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.

1. 3
rd
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
1
Civil Engineering Expert, Tractebel, Lyon, France
2
Expert in Civil Engineering, Bouygues Travaux Publics, Guyancourt, France
3
Project Engineer, Tractebel Engineering, Lyon, France
4
Civil 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

2. 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

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

4. 3rd
Conference on Technological Innovations in Nuclear Civil Engineering
TINCE 2016, Paris 5th
to 9th
September
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

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

6. 3rd
Conference on Technological Innovations in Nuclear Civil Engineering
TINCE 2016, Paris 5th
to 9th
September
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

7. 3rd
Conference on Technological Innovations in Nuclear Civil Engineering
TINCE 2016, Paris 5th
to 9th
September
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

8. 3rd
Conference on Technological Innovations in Nuclear Civil Engineering
TINCE 2016, Paris 5th
to 9th
September
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

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

10. 3rd
Conference on Technological Innovations in Nuclear Civil Engineering
TINCE 2016, Paris 5th
to 9th
September
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

11. 3rd
Conference on Technological Innovations in Nuclear Civil Engineering
TINCE 2016, Paris 5th
to 9th
September
[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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Conference on Technological Innovations in Nuclear Civil Engineering
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September
Please fill in the blanks at the end of this extended abstract (the additional blue lines and
potential page it may generate are not accounted in the number of pages)
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
Corresponding author: jacques.chataigner@gdfsuez.com