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).
