siler seismic-initiated events risk mitigation in lead

EUROPEAN COMMISSION 7th EURATOM FRAMEWORK PROGRAMME 2007-2013

THEME [Fission-2011-2.3.1] [R&D activities in support of the implementation

of the Strategic Research Agenda of SNE-TP]

SILER Seismic-Initiated events risk mitigation

in LEad-cooled Reactors

Grant Agreement N°: 295485

Deliverable title: Design of the foundation

Work

Pakage Deliverable

number Lead contractor Date

5 D 5.1 Numeria December 2013

Responsible person details name: Alberto Dusi telephone: +39 0372 36610 email: [email protected]

Starting date Due date Actual date Delay* Nature

Month 9 Month 21 Month 27 6 months PU Description of the activities: Task 5.4 is devoted to the structural issues that need to be addressed when adopting the

base isolation concept in nuclear plants.

Activities focus on the conceptual design of the LFR and ADS base isolated plants foundations

by identifying requirements, specifying performance needs and providing criteria for actual

design of base isolated nuclear power plants. The definition of a procedure for the

installation and replacement of the isolators is also addressed.

Task 5.4 will also provide input for WP6 “Recommendations for standardization”. SIGNATURES Author: A. Dusi, Numeria WP Leader: SCK-CEN Coordinator: M. Forni, ENEA

Project no: 295485

Project title: Seismic-Initiated events risk mitigation in LEad-cooled Reactors

Project acronym: SILER

Deliverable D5.1

DESIGN OF THE FOUNDATION AND ISOLATION SLAB

PROJECT N° 295485 PROJECT

TITLE SILER: Seismic-Initiated events risk mitigation in LEad-cooled Reactors

DOC Deliverable 5.1 DATE December 2013 REV. 00

DOC. NO: N2012-295485- DELIVERABLE D5.1

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INDEX

1 Foreword....................................................................................................................................... 4

2 Base isolated nuclear islands ........................................................................................................ 5

2.1 The accelerator driven system technology: MYRRHA nuclear island ................................... 5

3 Seismic input ................................................................................................................................. 9

4 Conceptual design of foundations .............................................................................................. 27

4.1 General design provisions ................................................................................................... 27

4.1.1 Applicability of existing European standards ............................................................... 28

4.1.2 Control undesirable torsional movements .................................................................. 29

4.1.3 Minimize different behaviour of isolating devices ...................................................... 29

4.1.4 Control of differential seismic ground motions ........................................................... 29

4.1.5 Ductility demand .......................................................................................................... 30

4.1.6 Arrangement of devices ............................................................................................... 30

4.1.7 Devices protection ....................................................................................................... 30

4.1.8 Measure against lightening: ......................................................................................... 30

4.1.9 Fail safe system ............................................................................................................ 30

4.1.10 Re-centering of the isolation system after an earthquake .......................................... 31

4.1.11 Design of a base isolated structure: method of analysis ............................................. 31

4.1.12 Design forces ................................................................................................................ 32

4.1.13 Combination of horizontal and vertical seismic loads ................................................. 32

4.1.14 Effects of P-delta loads ................................................................................................ 32

4.1.15 Soil containment walls ................................................................................................. 33

4.2 Numerical analyses .............................................................................................................. 34

4.2.1 Other issues investigated ............................................................................................. 53

4.3 Procedure for bearings installation, inspection and replacement ..................................... 55

4.3.1 Procedure for bearings installation ............................................................................. 55

4.3.2 Procedure for bearings inspection and maintenance ................................................. 55

4.3.3 Procedure for bearings replacement ........................................................................... 59

5 Conclusions ................................................................................................................................. 63

6 References .................................................................................................................................. 64


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1 FOREWORD

The SILER project aims at studying the risk associated with events triggered by earthquakes on

Generation IV Heavy Liquid Metal Cooled reactors, addressing the subsequent mitigation

measures and focusing on seismic isolation strategies and devices.

Aim of the SILER project WP5 is the design and manufacturing of new interface components

connecting the isolated and non-isolated parts of the plant, as well as the design of auxiliary

structures or components requiring a specific design in case of isolation of the system.

Within the activities of WP5, Task 5.4 is devoted to the structural issues that need to be addressed

when adopting the base isolation concept in nuclear plants. Indeed, the isolation system consists

not only of the isolator units but also of the entire collection of structural elements required for

the system to function properly.

Task 5.4, therefore, focused on the conceptual design of the LFR and ADS base isolated plants

foundations by identifying requirements, specifying performance needs and providing criteria for

actual design of base isolated nuclear power plants.

Criteria for the design of either the superstructure and substructure in DBE and BDBE conditions

have been assessed. In particular, aspects related to elastic behaviour requirements, control of

deformations, specifications/limitations on ductility demand on the superstructure, deformability

limits for piled foundations have been investigated by linear and nonlinear analyses.

The definition of a procedure for the installation and replacement of the isolators have also

addressed.

Task 5.4 activities will also provide input for WP6 “Recommendations for standardization”.

This report is the contractual deliverable D5.1 of the EC project n. 295485 “SILER: Seismic-Initiated

events risk mitigation in LEad-cooled Reactors”.


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2 BASE ISOLATED NUCLEAR ISLANDS

As agreed during the Annual meeting held in Bologna on October 30th

2012, Numeria’s task 5.4

activities are focused on the MYRRHA nuclear island, being the ADS base isolated plants

characterized by bigger dimensions and higher mass than LFR. However, results obtained can be

considered valid, at least from the conceptual and methodological points of view, also for LFR

plant.

2.1 The accelerator driven system technology: MYRRHA nuclear island

An accelerator driven system (ADS) is typically made of two main buildings or facilities in terms of

layout: the reactor building, housing the Reactor Vessel with the target, and the LINAC

Accelerator. While the reactor building is located at the end of the beam so that the beam aligns

with the target within the reactor vessel, the accelerator building, the associated RF workshop and

the lab are placed at the beginning of the beam. Adjacent to the reactor building, other facilities

are present, among which are the heat, ventilation and air conditioning system (HVAC) chimney,

the waste building and the spent fuel building. The rest of buildings of the facility are totally

independent of the Reactor building and the beam tunnel from the structural point of view.

The reactor building is a rectangular box-type structure, formed by shear walls, whose plan

dimensions are 90 x 49 m.

The next figure show a plan view of the MYRRHA (Multi-purpose Hybrid Research Reactor for

High-tech Applications) reactor building.

