arXiv subjects

S. D. Günay

Publications and source records attributed to S. D. Günay.

4 recordsLinked to original sources

arXiv2020

Actinide and Lanthanide Dioxide Lattice Dilatation Mechanisms with Defect Ingrowth

α-particle irradiated single-crystal isostructural oxide lattices (CmO2, AmO2, CeO2, UO2, NpO2, PuO2, ThO2) were modeled by molecular dynamics simulation method. Lattice parameter changes with IFP cation defects displayed exponential increases. These data were compatible with the available lattice parameter changes with α-particle dose experiments. Pre- and post- peaks emerged around principal peaks of irradiated oxide cation-cation radial distribution functions, which indicate obstruction and distortion type defects, respectively. Dependence of lattice dilatations on the number of obstruction type cation defects was examined. In a previous study, obstruction type uranium defects were found to be directly responsible for the UO2 lattice swelling and there was a linear relationship between them. It was also determined that this linear equation has two different slopes at low and high defect concentrations. In this paper, it was found that these phenomena were not specific to UO2 and applicable to all fluorite-structured actinide and lanthanide dioxides studied here. These findings provide clues about the existence of more general law.

cond-mat.mtrl-sci↗

arXiv2016

Thermal Recovery Mechanisms of UO2 Lattices

Thermal recovery of irradiated uranium dioxide lattice have been investigated by experimental groups and defects related to each recovery stages are estimated based on the indirect observations like activation energies. In this study, thermal recovery mechanisms of both neutron and alpha-particle irradiated uranium dioxide lattice is investigated by atomistic simulation methods. Isochronal and isothermal annealing have been applied to the irradiated samples. Following the same procedures of the experimental groups, activation energies are calculated with molecular dynamics simulations and compared with empirical data which show a good agreement. Migration energy barrier and the recovery energies of the obstruction type defects are calculated with molecular dynamics simulation and density functional theory. Recovery stages of the irradiated crystals are discussed however unlike the predictions of the most experimental groups, it is shown that neutron and alpha-particle irradiated crystal structures do not thermally recover for the same reason. All three recovery stages of neutron irradiated uranium dioxide crystal is attributed to oxygen interstitials while the three recovery stages of the alpha-particle irradiated crystals is considered to the oxygen - uranium, uranium and uranium interstitial defects respectively. Finally, the reason and the main driving force of thermal recovery of irradiated crystals is the obstruction type defects which give a lead to all other types of defects recover with themselves in each lattice recovery stages.

cond-mat.mtrl-sci↗

arXiv2015

Modeling Superionic Behavior of Plutonium Dioxide

The Bredig transition to the superionic phase indicated with the lambda-peak in Cp was highly expected for PuO2 as other actinide dioxides. However, least-square fit and local smoothing techniques applied to the experimental enthalpy data of plutonium dioxide in 80's could not detect a lambda-peak in specific heat that might be due to too scattered and insufficient experimental data. Therefore, this issue has not been yet put beyond the doubts. In the current article, a superionic model of PuO2 is developed with partially ionic model of a rigid ion potential. Thermophysical properties were calculated in constant pressure-temperature ensemble using molecular dynamics simulation. The Bredig transition with vicinity of a lambda-peak in specific heat was a successfully observed for the model system at about 2100K. Moreover, the experimental enthalpy change was well reproduced before and after the estimated transition temperature.

cond-mat.mtrl-sci↗

arXiv2014

Swelling Mechanism of Lattice with the Ingrowth of the Defects in UO2

Swelling of uranium dioxide with ingrowth of defects by irradiation is not fully understood. Experimental and theoretical groups have attempted to explain this phenomenon with various complex theories. In this study, experimental lattice expansion and super saturation of the lattice were well reproduced by molecular dynamics simulation method. From the resemblance with the experimental data, here it is manifested that only oxygen frenkel pairs were created in the fission induced lattice while alpha particle radiation causes both oxygen and considerable amount of uranium defects. Moreover, in this work, defects are divided into two sub-groups as obstruction and distortion and it is shown that obstruction type frenkel pairs merely responsible for the lattice swelling for both fission and alpha particle radiation. Evidently relative lattice expansion varies linearly with the obstruction type of survived uranium defects. Additionally, at high concentrations, some of the obstruction type uranium frenkel pairs forming double or triple structures with oxygens in their octahedral cages which increase the slope of the linear dependence.

cond-mat.mtrl-sci↗