Space nuclear propulsion: Humanity’s route to the solar system

Part I:  Space nuclear reactor safety

by Wesley Deason

Though humans have successfully traveled from the earth to the moon, our exploration of the remainder of the solar system has been limited to robotic space probes which, once set in their trajectory, were not designed to return to earth. The data returned from these probes has been of tremendous importance for our understanding of the solar system and regions beyond, but human exploration beyond earth’s orbit remains to be achieved. There are a number of concepts currently under study that would allow us to break out of earth’s gravity well. The most studied and discussed are nuclear electric propulsion and nuclear thermal propulsion. Before I jump into an explanation of those concepts and their respective differences, however, I want to address their similarity: Both are powered by a nuclear reactor.

The primary principle that drives the immense energy production of a nuclear reactor is the process of nuclear fission, in common terms the “splitting of atoms.” This process, induced in uranium through the absorption of a neutron, releases very large amounts of energy when part of the mass of the original uranium atom is converted into energy as the atom splits apart (E=mc2). The fission process also releases additional neutrons that can be used to invoke fission in other uranium atoms. If enough uranium atoms are present, the chain of fissions can be maintained at a steady rate and this configuration of uranium is said to have reached “critical mass.” Extended over a long period of time, this process allows a nuclear reactor to produce large amounts of energy. Fission energy becomes particularly useful and indeed necessary when large amounts of energy are required while availability of fuels or other energy sources is low. With this amazing energy generation capability, however, questions about its safety can, and should, be asked.

Is it safe to launch nuclear reactors into space?

Space reactors must be able to endure specific circumstances that are unique to their transport to outer space. Most importantly, the reactor must remain ”subcritical” until required by the mission to commence operation. One classic design requirement for space reactors is that the reactor remain subcritical after a water submersion (a launch accident scenario). Water around a submerged reactor behaves as a neutron moderator, a material which slows fast-moving neutrons. In order to meet this important design criterion, reactors will often contain a material that will absorb moderated or slowed neutrons before they can cause fission in the uranium fuel.

If there were a highly unlikely launch accident in which reactor fuel escaped containment, the environmental effects would remain minimal. Uranium, the fuel that drives modern reactors, is a naturally occurring radioactive element that has a half-life of around 700 million years (for the uranium-235 isotope). This means that it releases energy through radioactive decay at a very slow rate. Also, uranium is an alpha emitter. As discussed in my previous post on plutonium-238, alpha radiation is generally not harmful to humans, provided its emitters are not inhaled or ingested. The more highly radioactive constituents that comprise spent nuclear fuel would not be present before reactor operation commences in space.

Are nuclear reactors dependable and controllable for power generation in space?

To address the controllability and dependability of nuclear reactors, we must consider the methods and physical processes that allow a reactor to be controlled. The main concepts are the effects of negative temperature feedback and the active removal of neutrons through the use of neutron absorbing materials and leakage control.

Core arrangement – Space nuclear power by Angelo & Buden

Negative feedback within a nuclear reactor can come from two main effects, both of which are related to the slowing of the fission chain reaction due to a temperature rise. First is the commonly-known material property of thermal expansion. As a reactor core heats up, it will expand in size, causing the uranium fuel within to spread farther apart and absorb fewer neutrons for fission. Second, due to the neutron absorption properties of nuclei, when the temperature of uranium rises, it is more likely to absorb a neutron but not cause fission. From a safety and control perspective, negative temperature feedback can aid in preventing a reactor from producing too much power and overheating.

There are also methods to actively control a nuclear reactor by removing neutrons from the reactor. These include control rods, drums, shutters, and windows. Control rods and drums use boron, an element with a large neutron absorption ability, to remove neutrons from the reactor before they can cause fission of the uranium atoms. Control rods insert boron directly into the central region of the reactor to adjust power or shut it down. Control drums are a more popular alternative for compact space reactors; the drums contain an absorber section that is rotated towards or away from the reactor to adjust power. Shutters and windows are largely unique to space reactors as they take advantage of the vacuum of space. When these shutters or windows open, they allow neutrons to leak out of the system, thus slowing the chain reaction. These features, along with others specific to a selected reactor design, allow well-designed space reactors to maintain containment of radioactive materials in case of accident.

