Yuanzineng kexue jishu (Jan 2024)
Preparation Technology and Performance Mechanism of Multi-element (U, Zr, Nb)C Fuel
Abstract
High thrust nuclear-powered spacecraft is the key technology for deep space exploration and low Earth orbit transportation in the future, the nuclear thermal propulsion system has the advantages of high specific impulse, high thrust, long operating life and repeatable start-up, which can provide reliable power support for future deep space exploration missions. Multi-element (U, Zr, Nb)C fuel is one of the selected fuels for high-power space nuclear propulsion reactors due to its high melting point, high thermal conductivity, low fission gas release rate and excellent high-temperature mechanical properties. In this paper, multi-element (U, Zr, Nb)C fuel pellets were prepared by powder metallurgy process combining carbothermal reduction and liquid phase sintering. The mechanism and influence of process parameters on reaction kinetics, phase structure, thermal conductivity and microstructure of sintered pellets were studied. The results show that according to the calculation of reaction thermodynamics, the carbothermal reduction temperatures for UO2 and Nb2O5 are 2 122 K and 1 206 K at standard pressure, respectively; Under the sintering conditions of 1 800℃, 50 MPa and 1 h, the pellet density can reach 95.5%TD by adding 0.5% metallic uranium to form liquid phase sintering, liquid phase sintering accelerates the densification process of the pellets; When the raw material M/C (M is the total metal element) is 1:6.5, the stoichiometric carbide fuel with M/C ratio of 1 can be prepared, using ZrC powders as raw materials can not only reduce the reaction temperature, but also reduce the difficulty of adjusting the M/C ratio of the reaction product; The lattice constant of (U0.2, Zr, Nb)C is slightly higher than that of (U0.1, Zr, Nb)C; The thermal conductivity of carbide fuel is related to phase composition, density and temperature; The thermal conductivity of (U0.1, Zr, Nb)C and (U0.2, Zr, Nb)C fuels increases with the increase of the content of uranium carbide, and the increase of Zr content is beneficial to the increase of the thermal conductivity of the pellets, the thermal conductivity of (U0.1, Zr, Nb)C and (U0.2, Zr, Nb)C pellets are 17.89 W/(m·K) and 18.81 W/(m·K), respectively. With the increase of sintering density, the thermal conductivity of (U, Zr, Nb)C fuels increases, and the fuels with different compositions show different thermal conductivity changing trend under the same sintering density. The thermal conductivity of (U0.1, Zr0.55, Nb0.35)C and (U0.2, Zr0.5, Nb0.3)C pellets at 90%TD relative density are 16.43 W/(m·K) and 17.90 W/(m·K), respectively, and the thermal conductivity decreases by 8.2% and 4.8% respectively compared with that at 94%TD theoretical density. The pore distribution of the pellet is relatively uniform, there is no connected open pore, and the pore size is 1-3 μm.
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