tince2016 - steel concrete modules in civil work design of future nuclear power plant buildings -...

3rd Conference on Technological Innovations in Nuclear Civil Engineering

Full paper submission, TINCE-2016

Paris (France), September 5th – 9th, 2016

Steel Concrete Modules in Civil Work design of future Nuclear Power Plant buildings

Jacques Chataigner1, Denis Etienne2, Laure Simeoni3, Jean-Luc Tuscher4 1Civil Engineering Expert, Tractebel, Lyon, France

2Expert in Civil Engineering, Bouygues Travaux Publics, Guyancourt, France 3Project Engineer, Tractebel Engineering, Lyon, France

4Civil Engineering Expert, EGIS Industries, Montreuil, France Introduction

Design of reinforced concrete structures ongoing nuclear power plant projects frequently

faces issues related to more stringent design codes requirements that make necessary to take

into account at design stage severe accident design loads which, in turn, lead to very large steel

reinforcement demand. Subsequently design of these RC structures become very complex and,

at construction stage, constructability often raises problems linked to actual installation of large

densities of rebars which becomes in itself a complex task, expensive and time consuming. As a

consequence, a trend in nuclear civil work design is emerging which consists in substituting,

when very dense steel reinforcement ratios are expected from plain steel reinforcement bars

design, typical reinforced concrete structural elements by Steel Concrete (SC) structures. We will

successively develop in this paper the technical issues that can arises when using this design

process, try and identify the advantages and possible drawbacks that could be linked when using

such Steel Concrete modules in project where the overall design is based on Euronorms. Finally

will be quickly presented computational methodologies which may be used for their design and

describe some examples of structures for which this construction approach has been retained

3rd Conference on Technological Innovations in Nuclear Civil Engineering TINCE 2016, Paris 5th to 9th September

Figure 1 - Steel concrete module (also named double skin member below) principles and

terminology

Available Design Standards for SC Modules

SC modules for NPP structures design have, since several years, been proposed for structural

elements of NPP structures, which led to the development of specific design norms such as

[AISC] developed in USA, [JEAC] developed in Japan, [KEPIC] developed in South Korea.

However implementation of this construction technique in projects to be built in Eurocodes (EC)

environment may raise some difficulties as existing design norms quoted above have been

mostly developed on non EC basis. Such issues have been met in ongoing projects of future

reactors such as ASTRID, Gen IV Sodium Cooled fast Reactor under development in France, for

which the use of steel concrete modules is envisaged for some specific structures of the Nuclear

Island. In that case, lack of fully EC adapted design standards for SC modules necessitated the

preparation of a specific design and construction code in which appendixes dedicated to SC

modules had to be implemented.

The Eurocode 4 “Design of composite steel and concrete structures” applies to the design of

composite structures and members for buildings and civil engineering works. But this code deals

mainly with composite floor slabs, composite beams and composite columns; it does not deal

with composite walls consisting of two steel faceplates with structural concrete placed between

them. The lateral force resisting system in nuclear buildings consists generally in shear walls and

floors slabs.

A research project SCIENCE (Steel Concrete for Industrial, Energy and Nuclear Efficiency)

funded by the European Commission is now underway; It is managed and coordinated by the

Steel Construction Institute (UK); six other European companies participate in the project. It

ties

studs

Steel plates

Penetration (if any)

Concrete infill without reinforcement


started in July 2013 and will be completed in the first half-year of 2017. It includes tests on large

specimens and advanced numerical calculations of members and connections at ambient

temperature, at elevated temperatures (temperature of 180°C corresponding to the Loss of

Coolant Accident in pressurized water reactors) and under fire conditions. This aim of this

research project is to fill the gaps in the Eurocodes and to provide design rules for the SC

structures.

Typical SC structures studied in the frame of SCIENCE are mainly double skin member for walls

or slabs and single skin slabs for floors, as shown below:

Figure 2 -Typical connection of a double skin composite wall


Figure 3 Typical section of a single skin composite floor slab

The tests are undertaken on large specimens (the thickness of the specimens is like to those of

the elements of nuclear buildings and is generally more than 0.40 m) in several laboratories: VTT

in Finland, Karlsruhe Institute of Technology in Germany and Efectis in France.

From the tests results, are derived effective properties of SC elements which are used in Finite

Elements Modeling of SC structures and design equations for calculating the resistance of SC

elements. As the tests do not cover all the possible configurations of SC members, non-linear

finite elements analyses are undertaken in order to extend the experimental database.

