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 2.
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, 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.
Five research topics are covered as part of an approach combining experimental analysis of materials, modelling of material behaviour, and prediction using numerical simulation tools:
Mechanics of solid materials: continuous media – discrete media
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.
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).
- 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.
The goal is to understand and model multiphysics coupling in the field of nuclear fuel:
- Fuel-coolant interactions during the possible ejection of fuel, whose quantity and grain-size will determine whether a vapour explosion will occur or not.
- Behaviour of fission gases and their effect on the overall behaviour of the structure.
- The evolving cladding microstructure by diffusion/precipitation of hydrogen, oxygen or nitrogen under thermomechanical loading.
- Fluid or gas flows through porous continuous media or granular media, and particularly the loss of equilibrium in a fragmented medium, which is similar to the liquefaction of granular media.
Experimental identification by field measurements
This topic will come in support of the previously-mentioned topics, but nevertheless 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).
Here, the MIST laboratory aims at studying structural integrity from a global viewpoint. This mainly involves contributing to the definition and assessment of safety criteria required to prevent certain types of accidents occurring in the nuclear field.