In case of a severe accident on a nuclear power plant, UO2 fuel in the nuclear core may undergo high temperature, which enhance the migration of fission products and allow their release out of fuel pellets. Consequently, the radioactivity in the containment vessel increases and, potentially diffuse in the outside environment if the containment tightness fails. It is therefore extremely important to estimate the amount and kinetics of fission product as precisely as possible so that the consequences of the accident to the environment can be determined. In this framework, the french Institute of Radiological Protection and Nuclear Safety (IRSN) has initiated a study which deals with ab initio evaluation of the location and migration of fission products in nuclear fuel under such severe accident conditions.
More precisely, our interest is focused on caesium behaviour, as it has a significant fission rate, is very noxious, and may likely be released in large amount in case of a severe accident. We focus here on the solution and incorporation energies of caesium in several point defects in uranium dioxide.
Concerning the defects, various types of defects (point defects such as uranium and oxygen vacancies, interstitials, Frenkel pairs, Schottky defects, or more extended defects) are created in the crystal structure of UO2. Because of the important CPU time of calculation, we restrict our investigation to the formation energies of simple defects, and then incorporate caesium in them.
For the calculation, an ab initio method has been used. Such method can provide theoretical information about the location of the fission products in the crystal structure of uranium dioxide. Indeed, by calculating the formation energies of the defects, and the incorporation energies of fission products in various sites, one can determine the most stable incorporation site for each fission product. The theoretical value of the activation energy can then be calculated to finally propose a model for fission-product migration.
The ab initio calculations have been performed with VASP (Vienna Ab Initio Simulation Package) code using PAW (Projected Augmented Plane Wave) potentials in the GGA (Generalized Gradient Approximation) of the DFT (Density Functional Theory). Studies on caesium have already been performed by other authors such like Busker  and Crocombette  using respectively Mott-Littleton methodology and LDA (Local Density approximation). But in our work, a supercell of at least 96 atoms has been used to limit defects interactions due to the cell periodicity and, in case of a magnetic calculation, to show the antiferromagnetic structure of UO2.
We present here the results of our preliminary calculations on caesium’s incorporation energies in UO2.
(1) : CNRS, Laboratoire de Physique et Modélisation des Milieux Condensés, Maison des Magistères, 25 avenue des Martyrs BP166, 38042 Grenoble, France
(2) : IRSN