Low-Enriched Uranium Nuclear Thermal Propulsion: Propelling Humans to Mars

by Vishal Patel

NASA is developing capabilities of sending people to Mars in the 2030s under the “Journey To Mars” initiative. This plan has public and congressional support but many of the details have not been settled, as of yet. One major issue is that no decision has been made for the type of propulsion vehicle to get humans to Mars.

NASA Administrator Charles Bolden recently testified in the House Space Subcommittee on NASA’s fiscal year 2017 budget proposal. Bolden stated he supports nuclear thermal propulsion, and that he and most NASA people believe that “nuclear thermal propulsion will probably be the most effective form of propulsion to get [to Mars].”

Nuclear thermal propulsion (NTP) technology is a well-researched topic that has had cyclic cycles of funding starting from the initial Rover/NERVA program (1955–1972). In the Rover program, 22-fueled nuclear engines were tested, although no flight tests were performed. The program, along with other funded programs, was canceled due to political motivations rather than technical issues.

An NTP engine operates similar to a chemical engine in that hydrogen gas is heated up to high temperatures then expanded through a nozzle to accelerate the engine forwards. In a chemical engine, the heat is created from chemical reactions; in an NTP engine, the heat is created using a high power density nuclear reactor. Hydrogen is used to cool the nuclear core, which allows the gas to pick up the heat generated in the core.

An NTP engine operates similar to a chemical engine in that hydrogen gas is heated up to high temperatures then expanded through a nozzle to accelerate the engine forward. In a chemical engine, the heat is created from chemical reactions; in an NTP engine, the heat is created using a high power density nuclear reactor. Hydrogen is used to cool the nuclear core, which allows the gas to pick up the heat generated in the core.

When compared to chemical propulsion, NTP allows for faster transit times with longer launch windows to Mars, allows for less cosmic radiation exposure to astronauts, and increases payload capacity so that more supplies can reach Mars.

Historically, reactors created for NTP engines used highly enriched uranium (HEU) because it allowed for compact, high-power density reactors to be created. Unfortunately, since the days of the original NTP engines, HEU has come to bear a large social and political burden due to proliferation risks associated with it. Recent studies, however, have shown that low-enriched uranium (LEU) can be used to create an NTP engine, and Bolden stated that NASA is currently developing a “low-grade nuclear fuel” (low-grade referring to LEU) to enable these efforts.

LEU-NTP is a new type of nuclear propulsion that removes the use of HEU in the nuclear reactor typical in a space system. HEU’s absence enables private industry to play a leadership role in NTP development, allows non-nuclear weapons states to pursue NTP, reduces proliferation risks and costs, and conforms to the Department of Energy’s HEU reduction mission. All of this is done without sacrificing performance relative to historically tested HEU nuclear engine designs and without throwing away years of research by using many of the same sub-systems created in historical designs.

Research for LEU-NTP reactors is being performed at NASA Marshal Space Flight Center under the Space Capable Cryogenic Thermal Engine Program. Academic institutions and private industry are also pursuing this new technology.

Nuclear thermal propulsion, albeit a proven technology, will require a large multi-year research effort that will span several administrations to be successful. Research for NTP is funded within NASA for $20 million within the FY16 budget and in line to be funded again in FY17. Reactor cores without HEU are being developed, which enables public and private institutions to take part in further research.

With public and private backers, nuclear thermal propulsion will enable NASA’s vision to someday put humans on Mars.

Vishal PatelVishal Patel is a Ph.D. student at Texas A&M University studying nuclear engineering and is a research scientist at the Center for Space Nuclear Research at the Idaho National Laboratory. His current research focuses on reactor experiment uncertainty quantification, which enables him to moonlight as nuclear thermal propulsion reactor designer. He is an executive committee member of ANSTD.

21 thoughts on “Low-Enriched Uranium Nuclear Thermal Propulsion: Propelling Humans to Mars

  1. Abe Weitzberg

    Back to Wes Deason of April 11, 2016 at 08:43. The big problem with NTP is the idea that NASA should be moving any amount of HEU or LEU between NASA centers. NASA’s expertise with nuclear fuel and reactors of any kind is very limited to say the least. The reactor portion of any NTP system should be the responsibility of DOE with propulsion requirements coming from NASA. DOE and its contractors have no difficulty handling HEU without excessive costs. The infrastructure already exists. The cost of shipping to the launch site and providing security at the launch is a very small cost of the system and should not be a basis for choosing a technology.

  2. James Greenidge


    The Rover tests in the early ’60s reaped such success and performance of those prototype nuclear reactor drives that the team had high confidence that their progeny could land us on Mars in the ’80s.

