In plutonium-uranium mixed oxides fuel (U, Pu)O2-x used in Sodium Fast Reactors (SFRs), the combination of high temperatures and steep temperature gradients causes the migration of fabrication porosities towards the center of the fuel pellets.
This transport of matter occurs through an evaporation and re-condensation mechanism (E/C): when a medium is facing a pore, a part of its components evaporates spontaneously in an attempt to establish the equilibrium vapor pressure dictated by the local temperature, pressure, and chemical composition. Once in the gas phase, the evaporated species can migrate by atomic diffusion and condensate on another part of the surface facing the pore featured by suitable conditions (e.g., by a lower temperature). The consequence of this phenomenon is a local restructuring of the original microstructure since the matter is effectively displaced in the radial direction of the fuel pellet and a distillation of fuel constituents due to non congruent E/C.
Previous studies [1]–[3] demonstrated the impact of local temperature, stoichiometry, and enrichment in actinides such as plutonium and americium on the equilibrium pressures of the different species found in the fuel in such conditions. As a direct consequence, the velocity at which pores migrate depends on the same set of quantities. It is, therefore, necessary to accurately calculate the local chemical equilibria found in the fuel to properly characterize the kinetic of the central hole formation and fuel densification of (U, Pu)O2-x irradiated in SFR conditions. Indeed, calculating such phenomena is a crucial step in the estimation of the overall thermal regime of the fuel pin, especially at the beginning of irradiation.
The fuel performance code PLEIADES/GERMINAL [4] allows for the calculation of (U, Pu)O2-x behavior under irradiation in SFRs. In the code, the models accounting for fuel restructuring due to evaporation/condensation and constituent redistribution rely on the calculation of pore migration velocity based on the partial pressures of each species in the gas phase. Such pressures are evaluated by either empirical correlations or obsolete and simplified thermodynamical representation of the fuel, not (or wrongly) accounting, for example, for fuel burnup.
To overcome such limitations, it is necessary to upgrade the modeling employed in PLEIADES/GERMINAL by exploiting a more accurate thermodynamical representation of fuel under irradiation in SFRs. To this regard, the PLEIADES/GERMINAL has been coupled to the OpenCalphad [5] thermodynamic code and to the Thermodynamics of Advanced Fuels - International Database (TAF-ID) [6] database.
The goal of the proposed internship is to extend the multiphysics coupling between PLEIADES/GERMINAL and OpenCalphad to improve the modeling of (U, Pu)O2-x fuel restructuring. After a first familiarization with the modeling and theoretical approach, the candidate will integrate a new algorithm for the estimation of vapor pressures in the gas phase by OpenCalphad, compare the results obtained by the new approach to the legacy one, and perform a first validation of the coupled PLEIADES/GERMINAL/OpenCalphad/TAF-ID tool with analytical experiments
A successful internship could be followed by a Ph.D. thesis aimed at studying the mechanism of transport by evaporation/condensation by an advanced approach based on phase field modeling.
[1] K. Maeda et al., Journal of Nuclear Materials, vol. 389, pp. 78–84, 2009.
[2] Y. Ikusawa et al., ICONE22, Prague, Czech Republic, 2014.
[3] T. Ozawa et al., Journal of Nuclear Materials, vol. 553, p. 153038, 2021.
[4] M. Lainet et al., Journal of Nuclear Materials, vol. 516, pp. 30–53, 2019.
[5] B. Sundman et al., Computational Materials Science, vol. 125, pp. 188–196, 2016.
[6] C. Gueneau et al., Calphad, vol. 72, p. 102212, 2021.
Thermodynamics, C++ programming, condensed matter physics and chemistry.
MSc student in chemical, nuclear, mechanical, materials or aerospace engineering; Physics
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