated rubber bear-

ings. In 1986, Kajima Corporation (3-1, Motoakasaka 1-chome,

Minato-ku, Tokyo 107-8388, Japan) utilized this approach to

construct a two-story reinforced concrete (RC) acoustic labo-

ratory building in Japan [12]. This approach was also investi-

gated by the USA nuclear industry using laminated rubber

bearings [13]. More recently, a type of laminated thick rubber

bearing was adopted for the seismic isolation design of the

Japan sodium-cooled fast reactor (JSFR) [14].

Moving beyond design parameter modifications, other 3D

systems have been introduced. The GERB System consists of

helical springs that are flexible horizontally and vertically. This

system was used in the residential and industrial sector for

various applications. In Japan, a number of important devel-

opment studies have been completed related to 3D isolation of

nuclear facilities. A project was started in 2000 for the devel-

opment of 3D seismic isolation technologies for use in the Jap-

anese fast breeder reactors (FBR), under the sponsorship of the

Japanese Ministry of Economy, Trade and Industry. This was

motivated to achieve more economical designs for the FBR de-

signs thancouldbe achievedusing only thehorizontal isolation

systems. Threepromising ideas for 3D isolationwere examined

bytheFBRproject, i.e., “RollingSealTypeAirSpring,” “Hydraulic

3D Isolation System,” and “Cable Reinforced Air Spring” [2].

Kozo Keikaku Engineering Inc. (4-38-13 Honcho, Nakano-ku,

Tokyo164-0012, Japan) in conjunction with Shimizu Corpora-

tion (2-16-1 Kyobashi, Chuo-ku, Tokyo 104-8370, Japan) has

extended the basic ideas from these 3D isolation projects and

applied it to an actual three-story reinforced concrete apart-

ment building in Tokyo, Japan [3,4]. The 3D isolation system

installed in the building performed as expected in the 2011 East

Japan Earthquake.

Vertical isolation systems provide flexible supports in the

vertical direction by a combination of metallic or air springs

and supplemental damping devices. For example, the Euro-

pean FBR project considers isolating the reactor vault from the

horizontally isolated base mat using vertical springs [15]. For

the Japanese FBR, a vertical isolation system was explored

using a series of coned disk springs surrounding a central

vertical guide [2].

These vertical and 3D isolation systems and their potential

application to modern nuclear facilities are examined in this

paper. Moreover, a series of case study analyses of a modern

nuclear power plant (NPP) model are performed to examine

the benefits and challenges associated with 3D isolation

compared with horizontal isolation.

2. Choices of vertical and 3D isolationsystems for nuclear structures

2.1. Thick-rubber-layer bearing

3D isolation systems can be achieved by using thick rubber

layers for rubber bearings [16]. A sketch of thick rubber-layer

bearing is illustrated in Fig. 1. In 1986, Kajima Corporation

built a two-story RC acoustic laboratory building supported on

18 steel laminated natural rubber bearings [12]. The bearings

were designed to bemore flexible in the vertical direction than

other bearings used in Japan. Fourteen round steel bars were

used to provide damping. In addition, oil dampers were added

to reduce vertical and rocking motions during earthquakes.

The vertical isolation frequency is 5 Hz. The effectiveness of

the isolation system was demonstrated during both earth-

quakes and traffic vibrations.

Three dimensional isolation system using laminated rub-

ber bearings was also investigated for the USA nuclear in-

dustry [13,17]. The target horizontal and vertical frequencies

of the proposed system were 0.5 Hz and 3 Hz, respectively.

The investigation showed that rubber bearings could be

designed to provide isolation in the horizontal and vertical

directions.

Mitsubishi FBR Systems Inc. (2-34-17 Jingumae, Shibuya-

ku, Tokyo 150-0001, Japan) has proposed a thick rubber-layer



Nu c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 8 ( 2 0 1 6 ) 1 2 3 7e1 2 5 1 1239

bearing design for JSFR recently [14]. The target horizontal and

vertical frequencies are 0.29 Hz and 8 Hz, respectively. The

diameter of the bearing is set to 1,600mm, and the bearing has

10 layers of laminated rubber with each layer being 30-mm

thick. Okamura et al. [14] concluded that this bearing design

can reduce the response of the reactor system in comparison

with the previous design for JSFR. Tests of a 1/8-reduced-scale

bearing model (diameter, 200 mm) were carried out confirm-

ing the applicability of the bearings.

2.2. The GERB system

The GERB system consists of large helical steel springs ar-

ranged in an assembly that is flexible both horizontally and

vertically, as illustrated in Fig. 2. The spring assembly is

essentially undamped and typically would be used in

conjunction with supplemental dampers. The vertical fre-

quency is approximately three to five times the horizontal

frequency [18,19]. This is to avoid excessive movement in the

vertical direction due to variations in live load, wind, or other

lateral loads. The GERB system had been implemented in two

residential buildings in California, USA before 1994. The

buildings were shaken strongly in the 1994 Northridge Earth-

quake [20]. There is a limitation to this systemdue to the strong

coupling between vertical and horizontalmotion. If the bearing

Fig. 2 e Sketch of GERB system after ref [18,19].

Rolling seal rubber

Upper basement

Air compartment

Contact part

Stoper(damper)

Lower basement

To atmosphereElastomeric-based bearing

Fig. 3 e Sketch of rolling seal ty

offsets laterally, vertical vibrations will affect the lateral

displacement of the bearing due to geometric nonlinearities.