Fig.1 MYRRHA reactor building plan view

The reactor vessel has an outer diameter of about 8.14 m and a high of 11.2 m.


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Fig.2 MYRRHA reactor vessel – 3D view

Geometry, structural layout, materials and loads are taken from the SILER Deliverable D2.1, Part B

“Description of Systems: ADS”, issued on July 11, 2012 by Empresarios Agrupados Internacional,

S.A. [1] and its appendixes.

Two base isolation layouts have been proposed for MYRRHA: one is made of High Damping Rubber

Bearings (HDRBs) and one made use of Lead Rubber Bearings. Detailed information on the design

of the base isolated configurations can be found in [2,], while isolators characteristics are reported

in deliverables D4.1 and D4.2, [3], [4].

In the following, the main information relevant to the two base isolated configuration are

reported.

The whole MYRRHA reactor building has been isolated by making use of HDRBs or, alternatively

with the aim of reducing seismic displacements, of LRBs. As shown in the next Figure, the isolators

are placed at the base of the reactor building.

Base isolation level

Fig.3 MYRRHA reactor building in its base isolated configuration

The HDRB and LRB-based isolation systems proposed are made of an overall number of 339

elastomeric bearings of two types: 80 devices are of type A and 259 of type B.


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Bearings types and locations were chosen so as to have a good coincidence of the centre of mass

with the centre of stiffness. Bearings type A (in red in the next figure) are placed at the slab

corners (with a spacing of 2,0 m), while bearings type B are disposed according to the layout

shown in figure at an equal spacing of 4,0 m. Spacing has been defined taking into account

construction procedure as well as seismic isolators maintenance and replacing.

Figure 4 shows the isolation layout when using HDRBs or LRBs as isolating devices.

Fig.4 MYRRHA base isolated layout (in red are isolators type A)

Table 1 summarize the main design parameters of type A and type B HDRBs as per final design.

Tab. 1 – HDRBs main parameters

Type A Type B

Plan size Diam. 1600 mm Diam. 1050 mm

Displacement at DBE dbd (mm) 300 300

Horizontal stiffness (kN/mm) at dbd 9.88 4.18

Equivalent viscous damping (%) at dbd 10% 10%

Vertical stiffness (kN/mm) 8724 4229

Table 2 summarize the main design parameters of type A and type B LRBs as per their final design.

Tab. 2 – LRBs main parameters

Type A Type B

Plan size 1250 mm x 1250 mm Diam. 900 mm

Displacement at DBE dbd (mm) 161 161

Horizontal stiffness (kN/mm) at dbd 16.43 4.81

Equivalent viscous damping (%) at dbd 28.7 27.1

Vertical stiffness (kN/mm) 9105 3404


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As previously said, LRB-based isolation option was considered in order to limit seismic

displacements. LRBs are, actually, characterized by higher values of damping and stiffness than

HDRBs, thus effectively contributing to earthquake induced displacements.

It is, however, worthwhile recalling that when using devices with high values of damping, possible

adverse effects of damping in seismic isolated structures has to be carefully considered. The effect

of damping in higher mode response has been widely studied and results are published in

literature [5], [6]; analyses carried out on the MYRRHA nuclear island confirmed that the isolators

displacement and structural base shear may be reduced thanks to higher damping, but the floor

accelerations are increased.

As far as the numerical simulations specifically carried out in order to assess foundations’

requirements and performance needs are concerned, considering that structural forces are higher

when using LRBs, in the following reference is made only to the MYRRHA LRB-based isolated

configuration.

Nonlinear characteristics of the LRBs, given by the isolators manufacturer and based on the

experimental tests carried out are reported in the following figures. These bilinear curves have

been used in Finite Element (FE) analyses.

K b = 16.43 kN/mm

ξ b = 28.7 %

= 2645 kN

= 1447 kN

= 15.5 mm = 161 mm

VEbd

V1

d1 dbd

Forc

e

Displacement

Fig.5 Bilinear curve at DBE of LRB type A

Kb = 4.81 kN/mm

ξb = 27.1 %

= 774 kN

= 403 kN

= 15.3 mm = 161 mm

VEbd

V1

d1 dbd

Forz

a

Spostamento

Fig.6 Bilinear curve at DBE of LRB type B


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3 SEISMIC INPUT

According to the indications given in the contractual document [1], seismic input is defined by two

types of spectra, depending on the type of soil considered. The analyses have be carried out based

on these spectra.

For hard soils, the seismic input is defined by the spectra given in RG 1.60 [8] extended to Central

and Eastern USA, while for soft soils the elastic spectrum is defined according to EN 1998-1 [9] for

spectrum type 1, soil type E.

The following figures show the selected spectra.

Fig.7 Modified RG 1.60 elastic response spectrum – horizontal component

Fig.8 Modified RG 1.60 elastic response spectrum – vertical component


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Fig.9 EC8 elastic response spectrum – horizontal component

Fig.10 EC8 elastic response spectrum – vertical component

For each type of soil, and therefore for each type of spectrum, terns of artificial spectrum-

compatible time-histories have been developed by Empresarios Agrupados Internacional and are

reported in [7], Appendix F. The PGA considered is 0.3g in DBE conditions. In particular, the

following sets of accelerograms have been given by Empresarios Agrupados and have been used in

the numerical analyses:

� OBE (Operating Basis Earthquake) based on RG 1.60 spectra

� DBE (Design Basis Earthquake) based on RG 1.60 spectra


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� BDBE (Beyond Design Basis Earthquake) based on RG 1.60 spectra

� OBE (Operating Basis Earthquake) based on EC-8 spectra

� DBE (Design Basis Earthquake) based on EC-8 spectra

� BDBE (Beyond Design Basis Earthquake) based on EC-8 spectra

In addition to the accelerograms provided by Empresarios Agrupados, further synthetic

accelerograms have been defined by Numeria and SINTEC for DBE consistent with EC-8 spectra

(see next figures) and used the analyses. Considering that a base isolated structure has a relatively

low natural frequency, in the definition of the abovementioned set of accelerograms attention has

been paid to the fact the artificial ground motions contain low frequency components.


