# Phonon Scattering and Thermal Conductivity of Actinide Oxides with Defects
## 4 Conclusions  
In the present study, reverse non-equilibrium molecular dynamics (RNEMD) was used to explore thermal transport in two actinide oxides (UO\({}_{2}\) and PuO\({}_{2}\)) with oxygen vacancies, uranium/plutonium vacancies, and oxygen interstitials. The effects of these defects were observed to be detrimental across all actinide oxides simulated in this research. Even the smallest percentages of these defects have a notable impact on thermal conductivity. The vacancies of primary atoms (uranium and plutonium) reduced the thermal conductivities of oxide fuels the most significantly; for the case of PuO\({}_{2}\) of \(Nx=100\) with 0.5% plutonium vacancy, a 61% decrease in the thermal conductivity was observed. Oxygen interstitials were found to have an almost identical effect on the thermal transport in UO\({}_{2}\) and PuO\({}_{2}\) to oxygen vacancies; 0.1% oxygen interstitials decreased the thermal conductivity of UO\({}_{2}\) of \(Nx=100\) by 18.6%, while 0.1% oxygen vacancies decreased the thermal conductivity of the same sample by 19.4%. In phonon density of states (PDOS) plots with 1% of oxygen vacancies and interstitials, a similar degree of modification to the phonon spectra was observed; the both defect types do not alter LA phonon mode seriously, which is a dominant thermal energy carrier in actinide oxides.  
The thermal transport property of actinide oxides becomes highly diffusive by the presence of only one fission gas bubble. The presence of a single fission gas bubble (that is consisted of 30 Kr/Xe atoms) was found to generate the interfacial thermal resistance of \(2.1\times 10^{-9}\) K-m\({}^{2}\)/W. From the local PDOS plot of UO\({}_{2}\) with a fission gas bubble, it was observed that thermal transport is still diffusive even in the region far from the location of the fission gas bubble. It is interesting to note that changing the composition of a fission gas bubble (ratio of Kr and Xe atoms) does not alter the interfacial thermal resistance much, despite the large difference between their atomic masses.  
Figure 9: (**a**) Interfacial thermal resistances and thermal conductivities of UO\({}_{2}\) with fission gas bubbles as a function of the composition of fission gas. (**b**) Local phonon density of states (PDOS) in UO\({}_{2}\) with a fission gas bubble.  
**Author Contributions:** Conceptualization, J.P. and K.M.; methodology, J.P. and K.M.; validation, J.P., K.M., H.H. and A.R.; formal analysis, J.P., H.H. and K.M.; investigation, J.P., K.M. and A.R.; data curation, J.P., K.M., H.H. and A.R.; writing--original draft preparation, J.P.; writing--review and editing, J.P., K.M., A.R. and E.B.F.; visualization, J.P. and K.M.; supervision, J.P.; project administration, J.P. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Acknowledgments:** This work made use of the High-Performance Computing resource at Kennesaw State University. The authors would like to acknowledge NESEL (Nuclear Energy, Science, and Engineering Laboratory) at Kennesaw State University for supporting this research study.  
**Conflicts of Interest:** The authors declare no conflict of interest.