2.3. Rolling seal type air spring

Suhara [7] developed a 3D seismic isolation device that uses

laminated rubber bearing as a horizontal isolation device and

rolling seal type air spring as a vertical isolation device, as

illustrated in Fig. 3. The air compartment was about 1.4 m in

diameter and 3 m tall. The design air pressure is 1.6 MPa for

normal operating conditions and 2.0 MPa for the earthquake

conditions. This results in a vertical frequency of 0.5 Hz. The

issues of concern for this device were the ultimate strength of

the rolling seal (rubber) against internal pressure, deteriora-

tion of the seal due to aging, and the smoothness of motion

during combined axial and lateral motion. More information

on this system can be found in Suhara's [7] study.

Because the center of mass of the FBR is higher than the

isolation plane, a plant isolatedwith 3D bearings at its basewill

tend to pitch and roll. To suppress this motion, rocking sup-

pression systems were also investigated [7]. Rocking can in-

crease the horizontal displacements and accelerations in the

upper parts of the plant, and it can also increase the vertical

displacement demands for the isolator bearings located closest

to the outer edge of the plant. Thus, a variety of systems using

mechanical pantograph linkage apparatus [8], pulley systems,

and hydraulic systems that permit vertical and lateral motion

of the upper mat without pitch or roll have been explored. The

system cited by Inoue et al. [2] as the most favorable candidate

uses a separate system of interconnected hydraulic cylinders

placed around the perimeter of the plant to control rocking.

These are interconnected in a crossover arrangement whereby

out-of-phase motion on opposite sides of the plant is sup-

pressed while in-phase motion is not restricted. Small-scale

vibration tests have demonstrated that this system can sup-

press rocking motion. More information on the rocking sup-

pression system can be found in Shimada et al. [8] 2005.

Among the 3D isolation systems examined by the FBR

project, Inoue et al. [2] indicate that the rolling seal air spring

Upper cylinder

Air supply

Lower cylinder

Normalcondition

Deformation condition

pe air spring after ref [7,8].



Fig. 5 e Sketch of hydraulic three-dimensional isolation

system after ref [22]. (A) Isolator. (B) Accumulator unit.


in combination with the rocking suppression system was the

most promising system.

2.4. Cable reinforced air spring

Cable reinforced air spring provides both horizontal and ver-

tical isolation in a single air pressure activated device [2]. The

device is composed of two cylinders of different diameters, as

illustrated in Fig. 4. The inner cylinder is attached to the lower

mat, while the upper cylinder is attached to the upper mat. A

rubber sheet interconnects the two cylinders. Polyester fabric

and a set of load carrying wire cables reinforce the sheet. The

difference between the inner diameter of the outermost cyl-

inder and the outside diameter of the inner cylinder is roughly

twice the bearing horizontal displacement capacity.

The vertical restoring stiffness is provided by the bulk

modulus of the pressurized air in the device. The horizontal

restoring force comes from the difference in air pressure

acting on the U-shaped rubber gasket between the inner and

outer cylinders. As the upper cylinder moves towards the

lower cylinder, the rubber gasket hangs down lower on the

side with the narrower gap; on the opposite side, the gasket is

stretched and thus does not hang down so far. Thus, there is

net pressure on the gasket tending to make the inner cylinder

recenter inside the outer cylinder. For a proposed design, the

inner diameter is 6 m and the outer diameter is 8 m, resulting

in a lateral displacement capacity less than 1 m. The height of

the inner cylinder is 3 m. The internal air pressure is 1.4 MPa

for operational conditions. The vertical and horizontal effec-

tive isolated frequencies are 0.35 Hz and 0.27 Hz, respectively.

Issues of concern for this bearing are the ultimate strength

of the wire reinforced bladder, verification of the vertical and

horizontal restoring force characteristics, and smooth and

predictable behavior under 3D motion. Some experiments on

the bearing have been conducted. There is more information

on this type of bearing in the Kageyama et al. [21] study.

2.5. Hydraulic 3D isolation system

A hydraulic 3D isolation system consists of a natural rubber

bearing upon which a vertically oriented hydraulic cylinder is

placed, as illustrated in Fig. 5. The hydraulic cylinder is con-

nected to a small bank of accumulators that are partially filled

with pressurized nitrogen gas. The gas is pressure regulated to

provide sufficient vertical load strength for the isolator unit

under service operating loads. During the earthquake, the gas

compresses. By providing an appropriate volume of gas, the

accumulator acts like an air spring. The piping system

Fig. 4 e Sketch of cable reinforced air spring after ref [2,21].

between the hydraulic cylinder and the accumulator bank,

and a flow-restricting orifice in the piping system, generates

the desired viscous damping. For safety reasons, a noncom-

bustible hydraulic fluid is proposed.

This system is designed for the same load as the previous

rolling seal type air spring system, but the operating pressure

under service operations is raised to 15 MPa and under

earthquake conditions is raised to 20 MPa. The issues of

concern cited for this system were the leak-tightness of the

seal system between the piston and cylinder, friction char-

acteristics of the piston cylinder system with lateral loads

applied, and the confidence that the desired damping prop-

erties could be achieved. Some reduced scale tests were car-

ried out on this system. There is more information on this

system in the Kashiwazaki et al. [22] study.