  3. Li

    Guys, watching your talks, I’m not religious but I feel the need for some ritual, at least to “holly cow” (HC) and its collateral products…we have to state clear, nonproliferation now is over the borders of medical paranoia, is a field where political appointed people makes lots of money doing nothing, but preventing other to do something…I do not say that we do not have to take care how we are handling these sensitive materials, but wake up…NK made its bombs without US HEU or intellectual input; US made it in 3 years when they did know and measured very, very little, how childish one can be not to figure out that now any one of 80 countries active in nuclear physics if feels the external pressure, may not do it alone? HC; how childish one can be not figure out this?
    Fur US the biggest danger is ourselves, a disgruntled DOE or NAVY, MD of a forceful management application, and the work of terrorists is done, free for them, and higher impact; but nobody wants to openly talk about this. Why to treat better the employees, not to have them disgruntled, why to remove those managers, who are also politically so well-connected, it’s much easier to remove people who think, they seem to be the danger, may wake up the heard.
    So, back to space:
    LEU is good if one may have breed&burn capability, because allowed more actinide mass in space, otherwise the limit is about 2 critical mass. (1 mass burns another remains, make it Kg and multiply with IGWDay, that’s energy).
    Propulsion depends on mass expelled and speed it was expelled ( optimum is about 3 times the payload), while speed have to be as big as possible, but remember, if one accelerates too much needs to decelerate a lot to avoid a crash.
    Have breed and burn on board, have high burnup = impossible, need better materials…and that is what actual reactor (boiler) engineers to not get it.
    Using thermal propulsion adds minimal value, because exhaust speed is lower than for chemical, but well they use a single chemical material.
    Well we ate in the PRIMITIVE stage of nuclear power application, like 1000 years after the discovery of fire, compared with today fire applications/usage.

  4. Wes Deason

    One of the main concerns I’ve seen with use of HEU are the high physical protection Moving at least 10’s of kg of HEU between NASA centers—then modifying existing facilities to be able to store it—is concerning from a cost perspective.

    Concerning mass, given the already low thrust-to-weight of an NTR (whether HEU or LEU) and mission (very high altitude with low gravity loss), the biggest factor of mission performance is Isp. Because Isp performance between HEU and LEU is practically the same (Vishal Patel, Michael Eades, and I did research into this at CSNR and PDF ), its effect on mission performance and cost is small.

  5. Abe Weitzberg, PhD

    The U-238 is always a mass penalty for NTP, because there is insufficient burnup to produce significant amounts of plutonium which might help long-lived systems. Basically, it is a poison that always reduces performance. No matter how you try to explain it away, increased mass is always a penalty.

  6. Dr. Lyman J Petrosky

    “Decreasing enrichment reduces performance” is not a true statement in general. The power level and specific impulse of a NTP engine is governed by the thermal limits of the supporting materials used. Once critical, the engine power can be anything the structure can tolerate regardless of U enrichment. Engine operational life may be limited by the total fissile inventory; however, the fissile depletion in a NTP engine is usually not the governing limitation.

  7. Abe Weitzberg, PhD

    The primary purpose of any design is to meet performance requirements. Decreasing enrichment reduces performance. If the design cannot preform the mission, immersion criticality is irrelevant. You are correct that some detailed design information is essential for an educated evaluation.

  8. Dr. Lyman J Petrosky

    Decreasing U enrichment (toward LEU) and increasing moderator content strongly mitigates the immersion critically issue; however, it never quite reaches the category of simple. Even a LEU engine may require embedded poisons to be fully immersion proof. All of these questions depend strongly on the specifics of the engine neutronic design. The press release does not provide any details from which to make an educated evaluation.

  9. Abe Weitzberg, PhD

    Have you looked at the controllability issues with high volume fractions of hydrogen? They make the issue of preventing criticality during launch failures look simple.

  10. Dr. Lyman J Petrosky

    The need for HEU in the original NTP designs was generally driven by the engine’s small size and epithermal neutron spectrum due to the use of graphite & beryllium for moderation. While HEU is a proliferation concern, a more difficult problem with HEU is potential criticality in the event of a launch failure resulting engine submersion. In both regards LEU is much better. It was asserted above that LEU would result in a heavier engine; however, there is a tradeoff between U inventory, enrichment, and moderator mass. The best moderator is hydrogen, which is very light. Thus it is possible for a LEU NTP engine to be lighter than its HEU equivalent.

  11. Brian Mays

    IMO proliferation is perhaps the most over-hyped issue of all. My (joking) reaction to this is, “what, they’re afraid that the Martians will get their hands on weapons material?”

    Maybe they’ve heard about Project

  12. James Greenidge

    When you skim Space World and Aeronautics it’s sadly amusing to hear the aside misgivings of JUNO scientists and engineers that the project’s original concept power source was switched from RTGs to solar cell in a knee-jerk appropriations request adjustment during a political year to appear “nuclear-free green” PC friendly to constituents but left them with a severely underpowered underpackaged Jupiter probe.