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-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

0,4

0 5 10 15 20 25 30 35 40

acceleration (g)

time (s)

T1 N H1 DBE

Fig.11 Synthetic accelerogram N1 – horizontal component 1

Period [sec]43210

Res

pons

e A

ccel

erat

ion

[g]

0,75

0,7

0,65

0,6

0,55

0,5

0,45

0,4

0,35

0,3

0,25

0,2

0,15

0,1

0,05

0

Fig.12 Synthetic accelerogram N1: horizontal component 1 - response spectrum


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-0,4

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ele

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(g

)

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T1 N H2 DBE


Period [sec]43210

Res

pons

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ccel

erat

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[g]

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-0,4

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0,2

0,3

0,4

0 5 10 15 20 25 30 35 40

acc

ele

rati

on

(g

)

time (s)

T1 N DBE V

Fig.15 Synthetic accelerogram N1 – vertical component

Period [sec]43210

Res

pons

e A

ccel

erat

ion

[g]

0,850,8

0,750,7

0,65

0,60,55

0,50,450,4

0,350,3

0,25

0,20,150,1

0,050

Fig.16 Synthetic accelerogram N1: vertical component - response spectrum


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)

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Period [sec]43210

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[g]

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T2 N H2 DBE


Period [sec]43210

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ccel

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[g]

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(g

)

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T2 N V DBE


Period [sec]43210

Res

pons

e A

ccel

erat

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[g]

0,85

0,80,75

0,70,65

0,6

0,550,5

0,450,4

0,35

0,30,25

0,20,15

0,10,05

0



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T3 N H1 DBE


Period [sec]43210

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ccel

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[g]

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Period [sec]43210

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ccel

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[g]

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(g

)

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T3 N V DBE


Period [sec]43210

Res

pons

e A

ccel

erat

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[g]

0,850,8

0,75

0,7

0,65

0,6

0,55

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0,35

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Period [sec]43210

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[g]

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Period [sec]43210

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ccel

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[g]

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0,75

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Period [sec]43210

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(g

)

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T5 N V DBE


Period [sec]43210

Res

pons

e A

ccel

erat

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[g]

0,90,850,8

0,750,7

0,650,6

0,550,5

0,45

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4 CONCEPTUAL DESIGN OF FOUNDATIONS

In a base isolated nuclear plant, substructure (foundations) and structural element above the

isolation interface that must guarantee a rigid diaphragm are probably the more sensible and

expensive structures to be built (together with the lateral containment wall, for the part of the

plant underground, if the case).

As previously said, task 5.4 activities are focused on the MYRRHA nuclear island, being the ADS

base isolated plants characterized by bigger dimensions and higher mass than LFR, thus resulting

in a more severe set of forces to be considered in the conceptual design. However, results

obtained can be considered valid, at least from the conceptual and methodological points of view,

also for LFR plant.

The only standard, at present, specifically addressed to seismically isolated nuclear plant are the

guidelines issued by the Japan Electric Association (JEA), Nuclear Standard Committee of JEA,

“Design and Technical Guideline of Seismic Isolation Structure for Nuclear Power Plant”, JEAG

4614-2000, (in Japanese, only); in the United States of America, the U.S. Nuclear Regulatory

Commission is working on a document titled, “Technical Consideration for Seismic Isolation of

Nuclear Facilities”, whose latest draft version has been issued in November 2012.

Considering that no European Standard nor European guidelines for the design of seismically

isolated nuclear facilities exist, in the following, reference has been made to existing international

standards dealing with the design of base isolated structures, and, in particular, to the set of the

Eurocodes. Activities carried out in Task 5.4 aimed mainly at evaluating if additional requirements

with respect to those reported in the relevant European set of codes, dealing with civil buildings

and viaducts, have to be specified for nuclear plants.

4.1 General design provisions

An isolation system consists not only of the isolator units but also of the entire collection of

structural elements required for the system to function properly. The isolation system (Figure 41)

typically includes segments of columns and connecting girders just above the isolator units

because such elements resist moments (due to isolation system displacement) and their yielding

or failure could adversely affect the stability of isolator units.


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Isolation interface Isolator unit

Structural element that transfer

force between isolator units

Structure below the

isolation system

Fig.41 Typical isolation system

Based on the theory of base isolation and on the critical review of International Standards, as well

as on the results obtained from the activities carried out on MYRRHA, the following list of general

design provisions can be proposed for a base isolated nuclear plant.

4.1.1 Applicability of existing European standards

From the structural point of view, in the design of a base isolated nuclear plant the

fundamental requirements stated in the theory of base isolation [9], [10], [11] and in

internationally recognized standards, like Eurocode [8], shall be complied with.

These requirements result in a different structural arrangement with respect to the one

normally used in conventional (i.e. fixed-based) structure.

The next Figure shows an example of a base isolated petrochemical facility [12]; a

detailed view of the substructure elements is also shown.


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Fig.42 Base isolated Liquefied Natural Gas tanks

4.1.2 Control undesirable torsional movements

The effective centre of stiffness and the centre of damping of the isolation systems should

be as close as possible to the projection of the centre of mass on the isolation interface.

If there is a large distance between the centre of stiffness and centre of mass, its impact

on the behaviour of the seismic isolated structure shall be evaluated and it shall be

ensured that the seismic isolation device are able to provide the required seismic

isolation function.

4.1.3 Minimize different behaviour of isolating devices

The compressive stress induced in the isolating device by the permanent actions should

be as uniform as possible.

4.1.4 Control of differential seismic ground motions

Structural elements located above and below the isolation interface should be sufficiently

rigid in both horizontal and vertical directions.

A rigid diaphragm shall be provided above and below the isolating system, consisting of a

reinforced concrete slab or a grid of tie-beams, designed taking into account all relevant

local and global modes of buckling.

The foundation (and grout) directly below the isolator must have sufficient strength to

support concentrated bearing loads (for all possible lateral displacements of the isolator).

Similarly, the structure just above the isolator must have sufficient strength to transfer

concentrated bearing loads.

The devices constituting the isolation system shall be fixed to both end to the rigid

diaphragms defined above.


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4.1.5 Ductility demand

Limited ductility demand is considered necessary for proper functioning of the isolation

system. Limiting ductility demand on the superstructure has also the additional benefit of

meeting the performance objective of damage control.

No ductility demand shall be expressed on a base isolated structure, including a nuclear

plant.

4.1.6 Arrangement of devices

The seismic isolation devices shall maintain the required function throughout the in-

service period. To ensure that, it is necessary to inspect the base isolated structure

periodically.

Sufficient space between the superstructure and substructure shall be provided, together

with any other necessary arrangements, to allow inspection, maintenance and

replacement of the devices during the lifetime of the structure.