2.6. Coned disk springs

Providing vertical isolation for individual components within

a horizontally isolated NPP against the vertical components of




motion is an intermediate and perhaps more economical

approach to achieve 3D protection. In the Ministry of Econ-

omy, Trade and Industry program for 3D isolation, the concept

of a common deck isolation system was proposed. In this

system, a flat slab platform is constructed within the reactor

containment structure. It is assumed that the structure is

isolated to resist horizontal components of ground motion.

The common deck is supported on a series of vertical isolation

devices (lateral displacements of the deck relative to the plant

are restrained). The reactor vessel can be suspended through a

hole in the deck, and other components sensitive to vertical

motion can be supported on the deck. It is expected that the

isolation plane would be located at the elevation of the center

of mass of the items supported on the deck so that rocking

would not be an issue, and that the isolators would be posi-

tioned to avoid mass eccentricities in the vertical direction.

For the vertical isolation systems, a different approach

than what was considered for the overall plant was assumed.

In this case, stacks of large bore and coned disk springs (Fig. 6)

were considered [5,6]. For the cases cited, the springs were

configured to provide a vertical isolated frequency of 1 Hz.

This was selected to provide a more limited relative vertical

displacement movement for piping systems in the plant. The

spring units have an outer diameter of 1 m and an inner

diameter of 0.5 m. They are 27-mm thick and bent into a cone

shape. Seventy cone disks are used to make up one isolator;

five parallel stacks of 14 disks in series. These stacked cones

result in a total height of 2.2 mwith a stroke limit of ±100mm.

Damping is provided by separate steel yielding dampers. Dy-

namic analysis suggests that the vertical response is similar to

that observed for horizontal isolation; the spectral response

for frequencies higher than that of the isolation system are

lower than for the fixed base system, and the spectral

response around the isolation frequency is higher than for the

fixed base system. Inoue et al. [2] and Morishita et al. [5]

concluded that this system is effective.

Fujita et al. [23] carried out a set of shaking table tests with

small diameter natural rubber bearings providing isolation in

the horizontal direction. On top of these bearings, a set of

coned disk springs and yielding dampers were used to provide

full 3D isolation. The specimen had a horizontal isolated fre-

quency of 0.5 Hz and a vertical isolated frequency of 3 Hz. The

later fixed base frequency of the super structurewas 10Hz and

Damper

Common deck

Coned disk spring

Inside middle washer

Outside middle washerCenter guide

Fig. 6 e Sketch of coned disk spring after ref [5,6,23].

above 50 Hz in the horizontal and vertical directions, respec-

tively. Several levels of excitation were imposed. The results

indicated that the system worked as expected, with little

rocking. For a large amplitude El Centro motion, it is noted

that the floor spectral ordinates in the vertical direction are

significantly reduced for periods less than 0.15 seconds (6 Hz)

and much lower than the fixed base one at the natural fre-

quency of the fixed base building. Larger reductions were

obtained in the horizontal direction. Although there was a

consistent reduction in floor spectra in the vertical direction in

the high frequency of response range when the excitations

were moderate and large, it was noted that there was minor

amplification of response in the vertical direction for low

amplitudes of excitation. A summary of various vertical and

3D isolation systems is illustrated in Table 1.

3. Analysis results using different verticaland 3D isolation systems

3.1. Model summary

A representative but simplified stick lumped mass model is

introduced to represent a Generation III Pressurized Water

Reactor NPP in the case study analyses. Linear modal time

history analyses, conducted using SAP2000 (Computers and

Structures, Inc. 1646 N. California Blvd., Suite 600 Walnut

Creek, CA 94596, USA), are used to examine the effect of

different 3D isolation systems compared with a general hori-

zontal isolation system in reducing the floor response spectra

in the vertical direction. The 3D finite element model is illus-

trated in Fig. 7. The Nuclear Island consists of the reactor

containment building (RCB) and the auxiliary building, which

share a common basemat. The shape of the auxiliary building

is a rectangular type with maximum dimensions of

103.6m� 102.4m. It wraps around the RCBwith a gap equal to

5 cm and the radius of circular cavity for the RCB is 24 m. The

RCB consists of the containment shell (CS), the reactor

containment internal structures, and the reactor coolant

systems. The first horizontal frequency and the first vertical

frequency of the CS are 3.85 Hz and 11.02 Hz, respectively. The

first horizontal frequency and the first vertical frequency of

the primary shield wall (representative of the internal struc-

tures) are 8.13 Hz and 29.65 Hz, respectively.

There are 454 isolator elements in the model, as illustrated

in Fig. 7B. The input motion is illustrated in Fig. 8. It is

generated synthetically to be consistent with European Utility

Requirements for light water reactor NPPs, and is scaled to

have 0.5 g horizontal and 0.33 g vertical peak ground

accelerations.

The isolator properties used in the analyses are illustrated

in Table 2, where Case 0 is the fixed-base case; Case 1 uses

general horizontal isolation systems; and Cases 2e6 use

some form of 3D isolation. The vertical frequency of the

isolation system (fv) varies from 63.2 Hz to 0.432 Hz, while

the horizontal frequency remains constant at 0.432 Hz. The

damping for the response history analysis is defined as a

constant value of 0.04 over all the modes except for the

modes of isolators which are overwritten with the values

listed in Table 2.



Table

1e

Sum

mary

ofvariousverticalandth

ree-d

imensional(3D)isolationsy

stem

s.

