Space Fission Power Post #0: Introduction

Space Fission Power: A Series of Blog Posts and References

David Poston: spacenukes@gmail.com

The following series of blog posts and references intends to make the case that the US and/or other entities should be investing significantly in space fission power, and how best to establish the use of fission power in space. The first few posts are more philosophical in nature, and as such are more opinion and less fact, while the latter posts are based on historical and technical arguments. The meat of the content is in posts 6 and 7 (and the references referred to within), which detail the technologies that should be invested in now.  While I am the leader of the space fission reactor team at Los Alamos National Laboratory, the opinions in these papers are mine alone.  The papers, and especially the references, are largely a collection of work that I and others have done over the years; as such there are redundancy and transition issues between papers and themes. I have drawn upon the good work and ideas of many others in this text, but probably the two greatest contributors to this philosophy and content are Mike Houts and Lee Mason, and past/current members of the LANL space reactor design team, most notably Rick Kapernick and David Dixon. The actual list of contributors and space reactor enthusiasts would take pages, which I might eventually complete as part of this project.

My career has been dedicated to space fission power, from my PhD thesis in nuclear thermal propulsion, to working on the SP-100 reactor at GE, to my 17 years at Los Alamos trying to get reactors used in space. So far, I have been unsuccessful, which is why I am creating this series of blog posts as a different approach.  My experience helped me write these papers, but I am the last person to suggest that experience makes you more qualified to be an expert; all too often I’ve found cases where a young, bright student has better ideas and can perform better work than a person with a resume that shows 20 years of experience in a particular field.  Likewise there is “bias” in these documents, but my goal it that this bias is based on good information, technical facts, and logical conclusions.  My only true bias is that we should spending far more time and resources getting ourselves established in space, and the rest of my opinions stream from there.

Below is the list of blog posts. I’d appreciate it if anyone that comes across these documents (and chooses to read them) gives me feedback on how to change or improve.

Spacenuke Post #1: We need to explore and expand our presence in space

Spacenuke Post #2: Abundant power is the key to space exploration

Spacenuke Post #3: Fission power is the best option to produce abundant power in space

Spacenuke Post #4: Small steps are needed to utilize fission reactors in space

Spacenuke Post #5: First step: affordable, entry-level space fission systems

Spacenuke Post #6: Entry-Level Option: Fission Surface Power (FSP) for Mars/Moon

Spacenuke Post #7: Entry-Level Option: Low Power Space Reactor (LPSR) Systems

Spacenuke Post #8: Second Generation Space Fission Power (SFP) Systems

Spacenuke Post #9: Nuclear Thermal Propulsion (NTP)

Spacenuke Post #10: Ultimate Goal: High-Power Nuclear Electric Propulsion (NEP)

Space Fission Power Post #1: We Need to Explore and Expand our Presence in Space.

Our inherent desire to question and explore is at the root of being human.  This statement in itself, if you adhere to it, should be enough to justify a significant expenditure of societal resources on space exploration.  Unfortunately, it is not that simple. It is also human nature to have compassion for those in need, creating a constant struggle within individuals and society regarding the allocation of finite resources.  One must not only weigh the value of helping unfortunate individuals against helping society as a whole, but also the value of people alive today against people in generations come.  The comparison becomes more complex upon realization that an investment that increases the long-term viability of humanity means that the number of individuals helped could be countless, and the value of that investment priceless.  In this regard, the benefits of space exploration go far beyond our inherent curiosity—perhaps to the outright survival of the human race. 

Some of the benefits of space exploration are summarized below as they relate to four themes: curiosity and inspiration, science and technology, philosophy and enlightenment, and viability and preservation. The categorizing of the benefits, and to a larger extent what constitutes a benefit, is of course highly dependent on the individual.

Curiosity and Inspiration. 