After a large ground motion that might have caused the seismic response beyond the

design limit or the seismic devices, the utility shall consider removing the seismic

elements for testing and checking

4.1.7 Devices protection

Devices shall be protected against fire, flood, chemical or biological attack.

appropriate prescriptions have to specified in order to guarantee the protection of the

isolating devices. Efficient protection can, for instance, be fulfill thanks to the use of

special joints to cover the seismic gap.

4.1.8 Measure against lightening:

When rubber-based devices are used in seismic isolation, lightening protection measures

shall be taken because the rubber electrically isolates the building form the ground. For

protection against lightening, measures such as electrical connection between the

superstructure and the substructures to prevent the occurrence of any major potential

difference shall be taken. Ground cables shall have a sufficient length so as to

accommodate relative displacements of the isolated structure during a seismic event.

4.1.9 Fail safe system

Sufficient space shall be left around the devices to allow free movement with no

hammering.

If a fail-safe system is present, there shall be enough clearance between the seismic

isolation devices and the excessive displacement stopper so that the function of the

seismic isolation device is kept.

Effects of impact on the fail-safe system shall be considered in the design.


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4.1.10 Re-centering of the isolation system after an earthquake

Adequate reaction elements shall be provided in order to re-centre the devices, if residual

displacements (incompatible with the serviceability of the building and/or with the

correct behaviour of the isolating system) are likely to occur after an earthquake.

The position of the superstructure after an earthquake has to be checked. If the

superstructure has not gone back to the initial position, the utility shall consider moving it

back to the initial position or shall demonstrate that the residual displacement of the

seismic isolation would not affect required function of the devices.

4.1.11 Design of a base isolated structure: method of analysis

With reference to the characteristics of the isolated structure and of the isolation system,

the following types of analyses can, in general, be used:

1. linear static

2. linear dynamic (response spectrum modal analysis)

3. non linear dynamics

Non linear dynamics is recommended for applications to nuclear facilities. As a general

rule, seismic analysis of base isolated nuclear structures shall be conducted using time

history analysis. When seven of more records are used, the average value of the

parameter of interest is calculated from the analysis and is used for design verification in

the direction of interest (e.g., story shear in the x direction). This process is repeated for

ground motion records scaled to OBE and BDBE spectral accelerations.

Actually, time history analysis approach is not a particularly useful design tool due to the

complexity of results, the number of analyses required (to account for different locations

of eccentric mass), the need to combine different types of response at each point in time,

etc.. However, time history analysis is most useful when used to verify a design by

checking a few key design parameters, such as isolation displacement, overturning loads

and uplift and story shear force.

Although a large number of data are generated by these analyses, only a limited number

of response parameters is required for design verification. Response parameters of

interest include, for instance, peak isolation system displacement, peak story shear

forces, peak downward load on any isolator unit and peak uplift displacement of any

isolator unit.

Even if the international standards do not require the response of individual records to be

used for design, or design verification, when seven or more records are used, results

obtained by the numerical simulation carried out by Numeria within the framework of

Task 5.4 activities indicate that it is prudent to consider the implications of the results of

individual record analyses and it is helpful to use ELF formulas to evaluate RHA results

(serving as a sanity check on results).

It is recommended, for designers, to consider the consequences of the event that the DBE

(BDBE) displacement is exceeded in an actual earthquake. In this case, as reported in

SILER deliverable 5.2 [13], for a nuclear plants, displacements larger than BDBE

displacement would not cause damage to the bearings neither catastrophic failure of the


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superstructure thanks to the presence of a fail-safe system. Yielding and inelastic

response of either the substructure and the superstructure shall be avoided also for BDBE

earthquakes.

The structural components of the foundation and those constituting the rigid diaphragm

above the isolation system shall also be designed in such a way to elastically resist to the

impact on the surrounding horizontal fail-safe system, should the latter be present.

4.1.12 Design forces

In International Standards, forces to be considered for design of isolated structures are

different for design of the superstructure and design of the isolation system and other

elements of the structure below the isolation system (i.e., the foundation). In both cases,

however, use of the maximum effective stiffness of the isolation system is required to

determine a conservative value of design force.

In order to provide appropriate overstrength, peak design earthquake response (without

reduction) shall used directly for design of the isolation system and the structure below.

Design for unreduced design earthquake forces is considered sufficient to avoid inelastic

response or failure of connections and other elements for ground shaking as strong as

that associated with the Maximum Considered Earthquake (MCE).

Design earthquake response can be reduced by a modest factor for design of the

superstructure above the isolation interface, as suggested in the Eurocode.

4.1.13 Combination of horizontal and vertical seismic loads

When addressing the combination of horizontal and vertical seismic loads in the design of

seismic isolated structures, the use of the Square Root of Sum of Squares (SRSS) method

may results in non-conservative estimations (as in the design of conventional, non-

isolated structures). Therefore, an appropriate method, such as taking the sum of

absolute values, taking the algebraic sum of the time history of seismic loads in the

horizontal and vertical directions, and horizontal/vertical simultaneous input analysis,

shall be considered.

It is necessary to evaluate whether the coupling behaviour of vertical seismic force caused

by rocking motion due to horizontal and vertical ground motion would take place, [14].

4.1.14 Effects of P-delta loads

The effects of P-delta loads on the isolation system and adjacent elements of the

structure can be quite significant. The compression load, P, can be large due to

earthquake overturning (and factored gravity loads) at the same time that large

displacements occur in the isolation system.

Computer analysis programs (most of which are based on small-displacement theory)

may not correctly calculate P-delta moments at the isolator level in the structure above or

in the foundation below.

The next Figure illustrates moments due to P-delta effects (and horizontal shear loads) for

an elastomeric bearing isolation system. For the elastomeric system, the P-delta moment

is split one-half up and one-half down.


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Fig.43 P-delta effect

4.1.15 Soil containment walls

The surrounding walls that are constructed between the building and the surrounding soil

shall have a level of seismic safety that is as good as the base isolated structure. This

provision is intended to prevent damage or collapse of surrounding walls that might lead

to the loss of seismic isolation functions of the isolated structures; it shall not be

interpreted as a demand on the safety function of surrounding walls themselves.


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4.2 Numerical analyses

In order to check the performances of a foundation designed according to the requirements

reported in the previous chapter and to assess the validity of the given requirements/provisions as

well as their impact and implications in a real construction process, several non linear dynamic

analyses have been carried out by making reference to the MYRRHA nuclear island in its LRB-based

seismically isolated configuration (see paragraph 2.1).

Finite Element analyses have been made using SAP2000 commercial software [15].

Only the isolation system has been considered non linear in the model, whereas the reinforced

concrete structure was modeled as elastic.