Isolationsy

stem

Relatedrese

arch&

developmentefforts

Verticalfrequency

Horizo

ntal

freq

uency

Obse

rvations

Thickru

bber-layerbearing

[14]

2-story

RCaco

ustic

laboratory

buildingbyKajimain

1986;

Japanso

dium-cooledfast

reactor

3e8Hz

0.3e0.5

Hz

Effectiveness

demonstratedin

experiments

TheGERBSystem

[18,19]

2residentialbuildingsbuiltin

Californ

ia3e5timesth

e

horizo

ntalfrequency

eShakenstro

ngly

inth

e1994NorthridgeEarthquake,

stro

ngco

uplingbetw

eenvertical&

horizo

ntalmotion

Rollingse

altypeair

spring

[7,8]

Developmentofse

ismic

isolationsy

stem

for

nextgenerationNPPin

Japan

0.5

Hz

0.36Hz

Effectiveifco

mbinedwithro

ckingsu

ppressionsy

stem

Cable

reinforcedair

spring

[2,21]

0.35Hz

0.27Hz

Issu

es:

theultim

ate

strength

ofth

ewirereinforcedbladder,

etc.

Hydraulic3D

isolation

system

[22]

0.5e0.67Hz

0.54Hz

Issu

es:

leak-tightn

ess

ofth

ese

alsy

stem,etc.

Coneddisksp

rings[5,6,23]

METIpro

gram

for3D

isolationin

Japan

1e3Hz

0.5

Hz

Effective(m

inoramplifica

tionofresp

onse

inth

e

verticaldirectionforlow

amplitu

desofexcitation).

METI,MinisterofEco

nomy,TradeandIndustry;NPP,nuclearpowerplant;RC,reactorco

ntainment.


Modal frequencies for the different cases are illustrated

in Table 3. For the fixed-base case, the first two modes

correspond to the horizontal translation modes of the RCB.

The third mode corresponds to the horizontal translation

mode of the auxiliary building, and the 10th mode corre-

sponds to the vertical translation mode of the RCB. From

Case 1 to Case 4 (i.e., fv varies from 63.2 Hz to 3 Hz), the first

mode corresponds to the horizontal translation mode of

the isolators. However, the first mode shows a complex

characteristic of both horizontal translation and rocking

for Case 5 and Case 6. It means that rocking plays an

important role in the first mode when fv reduces to 1 Hz.

The first vertical translation mode for the different cases is

marked as shaded cell in Table 3. The modal direction is

also shown in Table 3 when the modal participating mass

ratio is greater than 1% in that direction. It can be found

that the first vertical translation mode is controlled by the

vertical stiffness of the isolators for Cases 2e6. Among

these cases, Case 4 maintains the horizontal period of the

isolators as the fundamental period while having the most

flexible vertical isolator properties.

3.2. Response of the isolators

As there are 454 isolator elements in the model, it is infea-

sible to illustrate the response results of all the isolators.

Therefore, the response of the center isolator is discussed in

this section. Response horizontal and vertical deformation

comparison of the center isolator among the different cases

are illustrated in Figs. 9Ae9D, respectively. It can be seen that

response horizontal orbits of the isolator are almost the

same for the caseswhen fv is between 63.2 Hz and 3 Hz, while

are different for the cases when fv is between 3 Hz and

0.432 Hz. As to the vertical deformation of the isolator, the

amplitude of the deformation increases with the decreasing

of fv.

Response isolator axial force comparison of the center

isolator among the different cases is illustrated in Figs. 10A

and 10B, while the shear force X and the shear force Y are

illustrated in Figs. 10C and 10D and Figs. 10E and 10F,

respectively. It can be seen that the isolator axial force shows

limited difference for the cases when fv is between 63.2 Hz

and 3 Hz, while it shows an obvious difference for the cases

when fv is between 3 Hz and 0.432 Hz. The isolator shear

forces are almost the same for the cases that fv is between

63.2 Hz and 3 Hz, while decrease with the value of fv for the

cases that fv is between 3 Hz and 0.432 Hz.

3.3. Response spectral accelerations of thesuperstructure

Vertical pseudo spectral accelerations of the fixed-base case

are illustrated in Fig. 11. The unit of pseudo spectral accel-

erations is set as g, the acceleration of gravity. It can be found

that the in-structure response for the CS near 11 Hz is highly

amplified, and for the Primary Shield Wall near 30 Hz is

amplified by the input motion. Referring to the modal fre-

quencies for the different cases, it is clear that the floor

spectra are amplified in the vertical direction at frequencies

corresponding to the vertical direction modes of the



Layout of the isolators

(A) (B)

3D View of the FE model

Top of the reactor containment building(EL. 101 m) Top of the primary shield

wall (EL. 58.2 m)

Bottom of the reactor containment building (EL. 23.8 m)

Fig. 7 e Three-dimensional (3D) finite element (FE) model. (A) 3D view of the FE model. (B) Layout of the isolators.

EL., elevation level.

0.60.40.2

00.20.40.6

0 5 10 15 20 25

Acc

el. (

g)

Time (s)

0.60.40.2

00.20.40.6

0 5 10 15 20 25

Acc

el. (

g)

Time (s)

0.40.30.20.1

00.10.20.30.4

0 5 10 15 20 25

Acc

el. (

g)

Time (s)

––– –

–– –

–––

(A) (B) (C)

Fig. 8 e Input motion. (A) X component. (B) Y component. (C) Z component. Accel., acceleration.

