Thomas Helfer

• PhD
• Atomic Energy and Alternative Energies Commission

The MFEM/MGIS application aims to efficiently use supercomputers in order to describe coupled multiphysics phenomena with a particular focus on thermo-mechanics. The authors primarily aim at describing the nuclear fuels at mesoscale (see example below), but MFEM/MGIS is versatile and can address more general cases. This open-source library is based...

Geotechnical safety analysis relies on numerical simulations capturing interacting physical phenomena. Systematic model development and selection are vital for interpretable results without black‐box aspects. Interpretability implies that models can be checked for plausibility and consistency as a whole and in terms of individual components by the...

In normal operating conditions of light water reactors (LWRs), fuel pellets exhibit cracks formed during the first reactor startup, resulting from thermal stresses induced by radial temperature gradients within the pellets, while the axial temperature gradient is negligible. The unique characteristic of nuclear fuel to generate its own heat through...

This video is about a combined torsion and pressure load of a bar using 20 node hexahedral elements with reduced integration and a hyper-viscoelastic constitutive law from the MFrontGallery project.

The aim of this contribution is to deeply introduce the ALCYONE Fuel Performance Code. ALCYONE is being implemented within the PLEIADES numerical framework and is dedicated to the multiphysics behavior of fuel rods in pressurized water reactors (PWRs). It has been a joint development between CEA, EDF and Framatome since 2005 and has been used since...

A novel experiment-driven modelling and simulation approach is proposed to study the formation of intergranular stresses and plastic strains during laser scanning of additively manufactured 316L stainless steel. Recently, laser scanning experiments were performed using a unique coupling between a continuous-wave laser and a scanning electron micros...

Wooden Panel Paintings (WPPs) stand as invaluable cultural artefacts from the past. These works present an intrigu- ing challenge in understanding their complex mechanical behaviour and ensuring their long-term preservation. The present study assumes as founding paradigm the unicity of each WPP, in terms of its material composition, historical back...

Implementing a constitutive model is a long, tedious and error-prone process, in particular for soils where a wide variety of phenomena must be taken into account. Moreover, the implementation must satisfy the interface requirements of the targeted solver. MFront is a popular code generator based on C++ mostly dedicated to mechanical behaviours whi...

Damage Mechanics and Local Approach to Fracture

Version 4.2 of TFEL is meant to stabilize the 4.x branches before a new development cycle that will start next year with the 5.x branch (based on C++-20). So does Version 2.2 of MGIS. Evolutions are focused on new features required by projects support TFEL developments, porting to GPUs of the TFEL/Math and TFEL/Material library, bugs fixing, and d...

Fuel cladding tubes made of zirconium alloys, are subjected in reactor to a complex loading history under nominal operating conditions. Furthermore, they exhibit a complex deformation behavior resulting from irradiation-induced growth, irradiation creep and thermal creep. For design and safety requirements, empirical models are usually used. To hav...

The mechanical integrity of the nuclear fuel cladding is critical for the safety of Pressurized Water Reactor (PWR) fuel elements, in normal and off-normal operating conditions. Stresses in the cladding result from many phenomena, but are mostly driven by the mechanical interaction of the cladding with the ceramic fuel. Hence, a comprehensive descr...

Highlights • Integrating innovative numerical analysis and non-invasive experimental tests • Numerical tool to optimize the conservation conditions of Mona Lisa's wooden panel • Analysing the internal stress state, and its safe limits, of the panel painting • FEM numerical modelling for the mechanical characterisation of the artwork • Assessment of...

Wooden Panel Paintings (WPP) are among the most significant historical and artistic artifacts from the Middle Ages and Renaissance and pose a challenge to conservators and scientists in both their comprehension and conservation. From a structural point of view, they can be considered as multi-layered objects, consisting of a wooden support and seve...

The nuclear fuel is produced by the ceramic process, which includes the grinding, pressing and sintering stages. Due to the presence of actinides in the fuel powder to be shaped, the grinding of the pellets to fit with the cylindricity requirements is prohibited after sintering. Therefore, the sintered pellets must be considered as «Net shape» or a...

Ductile fracture through void growth to coalescence occurs at the grain scale in numerous metallic alloys encountered in engineering applications. Classical models used to perform numerical simulations of ductile fracture, like the Gurson-Tvergaard-Needleman model and its extensions, are relevant for the case of large voids compared to the grain si...

