Postdoctoral Research Project 1: Behavior of Fission Froducts within Uranium Carbide Nuclear Fuel
2010-2012: Institut de Radioprotection et de Sûreté Nucléaire - Laboratoire d'étude du corium et du transfert des radioéléments
Supervisors: Dr R. Dubourg, Dr R. Ducher
This research project was related to the GEN IV project. The basic idea is to increase the yield of nuclear fuels as well as the possibility to incorporate long-lived Fission Product wastes in the fuel in order to consume and thus eliminate them. Generally, the UO2 nuclear fuel is used in nuclear facilities, but alternative uranium materials also exhibit interesting properties such as uranium carbide (UC) or uranium nitride (UN). Among these interesting properties, one can mention a higher density of metallic atoms that is related to the more compact nature of FCC structures. The aim of this study was to investigate fission product behavior within UC in a prospective approach. In operation conditions, the disintegration of uranium atoms in nuclear fuels emit an important quantity of energy as well as fission products with high kinetic energy. This progressively lead to damages in the fuel, changes in the composition and swelling of the fuel. In these conditions, it is of crucial interest to know what are the favored FP locations within the lattice and to quantify how fast they migrate in the material. We used the density functional theory with the Vasp program to model the elementary mechanisms concerning the behavior of the most common 15 FPs in UC. These parameters can then be used in mesoscale models to predict the fuel evolution in working conditions as well as in crise situation.
We first studied FP stabilities in two distinct scenario [1]. In the first one we consider preexisting vacancy defects within the fuel lattice and we evaluate the stability of the FP in it. This situation corresponds to an aged fuel with lot of defects in the lattice and we refer to the concept of the incorporation energy. In the second situation, there is no preexisting defect and the fission product is competing with lattice elements to reach a lattice site. The effect of light stoichiometry deviations is taken into account through the point defect model which was extended to the uranium carbide material. In this second approach, we refer to the concept of solution energies. Calculations showed that FPs always favor the occupation of U site for chemical or steric reasons. Indeed the U site corresponds to a carbide environment which strongly stabilizes metallic FPs. It is also the largest site and this is favorable for large FPs and non-bonding noble gases.
We then studied the potential migrations of FPs within the lattice [2]. To migrate within the lattice a vacancy in an adjacent site is a prerequisites. Since the site stability study showed that the FP favor the U site, we investigate migration of FP within the uranium sublattice of UC, considering an uranium vacancy in the direct vicinity of the FP. Here again, two distinct scenario can be considered 1. This vacancy is preexisting and we refer to migration energy, 2. This vacancy does not exist, and an additional energy is needed to create it, leading to the concept of activation energy. For all the FP, migration energies were computed and activation energies were determined with the simplified approach suggested by Andersson. To confirm the results obtained with the Andersson approach, we also determined the activation energies of two typical FPs in the framework of the more precised 5 frequency model, considering an ensemble of 5 possible displacements of the FP-Vacancy cluster. The results precisely corroborated those obtained with the simplified approach, providing a high degree of confidence in the results. Stoichiometry deviations were also taken into account.
As expected, noble gas FP only create weak interactions with the surrounding environment and thus show weak activation energies. On the contrary metals like Zr are strongly stabilized in a carbide environment and exhibit the highest activation energies. For other FPs, these observations are however counter-balanced by the size of the FP and the affinity for the carbide environment.
2010-2012: Institut de Radioprotection et de Sûreté Nucléaire - Laboratoire d'étude du corium et du transfert des radioéléments
Supervisors: Dr R. Dubourg, Dr R. Ducher
This research project was related to the GEN IV project. The basic idea is to increase the yield of nuclear fuels as well as the possibility to incorporate long-lived Fission Product wastes in the fuel in order to consume and thus eliminate them. Generally, the UO2 nuclear fuel is used in nuclear facilities, but alternative uranium materials also exhibit interesting properties such as uranium carbide (UC) or uranium nitride (UN). Among these interesting properties, one can mention a higher density of metallic atoms that is related to the more compact nature of FCC structures. The aim of this study was to investigate fission product behavior within UC in a prospective approach. In operation conditions, the disintegration of uranium atoms in nuclear fuels emit an important quantity of energy as well as fission products with high kinetic energy. This progressively lead to damages in the fuel, changes in the composition and swelling of the fuel. In these conditions, it is of crucial interest to know what are the favored FP locations within the lattice and to quantify how fast they migrate in the material. We used the density functional theory with the Vasp program to model the elementary mechanisms concerning the behavior of the most common 15 FPs in UC. These parameters can then be used in mesoscale models to predict the fuel evolution in working conditions as well as in crise situation.
We first studied FP stabilities in two distinct scenario [1]. In the first one we consider preexisting vacancy defects within the fuel lattice and we evaluate the stability of the FP in it. This situation corresponds to an aged fuel with lot of defects in the lattice and we refer to the concept of the incorporation energy. In the second situation, there is no preexisting defect and the fission product is competing with lattice elements to reach a lattice site. The effect of light stoichiometry deviations is taken into account through the point defect model which was extended to the uranium carbide material. In this second approach, we refer to the concept of solution energies. Calculations showed that FPs always favor the occupation of U site for chemical or steric reasons. Indeed the U site corresponds to a carbide environment which strongly stabilizes metallic FPs. It is also the largest site and this is favorable for large FPs and non-bonding noble gases.
We then studied the potential migrations of FPs within the lattice [2]. To migrate within the lattice a vacancy in an adjacent site is a prerequisites. Since the site stability study showed that the FP favor the U site, we investigate migration of FP within the uranium sublattice of UC, considering an uranium vacancy in the direct vicinity of the FP. Here again, two distinct scenario can be considered 1. This vacancy is preexisting and we refer to migration energy, 2. This vacancy does not exist, and an additional energy is needed to create it, leading to the concept of activation energy. For all the FP, migration energies were computed and activation energies were determined with the simplified approach suggested by Andersson. To confirm the results obtained with the Andersson approach, we also determined the activation energies of two typical FPs in the framework of the more precised 5 frequency model, considering an ensemble of 5 possible displacements of the FP-Vacancy cluster. The results precisely corroborated those obtained with the simplified approach, providing a high degree of confidence in the results. Stoichiometry deviations were also taken into account.
As expected, noble gas FP only create weak interactions with the surrounding environment and thus show weak activation energies. On the contrary metals like Zr are strongly stabilized in a carbide environment and exhibit the highest activation energies. For other FPs, these observations are however counter-balanced by the size of the FP and the affinity for the carbide environment.
References:
[1] First-principles study of the stability of fission products in uranium monocarbide, E Bévillon, R Ducher, M Barrachin, R Dubourg, Journal of Nuclear Materials, 426, 189-197 (2012). DOI: 10.1016/j.jnucmat.2012.03.014
[2] Investigation of the diffusion of atomic fission products in UC by density functional calculations, E. Bévillon, R. Ducher, M. Barrachin, R. Dubourg, Journal of Nuclear Materials, 434, 240-247 (2013). DOI: 10.1016/j.jnucmat.2012.11.030