A special attention is put on the behavior of the SC elements at elevated temperature. EDF

SEPTEN is the task leader for developing design methods and equations for SC elements at

elevated temperature and has entrusted Egis Industries with the corresponding parametric

studies. Non-linear finite element analyses are carried out on code ASTER; the characteristics of

the parameters are tuned with the test results and then extended to other configurations.

SC modules design standards vs Eurocodes

Though the complete design approach for Civil Work structures where SC modules are used

shall be based on finite element models and FE calculations (linear or nonlinear as well), simpli-

fied design methods for SC modules have already been developed (see [VAR2011] ) and may be

used at basic design stage; it must be nevertheless noticed that :

• most of these simplified methods will finally compare stresses or strength in constitutive

materials of SC modules to ultimate capacities of same materials as defined in US design

codes and requirements ; this point to highlight the fact that introducing such construction

methodology in Euronorm based projects requires at least some adaptations,

and

• have been developed mainly for structures of limited extension and exhibiting plane sur-

faces or straight contours (shear walls for examples) ,when it could interesting to use this

SC modules concepts for other substructures exhibiting significantly different features as

cylindrical ones or submitted to specific loadings (large thermal loads for example)

Therefore main adaptations that appeared necessary to implement the construction SC methods

in our ongoing projects were:

Preparation of specific design codes


Without waiting on the results of the research program SCIENCE, a specific chapter for double

skin composite elements has been included in the [RCC-GA] which provides rules for the design

and construction of the structures of ASTRID project, it is mainly based on the [AISC]. In this

chapter of the RCC-GA are provided specifications for the materials, general rules for the

thickness of the faceplates, the diameter of the connectors, the spacing of the connectors for

preventing the buckling of the faceplates, the spacing of the tie bars, the mechanical

characteristics of the SC elements to be considered in Finite Element modeling and rules for the

verification of the resistance of the sections. This document will be completed and improved from

the results of SCIENCE program and other publications. Examples of improvements or

adaptations to be brought to [RCC-GA] are listed below.

Initial requirements adaptations

• Adaptation of the requirements as regards material characteristics (concrete, steel

grades) as design methodologies proposed in existing codes such as [AISC] are generally

based on « closed form » equations that need to receive correct translation. Numerous

examples may be found in [AISC], [VAR2011] where such adaptations were required; we

will quote :

→ Spacing of the studs, defined in [AISC], § ,A-N9.1 as maximum dimensions

of an elementary cell of the steel sheet to avoid buckling in compression; proposed criteria

need to adapted in case the SC modules have a cylindrical shape (effect of curvature) or

are bi axially loaded, with one tensile stress component ,

Load combinations review

• Review of the definition of load combinations, as strength checks proposed in [AISC]

make use of load combination factored in accordance with US design norms, which may

differ from those proposed in Eurocodes and National Appendixes.

Design criteria revision

Revision (or check) of some design criteria to better fit with use of SC modules in

structures not specifically encompassed by existing standards. As a matter of fact, most of

design limit values of SC modules will refer to concrete or steel limit strength values as

given in US codes, which in some case could have different definition in Euronorms, this

requiring modifications; to be quoted as examples :

→ checking “through the wall” shear capacity of SC modules, makes use of

[AISC] ,formula (A-N9-17M) that proposes « closed form » equation:

V < Vconc = 0.125 (f’c )0.50 tc


a formula where it should be reminded that f’c corresponds to compressive strength of

concrete as per ACI norms, which slightly differs from same fck as given in EC2

→ same comment will apply to allowable maximum concrete compressive

stresses in SC modules when submitted to combined membrane forces {Nxx, ,Nyy , Nxy,}

and out of plane moments {Mxx, ,Myy , Mxy,} , or

σc =< 0,70 f‘c

according to [VAR2011], as reference is still made to f’c (ACI).

Computational methodologies

Basic and simplified methods

Design of structural members considered as SC elements may follow at conceptual or basic

design stages, basic and rather simplified methodologies, similar to those proposed in [AISC] and

[RCC-GA], or described more in detail in relevant available technical papers.

For instance, to carry out preliminary design of the Reactor Pit of an ongoing project, a specific

computational methodology has been developed, based on the principles presented in

[VAR2011], but already adapted to requirements from [RCC-GA].