Concrete exhibits a complex range of visco-plastic behaviour over its temperature range. At room temperature, concrete demonstrates basic creep behaviour (without loss of mass) when supporting an external load. In response to thermal fluctuations, there is an additional component of creep linked to moisture content and its transport (drying creep)....

Le code MANTA a l’ambition de permettre la réalisation de simulations complexes en mécanique sur des supercalculateurs actuels et futurs tout en préservant les fondamentaux des codes développés au CEA : adaptabilité au problème posé, robustesse des algorithmes, pérennité des modèles et du code. On expose les principes de développement de ce code de...

Bentonite materials are considered as the main sealing component in the closure of repositories for radioactive waste. This is particularly due to their swelling capacity upon hydration, which helps to close remaining gaps and voids at the sealing location and thus ensure the confinement of the disposed radioactive waste. In repository conditions,...

This document describes how to use MFront as a wrapper for an existing UMAT implementations, which is an interface introduced by the Abaqus/Standard finite element solver, of a small strain behaviour and how to introduce it in a material knowledge management project derived from MFrontGallery.

A novel approach of using existing implementations of constitutive material models in any numerical code interfaced with MFront. In this work, we developed a novel approach, which consists of using existing constitutive models already available in different finite element codes in any numerical codes interacting with MFront. We used for that the ve...

This talk presents various applications of MFront during the past year which are not described in the other talks of the 7th MFront User Meeting (21/10/2021). Some new features of Version 4.0 of the TFEL project and Version 2.0 of MGIS are discussed. Both projects are now based on the C++-17 standard. As a consequence, the TFEL/Math library have b...

The MFEM-MGIS library aims at efficiently use supercomputers in the field of implicit nonlinear thermomechanics. Our primary focus is to develop advanced nuclear fuel element simulations where the evolution of materials under irradiation are influenced by multiple phenomena, such as viscoplasticity, damage, phase transitions, swelling due to solid...

The characterization of the material behavior is crucial in the resolution of mechanical problems. For decades, constitutive laws were built thanks to parametric models depending on theoretical formalism [@truesdell_2004]. Identifying those parameters and fitting the data is one of the major difficulties. Therefore, modelling error and uncertainty...

Concrete exhibits visco-plastic behaviour when subjected to high temperature whilst supporting an external load; this component of plasticity is referred to as load induced thermal strain (LITS), which is also known as transient thermal creep (TTC). LITS phenomena can be important in pre-stressed concrete structures. Indeed, this can potentially le...

Implementing a constitutive material model is a long tedious and error-prone process, in particular for soils where a wide variety of models must be taken into account. Moreover, the implementation must satisfy the interface of the targeted solver. MFront is a popular code generator based on C++ mostly dedicated to mechanical behaviours which provi...

A non-equilibrium statistical thermodynamics-based framework for describing elasto-viscoplastic deformations in amorphous materials termed shear-transformation-zone (STZ) theory has been implemented in the material point method (MPM) and applied to the modeling of large-deformation geomechanical problems. Plane strain biaxial shear tests with diffe...

This document describes a new version of the Burger behaviour called Burger_EDF_CIWAP_2021 which describes various phenomea in concrete and discusses its implementation using the MFront code generator. The Burger_EDF_CIWAP_2021 aims at modelling the following phenomena in concrete: - Elasticity - Drying shrinkage - Basic creep - Drying creep Th...

This document gives some insights on how a user may extend the StandardElastoViscoPlasticity brick in MFront by adding a new stress criterion coupled with porosity evolution.

This report describes the implementation of porous (visco-)plastic behaviours that may be used to describe ductile failure of metals or the viscoplasticity of nuclears fuels. Aside from the standard implicit scheme, this report describes a new implicit scheme based on a staggered approach. The presented algorithms have been introduced in the Standa...

Constitutive equations describe how the internal state variables of a material evolve with changing external conditions or due to gradients of thermodynamic variables. Those state variables can describe many microstructural aspects of the material (grain size, dislocation density, hardening state, etc.) or be phenomenological in nature (equivalent...

The PLEIADES software environment is a simulation framework allowing the development of fuel performance codes dedicated to specific nuclear systems (PWR, SFR, and material testing reactors), experimental devices, and advanced fuel modeling. This chapter presents first the general features of the PLEIADES software environment to assess the fuel ele...

This talks will present some new features and improvements introduced during the development of Version 3.4 of the `TFEL/MFront` project and Version 1.2 of the `MGIS` project.