The rigid diaphragms above and below the isolating system consist of two reinforced concrete

slabs. Slabs cross sections have been assumed to be equal to those used in the conventional (i.e.

fixed-base) nuclear island. As demonstrated by the calculations made, slab sections used in

conventional design are more than sufficient to guarantee a proper functioning of the isolation

system.

As previously said, isolators are placed at the base of the reactor building, below the ground level.


Ground level

Fig.44 MYRRHA reactor building in its base isolated configuration

As far as the type of foundation is concerned, the following considerations can be made.

As in all the base isolated structures, the slab above the isolation interface will channel the plant

loads to the isolators, but then there is the need to distribute the isolators loads to the ground.

This requires either a second bottom slab or piles. The decision between the two depends purely

on the geotechnical characteristics of the ground, not on the isolators. In any case, attention has

to be paid to different vertical loads acting on isolators/pits, especially for very soft soil.

The foundation slab and the slab above the isolators were both modeled through shell elements

with high in-plan stiffness.


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Three different Finite Element Models (FEM) were used in the analyses carried out:

- Model 1: the nodes of the foundation slab are fully restrained, (see Figure 45)

- Model 2: the foundation mat rest on gap (compression only) links simulating the presence

of the soil, (see Figure 46)

- Model 3: the foundation mat rest on piles modeled with beam elements (see Figure 47)


Ground level

Fig.45 Base isolated MYRRHA reactor building – FE model 1


Ground level



Ground level



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Preliminary analyses were initially carried out, on the simple FEM shown in Figure 45, by using

response spectra in order to assess the main requirements of the base isolated plant foundation.

From the conceptual point of view, the leading criteria adopted for the design of the structural

elements of the substructure and of the foundations are their elastic behaviour and the control of

deformations.

The substructure conceptual design has been made considering the inertia forces directly applied

to it and the forces and moments transmitted by the isolation system, the superstructure being in

the linear elastic domain (q = 1).

As far as the superstructure is concerned, the analyzed nuclear plants are, because of structural

elements dimensions and arrangements, characterized by an inherently rigid body behavior;

therefore the rigid superstructure requirement is automatically satisfied.

As expected, base isolation turned out to be beneficial for the analyzed nuclear plant.

Results reported hereinafter are relevant to the configuration shown in Figure 45 (FE model n. 1).

TABLE: Modal Participating Mass Ratios

OutputCase StepType StepNum Period UX UY SumUX SumUY

Text Text Unitless Sec Unitless Unitless Unitless Unitless

MODAL Mode 1 2,211382 0,97093 0,02386 0,97093 0,02386

MODAL Mode 2 2,185047 0,02682 0,94387 0,99775 0,96773

MODAL Mode 3 1,763794 0,00224 0,02128 0,99999 0,989

MODAL Mode 4 0,177453 1,579E-07 0,00006587 0,99999 0,98907

MODAL Mode 5 0,17645 1,355E-10 0,0014 0,99999 0,99047

MODAL Mode 6 0,171761 1,235E-09 0,00028 0,99999 0,99075

MODAL Mode 7 0,167392 5,529E-08 0,00003525 0,99999 0,99079

MODAL Mode 8 0,164169 6,775E-08 1,039E-07 0,99999 0,99079

MODAL Mode 9 0,155325 5,022E-08 0,00000328 0,99999 0,99079

MODAL Mode 10 0,148675 2,643E-09 3,989E-09 0,99999 0,99079

Fig.48 HDRB base isolated MYRRHA reactor building – dynamic characteristics

TABLE: Modal Participating Mass Ratios

OutputCase StepType StepNum Period UX UY SumUX SumUY

Text Text Unitless Sec Unitless Unitless Unitless Unitless

MODAL Mode 1 1,969937 0,98092 0,01596 0,98092 0,01596

MODAL Mode 2 1,946583 0,01755 0,95871 0,99847 0,97467

MODAL Mode 3 1,510308 0,0015 0,01408 0,99998 0,98875

MODAL Mode 4 0,182395 8,432E-09 0,00194 0,99998 0,99068

MODAL Mode 5 0,177479 3,033E-07 0,000002121 0,99998 0,99069

MODAL Mode 6 0,172182 8,333E-10 0,00006707 0,99998 0,99075

MODAL Mode 7 0,167476 1,091E-07 0,0000142 0,99998 0,99077

MODAL Mode 8 0,164218 0,000000149 5,769E-08 0,99998 0,99077

MODAL Mode 9 0,155388 1,457E-07 0,000002565 0,99998 0,99077

MODAL Mode 10 0,148677 8,763E-09 3,033E-09 0,99998 0,99077

Fig.49 LRB base isolated MYRRHA reactor building – dynamic characteristics


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Result from linear analyses (i.e. response spectrum analyses) showed a significant reduction of

shear forces at the isolation level.

A comparison of overall horizontal forces (i.e. integrals of forces at all nodes at a given level)

obtained from DBE linear analyses carried out on the MYRRHA reactor building in its conventional

(fixed base) and base isolated configurations are shown in the next Figures.

In the following:

- NO ISOL indicates the fixed base configuration

- ISOL HDRB indicates the HDRB base isolated configuration

- ISOL LRB indicates the LRB base isolated configuration

- F hor = is the horizontal force (kN);

SectionCut OutputCase CaseType StepType F hor

Text Text Text Text

SCUT_Level93 EQx30y+eccX+0.3eccY+Vert Combination Max 1518306

SCUT_Level93 EQx30y+eccX+0.3eccY+Vert Combination Min 1515121

Section Cut Forces - Analysis - NO ISOl


Text Text Text Text



Section Cut Forces - Analysis - ISOL HDRB


Text Text Text Text KN



Section Cut Forces - Analysis - ISOL LRB

Fig.50 Horizontal forces at MYRRHA foundation level


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Results reported above have been confirmed also by time histories analyses carried out.

The comparison between results from nonlinear analyses is made in terms of accelerations

computed at selected joints at different elevations in the MYRRHA reactor building.

FEM nodes coordinates

z x y Label m. a.s.l. level

-14 65,75 24,5 8121 13 93

-10,65 65,75 24,5 11195 16,6 92

-7,05 65,75 24,5 11926 20,2 91

0 65,75 24,5 8562 27 0

8,15 65,75 24,5 454 35,4 2

12,35 65,75 24,5 21389 39,6 3

16,55 65,75 24,5 7639 43,8 4

21,75 65 24,5 13494 50 5

30 65 24,5 13997 57,6 7

37,8 65 24,5 14063 64,8 8

Fig.51 Control nodes identification

Fig.52 MYRRHA reactor building FEM, longitudinal cross section: control joints id #

For sake of conciseness, being the amount of data obtained from nonlinear analyses enormous,

the next figures show, as examples, the computed absolute accelerations (in X direction) at the


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control joints under the application of DBE “EC8” time history T1 to the MYRRHA fixed based

configuration.