    James Greenidge
    Queens NY

  13. Jim Hopf

    John is right. Given how important weight (and bulk) is with space travel, it makes no sense to carry all that U-238 around. For similar reasons, all reactors used in vehicles (e.g., submarines) have used HEU. I’m skeptical of the claim that they can use LEU w/o any significant loss of performance (U-238 weighs a lot).

    I’m sorry to be negative, but this is one more case where over-hyped issues are holding nuclear technology and its capabilities back. IMO proliferation is perhaps the most over-hyped issue of all. My (joking) reaction to this is, “what, they’re afraid that the Martians will get their hands on weapons material?” Outer space is a pretty difficult place to go to steal HEU.

    Seriously, the US govt. would be leading this program, and HEU under the strict control of the US govt., used in cooperation between DOE and NASA, will not be a significant proliferation risk (no more than our submarines are).

    Use of HEU would not significantly limit private companies’ or other nations’ ability to participate in this program. Design and technology development activities are completely unaffected (of course). Even component manufacture would be almost entirely unaffected. Creating the fuel material, and placing it into the reactor fuel rods, is a tiny component of the overall program.

    And let’s face it, almost all the nations that would be contributing significantly to this program (i.e., those with significant space technology, programs, and industries) are already nuclear weapons states. And most of the remaining nations that would contribute are developed nations that we trust (e.g., Germany and Japan, etc.).

    This is a non-issue. It’s all just one more example of how all common sense has left in the room, throughout the nuclear tech area. Please don’t surrender to the BS. I almost have to wonder if this is a justification for more funding/work for NASA scientists and engineers. Using (supposed) proliferation concerns as an excuse, they justify redoing all their NTP development work, to use LEU instead of HEU.

  14. James Greenidge

    This can’t be allowed to go unchallenged for the whole nuclear industry. This morning on a major feature on CBS Morning News, the reporter Jenkin Duncan was having a cow about how the Brussels attacks will effect “the protection and progress” of all nuclear plants — of course complete with irrelevant footage of Fukushima (just can’t get enough of masked bunny-suited workers!) and how “EASY” is to hack into plant computers and stealing radio materials and that the nuclear terrorist peril can only be reduced by abolishing such plants like nearby Germany (big sigh of relief for German’s foresight!!) I’m just waiting on any certified pro nuclear advocate orgs to knock on CBS’s door for a little correction if not equal time! Greenpeace sure isn’t!

    James Greenidge
    Queens NY

  15. Jeffrey Randorf

    I am interested in the weight penalty delta between HEU and LEU for space applications. Give Dr. Hassan my best!

  16. Liviu Popa-Simil

    It is a good paper, and it is an ongoing work.
    Please have few links that may bring some clarifications:
    Try to take part in: http://anstd.ans.org/nets-2016
    Look for the book:Mars: Prospective Energy and Material Resources; Chapters 7-10, or
    for more questions and potential solutions.
    Best regards

  17. John Tanner

    Weight is an over riding consideration in space travel. We must make an exception and allow high enriched uranium for space applications.

  18. Abe Weitzberg, PhD

    evaluator, please note that HEU in the 4th line should be LEU. can you make the change?

  19. Abe Weitzberg, PhD

    The LEU-NTP issue needs to be openly discussed and proposed concepts carefully evaluated. Your assertions about the potential proliferation benefits of LEU concepts need to balanced against the facts that for space applications HEU always carries a performance penalty relative to a similar HEU system. Also, space systems do not carry the same proliferation risk as terrestrial reactors. Inherent in your assertions is the belief that cermet systems will provide superior performance relative to the NERVA technologies that have been demonstrated. However, there is no evidence that cermet systems can perform at the high power densities required for such performance. There is also the belief that LEU systems can be regulated by the NRC and this will reduce cost relative to DOE regulation of HEU systems. Experts within DOE and NRC agree that their regulatory processes are essentially equivalent, except for more public involvement with the NRC, and therefore the costs and schedule would be equivalent for planning purposes.

    Right now, LEU concepts are just paper studies that have not been subject to non-advocate peer review. LEU cermet fuel has not been fabricated and cannot be tested at prototypic conditions for many years. The enriched tungsten required for these LEU concepts does not exist and may never be available economically. These and other issues need to be explored before one advocates for LEU-based rockets to Mars in preference to the NERVA technology that has already been demonstrated. NERVA HEU technology would be faster, and cheaper and possibly better if cermet fuels cannot sustain the high power densities required for acceptable performance.

  20. Dr. Lyman J Petrosky

    I worked for 2 years on recovering NERVA technology from the Westinghouse archives in the mid 80s (when Bush proposed a Mars mission) and also developed some novel NTP designs for small engines (conical fuel). If there is anything I can do to assist this current effort I would be interested.

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