Kiwi A Prime nuclear thermal rocket built and tested in the 1960s

Nuclear reactors, due to their ability to produce large amounts of energy at any location, will be the required energy source for future human space travel outside of earth’s orbit. Future installments in this series will focus on how nuclear reactors are applied in the two most-studied nuclear space propulsion technologies:  nuclear electric and nuclear thermal propulsion.



Wes Deason is a graduate student in nuclear engineering at Oregon State University working on the safety analysis of vented fuel systems for gas-cooled fast breeder reactors. He is a former summer fellow for the Center for Space Nuclear Research and the current student liaison for the Aerospace Nuclear Science and Technology Division of ANS.

11 Responses to Space nuclear propulsion: Humanity’s route to the solar system

  1. You may be presuming that the reader knows that you are describing the behavior of a fast reactor. Some of the characteristics that you describe are different for thermal reactors of which more people (including me and the general public) are familiar with and may be temporarily confused. Other than that your posting was informative and remained focused.

  2. Key point is that it would not be difficult to delay assembly until parts are in space thereby eliminating the threat caused from a failed launch of an indivudual part. Smaller rockets could take the parts to earth orbit, where they could be assembled into a complete unit. Not a hurdle any bigger than NASA has accomplished before.

  3. I think one component left out is ‘heat sinking’ a space reactor.
    Heat sinking in the vacuum of space is hard to do.
    NTR use reactors are bit better at temperature management than NEP use reactors. I would add the safety of the ‘party horn blower’ filled w/ gas medium fine carbon dust. I know it sounds silly but I think it would act more efficient than large area fluid filled capillary heat sink fin structures.
    It might even have less of mass penalty for space applications.

    Great nuclear space write-up piece keep up the good work.

  4. Nuclear electric propulsion has a promising future, but a fission nuclear reactor within a spacecraft needs massive radiators to get rid of waste heat. However, the use of moving and rotating magnetic fields can make heat to electrical energy conversion more efficient than ever, making possible to recover most of the thermal energy again into electricity thereby allowing the space nuclear radiators to be smaller and lighter.

  5. The response about heat sinking for NEP is of interest to me. If boiling UF6 is used both as the fuel and as the working fluid, one gets a much lighter reactor and reduced complexity (as well as the ability to remove the fisssion fragments continuously) but it is only about 25% thermally efficient rather than the 35% for a boiling water reactor or fast fission reactor. What is the trade-off for the mass of reactor and mass of the heat-sink/radiator?

  6. I agree with your premise that nuclear propulsion is our route to the solar system – and beyond. I also agree that precluding and preventing reactor operation (planned, or inadvertent via criticality in a launch, ascent, or reentry accident) prior to intended operation in space – except for zero-power testing prior to launch – represents an overwhelming safety attribute of nuclear reactors. However, I must take issue with your characterization of launch accidents as “highly unlikely,” and your cursory treatment of preventing inadvertent criticality. Historical data on launch vehicles, since the dawn of the space era up through the present, clearly illustrate their launch failure probability at ~2-3 x 10^-2, i.e., ~2-3%. This is far from “highly unlikely.” Given this unreliability for launch vehicles, any on-board reactor system must be designed to safely accommodate the full spectrum of launch accidents, and the associated explosion/overpressure, fragment/shrapnel, ground surface impact, and liquid and/or solid propellant environments that they can pose to an on-board nuclear reactor. For a fast spectrum reactor (most likely type to minimize reactor size), spectral-shift absorbers built into the system, outside any absorber materials located within any control mechanisms, are an obvious option to prevent inadventent criticality for a water immersion accident. Launch accidents, however, can result in mechanical and thermal damage, which can drastically alter the as-built geometry and integrity of an on-board space reactor. I am confident that a space reactor can be designed to safely accommodate the spectrum of possible launch accidents and environments that could potentially occur, but it’s not as straightforward as you tend to imply. I am much more concerned about suborbital, orbital, and Earth flyby reentry accidents, where safeguarding the fuel becomes an issue. Interested individuals should refer to the many papers and texts available within the open literature; the most recent being “Space Nuclear Safety,” A. C. Marshall (editor), Krieger Publishing Co., Malabar, FL, 2008.