It is human nature to be curious in all facets of life, and when possible to explore and expand into new frontiers. Curiosity is at the root of most paradigm shifting discoveries and advances in the human condition. Many would adhere to the philosophy that as soon as we stop asking questions we stop being human. It is because of our curiosity that space exploration is viewed favorably by almost everyone worldwide. Our relatively short history of space exploration has successfully answered many questions, while posing even more interesting ones. More so, exploration inspires people, particularly young people because their curiosity burns brightest.  It is no coincidence that the U.S. had a boom of technical innovation from the generation that grew up during the Apollo program. The Apollo program inspired many youth to pursue careers in math, science, and engineering with dreams of being a part of something even bigger when they were adults.  It can be argued that the current generation of American kids is becoming inferior in math and science, and perhaps more apathetic in general, because they lack such inspiration.  History also shows that the most successful civilizations continually explore beyond their known boundaries. Countries such as China, Russia and India must see some value in these respects, because in 2012 they continue to expand their human space programs, despite having smaller economies than the U.S.  Finally, in addition to inspiration, space exploration unifies all with a sense of pride and purpose, and a realization that our fates and those of future generations are to some extent are all linked together.

Science and Technology. 

Space programs enable the development and acquisition of technology, information, and resources that benefit all of humankind.  Technology spin-offs, whether direct or not, are usually the most talked about and documented benefit of space exploration.  Many compelling arguments have been made that justify investment in space exploration based solely on these tangible benefits. Some of the most obvious benefits are provided by satellites in orbit of the Earth, for example weather/climate prediction.  NASA specifically has a long list of technologies that were direct spin-offs from their space programs, ranging from transportation to health care.  The space program also had major influence on the development of the microprocessor and how computers are used.  In addition to technology, the scientific information learned from space exploration, i.e. the evolution of planets and stars, the predictability of solar flares, probabilities of asteroid impacts, etc., can be used to help us live within our environment and mitigate possible changes to our environment.  Beyond science and technology, space exploration has the potential to provide us resources as well, e.g. solar power beamed from space, rare materials from the moon or asteroids, or future resources that we are not even aware of.

Philosophy and Enlightenment. 

The most profound reason to explore the universe is to investigate the origin and nature of our existence. This is also the most contentious reason, because the issue of philosophy is entangled with religious beliefs.  A large fraction the people alive today believe that we already know the origin and purpose of our existence. A smaller fraction of those people could be correct, but they should have no objection to learning as much as we can about the physical nature of our existence.  In most cases, scientific discoveries could either confirm or shake the foundations of a specific faith (e.g. ancient gods lost followers when science displaced the use for that god).  Some of today’s religions preclude the possibility of life evolving elsewhere in the universe.  In this case, if life is found elsewhere they would have to rethink their faith, while if life is not found elsewhere then it would likely strengthen their faith and draw others to it (because this result would be in contrast to most scientific expectations). Likewise, those that believe everything ultimately has a natural explanation are not immune to this line of thinking; future discoveries, or lack thereof, could cause them to rethink their faith that science will explain everything.  Regardless, for many people enlightenment is the ultimate goal of life (even if enlightenment is simply knowing the proper questions to ask), and space exploration could help immensely in this endeavor.

Viability and Preservation.

The all-or-nothing benefit of space exploration is the long-term survival of the human race; although the extended timeframe of this benefit makes it very hard to quantify.  We know that our life on Earth is finite, but the preservation benefit of sustained civilization outside of the Earth could range from enormous to miniscule depending on whether viability of human life on Earth ends in <1 thousand years or >1 billion years.  There is a long list of potential calamities that could end human civilization, including asteroid/comet, super-virus, excess volcanism, socioeconomic collapse, environmental changes, weapons of mass destruction, or maybe something we’ve never envisioned.  Some of these initiating events can be mitigated or prevented as a result of space exploration; most notably the ability to deflect or destroy a potential extinction causing asteroid or comet.  The ability to deflect an asteroid could actually be developed within a decade using existing technology, the question is would we have enough warning time to successfully develop and deploy it.  Space exploration could also uncover currently unknown threats, such as looming changes in the behavior of the sun, or maybe astronomical threats such as nearby black holes, supernovae, dark matter, or something our current understanding of physics is not aware of. 

