tince2016 - post-fukushima seismic re-assessment of french reactor building - j. chataigner, a....

3rd Conference on Technological Innovations in Nuclear Civil Engineering

Full paper Submission, TINCE-2016

Paris (France), September 5th

– 9th

, 2016

Post-Fukushima seismic re-assessment of French Reactor Building

Jacques Chataigner1, Adrien Fuss1, Pierre-Yves Gourvellec1, Emilie Leroux1, Julien Massena1, Romain Ragouin1, Imad Tabet2.

1 Tractebel Engie / Coyne et Bellier, Lyon, France 2 EDF DIPDE, Marseille, France

Introduction

Following the consequent events that occurred at Fukushima Daiichi Nuclear Power Plant

after the 11th March 2011 Great East Japan Earthquake, the French electric utility company EDF initiated an extended seismic re-assessment program implementation to answer French Nuclear Safety Authorities requirements. This program, called SND (Séisme Noyau Dur), aims mainly at reaching safety objectives concerning the resilience of the SSC (Structures, Systems & Components).

End of 2015 matched with the end of the first whole re-assessment of an operating Reac-

tor Building (RB) in France, for which innovative thinking assessment solutions have been nec-essary to optimize current methodologies so as to assess in detail the civil work strength capaci-ty and in second step compute the induced floor accelerations under high level of earthquake, corresponding to site SND.

Context and key hypotheses for the study Exhaustive validation of any components and metallic parts fastened to the civil work of

the Inner Structures (IS) and containment structure (CS) required an exhaustive and refined FE modeling of the RB (Raft, IS, Containment, galleries below raft and other structures). This com-plete FE model enabled:

on one hand, to accurately account for coupling effects between main equipment and the supporting structures;

on the other hand, to analyze the RB concrete raft supporting both IS and containment. The IS are connected to the raft at the base of the reactor pit with an embedded link and at the base of the ring wall with a hinge connection. The raft, relatively thin, is therefore submitted to both IS and containment concomitant action which required careful attention when applying the seismic methodology.


TINCE 2016, Paris 5th to 9th September

Figure 1 : View of the global FE Model

Due to the existence of very specific geometrical features, as the equipment hatch, the pe-

ripheral zone of the raft, or the technical structures embedded in the base of the containment wall; various singularities proper to this RB have been scrupulously introduced in FE model, which made necessary volumetric modeling of raft and containment, this latter taking into ac-count in detail the actual 3D arrangement of the prestressing tendons:

Figure 2 : View off the dome tendons (left) and gamma tendons (right)

At stage of the definition of the main assumptions for the study, jointly with EDF,

TRACTEBEL made good use of its expertise in French Reactor Buildings design (among oth-ers):



A comprehensive analysis and then setting of the material properties considering the de-sign standards used at the initial design of the RB, the ones used for the re-assessment and the ones currently used in American re-assessment practice,

The modeling of the IS and containment; with their specific features as the reactor pit ge-ometry, connections under the floor supporting the primary loop equipment or the model-ing hypotheses associated to the global loads transfer between the ring wall and the re-actor pit (for instance),

The coupling between Civil Work structures and primary loop and main components, as well as with the steelworks structures, that have been modeled in a simplified way in the global model and have been re-assessed in detail by means of decoupling models,

Figure 3 : View of the main equipment in the IS (left) and some detailed models of the steel-

works (right)

The adjustment of prestress loads through by: 1. the analysis of the delayed deformations of containment wall using the strain meas-

urements provided by the monitoring system installed in the containment during the construction in order to quantify the real creep and shrinkage of the concrete;

2. the extrapolation at long term of the concrete delayed strains; 3. the accurate determination of the prestress losses due to creep and shrinkage.



Figure 4 : Extrapolation of the concrete delayed strains in containment wall at end of life

Seismic analysis and critical analysis for the resistance of structural elements After the constitution of the FE model described here above, seismic calculations based on

modal-spectral analyses have been performed to assess the structures strength capacity. Due to the high seismic level (ZPA>0.35g), methodologies presented in [ASCE05],

[IAEA03], [EPRI94] have been replaced by those proposed in [SEPT15] in order to account for potential cracked state of the structural elements. Therefore, iterative calculations have been carried including Young’s moduli of the structural walls which were representative of their cracked state. This step of the analysis aimed at getting a realistic seismic response of the build-ing and an accurate distribution of the forces regarding the cracked state of each structural member.

The basis of the analysis for the strength capacity re-assessment of concrete elements fol-

lowed the principles written in [SEPT15], itself inspired by [ASCE05], [IAEA03], [EPRI94]:

Strength checks of the walls and slabs of the RB have been carried out based on result-ing forces at their connection extracted from the seismic calculations. For the walls, the sliding mechanism has also been examined;

All strength checks finally led to the assessment, for all the structures, of a strength factor of Safety Fs as defined in [EPRI94];

Moreover, the detailing requirements rules of all the structures have been compared to the requirements of the standards. Whenever fulfillment of these requirements was vali-

dated, the use of an Inelastic Energy Absorption factor of Safety Fµ has been considered

in compliance with the values proposed in [SEPT15];

At the end of the analysis, the final Safety factors F have been provided of all the mem-bers of the structures and each failure mode considered. Consequently, the elements with the lowest resistant margin could be easily identified.

