· 56 · IMP & HIRFL Annual Report 2018 formation of such carbonyl complexes.

Reference [1] Y. Wang, S. W. Cao, J. C. Zhang, et al., Physical Chemistry Chemical Physics, 21(2019)7147. [2] Y. Wang, Y. Wittwer, J. Zhang, et al., PSI Annual Report, (2019).

3 - 20 Progress in Fabrication of UC Ceramic Nuclear Fuels

Tian Wei and Qin Zhi

Uranium dioxide nuclear fuel has been widely used in pressurized water reactors (PWR), because of its high melting point, isotropic expansion, excellent radiation behaviors and mechanical properties[1]. However, it is easy to embrittlement and the thermal conductivity is quite low. Compared with traditional uranium oxides nuclear fuel, the uranium carbide has extremely high hardness and no phase transition occurs in wide temperature range[2]. The thermal conductivity, density and uranium fraction of UC is 21.7 W/(m·K) (1 237 K), 13.63 g/cm3 and 95.2% respectively, which are much higher than that of UO2. Uranium carbide ceramic fuel has been considered as a potential candidate nuclear fuel for the next-generation fast-neutron reactors, especially for the Accelerator-Driven Systems (ADS). Based on the accelerator driven recycling used nuclear fuel, uranium carbide can recrystallize with plutonium (Pu) and minor actinides (MAs). Therefore uranium carbide is chosen as the fuel format in ADS. Uniform sized ceramic UC microspheres with a diameter of (675  10) µm were successfully fabricated by an improved microwave-assisted rapid internal gelation process combined with carbothermic reduction (Fig. 1).

First of all, the nanoparticle carbon was dispersed into the HMUR stock solution, and the C-UO3·2 H2O gelled microspheres were prepared using an improved microwave-assisted internal gelation process without cooling the initial stock solutions. Next, the gelled microspheres were subjected to a carbothermic reduction process to obtain ceramic UC microspheres. This method will be potentially used for the fabrication of MAs and Pu-containing ceramic nuclear fuel microspheres at large scale in glove-box for the ADS project in future.

Fig. 1 (color online) Ceramic UC spheres fabricated by the improved microwave-assisted rapid internal gelation process. (a) optical image of the ceramic UC microspheres; (b) SEM image of the ceramic UC microspheres; (c) micro-morphology of the ceramic UC microspheres..

References [1] A. K. Sengupta, R. Agarwal, H. S. Kamath, et al., Comprehensive Nuclear Materials, Elsevier, Oxford, (2012), 55. [2] R. E. Rundle, N. C. Baenziger, A. S. Wilson, et al., J. Am. Chem. Soc., 70(1948)99.