# Phonon Scattering and Thermal Conductivity of Actinide Oxides with Defects
###### Abstract  
In the present study, we examine the effect of point defects and fission gases on thermal transport in representative actinide oxides used in modern reactors. In particular, oxygen interstitials and Kr/Xe fission gas bubbles are of primary focus. Reverse non-equilibrium molecular dynamics is employed to investigate thermal transport in UO\({}_{2}\) and PuO\({}_{2}\) with oxygen interstitials at the defect concentrations of 0.1%, 1%, and 5%. The results show that any alteration to the lattice structures of these fuels reduce their thermal conductivities significantly. For the largest UO\({}_{2}\) structure simulated in the present study, for example, 0.1% oxygen interstitials decreased the thermal conductivity by 18.6%. For the case of the effect of fission gas bubbles, serious modification to phonon dispersion in oxide fuels is caused by the presence of a single fission gas bubble, resulting in a large temperature drop in their temperature profiles. The average interfacial thermal resistance across a fission gas bubble (comprised of 30 Kr and/or Xe atoms) is estimated to be \(2.1\times 10^{-9}\) Km\({}^{2}\)/W.  
thermal conductivity; actinide oxides; defects; phonon scattering 2020  
interstitials are major byproducts of the nuclear fuel cycle; since oxygen interstitials have negative formation energy, oxide fuels can be oxidized easily and become hyper-stoichiometric. Therefore, it is critical to analyze their effect on the thermal/mechanical behavior of oxide fuels to understand radiation damage and non-stoichiometry. The effect of oxygen interstitials on actinide oxides and their diffusion process have been important research topics for a long time. In 1978, [PERSON] (1981) showed summarized experimental data for diffusion coefficients of oxygen interstitial in UO\({}_{2}\)\(\pm\) x. In 1981, [PERSON] and [PERSON] [PERSON] and [PERSON] (1981) reported that oxygen interstitials and vacancies contribute significantly to oxygen diffusion in stoichiometric UO\({}_{2}\). More specifically, the thermal transport properties of actinide oxides with various defects have been investigated by many researchers. For example, [PERSON] et al. [PERSON] _et al._ (2017) explored point defect scattering from oxygen interstitial in hyperschiometric SIMFUEL (simulated spent nuclear fuel). [PERSON] et al. [PERSON] _et al._ (2018) reported thermal conductivity reduction in UO\({}_{2}\) caused by He2+ ion irradiation. However, systematic investigations on the effect of oxygen interstitials on the thermal properties of actinide oxides are rare. Recently, [PERSON] et al. [PERSON] _et al._ (2018) studied the thermal conductivity of hypo- and hyper-stoichiometric UO\({}_{2}\) with different defect concentrations at different temperatures. Along with the formation of oxygen interstitials, oxygen vacancies can come along. [PERSON] _et al._ (2018) explored how fission induces these oxygen Frenkel pairs, which are generated when an atom is dislodged, creating a vacancy defect, and then relodged into the structure as an interstitial defect site. It was reported that Frenkel pairs are produced in multiple stages as the fission dose is increased; once isolated Frenkel pairs are produced at a constant rate, newly produced displaced atoms are trapped at existing defect sites, resulting in the formation of interstitial clusters.  
In the present study, we provide a microscopic understanding of the effect of oxygen interstitials and fission gases on the thermal conductivities of oxide fuels. Molecular dynamics (MD) is used to estimate the thermal conductivities of oxide fuels with different defect types and concentrations. Interfacial thermal resistances caused by fission gas bubbles are analyzed. Moreover, phonon density of states are calculated to better understand phonon scattering caused by such defects.