In this talk, we will present a [`python`](https://fenicsproject.org/) package called mgis.fencis (https://thelfer.github.io/mgis/web/mgis_fenics.html) which provides a binding between MFront and FEniCS, a finite-element library dedicated to the automated formulation and resolution of PDEs.

This talk presents an extension of MFront’ StandardElastoViscoPlasticity brick to constitutive equations developed for porous materials. Porous plasticity models describe the nucleation, growth and coalescence of voids, accounting for the porosity as an additional state variable. These models are used to perform numerical simulations of ductile fra...

The report describes the new functionalities of Version 3.4 of the TFEL project.

MFront is an open-source project started by CEA and EDF aiming at buildinga rigorous material knowledge management strategy and ease the transfer ofthis knowledge from R&D laboratory to engineering studies. Implementations ofadvance mechanical constitutive laws is one of its most salient usage. MFronthas three main objectives: •ease of use •portab...

Argillaceous formations are currently being considered in several countries as a possible host medium for deep geological disposal of high- and intermediate-level and long-lived nuclear waste (Delage et al., 2010). They exhibit desirable features such as low hydraulic conductivity, low molecular diffusion, and significant radionuclide retention cap...

This report shows how to integrate the Méric-Cailletaud behaviour under the small strain and finite strain assumptions. This article shows how to implement the Méric-Cailletaud single crystal viscoplastic behaviour (Méric and Cailletaud 1991) in MFront. Such an example illustrates: - The usage of StandardElasticity brick. - The usage of the funct...

The phase field method has been widely adopted in brittle fracture analysis for its ability to handle complex crack topology. This paper presents a novel efficient and robust phase field algorithm for quasi-static brittle fracture analysis. This algorithm overcomes two major issues that affect significantly the numerical cost of the method: the tre...

We report a set of compression test experiments carried out on sintered uranium dioxide at 1500°C, at constant strain rates and under controlled oxygen partial pressures. The macroscopic data indicate an initial strain hardening stage followed by a quasi steady–state period during which changes in the stress level are sluggish. The data are further...

The purpose of this report is to describe the most common method used in MFront for computation of the consistent tangent operator when an implicit scheme is used. This method is based on the fact that the consistent tangent operator may be deduced from the jacobian of implicit system in a straightforward manner.

The present demo of the mgis.fenics module illustrates how multi-physics problems can be coupled and solved in a staggered fashion. This problematic is illustrated on a phase-field approach to brittle fracture in which two different mechanical problems are involved: - a displacement problem ($u$-problem) involving a non-linear elastic constitutive...

This demo of the mgis.fenics module is a direct transposition of the transient thermo-elasticity demo using a pure FEniCS formulation. We will show how to compute fully coupled thermo-mechanical problems using MFront , which can pave the way to more complex thermo-mechanical behaviours including plasticity for instance.

This tutorial shows how to use the mgis.fenics to solve a non linear stationary heat transfer problem.

This tutorial shows how the mgis.fenics can be used to perform small strain plastic computations.

This demo is dedicated to the resolution of a finite-strain elastoplastic problem using the logarithmic strain framework proposed in Miehe, Apel, and Lambrecht

The ability to easily integrate user-defined constitutive equations plays a major role in the versatility of (mechanical) solvers. Constitutive equations describe how the internal state variables of a material evolve with changing external conditions or mechanical loading. Those state variables can describe many microstructural aspects of the mate...

This paper describes how MFront can be used as a wrapper to integrate an existing implementation of a constitutive equation in a solver supported by MFront. More precisely, this document shows how to wrap the bentonite behaviour developed by D. Mašín (see [1, 2]) for its use in the thermo-hydro-mechanical model of the OpenGeoSys’ solver (OGS, see...

This page describes how to implement a non-associated plastic behaviour based on the Mohr-Coulomb criterion. The algorithm mostly follows the work of Nagel et al. (2017) and relies on an apex smoothing introduced by Abbo and Sloan (1995). This implementation has been introduced in OpenGeoSys. This document shows: • how to implement a plastic behavi...

This tutorial show how to implement a new stress criterion for: 1. Its inclusion in the TFEL/Material libray 2. Extending the StandardElastoViscoPlasticity brick in MFront Running this tutorial requires the current development version of MFront.

This example is a direct continuation of our previous 2D example on non-linear heat transfer using the [`mgis.fenics` python module.

In this demo of the mgis.fenics module, we expand on the stationnary nonlinear heat transfer demo and consider a transient heat equation with non-linear heat transfer law including solid/liquid phase change. This demo corresponds to the TTNL02 elementary test case of the code_aster finite-element software.

