An initial step [1] allowed the calculation of the contribution of oxygen ions to Q* using classical molecular dynamics (CMD). However, these classical calculations overlook the contribution of localized electronic defects (or small polarons) U3+ and U5+ that accompany oxygen defects to ensure electroneutrality. Our project therefore aims to calculate this polaronic contribution.

To achieve this, other atomistic techniques are necessary since the available empirical potentials do not allow for the treatment of electron movement. We will therefore focus on ab initio calculations, possibly complemented by CMD. Both macroscopic polaronic transport parameters will be calculated: the diffusion coefficient D and the heat of transport Q*.

The work will include the following steps:

• State of the Art
  • Review the quantum theories of the Seebeck effect of small polarons and their implications for atomistic calculations.
  • Inventory the available atomistic techniques for characterizing D and Q*. Particular attention will be given to the dynamic field of thermoelectric materials.
• Calculate the migration barrier and diffusion prefactor for the polarons in UO2 using ab initio methods.
• Compare with available measurements in UO2.
• Test the available methods for calculating the heat of transport and select the most relevant methods. This very innovative subject requires exploratory work, and may involve developing a method tailored to small polarons, should the current state of research be insufficient.
• Determine, from calculated atomistic quantities, the relevant macroscopic parameters for material simulations in the field of nuclear fuel.
• If available, compare the results with measurements.

The work environment is particularly stimulating.

• The position is based in Cadarache, within the Nuclear Fuel Department, which offers the opportunity to interact with most of the specialists involved in this type of study (experiment, theory, ab initio or MD simulation, scientific application developers).
• Depending on the needs, you will be trained on ab initio calculations on small polarons by experts from the CEA Bruyères-le-Châtel site.
• You will benefit the opportunity to present your results at conferences and publish them.
• This position will allow you to participate in a multi-scale approach applied to nuclear materials and to see how the most fundamental calculations contribute to a realistic simulation of material evolution under extreme conditions.

References (doi.org):
[1] Bareigts et al.  10.1016/j.ces.2023.119141
[2] Konarski et al.  10.1016/j.jnucmat.2019.03.021

• You have a PhD in materials science or solid state chemistry, a taste for and skills in theory (statistical, quantum and solid state physics) and simulation.
• You have solid skills in ab initio calculations (DFT).
• You would like to extend them to other techniques and produce macroscopic quantities useful for "multi-scale" simulations.
• You are motivated by upstream research applications and would like to work in a rich, multidisciplinary environment (theory, simulation, software, experimentation).