Lockheed Martin and BWXT will develop a nuclear thermal propulsion demonstration spacecraft for NASA/DARPA’s DRACO program. (credit: Lockheed Martin)
Repurposing nuclear reactors used in space propulsion for high-density power on the Moon and Mars
by Ajay Kothari Monday, November 11, 2024
In the recent NASA Space Technology Mission Directorate Shortfall Ranking Report, “High Power Energy Generation on Moon/Mars” is rated #2 of 187 considered in the overall list. But rather than just transporting a fission reactor to Moon, what if we use it for propulsion to the Moon and landing on the lunar surface, and then repurpose it for energy on the lunar surface for habitats? This could provide a two-in-one solution.
The energy density of a fission nuclear reaction is almost a million times greater than the chemical energy of propellants we use today for space transportation. If we can use this smartly, it solves many problems: not just space propulsion, but more importantly being able to repurpose the reactor part for abundant energy on Moon or Mars surface. One Gen IV Reactor concept offers this possibility with reduced reactor weights since it does not operate at high pressures, and liquid state of fueled molten salt sends the energy where needed more easily. The issue to be addressed is high-temperature containment, and that has been ameliorated partially by the recent (over the last few decades) advent of high temperature materials such as C/SiC and C/ZrOC. Why not take advantage of all of the above to formulate a solution?
Such a vision vehicle is designed that is transported to LEO by Starship. It is compared with the 8 to 20 refueling flights need for Starship for HLS, which will have a low probability of success with numerous attempted flights. This concept requires only one flight. An Uranium Molten Salt Reactor (UMSR) Stage, where uranium 233 is dissolved in hot molten salt such as FLiBe (fluoride-lithium-beryllium), is compared with an equivalent hydrogen-oxygen stage for the same mission and the same total payload. Unlike solar power, the intent here is also to provide uninterrupted power during the lunar night as well.
High power—many tens to hundreds of megawatts—energy production is absolutely essential for future habitats, machines, and infrastructure development that only fission power can provide at present. One shortcoming of the design shown herein is that it could also be designed to have multiple reactors and rockets in this stage, similar to Falcon 9 or Starship. While this would negate some of the economy of scale, the advantage of being able to distribute such power stations at many (say, 10 to 20) locations may far outweigh the potential disadvantage.
This concept will provide continuous power on the Moon and can be extended in future to Mars. There is also potential to apply this technology of power production terrestrially, which can solve the climate crisis and ensure sufficient energy for humanity
A possible scenario is for the Starship to carry this nuclear propulsion device in its payload bay and deposit it in high LEO (approximately 500 kilometers perigee). The device gross weight of 103 ton is very much within the capacity of just one Starship flight without any refueling requirement. The device will then start its reactor by introducing a neutron-producing element in the reactor. Uranium 233 (U233) dissolved in the molten salt will start absorbing some neutrons in its nucleus and start to fission instantly. With 2.48 as neutron economy (the number of neutrons being released that can cause fission compared to the number needed to maintain the chain reaction), the chain reaction will continue while controlled by the graphite rods. The molten salt is heated to about 1200 degrees Celsius and such heat is then transferred to the heat exchanger located below it, as shown in the figure below. The non-radioactive molten salt in the heat exchanger will heat the liquid hydrogen to a gaseous state, with high pressure provided by a turbopump located near the thrust chamber. Such hydrogen is calculated to provide 662 seconds of specific impulse, lower than the 841 seconds promised by NERVA but higher than chemical alternatives like liquid hydrogen and liquid oxygen.
Diameters: Designed to Starship 9 m (not used: New Glenn 7 m, FH fairing 5.2 m)
A mass flow rate of 67 kilograms per second of hydrogen will provide 44 tons of thrust to the 103-ton vehicle, giving it acceleration of 0.4 to 0.7 g for a ten-minute burn at translunar injection (TLI). Any variation in burn time within a range of 5 to 20 minutes will have almost linear effect on thrust required. This thrust can be varied by using the methods described below as needed for TLI first at the perigee, then lunar orbit insertion upon reaching the vicinity of the Moon, and then landing on the surface with a 20-ton payload and 13-ton reactor, in addition to the four-ton liquid hydrogen tank (which could also possibly be used for a habitat, though that is not the primary goal here.) The reactor and heat exchanger are then removed from the rocket and repurposed for use for energy production.
Instantaneous power supplied can be varied as needed by changing the pump speed to change the molten salt flowing speed that goes to the heat exchanger and inserting or removing the graphite rod moderators. This would of course be tied to the hydrogen fluid flow, resulting in different thrust levels for Hohmann transfer impulse at perigee (always sufficiently outside the atmosphere, beyond the nuclear safe distance of about 500 kilometers) once the rocket payload is in LEO. The number of Hohmann transfers to reach LOI can also be varied, as time is not of critical importance.
The U233 dissolved in the molten salt is 1–2% by mass. It is normally so small that any additional U233 could also have a large impact on rate of energy production. Since the energy produced per fission of U233 atom is 200.1 MeV, which when multiplied by Avogadro’s number and converted to gm-mole yields 2.6 megawatts of energy for a full year for only one kilogram of fuel: a very attractive solution for energy on the Moon.
To begin with, U233 would be bred from thorium 232 (Th232) on the Earth. This U233 can be stored and distributed for such use.
Comparison is made between the nuclear (UMSR, left) and hydrogren-oxygen (hydrolox, right) vehicles performing the same mission. Since the UMSR stage is carrying both a payload and the reactor for use on Moon (repurposing), the hydrolox stage is sized for an equivalent payload plus the reactor.
Both vehicles are designed and sized for a delta-V of TLI (3.2 kilometers per second), LOI (700 meters per second), and landing (1.9 kilometers per second). There is no requirement for takeoff from the Moon and return to Eartj. It requires about 61 tons of liquid hydrogen for propellant whereas the hydrolox vehicle needs 102 tons of liquid hydrogen and liquid oxygen. It has diameter of 8.5 meters to allow it to fit into the nine-meter-diameter Starship.
There is one issue however: the length of the UMSR vehicle almost 50% longer than the Starship payload bay. This will need to be discussed with SpaceX.
One kilogram of Th232 or U233 can provide 2.6 megawatts of power for a year on Moon, which is going to be crucial for melting regolith to make bricks (for landing/takeoff pads), building road infrastructure, and providing continuous power for habitats, life support systems, and operating rovers.
Although the reference mission evaluated here is for a full operational vehicle, the development can be done incrementally. At first, a smaller vehicle could be sized so that TLI is provided by external (chemical) means, and the nuclear reactor comes alive for lunar orbit insertion and landing only, providing a delta-V of about 2,600 meters per second.
It appears that the repurposing of reactor and heat exchangers will this add a lot to the sustainability of activities on the Moon as well as on Mars.
Dr. Ajay Kothari is President and Founder of Astrox Corporation, an Aerospace R&D company located in suburban Washington, DC. His PhD and MS in Aerospace Engineering are from the University of Maryland and BSc in Physics from Bombay University. He is Associate Fellow of AIAA and member of AIAA Aerospace Power Technical Committee. He has over 40 professional publications and has been PI on more than 35 NASA and DOD contracts.
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