The ultimate defense against human extinction would be to establish permanent, self-sustaining colonies of humans beyond the Earth.  The path to this type of existence does not require a huge leap in science and technology; most engineers agree that abundant, reliable energy (probably nuclear) is the key to expanding into space. In the near term (decades) exploration could focus on where and how to develop sustainable communities away from the earth, including quasi-sustainable outposts on the moon and Mars.  In the mid-term (centuries) sustainable outposts could be created on Mars, Titan, asteroids, etc. that could be considered planetary lifeboats, as a safeguard against major calamities that could end human civilization.  In the long term (millennia), the concept of the “planetary lifeboat” could transform to a “celestial Mayflower”, taking us to new worlds outside of our solar system.  The benefits of this scenario are not limited to merely saving the human race.  Even if humanity continues to thrive on Earth, there would be the possibility for a nearly unlimited number of humans to experience existence (in addition to the increased population that Earth could support by importing resources) and expand the extent of human condition (e.g. well-being, knowledge, and enlightenment).  If new opportunities and experiences emerge, people will migrate to them, just as they did to the New World ~500 years ago.

Space Fission Power Post #2: Abundant Power is the Key to Space Exporation.

On Earth, life and humanity has benefited from the energy of the sun (plants, animals, fuels, heat, etc.), but in space you’re essentially on your own.  Power is the single most important element to the survival of both spacecraft and humans in space, and an abundance of power is essential to providing safe and reliable missions – without power there is no existence. NASA science missions have been extremely successful since the 1960s, but since then there has been no fundamental change in where we can go and how much power we have when we get there. We could increase the benefits of science missions by orders of magnitude with a robust, low-mass source of abundant power. For human exploration, the need for abundant power provides a different paradigm that we’ve come to know on Earth.  An astronaut is not concerned with the over-hyped health effects of radiation (from a reactor, or the sun and cosmic rays); rather, an astronaut’s concern is whether the power system will provide power when it is needed. In addition, these astronauts will want to be “power rich”, a term frequently used by NASA astronauts (e.g. discussion I’ve had with Scott Horowitz, Ed Lu, John Grunsfeld, and Franklin Chang Diaz) when discussing human space exploration.  The most important asset is the one that you’d prefer to be the most in abundance. Power can be binned into three areas when discussing space exploration: electricity, propulsion and heat.

Electricity

Electricity is the most valuable asset that space power systems can provide. Almost all terrestrial technologies have evolved within the paradigm of available electricity, and with enough electricity almost anything can be accomplished.  It is hard to imagine any spacecraft or human outpost in which electricity will not be the lifeblood of the mission.  Nomads and Pioneers could live off the land to explore and settle the Earth; space explorers can potentially do the same thing sans one component: electricity.  Some things can be done more efficiently without our favorite energy “middleman,” but there is almost nothing we can’t do if we have abundant electrical power.  For near-term scientific missions, increased electricity allows more capable instruments, increased instrument duty cycles, onboard scientific analysis, higher data-rate communications, and smaller antennas. For human missions, abundant electricity enables science and exploration, but more importantly it is the foundation of their safety and life support.

Space Propulsion

The first order physics of space propulsion is very simple – throw something off the back off your spaceship and you will go faster (Newton’s third law of motion).  In this respect, the brute force approach to space propulsion is to launch as much propellant as you can into Earth orbit (and if launch costs can be made affordable this is a great option for relatively low delta-V missions, like a conjunction class mission to Mars).  If launch costs are high, and/or you need to get somewhere fast, then the key to making propulsion effective is to gain as much thrust as you can for every unit of mass that you eject (Specific Impulse).    On top of this, the thrust to weight of your propulsion system and spacecraft is very important.  Ultimately, a power source can provide a propulsion system in three ways: the byproducts of the power source are used directly as the propellant (direct propulsion), the power source provides heat directly to the propellant (thermal propulsion), or the power source creates electricity that is used to accelerate an ionized propellant (electric propulsion).  In all cases, abundant energy/power is needed to enable effective human space propulsion (with due concern given to the effort required to place those energy sources and propellants into Earth orbit).