Fig.53 NO ISOL _ Abs acc_TH1 DBE_EC8_Ux_joint 8121




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In order to allow for a comparison between conventional and base isolated configurations, the

computed absolute accelerations (in X direction) at the selected control joints under the

application of DBE “EC8” time history T1 to the MYRRHA reactor building, isolated with LRB, are

reported in the next figures.

Fig.63 ISOL LRB_ Abs acc_TH1 DBE_EC8_Ux_joint 8121

Fig.64 ISOL LRB _ Abs acc_TH1 DBE_EC8_Ux_joint 11195


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As it can be seen by the results reported above, accelerations trend is substantially different in the

two MYRRHA configurations considered. In the conventional structure, the accelerations increase

along the building height, going from values of 4 ÷ 6 m/s² in the lower part of the reactor building

up to 15 m/s² at the roof level. Conversely, accelerations in the base isolated structure show an

almost constant distribution along the height, with values just above 1 m/s², reaching only the

upper and more deformable part of the structure values of 3m/s².

LRB time-dependent displacement graphs and typical force-displacement curve can be seen by

referring to the next images, showing the DBE and BDBE results for an LRB type B (link 38).


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Fig.73 MYRRHA reactor building isolation system plan view – Identification of link 38

Fig.74 Link 38: horizontal and vertical displacements for DBE “EC8” time history #1


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Fig.75 Link 38: vertical and shear forces for DBE “EC8” time history #1

Fig.76 Link 38: force-displacement curve for DBE “EC8” time history #1


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Fig.77 Link 38: horizontal and vertical displacements for BDBE “EC8” time history #1

Fig.78 Link 38: vertical and shear forces for BDBE “EC8” time history #1


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Fig.79 Link 38: force-displacement curve for BDBE “EC8” time history #1

As far as the type of foundation is concerned, the following considerations can be made.

The bottom slab will channel the plant loads to the isolators, but then there is the need to

distribute the isolators loads to the ground. This requires either piles or a second bottom slab.

The decision between the two depends purely on the geotechnical characteristics of the ground,

not on the presence of the isolators.

In any case, attention has to be paid to different vertical loads acting on isolators/pits, especially

for very soft soil.

Analyses carried out on Finite Element model n. 3 (see Figure 47), i.e. considering a foundation

with piles, pointed out that the requirement on the elastic behaviour of the substructure has to be

strictly satisfied: plasticization could led to unexpected variations in loads distribution.

An interesting results was obtained from the dynamic analyses carried out. It is generally

prescribed that the isolation system shall be within two rigid diaphragms, thus giving the

possibility to consider the isolated structure as a Single Degree of Freedom (SDOF) system.

A series of parametric analyses were carried out by Numeria by considering the superstructure as

rigid, while investigating the effect on the dynamic behaviour of deformable substructures. In all

cases, superstructure and substructure are supposed to be elastic (no ductility).

Results showed that a certain deformability in the substructure can be accepted, provided that

deformations do not imply the exceedance of the elastic range of the piles. Piles deformability

could results in slightly higher displacements of the isolated structure or, when the substructure

deformability give rise to uneven deflections in the isolating devices, to torsion, whose effect turns

out in an increase of horizontal displacements of the devices themselves. Torsion results therefore

in an economic disadvantage because of the bigger plan dimensions of the isolators needed to

accommodate the increased displacements, but torsion in itself is not an issue provided that

elastic behaviour is guaranteed.


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As an example of the preliminary design carried out, in the following figure is shown a possible

substructure layout for MYRRHA. Bearings are placed on a forest of 2 meter height columns whose

plan dimensions have been determined from calculation; attention has been paid to elastic

deformations check and to the need of guarantee accessibility for isolators inspection and

maintenance.

Fig.80 MYRRHA substructure layout

The following images schematically illustrate the MYRRHA facility in the base isolated

configuration.

Fig.81 Base isolated MYRRHA nuclear plant: 3D view from ground level (in red is the outside shaft

for accessing to the base isolation level)


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Fig.82 Base isolated MYRRHA nuclear plant: 3D view from underground level (in red is the outside

shaft for accessing to the base isolation level)

Fig.83 Base isolated MYRRHA nuclear plant: 3D view from underground level


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Fig.84 Base isolated MYRRHA nuclear plant: 3D view of the superstructure and substructure

Fig.85 Base isolated MYRRHA nuclear plant: 3D view of the isolator layout and of the substructure


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4.2.1 Other issues investigated

Long-term deformations of the superstructure including shrinkage, creep and post-tensioning

effects shall be evaluated, and the induced displacements evaluated, as they are for any other

structure, e.g. bridges.

The problems of shrinkage have to be managed in all structures, whether or not on isolated.

The amount of shrinkage depends on the curing process, particularly the ambient moisture,

though the dimensions also play a role.

In the worst case, if no precautions are taken, shrinkage of the concrete slab above the isolators

could produce deformations on the isolators of about 1.3 ÷ 1.5 cm (shrinkage strains calculated

according to EN1992-2, Appendix B).

As a consequence of these even small deformations, the isolators will be subjected to a quite

significant horizontal load (40 ÷ 100 kN).

Like in all huge dimensions concrete casting, a set of precautions must be taken to minimize the

effects of shrinkage.

Fig.86 Huge concrete slab

It can also be suggested to wait some time before finalizing the installation of the isolators as most

of the shrinkage occurs in one year. This will help in centering the isolators after dimensions are

final; if necessary, mock isolators can be installed initially, to be replaced by the actual isolators at

a later stage in the construction, thus also allowing more time to the isolators suppliers while still

maintaining the construction deadlines.

The effects of shrinkage and thermal deformation can be limited by installing pre-deformed

bearing on site. Pre-deformation is imposed during the testing phase by the laboratory press.


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Deformed bearings are then sent to the construction site and installed. Safe removal of the special

mechanical lockers used to keep the isolators in the deformed configuration can be made once

thermal/shrinkage effects causes the separation of the upper and lower locking plates.

Numeria is investigating if the systems are patentable, therefore sensitive information are not

included in the present report.