Table 2 e Isolator properties used in the analyses.

Caseno.

Supposed isolation system Vertical properties Horizontal properties

Frequency fv(Hz)

Stiffness kv(kN/m)

Dampingxv

Frequency fh(Hz)

Stiffness kh(kN/m)

Dampingxh

Case 0 Fixed base

Case 1 Lead rubber bearing 63.2 1.68E þ 08 2% 0.432 7.62E þ 03 20%

Case 2 Thick rubber-layer bearing 8 2.70E þ 06 4%

Case 3 Thick rubber-layer bearing 5 1.05E þ 06 10%

Case 4 Coned disk spring/GERB system þ LRB 3 3.79E þ 05 20%

Case 5 Air spring/hydraulic system þ LRB 1 4.21E þ 04 20%

Case 6 0.432 7.62E þ 03 20%

LRB, Lead-plug Rubber Bearing.


Containment Shell and the Primary ShieldWall. As illustrated

in Fig. 12, vertical floor spectral accelerations of the Contain-

ment Shell are amplified for the cases when fv is between

63.2 Hz and 5 Hz, reduced for the case when fv is 3 Hz at the

frequency of 5~20Hz, and reduced for the case when fv is no

more than 1 Hz over a broad frequency range. Comparison of

vertical floor response spectra at three specific positions for

the different cases is illustrated in Fig. 13. It can be seen that

the vertical floor response spectra have little difference for the

fixed-base case and the case when fv is 63.2 Hz. At the top of



Table 3 e Modal frequencies for the different cases.

Mode no. 1 2 3 4 5 6 7 8 9 10 11

Case 0 Freq. (Hz) 3.850 3.852 5.926 6.282 7.373 8.382 9.444 9.915 10.706 11.020 11.617

direction UYa UX UY UX RZ RZ UX RZ UY UZ

Case 1 Freq. (Hz) 0.432 0.432 0.445 3.677 3.691 7.747 7.869 7.989 9.577 10.023 10.690

direction UY UX RZ RX RY RX RY RY UZ

Case 2 Freq. (Hz) 0.432 0.432 0.445 3.305 3.410 6.503 6.888 7.878 7.971 8.563 9.143

direction UY UX RZ RX RY RX RY UZ RY RX

Case 3 Freq. (Hz) 0.431 0.431 0.445 3.083 3.219 4.407 4.793 4.855 7.876 8.146 8.362

direction UY UX RZ RX RY RY RX UZ RX

Case 4 Freq. (Hz) 0.430 0.430 0.445 2.432 2.613 3.005 3.520 3.647 7.876 8.039 8.194

direction UY UX RZ RX RY UZ RX RY

Case 5 Freq. (Hz) 0.407 0.412 0.445 0.908 0.995 1.018 3.302 3.451 7.875 7.961 8.118

direction UY UX RZ RY RX UZ

Case 6 Freq. (Hz) 0.288 0.307 0.440 0.444 0.555 0.583 3.288 3.441 7.875 7.950 8.111

direction RX UX UZ RZ UX RX

a RX, RY, and RZmean rotation in X, Y, and Z direction, respectively. UX, UY, and UZmean translation in X, Y, and Z direction, respectively. The

direction is shown only when the modal participating mass ratio is > 1%.

Freq., frequency.

Shaded cell: the first vertical translation frequency.

0.1

0.05

0

0.05

0.1

0.1 0.05 0 0.05 0.1

Isol

ator

def

orm

atio

n Y

(m)

Isolator deformation X (m)

fv = 63.2 Hz fv = 8 Hzfv = 5 Hz fv = 3 Hz

0.1

0.05

0

0.05

0.1

0.1 0.05 0 0.05 0.1

Isol

ator

def

orm

atio

n Y

(m)

Isolator deformation X (m)

fv = 3 Hz fv = 1 Hz fv = 0.432 Hz

0.0120.0090.0060.003

00.0030.0060.0090.012

0 5 10 15 20

Isol

ator

def

orm

atio

n Z

(m)

Time (s)

fv = 63.2 Hz fv = 8 Hzfv = 5 Hz fv = 3 Hz

0.10.080.060.040.02

00.020.040.060.080.1

0 5 10 15 20

Isol

ator

def

orm

atio

n Z

(m)

Time (s)

fv = 3 Hz fv = 1 Hz fv = 0.432 Hz

– –

–– –

–– –

––

––––

–––

(A) (B)

(C) (D)

Fig. 9 e Response deformation comparison of the center isolator among the different cases. (A) Horizontal deformation for

Case 1e4. (B) Horizontal deformation for Case 4e6. (C) Vertical deformation for Case 1e4. (D) Vertical deformation for Case

4e6. fv, vertical frequency of the isolation system.


the CS, the case when fv is 63.2 Hz has the maximum vertical

floor response spectra near 11 Hz for the input motion. At the

top of the primary shield wall, the case when fv is 63.2 Hz has

the maximum vertical floor response spectra near 20 Hz,

while other cases take the local maximum spectra at other

frequencies. At the bottom of the CS, the case when fv is 3 Hz

has the maximum vertical floor response spectra near 3 Hz.

