USES OF LIQUID-METAL AND WATER HEAT PIPES IN SPACE REACTOR POWER SYSTEMS

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Water and liquid-metal heat pipes are considered for redundant and passive cooling of nuclear reactors and redundant operation of light-weight heat rejection radiators of space reactor power systems. The systems operate continuously for many years independent of the Sun, are enabling of deep space exploration and could provide 10’s of kilowatts of electrical power for lunar and Martian outposts. Space reactors typically operate at 900 K – 1400 K could be cooled using liquid-metal heat pipes with either sodium (900 –1100 K) or lithium (1100 K–1400 K) working fluids. The heat rejection temperatures dictate the working fluid of the radiator heat pipes; potassium for 600 – 800 K, rubidium for 500 – 700 K, and water for < 500 K. These working fluids would be frozen prior to starting up the power systems in orbit or at destination. The startup of water and liquid-metal heat pipes from a frozen state is complex, involving a number of non-linear mass and heat transport processes that require implementing appropriate procedures, depending on the type of working fluid. This paper reviews operation and design constraints pertinent to the uses of water and liquidmetal heat pipes in space reactor systems, and the modeling capabilities of the startup from a frozen state. In addition to models validation, results of design optimization of liquid-metal and water heat pipes in a number of space reactor power systems are presented and discussed.