The soil structure interaction in the vertical direction shall not be neglected. This interaction would

affect the main vertical modes and therefore the positions of the peaks on the horizontal response

spectra. The soil damping will directly affect the amplitude of these peaks. The vertical stiffness of

the isolation system, generally considered of secondary importance, might prove a valuable tool to

displace the peaks on the horizontal spectra as well as on the vertical spectra.


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4.3 Procedure for bearings installation, inspection and replacement

4.3.1 Procedure for bearings installation

Procedures for bearings installation are reported in SILER deliverable D6.3.

4.3.2 Procedure for bearings inspection and maintenance

In the following, the procedure for inspection and maintenance activities for the elastomeric

seismic devices (HDRB and/or LRB) is reported.

The Standards used as references are:

- EN 1337-10:2003 - Structural Bearings - Part 10: Inspection and maintenance

- EN 15129:2009 – Anti-seismic devices

Inspections are classified as “Principal Inspections” and “Routine Inspections”. Inspections have in

any case to be made after a seismic event or a flood.

“Routine Inspections” should be carried out by the Authority as part of the other routine

maintenance activities.

In the routine inspection the following properties shall be checked:

- condition of elastomeric external lateral surface;

- actual horizontal movement in both principal directions;

- entity of rotation of upper elements (by means of gauge to measure the tilting

clearance);

- condition of the concrete under the isolator.

The air temperature shall be measured near of the isolator by means of alcohol or mercury

thermometer or similar and shall be recorded.

Any non-conformity encountered as part of either a "Principal" or "Routine" inspection should be

dealt with as described in the following.

All inspections should be recorded, and in the case of a "progressive" defect, photographic records

should be kept to enable the progression to be assessed.

Sequence of Operations:

- visual inspection of the bearing general condition

- visible defects (cracks, incorrect position, etc);

- check on the conditions of sealing and fastening;


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- inspection of the isolator displacement;

- inspection of isolator upper element rotation;

- conditions of corrosion protection;

- visible defects in structural parts adjacent to the isolator;

- recording of the collected data, temperature, time and date.

Measuring inspection tools and ancillary equipment:

- plans and drawings, manuals, product specification, etc.;

- scaffolds and working platforms (if needed);

- lighting equipment (if needed);

- mirror;

- tools for covers removal;

- cleaning devices;

- instrumentation for measuring the protective layers thickness;

- thickness gauge;

- ruler;

- spirit level;

- goniometer;

- gauges;

- instrumentation for measuring the thickness of the corrosion protection;

- instrumentation for cracks measurement;

- thermometer;

- camera.

In the following, a list of the most important elements to be inspected is reported, along with

inspection frequency, level of acceptability and measures to be taken in case of non-conformity.

1. Condition of elastomeric external surface:

- frequency: first inspection after one year, then every two

years

- what to look for: cracks, fissures, protuberances of the

elastomeric layers

- level of acceptability: minor protuberances of elastomeric layers

between two internal steel reinforcement are

normal in service condition. Except these

protuberances, the external surface should be


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smooth and without defects. Irregular

protuberances of elastomeric layers indicate

that the internal steel reinforcement are bad

positioned or that an excessive vertical load is

applied to the isolator

- action in case of non conformity: more frequent inspections.

2. Displacement:


years

- what to look for: relative positions between the top and bottom

plates

- level of acceptability: 1) residual displacement as indicated by the

structural designer

2) no major anomalies in the relative position

- action in case of non conformity: reduce or eliminate unexpected residual

displacements by re-centering of the devices.

3. Rotation:


years

- what to look for: tilting clearance

- level of acceptability: rotations as indicated by the structural designer

- action in case of non conformity: check with isolators manufacturer.

4. Corrosion protection:


years

- what to look for: rust

- level of acceptability: rust extent less than 2% of the total surface

- action in case of non conformity: paint touch-ups.

5. Bolts and fixing:


years

- what to look for: tightening of bolts


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- level of acceptability: none

- action in case of non conformity: Re-fix / tighten bolts. Any bolts or other form of

connections should be checked to ensure that

it has not become loose or otherwise inactive.

6. Damage to steel parts:


years

- what to look for: distortion / any general damage

- level of acceptability: no damage

- action in case of non conformity: 1) repair/replace damaged parts

2) more frequent structure’s inspections for

possible anomalies.

7. Condition of adjacent structural elements:


years

- what to look for: cracks/settlements in the bedding material

- level of acceptability: -

- action in case of non conformity: more frequent structure’s inspections for

possible anomalies.

According to the results of the inspection one of the following actions must be taken:

- no action;

- additional measurements (under extreme temperatures, different loads, etc.);

- additional tests;

- repair (replacement of the entire isolator or of a part, renew of corrosion protection,

sealing with mortar, etc.).


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4.3.3 Procedure for bearings replacement

In the early 80’s of the last century, EDF built the Cruas NPP, the first nuclear plant seismically base

isolated, but the isolation system was not designed to be replaceable. The French Safety Authority

requested in the 90’s to demonstrate that such replacement was possible: a replacement

operation was therefore successfully carried out on a single pedestal supporting 2 isolators.

Even if isolators are designed to resist the most severe conditions in terms of performance and

lifetime, an unlikely event requesting the removal and subsequent replacement of an isolator has

to be considered.

Bearings replacement shall therefore be taken into consideration during the design of the

structure.

In civil buildings or bridges the isolator replacement has already been done several times and can

be considered a routine operations. For buildings, the deformability of the superstructure helps in

the removal of the old isolator and in the insertion of the new one; operations are normally done

by using hydraulic jacks.

The replacement of an isolator is an operation that requires the temporary transfer of vertical

actions acting on the device. Such actions can be transferred directly from the jacks to the

substructure, or indirectly, placing the jacks on an “ad hoc” temporary structure (see next Figure).

Fig.87 Example of set-up for isolator replacement in building

The peculiarities of a base isolated nuclear plant (e.g. the huge isolated mass, the sizes of the

isolators, the fact that the base slabs are very stiff up to a point that they can be considered

almost non deformable) make the bearing replacement a very challenging task that requires a

specific procedure.


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Fig.88 Vertical loads distribution on ADS isolated slab

Dedicated technologies were implemented on recent projects (JHR and ITER project) to make such

replacement easier by the introduction of a fused mortar layer underneath the bearing.