The principles, briefly summarized, of the adopted method necessitated:

→ to determine in each elementary elements for the overall reactor pit structure the principle

stresses (or forces ) in each inner or outer steel sheet (or inner and outer notional halves

of the concrete core )

→ to compare then the resulting stresses or forces to design allowables as given in relevant

norms.

Additional specific calculations were also automatically carried out as stud spacing and ties cross

sectional areas. Method proved to be applicable and led to validate the proposed design (see

Figure 4).

Fig 4 – ASTRID SC Reactor pit – Developed view of “envelope (1)” principle stresses (MPa) in outer steel

plate (1) For all design load cases


More refined computational methods

Design principles validated through basic and simplified methods described here above shall

afterwards be validated at Detailed Design stage using FE models in which proper modeling of

the behavior of each constitutive material of the SC modules shall be implemented. Available

softwares such as Code ASTER, ANSYS or LS Dyna make possible the performance of refined

calculations, which for some load cases may need non-linear finite element analyses.

• Ongoing validation test programs

The objective of the research program SCIENCE is to issue a design guide, written in a

Eurocode format, for the SC structures. It will provide guidance for the design at the execution

stage and for the design of the final composite structure, including also rules for designing the

shear connectors, the connections and the structures at elevated temperature.

Application of SC modules technique –––– Examples

Examples will be provided below from projects of future reactors such as ASTRID, Gen IV

Sodium Cooled fast Reactor under development in France, for which designers proposed, in the

feasibility phase, to make the use of steel concrete modules in specific zones of buildings

belonging to the Nuclear Island. As previously reminded, introduction of SC modules technology

seemed beneficial for this project design and constructability in zones were standard RC design

would lead to extremely dense reinforcement ratios or needed anyway to use thick steel

shuttering for erection of the structure that would be afterwards misemployed.

Substructures that are concerned are:

Reactor pit in Reactor Building Inner Structures

The large and heavy ASTRID reactor vessel is supported through an annular metallic structure by

a cylindrical concrete structure, with a mean radius R = 10,40 m, thickness 1,50 m and height

approximately 16,50 m; it carries the reactor weight down to the common concrete raft of the

Nuclear Island. Governing load case for the longitudinal reinforcement of this independent

substructure consists of an accident situation which develops, in the reactor pit, a large overall

upwards tensile force. Issues in relation with huge steel reinforcement demand together with

design of complex metallic embedded parts to transfer this overall force from vessel to steel

reinforcement led to explore a more rational steel construction solution that was finally analyzed


at Conceptual Design stage. Eventually, using computational method as presented in paragraph

before, Reactor pit steel concrete structure has been checked for other load cases such as

normal operating conditions with stationary through the wall thermal gradients or seismic overall

actions. Design based on SC module methodology proved to be feasible. Eventually the analysis

led to a design consisting of a large and unique steel concrete structure, anchored in the concrete

raft, and that the Civil work Contractor would fill with concrete in a single concreting phase,

except for its upper part, were connection to the metallic annulus supporting the reactor vessel

has to be carried out in a second phase of work. It is also expected that this solution will

significantly ease the erection of this specific structure as installation of huge reinforcement in a

rather congested zone would otherwise be a complex and time consuming task.

Figure 5 - 3D cross sectional view of reactor pit.

Upper cylindrical vault of Reactor Building.

The upper structure of the rectangular ASTRID Reactor Building is closed by a cylindrical vault

with horizontal axis that plays both roles of containment structure and APC shell; dimensions of

this vault, length 53,0 m approx., span 43,0 m and thickness 1,80 m led to propose, in order to

enable its construction without any use of propping inside the building, a design of this vault

based on:

• thick steel sheet (skin) on its inner face that will be used as formwork,

• steel frames as supports of the steel sheet (and also of layer of fresh concrete in

concreting phases)

Reactor pit SC module

Surrounding concrete internal structures

Nuclear Island common raft


• upper 1,80m thick reinforced concrete layer

Resulting design corresponds to a SC structure which in this specific case has a single metallic

skin on its inner surface.