Considering that the natural vertical frequency of reactor



6.0E + 6

4.0E + 6

2.0E + 6

0.0E + 0

2.0E + 6

4.0E + 6

6.0E + 6

0 5 10 15 20

Isol

ator

axi

al fo

rce

(N)

Time (s)

fv = 63.2 Hz fv = 8 Hz fv = 5 Hz fv = 3 Hz

6.0E + 6

4.0E + 6

2.0E + 6

0.0E + 0

2.0E + 6

4.0E + 6

6.0E + 6

0 5 10 15 20

Isol

ator

axi

al fo

rce

(N)

Time (s)

fv = 3 Hz fv = 1 Hz fv = 0.432 Hz

8.0E + 56.0E + 54.0E + 52.0E + 50.0E + 02.0E + 54.0E + 56.0E + 58.0E + 5

0 5 10 15 20

Isol

ator

shea

r for

ce X

(N)

Time (s)


8.0E + 56.0E + 54.0E + 52.0E + 50.0E + 02.0E + 54.0E + 56.0E + 58.0E + 5

0 5 10 15 20

Isol

ator

shea

r for

ce X

(N)

Time (s)

fv = 3 Hz fv = 1 Hz fv = 0.432 Hz

8.0E + 56.0E + 5

4.0E + 52.0E + 50.0E + 02.0E + 54.0E + 56.0E + 58.0E + 5

0 5 10 15 20

Isol

ator

shea

r for

ce Y

(N)

Time (s)


8.0E + 56.0E + 54.0E + 52.0E + 50.0E + 02.0E + 54.0E + 56.0E + 58.0E + 5

0 5 10 15 20

Isol

ator

shea

r for

ce Y

(N)

Time (s)

fv = 3 Hz fv = 1 Hz fv = 0.432 Hz

– –

––

– –

–

–

–

–

–––

–––

–––

–––

(A) (B)

(C) (D)

(E) (F)

Fig. 10 e Response force comparison of the center isolator among the different cases. (A) Isolator axial force for Case 1e4. (B)

Isolator axial force for Case 4e6. (C) Isolator shear force X for Case 1e4. (D) Isolator shear force X for Case 4e6. (E) Isolator

shear force Y for Case 1e4. (F) Isolator shear force Y for Case 4e6. fv, vertical frequency of the isolation system.


systems is about 10 Hz, it can be concluded that vertical floor

response spectra can be effectively reduced for the cases

when fv is no more than 3 Hz.

Although the input motion has X component and Y

component in a horizontal direction, the horizontal floor

response spectra in X direction can present the characteris-

tics of the response in a horizontal direction and was chosen

as the output variable for the horizontal responses. Hori-

zontal floor response spectral accelerations in X direction of

the fixed-base case are illustrated in Fig. 14. Similar to the

vertical direction, it can be found that the floor spectra are

amplified in the horizontal direction at frequencies

corresponding to the horizontal direction modes of the CS

and the primary shield wall. For the cases with isolators,

there is a substantial reduction of the horizontal floor spectra

over a broad frequency range, as illustrated in Fig. 15. A

comparison of horizontal floor response spectra at three

specific positions for the different cases is illustrated in

Fig. 16. Horizontal floor response spectral accelerations in X

direction of the CS increase with elevation near 11 Hz for the

cases when fv is 63.2 Hz and 8 Hz, while this trend tends to

disappear for other cases. This is considered to be the

coupling effect between the vertical and the horizontal re-

sponses for the cases when fv is 63.2 Hz and 8 Hz, while the



0

2

4

6

8

10

12

0.1 1 10 100

PSA

(g)

Frequency (Hz)

Input motion EL.101 m EL.66.7 mEL.47.5 m EL.34.7 m EL.23.8 m

00.20.40.60.8

11.21.41.61.8

2

0.1 1 10 100

PSA

(g)

Frequency (Hz)

Input motion EL.58.2 m EL.47.5 mEL.34.7 m EL.23.8 m

(A) (B)

Fig. 11 e Vertical floor response spectral accelerations of the fixed-base model. (A) Containment shell. (B) Primary shield

wall. EL., elevation level; PSA, pseudo spectral accelerations.


vertical response is mitigated for the cases when fv is < 5 Hz.

One interesting point is that for the case when fv is 1 Hz, the

horizontal floor spectra near 0.432e0.8 Hz (frequency of the

isolators) tends to increase with the elevation and the trend

changes at other frequencies. For the case when fv is

0.432 Hz, the horizontal floor spectra near 0.288 Hz (the

0

2

4

6

8

10

12

0.1 1 10 100

PSA

(g)

Frequency (Hz)

Input motion EL.101 mEL.66.7 m EL.47.5 mEL.34.7 m EL.23.8 m

00.5

11.5

22.5

33.5

44.5

5

0.1 1

PSA

(g)

Frequ

Input motionEL.66.7 mEL.34.7 m

0

0.5

1

1.5

2

2.5

0.1 1 10 100

PSA

(g)

Frequency (Hz)


00.10.20.30.40.50.60.70.80.9

1

0.1 1

PSA

(g)

Freque

Input motionEL.66.7 mEL.34.7 m

(A) (B)

(D) (E)

Fig. 12 e Vertical floor response spectral accelerations of the co

fv¼ 8 Hz. (C) fv¼ 5 Hz. (D) fv¼ 3 Hz. (E) fv¼ 1 Hz. (F) fv¼ 0.432 H

system; PSA, pseudo spectral accelerations.

natural frequency of that case) tends to increase with the

elevation, while the horizontal floor spectra decreases first

and then increases as the elevation increases at the fre-

quencies greater than 0.5 Hz. This is due to the rocking effect

of the superstructures. Thus, rocking restraining devices are

necessary for such cases.