Heat

Heat is needed to keep things warm and to drive chemical processes – these functions can be completed via electricity, but they are done more efficiently without it (because heat generally creates the electricity in the first place).  In space, thermal heating is needed for systems ranging from the smallest spacecraft to large human outposts.  Where adequate sunlight is not available, NASA has routinely used the Radioisotope Heater Unit (RHU) to keep components warm in space, while waste heat from a large thermodynamic power system could keep entire spacecraft or human outposts warm at essentially no “cost” to the infrastructure.  Process heat is very important to create practical and sustainable human outposts elsewhere in the solar system.  Many technologies have been envisioned, and in some cases developed to transform in-situ materials to usable consumables (air, water, rocket fuel) and/or construction materials (ceramics and metals).  Again, if a large-scale power plant is used to create electricity, the process should reject enough “free” heat to meet most of these needs.

In the very long term, we would need to use in-situ resources to power a sustained space-faring existence. These in-situ resources could simply be the materials to fuel our previously developed technologies, or the in-situ resource might provide a unique energy alternative (using site specific chemical or geothermic resources). In all cases, an Earth-based energy technology that can provide abundant power is needed to 1) get us to our destination and 2) establish a semi-autonomous outpost/colony which can eventually evolve to a self-sufficient location.  For more information on the need for, and possible uses of abundant power to enable our expansion into space, I recommend the writings of Robert Zubrin.

Space Fission Power Post #3: Fission Power is the Best Option to Produce Abundant Power in Space.

Abundant power is needed to significantly explore and expand into space. A robust power source is needed that can provide high energy and power density while being independent of location, environment and application.  The primary energy alternatives that are available in the near-term are discussed below.

Solar Power

Using the sun as an energy source in space is often the preferred option, but is limited in power density and is location specific; if you’re ever in the sun’s shadow or in a dusty/cloudy environment, or as you move deeper into the solar system the sun becomes a poor energy source.  Even on Earth, the sun is not practical as a source of baseload electricity.  Solar power on the Moon is significantly hampered by the 28 day cycle (i.e. 14 days of darkness).  The technology to efficiently and reliably store energy in the deep cold on the lunar night presents substantial challenges, and would likely be more complex and heavier than the solar power system itself.  On Mars, the diminished sunlight decreases the value of solar power, but it is the dust, and more importantly dust storms that make solar power unattractive.  The potential of a protracted dust storm will require more baseload power and a longer term storage system, presenting the same challenges as a lunar system (in addition to the decreased solar insolation during the day). Finally, the lower power density of sunlight beyond the asteroid belt would make the required size of a solar powered system excessive for a high power mission.

Chemical Power

Since the dawn of the human race, our favorite energy alternative to the sun has been to burn things (primarily wood until the industrial revolution). On Earth we are surrounded by materials laden in hydrogen and carbon, fortuitously surrounded by a gas that contains oxygen (air).  When we move about, we generally worry about our fuel supply but take the oxidizer for granted.  In space, we have to bring both fuel and oxidizer with us, or have a means of generating either in-situ (which requires power). Space does not change fundamental chemistry (except for gravitational effects on combustion), but using chemical power in space is extremely inefficient because of all the power needed to transport or create constituents in-situ. In addition, storing these constituents in the extreme temperatures of space presents problems in logistics and reliability.

Energy Storage

Energy-storage systems (e.g. batteries) have a role for almost any space mission, but generally only in the very early stages. The advantage of batteries is that we can use low-cost Earth-based technologies to generate the energy, and then extract it from the storage technology to utilize it in space. There is considerable ongoing research in fuel-cells and alternative means of storing energy, which could be much more mass efficient than traditional batteries; however, there is no chance that a chemical energy storage system would ever be practical to power ambitious space exploration. Some nuclear sources, such as anti-matter and even ²³⁸Pu might fit within the definition of an energy storage system, but they are better classified as nuclear power sources.

