1 t0 10 Kilowatt Electric HEU Space Reactor
Figure 1 - 1 kWe Space Reactor with HEU core
The quickest reactor to production for space flight is the 1 to 10-kilowatt electric (kWe) HEU space reactor (the original Kilopower design) that would be almost identical to the reactor tested in the “KRUSTY” experiment, (Note, an LEU 1-kWe metal core reactor is too heavy to be of use).
Figure 1 shows the basic layout of a 1-kWe space reactor with a highly enriched Uranium (HEU) core. With an HEU core, the reactor is small enough and light enough to fit a range of deep space missions. It is also small enough to have the ability to rideshare on a mission where the launch vehicle was shared by multiple spacecraft.
The 1 kWe space reactor would have eight heat pipes attached to eight 125 watt Stirling engines. The reactor would have all the qualities of a SpaceNukes design including load following, high uranium atom density, low burnup, and low power density. It is believed that this design could go decades without any control commands except for minor reactivity changes to account for small decreases in temperature.
The 1-kWe HEU space reactor could be ready for flight in approximately 3 years. No major technical issues remain; the long pole in the schedule would be the launch safety approval. The reactor is approximately 400 kg and is 58 cm in diameter and 145 cm in length (without heat rejection). The 1-kWe reactor can be readily upsized to 10- kWe by increasing the weight and size of the reactor to approximately 1300 kg with an HEU core. We believe this design can be produced for deep-space applications without additional terrestrial nuclear testing.
10 t0 30 Kilowatt Electric LEU Surface Reactor
A surface reactor is one of the main applications of nuclear reactor technology in space. The need for higher output on Lunar or Martian surfaces will be essential to making propellant and for providing power for human habitation. The SpaceNukes 30-kWe surface reactor is an extension of the basic Kilopower technology and is shown in Figure 2.
Figure 2 - 35 kWe Surface Reactor with LEU core
The design uses 500 kgs of low enriched uranium core to power sodium-filled heat pipes each connected to an individual Stirling Engine. In early HEU Kilopower surface reactor concepts, the electric power of the reactor core was limited to 10-kWe in order to keep uranium atom burnup to less than 0.5% (at about 1% burnup, fission gas swelling of the metal fuel can occur.) By using LEU instead of HEU, the burnup can be extended (more total uranium atoms are available) and the power raised to 30 kWe. In reactor design, these “knees in the curves” are important indicators of where reactor design can be optimized.
The weight of the LEU surface system would be heavier than a 10 kWe HEU system, but the increase in power more than offsets the increased weight. The alpha (power to weight ratio) of the 10 kWe HEU surface reactor is about 8 watts/kg. The 30 kWe LEU system would also have an alpha of about 8 watts/kg. The 30 kWe LEU surface reactor has a flight development schedule of approximately 4 to 5 years.
100 t0 200 Kilowatt Electric LEU Surface Reactor
Figure 3 - 200 kWe LEU Surface Reactor with Top Shield and Intercooler Fan for Mars
As the power needs grow for surface applications, the reactor technology needs to grow to meet the increase in demand. A 100 to 200 kWe LEU surface reactor is one of the next steps in the evolution of the space reactor from the solid metal fuel reactors based upon the original Kilopower technology.
This design uses approximately 2 metric tons of uranium oxide fuel in a stainless-steel monolith. The monolith has both holes for gas flow and for containing the fuel. It is anticipated that the monolith would be printed as one solid block. The block would be about 50 cm in diameter and about 1 meter high.
This reactor concept is in the conceptual design phase and would require more engineering before a valid schedule could be developed. Given that the reactor is a fast neutron reactor with a monolithic fuel block, the reactor would self-regulate in the same fashion as the Kilopower reactor concepts. The practical evolutionary limit of this technology would be in the range of a few MWe.