This talk gives an overview of the MFront code generator with focus on: - Generic behaviours - Coupling of MFront and OpenGeoSys through MGIS

Release notes of Version 3.3 of TFEL/MFront

When subjected to high temperatures, concrete exhibits a load-dependent thermal strain known as load induced thermal strain (LITS) or transient thermal creep (TTC). LITS phenomena can be important in pre-stressed concrete structures, as its evolution under transient thermal conditions can potentially lead to both loss in pre-stress and residual ten...

This introduction is meant to enlight how the use the MGIS library in C++ to integrate a mechanical behaviour. This document targets solver developers who wish to integrate MGIS.

This document is meant to show how FEniCS and MFront can be combined to build almost arbitrary complex non linear solid mechanics simulation.

French GENIV sodium-cooled fast reactor (SFR) system could include Americium (Am) recycling in the radial fertile blankets located in the outer core regions. An issue related to this option is the lack of knowledge regarding the in-pile behaviour of the Am-bearing blanket (AmBB). The DIAMINO separate-effect experiment was irradiated in the OSIRIS M...

This article shows the implementation of an isotropic linear viscous model based on Skorohold-Olevsky Viscous Sintering (SOVS) model using MFront.

MFront is a tool which allows easy implementation of arbritrary complex mechanical behaviours in an efficient way. Those implementations are portable between various finite element solvers and solvers based on FFT. MFront is part of the open-source TFEL project. The purpose of this paper is to highlight a selected set of features introduced in Vers...

Description of the current status of the TFEL/MFront project and description of the new functionalities of version 3.2.

The PLEIADES software environment is devoted to the thermomechanical simulation of nuclear fuel elements behaviour under irradiation. This platform is co-developed in the framework of a research cooperative program between Électricité de France (EDF), AREVA and the French Atomic Energy Commission (CEA). Within this plateform, a unified software env...

The viscoplastic behavior of uranium dioxide (UO2) single crystal is of great interest to perform predictive multiscale modeling of the nuclear fuel. Here, a viscoplastic model is built considering dislocation glide in ½<110>{100} and ½<110>{110} slip systems. The constitutive law parameters are determined adjusting the temperature dependency of th...

The ALCYONE multidimensional fuel performance code co-developed by the CEA, EDF and AREVA NP within the PLEIADES software environment models the behavior of fuel rods during irradiation in commercial pressurized water reactors (PWRs), power ramps in experimental reactors or accidental conditions such as loss of coolant accidents (LOCAs) or reactivi...

We investigate in this paper analytically and numerically by means of 3D simulations the viscoelastic behavior of concrete and mortar subjected to creep loading and moderate temperatures at mesoscale. These heterogeneous materials are assumed to be composed of thermoelastic aggregates distributed in a linear thermoviscoelastic matrix; moreover, the...

The ALCYONE multidimensional fuel performance code co-developed by the CEA, EDF and AREVA NP within the PLEIADES software environment models the behavior of fuel rods during irradiation in commercial Pressurized Water Reactors (PWRs), power ramps in experimental reactors or accidental conditions such as Loss Of Coolant Accidents (LOCAs) or Reactivi...

This paper focuses on a new algorithm for the quasi-static simulation of unstable crack propagation in time-independent brittle material. The proposed approach is based on a combination of the standard Newton's algorithm with a fictive path loading algorithm. First, a time step refinement process, associated to the Newton's algorithm, enables us to...

This paper reports on preliminary studies performed regarding the insertion reliability of the RBC-type Control Rods designed for the ASTRID Sodium-cooled Fast Reactor. At this stage, the primary aim of the analysis is to evaluate the mechanical behavior of RBC Control Rods under Emergency Shutdown conditions, for which reactor core structures are...

MFront is a tool which allows easy implementation of abritrary complex mechanical behaviours in an efficient way. Those implementations are portable between various finite element solvers and solvers based on FFT. MFront is part of the open-source TFEL project which also provide an useful point-wise solver called MTest. The purpose of this paper is...

Depuis les travaux de Marigo et Bourdin, la modélisation par champ de phase de la fissuration des matériaux fragiles a gagné en efficacité et en robustesse, notamment par les travaux de Miehe. Celui propose une implémentation explicite dans un sens que nous précisons dans ce papier. Ce papier compare différentes implémentations purement implicites...