The methodology comes from bridges applications, where bearing replacement is a common

practice. The deck is lifted using hydraulic jacks; the bearings are then unloaded and can be easily

removed and replaced. The bearings are than re-loaded by jacking down the deck. Given the

vertical deformability of the bearings, the deck will then sit in a slightly lower position with

respected to the initial one. In order to avoid structure’s drops, the following procedure can be

adopted. Flat jacks are inserted under the new bearings and then injected in order to transfer the

load from the temporary hydraulic jacks; once the loads are transferred onto the new bearing,

then temporary hydraulic jacks are removed.

Fig.89 Typical flat jack (from NUVIA)

However, this methodology cannot simply be transferred to a nuclear power plant because of:

- the number of bearings: the replacement has to be conducted bearing per bearing or

group of bearings per group of bearings, the influence of the high hyperstaticity of the

structure has to be evaluated;

- the safety requirements related to the structure’s functioning.


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Hence, the procedure proposed for replacement in a nuclear plant requires a different procedure

for load transfer as the structure cannot be lifted. Bearing must be first unloaded: load transfer

can be obtained by means of auxiliary hydraulic jacks. Subsequently, the bearing shall be removed.

It is strongly recommended to integrate a fuse layer in the bearings themselves in order to avoid

any demolition of the concrete to allow the removal of the isolator. Such a fuse layer can be easily

designed and included, for instance at the base of the bearing, prior to its installation.

Once the “old” bearing is removed, a new isolator can be placed into position. Connection systems

have to be designed in order to allow for an easy installation; use of steel counterplates is

recommended (see next Figure).

Fig.90 Typical LRB for ADS (from SILER deliverable D4.2)

The newly installed isolator must then be reloaded using a “lost” flat jack to be injected with grout

or epoxy.

It has to be underlined that, in order to allow for an easy replacement not requiring destructive

operations, the design and the manufacturing of the isolator as well as the its connections to the

concrete structure has to be made by taking into account the proposed replacement methodology

and its consequences. Thus the design of the seismic isolation system shall consider:

- the presence of a fuse layer in the bearing;

- the effect of loads caused by the shoring of the upper slab during the replacement of an

isolator;

- an appropriate layout in order to guarantee accessibility and facilitate isolator

replacement;


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- a construction-wise solution in order to avoid any plinth destruction in case of replacement

(using fuse layers easy to destroy or, better, manufactured and designed to be easily

removed);

- the capacity to accommodate the isolator distortion at the replacement date. Indeed, the

upper and lower bearing plates will not be vertically aligned due to shrinkage of the slab

and thermal effects;

- the new isolator loading transfer and sequence: the new isolator has to recover the “same”

vertical compression stress as the initial one.


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5 CONCLUSIONS

Considering that no European Standard nor European guidelines for the design of seismically

isolated nuclear facilities exist, in the following, reference has been made to existing international

standards dealing with the design of base isolated structures, and, in particular, to the set of the

Eurocodes. EN 1998: “Design of structures for earthquake resistance”, applies to the design and

construction of buildings and civil engineering works in seismic regions. It covers common

structures and, although its provisions are of general validity, special structures, such as nuclear

power plants, large dams or offshore structures are actually beyond its scope. Activities carried

out aimed mainly at evaluating if additional requirements with respect to those reported in the

relevant European set of codes, dealing with civil buildings and viaducts, have to be specified for

nuclear plants.

Criteria for the design of either the superstructure and the substructure in DBE and BDBE

conditions have been assessed. In particular, aspects related to elastic behaviour requirements,

control of deformations, specifications/limitations on ductility demand on the superstructure,

deformability limits for piled foundations have been investigated by linear and nonlinear analyses.

Scenarios for isolated versus fixed base foundations have been studied and compared.

The foundations in the base isolated plant become more complicated due to a second foundation

level and the introduction of the seismic gap. It is assumed that in total 10% higher costs are to be

expected for the foundations.

Considering the overall cost of construction (i.e. the whole plant), it has been found that the upper

limit is additional costs in the order of 2%, whereas in the best case some 5% of the total costs can

be saved.

It has been further demonstrated that the additional costs, if any, can be compensated by the gain

of safety and related overhead cost issues.

No additional demand on maintenance is anticipated for the isolated case.


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6 REFERENCES

[1] SILER Deliverable D2.1, Part B - “Description of Systems: ADS”; Empresarios Agrupados

Internacional document n. 092-260-F-C-00102 - rev. 2, (July 2012).

[2] SILER Deliverable D2.2, “Description of the Design of Seismic Isolator”; ENEA & Numeria,

(December 2012).

[3] SILER Deliverable D4.1, “Development of full scale prototypes of High Damping Rubber

Bearings and experimental validation”; FIP Industriale & Numeria, (March 2014).

[4] SILER Deliverable D4.2, “Development of full scale prototypes of Lead Rubber Bearings and

experimental validation”; FIP Industriale & Numeria, (March 2014).

[5] Kelly, J.M., The Role of Damping in Seismic Isolation, Earthquake Engineering and Structural

Dynamics, 28, 3D20 (1999).

[6] Politopoulos, I., A Review of Adverse Effects of Damping in Seismic Isolation, Earthquake

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[7] USNRC RG 1.60. Design Response Spectra for Seismic Design of Nuclear Power Plants.

[8] EN 1998-1:2004 Eurocode 8 – Design of structures for earthquake resistance. Part 1: General

rules, seismic actions and rules for buildings.

[9] Kelly, J.M. “Earthquake-Resistant Design with Rubber”, Springer-Verlag (1993).

[10] Skinner, R.I., Robinson W.H. and McVerry, G.H. “An Introduction to Seismic Isolation”, John

Wiley & Sons Ltd (1993).

[11] Naeim, F., Kelly, J.M. “Design of seismic isolated structures – From theory to practice”, John

Wiley & Sons Inc (1999)

[12] Dusi, A., Fuller, K.N., Chee Cheang. T., Mezzi, M. Some Recent Applications of Base Isolation

Using High Damping Rubber Bearings, Proc. 8th Pacific Conference on Earthquake

Engineering (8PCEE); Singapore (2007)

[13] SILER Deliverable D5.2, “Design of horizontal fail safe system”; ENEA & Numeria, (December

2013).

[14] Politopoulos, I., “Response of seismically isolated structures to rocking type excitations”,

Earthquake Engineering & Structural Dynamics, Vol 39, pp.325-342 (2010).

[15] SAP2000 Advanced v. 16. Static and Dynamic Finite Analysis of Structures. Berkeley, CA, USA,

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siler seismic-initiated events risk mitigation in lead

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