10 100ency (Hz)

EL.101 mEL.47.5 mEL.23.8 m

0

0.5

1

1.5

2

2.5

3

0.1 1 10 100

PSA

(g)

Frequency (Hz)


10 100ncy (Hz)

EL.101 mEL.47.5 mEL.23.8 m

00.10.20.30.40.50.60.70.80.9

1

0.1 1 10 100

PSA

(g)

Frequency (Hz)


(C)

(F)

ntainment shell for the isolated model. (A) fv¼ 63.2 Hz. (B)

z. EL., elevation level; fv, vertical frequency of the isolation



0

2

4

6

8

10

12

0.1 1 10 100

PSA

(g)

Frequency (Hz)

Input motion fv = 63.2 Hzfv = 8 Hz fv = 5 Hzfv = 3 Hz fv = 1 Hzfv = 0.432 Hz Fixed base

0

0.5

1

1.5

2

2.5

0.1 1 10 100PS

A (g

)Frequency (Hz)


0

0.5

1

1.5

2

2.5

0.1 1 10 100

PSA

(g)

Frequency (Hz)


(A) (B) (C)

Fig. 13 e Comparison of vertical floor response spectral accelerations for the different cases. (A) At the top of the

containment wall. (B) At the bottom of the containment wall. (C) At the top of the primary shield wall. fv, vertical frequency of

the isolation system; PSA, pseudo spectral accelerations.

0

2

4

6

8

10

12

0.1 1 10 100

PSA

(g)

Frequency (Hz)

Input motion EL.101 m EL.66.7 mEL.47.5 m EL.34.7 m EL.23.8 m

02468

101214161820

0.1 1 10 100

PSA

(g)

Frequency (Hz)

Input motion EL.58.2 m EL.47.5 mEL.34.7 m EL.23.8 m

(A) (B)

Fig. 14 e Horizontal floor response spectral accelerations in X direction of the fixed-base model. (A) Containment shell. (B)

Primary shield wall. EL., elevation level; PSA, pseudo spectral accelerations.


3.4. Response displacements and spectral displacementsof the superstructure

Comparisons of response displacements at the top and at the

bottom of the CS are illustrated in Figs. 17 and 18, respectively,

for the different cases. Response displacement difference be-

tween the top and the bottom of the CS is illustrated in Fig. 19.

For the caseswhen fv is between 63.2 Hz and 3 Hz, the response

displacement of the superstructures is in translationmode. For

the caseswhen fv is 1Hzor 0.432Hz, the responsedisplacement

of the superstructures shows a rocking mode, and the vertical

response displacement has a larger amplitude while the

displacement difference between the top and the bottom has a

smaller amplitude compared with the other cases. It is in

agreement with the isolation characteristics in the vertical di-

rection for the different cases. Comparing the response dis-

placements for thevertical andhorizontal directionsat thebase

of the structure, it suggests that the vertical displacements of

the bearings are as large or even larger than the horizontal

displacements in the case where fv is 0.432 Hz.

Comparisons of response spectral displacements at the top

and at the bottom of the CS are illustrated in Fig. 20 and in

Fig. 21, respectively, for the different cases. In the horizontal

direction, the response spectral displacement is reduced at

the frequencies over 0.7 Hz, is amplified at frequencies less

than 0.7 Hz, and shows different trends at the top and at the

bottom for the different cases because of the rocking effect. In

the vertical direction, the response spectral displacement is

amplified near the frequency fv, but is negligible at the inter-

ested frequencies over 10 Hz.

4. Discussion

Verticaland3Disolationsystemsandtheirpotentialapplication

to modern nuclear facilities are investigated in this paper. Two

approaches to isolate vertical groundmotion canbe selected for



0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100PS

A (g

)Frequency (Hz)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


(A) (B) (C)

(D) (E) (F)

Fig. 15 e Horizontal floor response spectral accelerations in X direction of the containment shell for the isolated model. (A)

Vertical frequency of the isolation system (fv )¼ 63.2 Hz. (B) fv¼ 8 Hz. (C) fv¼ 5 Hz. (D) fv¼ 3 Hz. (E) fv¼ 1 Hz. (F) fv¼ 0.432 Hz.

EL., elevation level; PSA, pseudo spectral accelerations.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


0

0.1

0.2

0.3

0.4

0.5

0.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.1 1 10 100

PSA

(g)

Frequency (Hz)


(A) (B) (C)

Fig. 16 e Comparison of horizontal floor response spectral accelerations in X direction for the different cases. (A) At the top of

the containment shell. (B) At the bottom of the containment shell. (C) At the top of the primary shield wall. fv, vertical

frequency of the isolation system; PSA, pseudo spectral accelerations.




0.20.15

0.10.05

00.05

0.10.15

0.2

0 5 10 15 20

Hor

izon

tal d

ispl

acem

ent (

m)

Time (s)

fv = 63.2 Hz fv = 8 Hz fv = 5 Hzfv = 3 Hz fv = 1 Hz fv = 0.432 HzFixed base

0.120.1

0.080.060.040.02

00.020.040.060.08

0.1

0 5 10 15 20

Ver

tical

dis

plac

emen

t (m

)

Time (s)


– ––

––––

––

–

(A) (B)

Fig. 17 e Comparison of response displacement at the top of the containment shell. (A) Horizontal (X direction). (B) Vertical.

fv, vertical frequency of the isolation system.