  7. You can ask students working on these issues @

  8. I think the following story link can answer your fears.
    The MK-6 re-entry vehicle 10 ft. tall & 5 ft wide weight 2100 lbs. containing 9 (MIRV) W-53 warhead atop a titan missile EXPLOSION which popped the silo’s 750-ton steel and concrete lid off, spewing table-size chunks of twisted steel and concrete into the air and injuring a total of 22 airmen, two seriously. A 60-man Air Force emergency response team was above ground near the silo at the time. This has happened a few times already and in every incident neither nuclear bomb explosion or radioactive components leaked on to public or to groundwater.

    also: Read THE NUCLEAR ROCKET by James Dewar
    In his book he describes explosive intended destruction of rocket nuclear reactors project ROVER-NERVA. Toonerville Trolley KIWI-TNT 1000MW reactor exploded for effect and data with no major pollution or radiation poison it took 25 (workers) crew, 2 months at $88 thousand dollars to complete clean-up of area.
    There really is no technical reason NOT to use the nuclear ‘Hammer’ in space as a tool.
    Way too many people use fake argument to discredit this technology this has already been tested and the results are consistent with efficient safe use for space.

  9. James Wendte


    After you graduate, I suggest you go to work for a utility that has a water cooled reactor(BWR,PWR). Studies have been made on space power reactors since 1960. Only one was built and it only ran for a few minutes. Also stay from “advanced” reactors, i.e. gas cooled and liquid metal reactors. All of these reactors have limited funding time periods. If you get layed off at age 40 or above, the experience you get from working on these projects is worthless. All prospective employers care about is your experience on BWR’s and PWR’s.

    I used to work on a liquid metal reactor that was cancelled. The experience was worthless as far as getting a job on a BWR or PWR was concerned. Fortunately I worked for GM for several years before working at Argonne, and was able to get a job working for a GM division.

    Stay away from those “advanced” reactor projects, including space reactors. The only thing they are good for is your ego.

  10. James Greenidge

    Thank you for opening this topic up! It’s bad enough it’s a d__ shame NASA’s green-PC attitude can’t be dragged into a positive no-hazard discussion of nuclear anything nowadays! (notice how NASA falls all over themselves mentioning that their solar-powered probes are — but how much do they even whisper New Horizons and Cassini are nukes? Heck, NASA sounds almost plum apologetic about it!). In college we had NASA’s Dr. Jastrow of Columbia preaching nuclear drives all through the 60′s and how it had been hoped post-Apollo to’ve had eight-month manned round-trips to Mars in the _1980s_ based just on Rover technology! (Would we be on Titan today??) How the fear-mongers have won setting us back! !
    Awaiting your next chapter!

    James Greenidge
    Queens NY

  11. I lament the history of which James Wendte writes. And as for “NASA’s green-PC attitude”, I trust thatJames Greenidge considers both “green” and “politically correct” to be terminological inexactitudes. Taking “green” to mean “environmentally benign”, it is clear that the USA’s failure to replace coal burning with the energy from splitting atomic nuclei has caused decades of continuing air pollution, mountaintop destruction, and now pollution of underground water by fracking for gas. Quite a lot of the dogma that is called PC, in particular the opposition to all things nuclear, is not only incorrect, but by being held unquestionably it violates the essential definition of “liberal”.

    In another place, I note that one of the pioneers of the IFR research, in 2004 received a science medal and money prize at the hands of Vladimir Putin, which probably means that Russia is posed to beat the pants off the USA in a technology that we pioneered and ought to have deployed by now.