Advanced nuclear power engine: A brief overview of gas core reactor for space exploration

Space travel requires propulsion with high specific impulse. Gas core reactors using plasma nuclear fuel operate at high temperature (${10}^4$-${10}^{5}K$) and theoretically produce higher specific impulses ($2500$-$7000s$) than solid core nuclear thermal rockets. Ground-based reactors have higher total energy conversion efficiency ($70\%$). It also can exhaust all actinides and have negative density coefficients of reactivity. This paper reviews the evolution of gas core reactors for space exploration in the United States and the Soviet Union, including nuclear light bulb, open-cycle gas core nuclear rockets and gas core reactors for power generation. In terms of reactor physics, the selected materials and selection criteria for fuel and moderator-reflector are compiled, and the available neutron analysis methods are summarized. In terms of reactor physics, fluid thermal properties, radiation heat transfer models and thermal protection methods are briefly introduced. Fuel loss as the representative challenge is analyzed for the main causes of generation. In this paper, the principles, related studies, and advantages and disadvantages of four fuel confinements are reviewed. Finally, the principles and start-up process of the two start-up methods are discussed. Start-up is another challenge for this reactor design. Early this century, gas core reactor research stagnated due to the lack of thermal property data and the ability of high-temperature hydrodynamics simulation. Nowadays, with the increase of computing power and the breakthrough of computational fluid dynamics, these challenges are expected to be overcome.