Thermal Conductivity of Uranium Nitride and Carbide

We investigate the electronic thermal conductivity of alternative fuels like uranium nitride and uranium carbide. We evaluate the electronic contribution to the thermal conductivity, by combining first-principles quantum-mechanical calculations with semiclassical correlations. The electronic structure of UN and UC was calculated using Quantum Espresso code. The spin polarized calculations were performed for a ferromagnetic and antiferromagnetic ordering of magnetic moments on uranium lattice and magnetic moment in UC was lower than in UN due to stronger hybridization between 2p electrons of carbon and 5f electrons of uranium. The nonmagnetic electronic structure calculations were used as an input to BolzTrap code that was used to evaluate the electronic thermal conductivity. It is predicted that the thermal conductivity should increase with the temperature increase, but to get a quantitative agreement with the experiment at higher temperatures the interaction of electrons with phonons (and electron-electron scattering) needs to be included. 1. Introduction The recent tragic accident in Fukushima clearly illustrates the risks associated with the present design of reactors based on uranium dioxide (UO2) fuel and justifies research towards a safer fuel. Pioro et al. [1] demonstrate that the traditional urania fuel is not suitable for some designs of new generation reactors due to its low thermal conductivity (e.g., the estimated fuel centerline temperature for super critical water reactor (SCWR) surpasses the industry accepted limit of 1850°C (2123？K). Uranium nitride (UN) and uranium carbide (UC) have been proposed as possible safer fuels. UC and UN are advanced types of nuclear fuel since they have not only higher thermal conductivity but also lower linear expansion coefficients and are more compatible with fuel cladding materials [2]. The thermal conductivity of UN is compared with UO2 in [3]. The researchers show that in contrast to a typical ceramic (e.g., UO2) the thermal conductivity of UN increases with temperature due to a large electronic transport. Additionally, UN melting temperature increases with pressure, which prevents dissociation of nitrogen gas and can be as high as 3035？K at 1？atm [3]. UC also has an advantage over UO2 as a fuel for fast reactors due to about 2.6 times higher thermal conductivity [2, 3]. The melting temperature of UC is slightly lower (~2623？K [4], 2638 (165) K–2780 (25) K [2]) but still well above 2000？K. We present here preliminary studies of the thermal conductivity of these advanced types of nuclear fuel. 2.