DFT and experimental study on the thermal conductivity of U3O8 and U3O8-X; (X=Al and Mo)

Abstract

In this article, a theoretical and experimental investigation on the phonon thermal conductivity (κL) of α-U3O8 in the temperature range of 300 to 1000 K is reported. The theoretical calculations of the κL were performed via the density functional theory (DFT) unified with the Boltzmann transport equation (BTE). An extensive investigation of the mode-wise scattering rate, group velocity (vg), and the Grüneisen parameters (γ) were also conducted. The theoretically predicted thermal conductivities of U3O8 were compared with the measured values obtained through the transient method. Spark plasma sintering technique (SPS) was applied for sample preperation . In addition to the thermal properties of U3O8, an investigation on the effects of the addition of aluminum (Al) and molybdenum (Mo) is also reported. Our theoretical prediction corroborated the larger directional dependence of κL in U3O8, for example, at 300 K, they are 0.97, 0.67, and 3.05 W.m−1.K−1 along (100), (010), and (001) directions, respectively. The average κL values of U3O8 decreases as a function of temperature from 1.57pleasedelete to 0.49 W.m−1.K−1 when the temperature increases from 300 to 1000 K. The reason for the low thermal conductivity in U3O8 has been explained by evaluating the mode-dependent scattering rates and γ. Also, the prediction of vg corroborated the larger directional dependence of κL. Finally, the effect of Al (with 15, 20, and 30vol% of U3O8 in Al matrix) and Mo (with 8.2 and 12.5vol% of Mo in U3O8) on the thermal conductivity of U3O8 has been reported. At 300 K, thermal conductivities of U3O8-Al samples with 15, 20, and 30vol% of U3O8 dispersions are 50.91, 33.79, and 16.53 W.m−1.K−1,respectively. Moreover, the samples with 8.2 and 12.5 vol% of Mo reported 59% and 104% respective thermal conductivity enhancements at ~300 K. Ultimately, this work determined the possibility of sintering U3O8 based nuclear fuels using SPS and demonstrated the thermal conductivity enhancement of U3O8 with Al and Mo additives.