Energy Conversion Technologies for Advanced Radioisotope and Nuclear Reactor Power Systems for future Planetary Exploration

 

M. S. El-Genk

 

Future exploration of the solar system, particularly the outer most plants will depend on developing robust advanced nuclear power systems capable of providing not only electricity but also co-generation heat for thermal management of the spacecraft while in route, at destination, and for the instrumentation and science missions on board.  Going beyond Mars, solar power is not an enabling option due to the weak solar isolation and the prohibitive mass and size of the solar power system.   Conversely, advanced radioisotope and nuclear reactor power systems are enabling for future planetary exploration and science missions, requiring various power levels from a few mWs to tens or even hundreds of kWe.  The nuclear electrical propulsion initiative announced in February 2002, to move NASA’s exploration effort from “ the buggy and horse era to the railroad age” is aimed to:

(a)    Develop advanced radioisotope heat sources for radioisotope power systems (RPSs) to enable deep space exploration where there is little or no solar energy, and to provide electrical and thermal powers for long duration surface and subsurface exploration missions on Mars and further away planets in the solar system; and

(b)    Develop Nuclear Reactor Power Systems (NRPSs) to enable interplanetary nuclear electric propulsion missions to reduce the flight time by 50% or better, increase delivered cargo, remove dependency on gravity assist maneuvering, and facilitate multiple destination missions.   

Unlike RPSs, nuclear reactor power systems could operate at different power levels and are capable of multiple restarts.  They can also provide high electrical power in the order of tens to hundreds, or even thousands of kilowatts electric, both on-board and at destination, for increased data transmission rate, house keeping and science experiments while in flight, and for surface and subsurface operations at destination.  RPSs and NRPSs operate typically at a source temperature of 1200 – 1300 K, however, the temperature of the heat rejection radiator and total mass of the system will depend on the type of conversion technology used. 

This talk will review the recent advances in energy conversion technologies being considered for use in RPSs and NRPSs for NASA’s future missions.  These include skutterutides Segmented Thermoelectric (STE), Alkali-Metal Thermal-to-Electric Conversion (AMTEC), Free Piston Stirling Engine (FPSE), and Brayton Engine.  While a high conversion efficiency reduces the mass of either the radioisotope heat source or those of the nuclear reactor and radiation shield, it may result in a heavier power system because of the high mass of the converter and/or of the heat rejection radiator.  Other parameters that affect the choice of the suitable conversion technologies for these power systems include redundancy, modularity, no single point failures, integration with the nuclear heat source, size and mass of the heat rejection radiator, load-following characteristics, power management and distribution requirements, and radiation hardness.  These parameters for the different energy conversion technologies and their impact on the system’s specific mass in kg/kWe will be compared and discussed.

 

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