TRACTEBEL finalized this analysis by means of additional verifications and methods, among

others:



A comparative study of the reinforcement needs in the containment, taking into account or not the prestressing tendons participation as passive reinforcement, thus enabling to justify specific zones of the containment wall where margins were low. In the critical are-as (equipment hatch and some areas of the raft for example), the choice of volumetric modeling associated to tendon participation has been therefore decisive in comparison to a shell modeling that could have denigrate some stress concentration;

A comprehensive analysis of the load transfer in the IS when representing the cracked state of its shear walls, in compliance with [SEPT15] method;

A detailed analysis of the behavior of the structural elements under concomitant forces;

Local additional verifications for specific parts:

a. Base of reactor pit under shear and torsion forces;

Figure 5: Shear stress distribution at the base of the reactor pit

b. IS ring wall at different levels and its hinged link to RB raft when submitted to combined loads;

c. Many sections have been studied through calculation of local forces equilib-rium, taking into account the actually installed reinforcement (anchorages length and available sections) and the concomitant forces;

d. Usual local verifications as punching for instance;

e. A detailed analysis of the containment wall gusset structural strength capac-ity when submitted to additional loads due to collapse failure of the pre-stressing gallery;



Figure 6: Cracking plane analysis in the gusset area

f. Other verifications adapted to the areas analyzed.

Independently of the study realized with EDF, TRACTEBEL strengthened the conclusions

of its strength assessments with different calculations involving other seismic methodologies, in order to identify the influence of the methodological choices on the resistance conclusions for similar studies.

Floor spectrum response calculation

The re-assessment of the RB including the resistance of all ND equipment, the resistance

study has been carried out in parallel with a floor spectrum response study. Beforehand the floor spectrum calculation, a specific zoning of the dynamic amplification of

all floors of the reactor building has been performed (cf. Figure 7), using spectral acceleration. This zoning enables to select, in the FE model, the most representative nodes as regards the dynamic response of the floors.

Figure 7 : Zoning (left) and floor response spectrum (right)



A specific analysis has been applied in order to take into account the fact that the raft is both flexible and embedded. This analysis has been carried out with the operator CALC_MISS of Code_Aster (réf. [SOFT1]) which uses the substructuring method to solve the soil-structure interaction problem in the frequency domain according to the three following steps (Cf. Figure 8):

1. The free-field seismic excitation is calculated at the base of the foundation through

a linear deconvolution ;

2. The complex stiffness matrix of the foundation (impedance matrix) is determined taking into account both the embedded foundation and the flexibility of the raft that is for the six degrees of freedom of all nodes of the foundation. The dimension of the calculated impedance matrix is therefore 6nx6n, n is number of nodes of the mesh of the foundation.

3. The global mass and stiffness matrices of the total system are assembled and the system of linear equations of motion is solved in the frequency domain using the FFT method.

Figure 8 : Solving the soil-structure interaction problem using the substructuring method

In parallel of this substructuring approach, a wide range of tests have been carried out by TRACTEBEL’s engineers using alternative calculation approaches in order to reach a satisfacto-rily confidence level in results obtained with Code_Aster CALC_MISS operator. These parallel analyses showed that the substructuring method allows a better taking into ac-count of both the damping and stiffness of the soil, which values change with the frequency of the study (whereas they are fixed, in the “classical” transient analysis). Therefore, the substruc-turing analysis can provide significant margin on the level of acceleration of floor spectrum, mainly at high frequency, where the damping of the soil is, in general, more important.



Conclusions

This first fully exhaustive re-assessment of a still operating RB building under SND condi-

tions highlighted some adjustment needed in existing methodologies used in the field of seismic reevaluation so that they could become fully applicable to RB structure. The modeling of specific structures belonging to RB, as for example as its containment and its main singularities, has to be made carefully so that the efforts used during the strength capacity analysis be representa-tive of the actual load transfers.

With these issues in mind, Tractebel engineers prepared after completion of their work a “feedback” report in order to focus on specific points and to present some suggestions to adapt the future SND strength reassessment program to other Reactor Buildings.

Nevertheless, wide and strong knowledge of design codes, of geometry and as built doc-umentation, and finally of load transfers in structural behavior of these structures are essential to reach optimized re-assessment of existing Reactor Buildings at seismic levels as high as those met in SND. References [ASCE05] ASCE/SEI43-05, Seismic design criteria for Structures, Systems, and Components in

Nuclear facilities, American Society of Civil Engineers (2005) [IAEA03] Safety Reports Series n°28, Seismic Evaluation of Existing Nuclear Power Plants, In-

ternational Atomic Energy Agency (2003) [EPRI94], Methodology for developing seismic fragilities, TR-103959, Final report (1994) [SEPT15] Guide méthodologique pour l’évaluation de la capacité sismique des structures en

béton armé, béton précontraint et des charpentes métalliques, EDF SEPTEN (2015) [SOFT1] Code_Aster v12.5 - Software package for civil and structural engineering, finite element

analysis, and numerical simulation in structural mechanics

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: [email protected]