Simulation challenges
"Modelling & simulation plays a role in all stages of R&D on fuel: design, fabrication, preparation and interpretation of experiments, and the use of characterisation results. A great deal of physics and experimental data is needed to build a good model which has the desired prediction capacities […]" explained Carole Valot, an engineer working for the CEA Nuclear Energy Division. This emphasises the continuous interplay between experimentation and simulation, as each "feed" into the other.
Understanding, modelling and simulating fuel behaviour is an undertaking just as complex as the subject itself which involves the expertise of the nuclear materials and the physical chemistry departments, in addition to that of the teams working at the fuel studies department.
We need to be capable of predicting a wide range of phenomena: creation of new elements (e.g. fission products), formation of cavities and bubbles in the material, cracking pellets, migration of material1, deformation of the pellet, cladding, and assemblies.
From the scale of an atom to that of a grain in order to connect the fundamental physical phenomena and experimental data
To adress these challenges, the CEA has an unrivalled simulation platform which is used to develop a whole range of models for time periods ranging from a picosecond to a year, and scales of size ranging from the electron cloud to the pellet or even the assembly. "
Our aim is for all these scales to communicate. However, they require the involvement of experts from very different fields (physics, chemistry, temperature control, thermodynamics, mechanics) who can sometimes struggle to understand one another. This is one of the challenges of multi-scale simulation!", says the researcher.
1 Formation of a hole in the centre of irradiated pellets in fast reactors
2 The material consists of a fine powder (grains of a few microns) agglomerated by sintering.
In practical terms, two complementary approaches are used. The first is dedicated to understanding the basic physical phenomena, starting with the scale of an atom and gradually building up to the grain2. Basic research uses models of the material physics and codes developed by university teams, although these need to be adapted because they do not necessarily take account of the problems specific to nuclear fuels. "We do not have the resources to develop everything. To keep up to date, we use the best technology from other sources and we collaborate with other researchers at the CEA and in the academic world."
The second approach looks at the problem the other way round. It involves developing - with EDF and AREVA - performance codes which simulate pellet or rod behaviour under irradiation (relying on the numerical methods developed within the scope of academic collaborations) and enriching them with observation data. "These two simulation approaches coincide at grain level, which is at the end of the day a key scale, a receptacle for a vast amount of information", said Carole Valot.
A multitude of models and data
At Cadarache, the PLEIADES platform for simulating fuel behaviour combines performance codes from all reactor technologies, especially two major codes: Alcyone for PWRs and Germinal for FRs. It contains all the physical models and the CEA databases, along with those of its partners. "One of the CEA's strongest assets" concluded Carole Valot.