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Research units

Micromechanics and Structural Integrity Laboratory (MIST)

Last update on November 2018 

The Micromechanics and Structural Integrity Laboratory (MIST) jointly funded by the CNRS and IRSN could be described as a "without walls" laboratory. It pools research resources to study the behaviour of materials and structures exposed to harmful environments. Resources are provided by PSN-RES at Cadarache for IRSN, and by the Mechanics and Civil Engineering Laboratory (LMGC) in Montpellier for the CNRS and University of Montpellier.

Context and research themes


The MIST laboratory aims at studying the integrity of heterogeneous and evolving structures. This requires understanding and predicting the behaviour of materials in harmful environments such as high thermomechanical loads, or natural or induced ageing. These environments especially exist in the nuclear industry: reactor cores, containment structures, waste repositories, etc.


The MIST laboratory is especially aiming to solve two types of problems: 

  • High-speed dynamics. Modelling the behaviour of nuclear materials and structures under accident conditions can sometimes be greatly influenced by the analysis of the fast-evolving phenomena involved. This can involve global or local dynamic problems for which their analysis and understanding requires cross an experimental step, thus making it possible to obtain the necessary input data for already-operational computer codes.
  • Microstructural changes. The objective is to predict the behaviour of materials and heterogeneous structures when exposed for lengthy durations in an environment that has a very harmful effect on their integrity: variation in the concentration and properties of phases, influence of each phase on the overall behaviour, influence of solid and gaseous precipitates, influence of multimodal grain-size distribution, etc. This issue relies on modelling and numerical simulation in the nuclear field because it is impossible to obtain direct experimental measurements.

Several research topics are covered as part of an approach combining experimental analysis of materials, modelling of material behaviour, and prediction using numerical simulation tools.
On the one hand, this involves understanding the non-linear thermomechanical behaviour of materials composing fuel rods (the fuel and the cladding), and predicting such behaviour by means of modelling tools, knowing that: 
  • Fuels are classified as high-temperature porous ceramics. Their behaviour is considered to be hydro-poro-mechanical under compression, and includes elasticity, creep and plasticity. Under tension, these ceramics are considered to be elastic brittle materials with a low ultimate tensile stress. 
  • Cladding can be compared to multi-layered metal matrix composites and functionally graded materials inside layers (presence of hydride plates). They are considered as either continuous solids (with internal discontinuities such as cracks or phase transitions), or granular media.

Another goal is to understand the thermo-chemical-hydro-mechanical behaviors of the cementitious materials which comprise the solid pieces of the nuclear reactors (containments, rafts, etc.). Concrete is a porous multiphase heterogeneous composite material made up of a granular skeleton coated with a hydrated cement paste. It demonstrates dissymetric behaviour in traction and compression (high resistance to compression and low resistance to traction). Its behaviour is considered poro-viscoelastic, compounded by deferred deformations (drying shrinkage, creep, pathologies) and damage from micro-cracks.

To do this, the MIST Laboratory is divided into three Research Operations: 

Scale transitions: microstructural descriptions, equivalent behaviours, propagation of uncertainties
Various scale transitions will be investigated in support of material mechanics so as to solve problems such as fragmented assembly stability and granular plasticity under complex loading. Different types of scale transitions exist: 
  • Micro-meso scale transitions (from the grain to grain distribution) and meso-macro transitions (from grain distribution to consideration of macro-heterogeneities: cracks, precipitates, hydrides, and plutonium inclusions, deferred deformations of concrete such as internal swelling reactions);
  • Lower scales to cover the mechanics of continuous or discrete media (from molecular dynamics to microscopic properties) and direct micro-macro transitions when micro-meso-macro scale separations are abusive.

Cracking Fragmentation: performance of materials and structures in relation to microstructural changes

The goal is to set up cohesive zone models to use when modeling cracking and fragmentation of heterogeneous materials. Another aim is to develop the corresponding numerical methods:
  • Identification of cohesive zone models applied to nuclear materials (fuel cladding, nuclear fuel, cementitious materials, steels);
  • Determination of intra- and inter-phase breach criteria. This essentially experimental topic concerns heterogeneous metal alloys, as well as cementitious materials;
  • Development of accurate numerical methods to model multi-cracking and breach while taking into account complex heterogeneities and multi-physics couplings;
  • Optimization of numerical methods related to cracking and fragmentation of heterogeneous materials. 

Multiphysics coupling: influence of harmful environments on the thermomechanical behaviour of materials
The goal is to understand and model multiphysics coupling in the fields of nuclear fuel and the aging of concrete that is healthy or potentially affected by physicochemical pathologies: 
  • Behaviour of fission gases and their effect on the overall behaviour of the nuclear fuel;
  • The evolving cladding microstructure by diffusion/precipitation of hydrogen, oxygen or nitrogen under thermomechanical loading; 
  • The evolving microstructure of concretes by distribution/dissolution/precipitation of chemical species under complex thermomechanical loading;
  • the coupling of cracking and species distribution in a porous granular medium;
  • the flows of fluids or gases through porous continuous media or granular media, and especially the breach of equilibrium in a fragmented medium which is akin to the liquefaction of granular media.

It should be noted that the three research operations use experimental identification techniques involving field measurements. This topic remains an important research topic in its own right due to the originality of the expected applications. This involves developing experimental methods capable of determining complex volume behaviour laws for heterogeneous materials, cohesive-zone models, and for various inverse problems. These methods are mainly based on mechanical imagery (correlation of images or infrared thermography).

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Cross section of a fuel rod taken perpendicular to its axis. Left the fuel porous ceramic

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