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

Abstract

One of the consequences of the fission process in UO 2 fuels in nuclear reactors is the eventual formation of metallic fission product inclusions and precipitates. Here the stability and clustering behavior of one particular metallic fission product -Ru -is investigated using density functional theory in combination with classical thermodynamics. In particular, the solution energies of individual Ru atoms, dimers, and trimers at interstitial, uranium and oxygen vacancy, divacancy, and Schottky defect sites are calculated. Ru is predicted to be insoluble in most cases, but is soluble in uranium vacancy sites under hyper-stoichiometric conditions (UO 2+x ). Density of states analysis reveals the metallic nature of even the smallest Ru aggregates. Finally, by analyzing the binding characteristics of Ru in UO 2 , metallic dimers in Schottky defects are identified as the probable nucleus of metallic precipitates in UO 2 .

First-principles DFT+Uinvestigation of charged states of defects and fission gas atoms inCeO2

“…In Eq. 1 , accessible under different stoichiometry conditions, we use a method based on the study of Na-Phattalung et al [62] and Hong et al [63] considering the standard enthalpy of formation for various phases of the Ce-O system. For CeO 2 , the standard enthalpy of formation is defined as…”

Understanding xenon and vacancy behavior in UO2, UN and U3Si2: a comparative DFT+Ustudy

“…DFT is a powerful tool that is used to predict thermophysical properties. The chemical potential, which is a variable physical quantity influenced by the zero-point vibration energy, temperature and pressure, [53][54][55] is critical in determining the stability of defects. As such, it has been widely investigated within the framework of DFT under different chemical environments.…”

“…62 Then, the generalized gradient approximation (GGA)+U method is usually employed to optimize the chemical potential range of UO 2AEx by considering U 3 O 8 and UO 3 . 53 Moreover, the experimentally measured formation enthalpies of UO 3 , U 3 O 8 , U 4 O 9 and UO 2 can also be used to determine the similar variation range of the chemical potential. 63 UN and U 3 Si 2 are usually treated as line compounds in binary phase diagrams, and the reference of single-atom energy or the standard state of an element can be used to estimate the chemical potential simply in DFT calculations.…”

The role of charge and ionic radius on fission product segregation to a model UO2 grain boundary

“…16 Details of the calculations are the same as in our previous work. 17,18 For efficiency, only the two lowest energy sites discussed above were examined in a supercell containing a grain boundary structure that was reduced in size by half along both the y-and z-axes indicated in Figure 1, but which maintains the essential geometrical features of the R5 grain boundary. Despite the small system size, the fully relaxed structure is quite similar to that found using the potential.…”

“…When multiple fission products interact, their charge state might change dramatically, as we have indeed seen in the clustering of Ru within the UO 2 matrix. 17 Some of these limitations can be circumvented by using DFT, as we have done for Xe and Ru, but new limitations related to system size and computational efficiency arise. Even so, at least for the cases of Xe and Ru, we find that the charge state of the substitutional species is relatively constant regardless of where it is placed within the system, suggesting that the fixed charge assumption of the potential is physically reasonable, at least for this scenario.…”