0.120.1

0.080.060.040.02

00.020.040.060.08

0.1

0 5 10 15 20

Hor

izon

tal d

ispl

acem

ent (

m)

Time (s)


0.120.1

0.080.060.040.02

00.020.040.060.080.1

0 5 10 15 20

Ver

tical

dis

plac

emen

t (m

)

Time (s)


– ––

––––

–––––

(B)(A)

Fig. 18 e Comparison of response displacement at the bottom of the containment shell. (A) Horizontal (X direction).

(B) Vertical. fv, vertical frequency of the isolation system.

0.15

0.1

0.05

0

0.05

0.1

0.15

0 5 10 15 20

Hor

izon

tal d

ispl

acem

ent (

m)

Time (s)

fv = 63.2 Hz fv = 8 Hz fv = 5 Hzfv = 3 Hz fv = 1 Hz fv = 0.432 Hz

0.0050.0040.0030.0020.001

00.0010.0020.0030.004

0 5 10 15 20

Ver

tical

dis

plac

emen

t (m

)

Time (s)

fv = 63.2 Hz fv = 8 Hz fv = 5 Hzfv = 3 Hz fv = 1 Hz fv = 0.432 Hz

– –––––

–

–

(A) (B)

Fig. 19 e Response displacement difference between the top and the bottom of the containment shell. (A) Horizontal

(X direction). (B) Vertical. fv, vertical frequency of the isolation system.




0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.1 1 10 100Hor

izon

tal s

pect

ral d

ispl

acem

ent (

m)

Frequency (Hz)

Input motion fv = 63.2 Hz fv = 8 Hzfv = 5 Hz fv = 3 Hz fv = 1 Hzfv = 0.432 Hz Fixed base

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.1 1 10 100

Ver

tical

spec

tral d

ispl

acem

ent (

m)

Frequency (Hz)


(A) (B)

Fig. 20 e Comparison of response spectral displacements at the top of the containment shell. (A) Horizontal (X direction). (B)

Vertical. fv, vertical frequency of the isolation system.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.1 1 10 100

Hor

izon

tal s

pect

ral d

ispl

acem

ent (

m)

Frequency (Hz)


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.1 1 10 100

Ver

tical

spec

tral d

ispl

acem

ent (

m)

Frequency (Hz)

Input motion fv = 63.2 Hz fv = 8 Hzfv =5 Hz fv = 3 Hz fv = 1 Hzfv = 0.432 Hz Fixed base

(A) (B)

Fig. 21 e Comparison of response spectral displacements at the bottom of containment shell. (A) Horizontal. (B) Vertical. fv,

vertical frequency of the isolation system.


consideration for the next generation nuclear power plant. One

is where the entire building utilizes 3D base isolation technol-

ogy, and the other iswhere vertical isolation is provided for key

components within a horizontally isolated building. While no

NPPhasbeenconstructedusingverticalor3Disolationsystems,

there have beenmany efforts to develop them.

Moreover, a series of case study analyses of a Generation

III pressurized water reactor NPP model are performed to

examine the benefits and challenges associated with 3D

isolation compared with horizontal isolation. It is found that:

(1) the vertical response spectrum of the superstructure is

amplified near the frequency of the vertical isolation system.

But compared with the general horizontal isolators, isolators

that have vertical frequency fv of no more than 3 Hz can

effectively reduce the vertical in-structure responses for the

studied NPP model.; (2) among the studied cases, the case

when fv is 3 Hz is the one that can keep the horizontal period

of the isolators as the first period while having the most

flexible vertical isolator properties. For the cases when fv is

between 63.2 Hz and 3 Hz, the response displacement of the

superstructures is in translation mode. Thus, a 3D isolation

system which has a vertical frequency of about 3 Hz is

feasible for the studied NPP model; (3) Comparing the

response displacement for the vertical and horizontal di-

rections at the base of the structure suggests the vertical

bearing displacements are as large as, or even larger than,

the horizontal displacements in the case fv being 0.432 Hz;

and (4) when the vertical frequency of isolators reduces to

1 Hz, the rocking effect is obvious and maximum horizontal

in-structure responses can decrease first and then increase

with increasing elevation. The fundamental frequency of the

system is no longer controlled by the horizontal stiffness of

isolators. Rocking restraining devices will be required for

such cases.

For structures containing equipment and other items

sensitive to accelerations, such as in NPPs, considerable

attention should be placed on bearing stability and the hori-

zontalevertical coupling behavior that can occur in different

isolation systems to a varying degree. Linear analysis was

used in this study; however, results show that future research




on this subject will benefit from nonlinear analysis and an

advanced bearing model that can account for the coupling

behavior. Moreover, European Utility Requirements motions

are used for this study, but analysis considering long-period

motions or other ground motion characteristics such as

those described in Nuclear Regulatory Commission RG1.60 is

expected for future work.

Conflicts of interest

All authors have no conflicts of interest to declare.

Acknowledgments

The authors wish to gratefully acknowledge the support for

thiswork by theNational Natural Science Foundation of China

under Grant Numbers 51478357 and 51308418, National Sci-

ence and Technology Pillar ProgramNumber 2015BAK17B04, a

cooperative research program funded by KEPCO Engineering

and Construction Company Inc., and a research program

funded by the Electric Power Research Institute.

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