Design of the structure, against loads it has to withstand, normal and accident loads or external

hazards such as airplane crash, has been carried out with composite methodology, i.e. partly as

a SC structure in agreement with [RCC-GA] specific appendix for SC component and partly as a

typical RC structure to determine its upper reinforcement and through the wall reinforcement

required to resist punching effects from external hazards. Resulting design is illustrated in figure 6

below. It shows that inner metallic sheet of the vault, provided it is given a sufficient thickness, is

finally able to sustain the load from full thickness of fresh poured concrete at construction stage

and to be used as inner reinforcement for impact loads, thus leading to a feasible and economical

design solution of this substructure.

Figure 6 –Reactor Building vault tor Building sooverall view and detail

Substructures as steam generators bunkers in Steam Buildings.

• At the conceptual design phase, each one of the four steam generators is enclosed in a

cylinder of internal diameter of nine meters and of thickness 1 meter, in order to resist

severe hypothetical accidents leading to very high internal pressure and high temperature.

A Steel Concrete structure was envisaged consisting of face plates of thickness 20 mm in

steel grade S355, a concrete core of class C40/50, NELSON studs of diameter 25 mm in

steel grade S235 J2G3+C450, tie bars of diameter 40mm in steel grade of characteristic

resistance of 500 MPa. The design was confirmed by transient dynamic analyses carried-

out on a 3D finite element modeling with the computer code ANSYS using volume

elements.


Other possible uses of SC modules in NPP civil work

A tentative list of specific structural elements of NPP buildings for which use of SC modules

construction methodology could be envisaged in future projects maybe as follows:

→ some reinforced concrete slabs, as for instance the annulus shaped slabs around the

reactor pit, which would necessitate large radial/ortho-radial reinforcement if designed as

reinforced concrete members could be replaced by single skin SC slabs,

→ slabs which would require high propping, or slabs and walls (then with double skin design)

in which a high density of steel anchor plates are embedded to fasten small items of

equipment,

→ All zones of the structures where large tensile forces must be transferred from special

metallic anchors to steel reinforcement of the supporting reinforced concrete element as

this load transfer requires then complex steel reinforcement the design of which does not

lie in most cases in the strict sense in the frame of current design standards.

Finally it shall be noticed that a significant advantage of SC members lies in the fact that small

items of equipment (cable trays, piping, air duct…) can be attached anywhere on the steel

faceplates, without any need for additional embedded plates, that are on the contrary required

in reinforced concrete structures of the nuclear buildings.

Conclusions

Design of specific large structural elements of European nuclear facilities ongoing projects, based

on the use of SC modular structures, have been proposed. Given that the Civil Work design of

these structures had to be achieved on a Eurocodes basis, it appeared that necessary

adjustments or additional requirements had to be brought in existing design codes dedicated to

SC structures as they could be not fully in agreement with Euronorms. Therefore, a European

Research project SCIENCE has been launched, the aim of which was to fill existing lacks and

provide a complete set of Euronorms compliant design rules for SC structures.

Nevertheless introduction of SC modular structures was proposed in the design of some

particular substructures of future reactors such as ASTRID project ; studies that were carried out

at conceptual stage demonstrated that this construction method should be on one side beneficial

for the Civil Work Design, making it more rational, and on the other side, should enable, without

any impact on the strength margins, to significantly simplify the construction of these complex

structural elements and reduce the construction schedule.

References


[AISC] - AISC N690-12/ANSI/AISC N690s1-15 - Specification for Safety-Related Steel Structures for Nuclear Facilities - Appendix N9 -Steel-Plate Composite (SC) Walls [EC4] EN 1994-1:2004 Eurocode 4 - Design of composite steel and concrete structures - Part 1-1 General rules and rules for buildings [JEAC] - JEAC - 4618-2009 -Technical Guidelines for Aseismic Design of Steel Plate Reinforced Concrete Structures [KEPIC] - KEPIC - SNG - Steel- Plate Concrete Structures - 2010 Edition (Rev. 1) [RCC-GA] Rules for Design and Construction of Civil Works of ASTRID [VAR2011] - Steel-plate Composite (SC) walls for safety related nuclear facilities: Design for

in plane and out-of-plane demands – A.H. VARMA, S. R. MALUSHTE , K. SENER, Z. LAI - SMiRT 21 - 2011,


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Preference: � Poster � Oral Topic: � 1 - Advanced Materials � 2 - Design and Hazard Assessment

� 3 - Civil Works Construction � 4 - Long Term Operation & Maintenance � 5 - Dismantling of civil works & Civil Works in Hostile Environment � 6 – Geotechnical Design & Construction & Fluid Structure Interaction

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