Investigation of molybdenum and caesium behaviour in urania byab initiocalculations

Abstract

A theoretical study of molybdenum and caesium solution in uranium dioxide is carried out. Calculations are performed using the density functional theory with the projector-augmented-wave method as implemented in the Vienna ab initio simulation package (VASP). Correlation effects are taken into account within the DFT+U approach. Molybdenum is preferentially inserted in uranium-oxygen divacancies for understoichiometric urania and uranium vacancies for overstoichiometric urania. The favourable sites for caesium solution are the Schottky defect for understoichiometric urania and U vacancies and U-O divacancies for overstoichiometric urania. Using the stability of many binary and ternary compounds in comparison to soluted atoms, we show that caesium and molybdenum are insoluble in uranium dioxide whatever the stoichiometric regime.

Fission products stability in uranium dioxide

“…Finally, Ce displays a very low energy and belongs to the third group. From our previous calculations [14][15][16], we can include Cs in the first group, Ba and Mo in the second group, and Zr in the third one.…”

“…Xs = divacancy in UO 2Àx ), E sol Ce2Zs the solution energy of cerium in urania in the most stable site Zs as calculated in section 3.2 and E sol O2Ys the solution energy of oxygen in urania in the most stable site Ys. Note that the solutions energies of Ba, Zr, Cs and Mo are extracted from [14,16]. It should be noted that for SrUO 3 , the following reaction is considered:…”

“…In order to better describe the localization of the f electrons of uranium in this compound, the GGA+U functional can be considered [12,13]. Very recently, we showed that the use of GGA+U functional leads to a better agreement with experimental data for solution energies of fission products, namely Ba, Zr, Cs and Mo in urania [14][15][16]. In this contribution, we extend our calculations of solution energies with GGA+U functional to He and other important FP for safety like Kr, Xe, I, Te, Ru, Sr and Ce.…”

“…Cerium oxide has a peculiar behavior since it is almost completely miscible with UO 2 . Since we are not able to take into account all the chemical complexity of both metallic and gray phases, we will consider prototypic phases which are expected to give a represen- tative model of complex phases together with the phases studied in our previous contributions [14][15][16] that can be observed in oxide fuels.…”

Solubility and clustering of ruthenium fission products in uranium dioxide as determined by density functional theory

“…This insolubility was also predicted by a subsequent computational study 13 which presented the dependence of the solubility of fission products on the thermodynamic conditions of the fuel such as stoichiometry, defect concentration, and temperature. In addition, over the last decade, the chemical stability of various fission products that exist in gaseous or oxide form in UO 2 -including He, I, Cs, Sr, Ba, Zr, Mo, and Xe -have been determined using DFT [13][14][15][16][17] . More recently, the stabilities of selected metallic fission products were also examined 18 .…”