Uranium dioxide nuclear fuel has been widely used in nuclear power plant pressurized water reactors. It has high melting point, expansion isotropic characteristics and good irradiation behavior and mechanical properties, but it has problems such as low thermal conductivity and easy embrittlement. Uranium Carbide (UC) nuclear fuels have extremely high hardness and do not undergo phase transitions over a wide temperature range, and thus can withstand higher service temperatures. The thermal conductivity of uranium carbide is 21.7 W/(m·K) ( 1237K), density is 13.63 g/cm 3The uranium content is 95.2%, which is higher than that of uranium dioxide. The uranium carbide nuclear fuel is considered to be an ideal candidate nuclear fuel for the fourth generation reactor. Based on the exploration of the closed fuel cycle in the advanced nuclear energy system driven by the accelerator, the uranium carbide can also be combined with strontium. (Pu) and some secondary nuclides (MAs) form a binary mixed eutectic system. Therefore, researchers choose uranium carbide as a form of regenerative nuclear fuel.
Recently, researchers from the Institute of Modern Physics of the Chinese Academy of Sciences, the researchers of the Department of Change and Research, successfully prepared UC ceramic nuclear fuel pellets by sol-gel method combining instant-no-cooling mixing and microwave heating; and successfully prepared the material by Pechini-type polymerization chelation. Single UC powder.
Researchers and the Swiss Paul Scherrer Institute (PSI) have jointly developed a rapid sol-gel process platform combining room temperature instant-no-cooling mixing with microwave-assisted heating, and successfully used the platform to prepare uranium carbide nuclear fuel pellets ( Figure 1) Firstly, the carbon black is uniformly dispersed in a gel solution (HMUR) at a nanometer level by ultrasonic dispersion, and then a C-UO32H2O gel sphere containing carbon black is prepared by a sol-gel method. Finally, it is converted into a phase-uniform UC ceramic pellet by carbothermal reduction reaction. The prepared UC ceramic pellet has a particle size of 675±10μm and a density of 92% or more of the theoretical density. The platform will be directly applied. Batch preparation of regenerated carbide nuclear fuel pellets in a closed fuel cycle. The researchers also used Pechini-type polymerization chelation to chelate uranyl ions (UO22+) with citric acid (CA) to form a stable UO22+-CA complex; With the evaporation of the solvent and the polymerization cross-linking reaction between the complex and mannitol, a porous porous precursor material is obtained; then the UO2/C nanocomposite is obtained by in-situ carbonization; UC powder (shown in Figure 2). This method reduces the migration distance between reactants by uniform mixing of U and C at the atomic level, and achieves the preparation of UC powder at a relatively low temperature (1400 ° C). Work has a certain application prospect for low temperature synthesis of carbide fuels containing Pu and MAs.
The research was supported by the Chinese Academy of Sciences' Strategic Pilot Science and Technology Special (Class A) 'Future Advanced Nuclear Fission Energy - ADS Transmutation System' project and the National Natural Science Foundation of China (Advanced Design, Preparation and Performance Study of Metamorphic Fuel Components). Published in the international journals Ceramics International and Journal of the America Ceramic Society, the first authors of the article are Tian Wei and Guo Hangxu.
Figure 1: Instantaneous no-cooling mixing-microwave heating sol-gel method for preparing UC ceramic nuclear fuel pellets. a: UC ceramic nuclear fuel pellets; b: SEM photo of UC ceramic nuclear fuel pellets; c: UC ceramic nuclear fuel pellets Microscopic appearance
Figure 2: Preparation of UC powder by Pechini-type polymerization chelation. First, citric acid (CA) is chelated with uranyl ions to form a stable UO22+-CA complex; as the solvent evaporates and the complex is polymerized with mannitol. The cross-linking reaction occurs to obtain a porous porous precursor material; then, the UO2/C nanocomposite is obtained by in-situ carbonization; finally, the UC powder is obtained by carbothermal reduction.