This paper focuses on the numerical simulation of crack initiation and growth in ceramic materials. This work is devoted to nuclear fuel modelling under irradiation and more precisely to fuel pellet fragmentation assessment at macroscopic and microscopic scales. Simulation tools are developed in the framework of a cooperative program between the CE...

Concrete is a heterogeneous material made up at mesoscale of linear elastic aggregates distributed in a mortar matrix whose behavior is time and temperature dependent. The Interfacial Transition Zone (ITZ) between aggregates and matrix also influences the overall behavior. We investigate here analytically and numerically by means of 3D simulations...

This paper presents 2D generalized plain strain simulations of the thermo-mechanical response of a pellet fragment and overlying cladding during a RIA transient. A fictitious hydride blister of increasing depth (25 to 90% of the clad thickness) is introduced at the beginning of the calculation. When a pre-determined hoop stress is exceeded at the c...

This paper provides an overview of the Licos fuel performance code which has been developed for several years within the platform pleiades, co-developed by the French Alternative Energies and Atomic Energy Commission (CEA) and its industrial partners Électricité de France (EDF) and AREVA. CEA engineers have been using Licos to back multidimensional...

In this paper we investigate analytically and numerically the creep behavior of concrete at mesoscale. The simulations are carried out with the finite element (FE) code (Cast3M) on 3D concrete specimens consisting in about 4600 polyhedral aggregates of various size and shapes randomly distributed in a box. Both matrix and Interfacial Transition Zon...

Fixed point iterations are still the most common approach to dealing with a variety of numerical problems such as coupled problems (multi-physics, domain decomposition, ...) or nonlinear problems (electronic structure, heat transfer, nonlinear mechanics, ...). Methods to accelerate fixed point iteration convergence or more generally sequence conver...

This paper shows how the Lagrangian logarithmic strain framework proposed by Miehe et al. can be used to extend monodimensional fuel performance codes, written in the framework of the infinitesimal strain theory, to be able to cope with large deformation of the cladding, such as the ones observed in reactivity initiated accidents (RIA) or loss-of-c...

The PLEIADES software environment is devoted to the thermomechanical simulation of nuclear fuel elements behaviour under irradiation. This platform is co-developed in the framework of a research cooperative program between Électricité de France (EDF), AREVA and the French Atomic Energy Commission (CEA). As many thermomechanical solvers are used wit...

Ce papier est dédié au générateur de code open-source nommé MFront [7]. Plusieurs lan-gages dédiés (domain specific languages), basés sur le C++ [27], permettent d'écrire de manière simple, compacte, efficace et robuste, l'implantation de lois de comportement mécanique. Après une vue d'en-semble, nous décrivons certaines des techniques de programma...

In this paper, the behavior of a dense UO2 (porosity less than 2%) was studied experimentally on a range of temperatures (1100–1700 °C) and strain rates ( /s) representative of RIA loading conditions. The yield stress was found to increase with strain rate and to decrease with temperature. Macroscopic cracking of the samples was apparent after the...

This work presents an experimental characterization of uranium dioxide (UO2) in compression under Reactivity Initiated Accident (RIA) conditions. Pellet samples were tested at four temperatures (1100, 1350, 1550 and 1700°C) and at a strain rate varying over 4 decades (10-4–10-3–10-2–10-110-4–10-3–10-2–10-1/s). The experimental results show that the...

This paper presents two closed-form solutions that can be used to enrich the mechanical description of fuel pellets and cladding behavior in standard one-dimensional based fuel performance codes. The first one is concerned with the estimation of the opening of a radial crack in a pellet fragment induced by the radial thermal gradient in the pellet...

A critical look at the conceptual designs of control and shutdown rods and absorber elements, along with the lessons learnt from the operation of French fast reactors (Phénix and Super-Phénix especially) and the associated irradiation tests, has yielded improved and even innovative absorber assembly design concepts which are presented in this paper...

Concepts and design activities for ASTRID prototype’s control and dshutdown rods

Future GENIV sodium-cooled fast reactor (SFR) may include Americium (Am) recycling in radial blankets positioned in the outer core regions. Indeed, Am recycling on a UO2 matrix has the advantage of providing flexible breeding gains, Am burning and making blanket reprocessing possible in a standard UPuO2-x fuel flux. Two separate-effect experiments...

The PLEIADES software is a joint project developed by the CEA (Commissariat à l'Energie Atomique) and EDF (Electricité de France) and supported by Framatome-ANP aimed at modelling the behaviour of nuclear fuels. Based on an innovating architecture providing the integration of numerical components, the PLEIADES code will capitalize within the same s...