Nuclear Power

Nuclear power can be used to provide electricity, propulsion and/or heat in space.  The major benefit of nuclear technology is that power can be provided in a robust form that is independent of location or application.  There is a great deal of experience in space radioisotope power systems, and limited experience in space fission power systems. On Earth, the physics of fission are well understood, and engineered systems are well established. Other nuclear power types like fusion, anti-matter, or options like triggered isotopes are not well developed for either terrestrial of space application.

“Nuclear power in space” encompasses a wide range of sources, technologies and applications.   The physical sources of nuclear energy that can be utilized are radioactive decay, fission, fusion, and antimatter.  The technologies to harness these forms of energy are almost limitless, but the list of practical near-term technologies rather small.  The applications of nuclear technology include electricity, propulsion, and/or heat (for warmth or to drive chemical processes).  There are numerous combinations of nuclear power sources, technologies, and applications that can be used to enable our exploration and expansion into space.  In the end, the attractiveness of nuclear energy in space is the ability to provide robust, long-lived, high power density (kW/kg) systems at any location.

The current forms of nuclear power that we know of and can reproduce on some level are radioactive decay, fission, fusion and anti-matter. 

Energy Source	Energy Density	Max Power Density
LH2-LOx Combustion	13 MJ/kg	limited only by engineering
Pu-238 Decay	2,100,000 MJ/kg	0.54 kW/kg
U-235 fission	82,000,000 MJ/kg	limited only by engineering
D-He3 fusion	354,000,000 MJ/kg	limited only by engineering
Antimatter	90,000,000,000 MJ/kg	limited only by engineering

The 2 key points of the above table is that 1) nuclear power sources offer enormously higher energy densities than chemical systems, and 2) that power density is limited for Pu-238.   Fission, fusion and antimatter can all provide energy and power densities beyond what we could feasibly utilize in the foreseeable future.  However, discussions of fusion and anti-matter are essentially moot for decades or perhaps centuries to come, because even if we engineer the technologies required to make these power sources practical, we will not have the technologies to engineer the high power density systems needed for space application. Restated, fission has the ability to provide energy densities so high that it will take many generations of technology advancement (materials and fabrication) until we could take advantage of a higher energy density power source (even if that source itself was off the shelf). Therefore, since fission technology is established and well-understood, it is the obvious technology to focus on for space nuclear technology development.

It is possible that once we establish large scale settlements on Mars or elsewhere, that we could shift away from nuclear power to an in-situ source (geothermal power, solar power with an in-situ energy storage mechanism, wind power where there is an atmosphere, or perhaps nuclear if we could mine thorium, uranium, D, He-3, etc.). Until then, we need to have a technology that can enable the energy intensive missions and operations that could get us to that end goal, and that power source is fission power.

Space Fission Power Post #4:Small Steps are Needed to Utilize Fission Reactors in Space

There are generally two ways to establish a truly enabling technology that goes well beyond current capabilities: 1) a nationally mandated program that utilizes a significant fraction of resources over an extended time period or 2) an evolution of the technology via smaller, shorter, more affordable steps.  Nearly every technology used today evolved via path #2: automobiles, airplanes, computers, etc.  The same is true of all technologies used in space exploration today: solar panels, lightweight structures, power conversion technologies, control systems, propulsion systems, etc.  Unfortunately, fission technology has not progressed in a similar matter.  In the 1960s, space fission technology was being successfully developed with frequent design, built, and test iterations, which is arguably the best way to develop any technology. With the SNAP and ROVER/NERVA programs, significant progress was being made in all regimes of space nuclear power and propulsion. In the 1970s, an irrational fear of nuclear radiation began to develop that led the creation of cumbersome regulations and bureaucracies; these in turn made nuclear operations increasingly difficult and expensive.  In the 1980s, a major space reactor development program (SP-100) was attempted, but the cost and risk of ground testing a prototype became so high that the sponsoring agencies could not maintain support of the project. Not only was the ground test prohibitively expensive, the burdensome nuclear regulations had forced so many design changes in the test article that the “prototype” would have been far from prototypical.

Unfortunately, the environment for nuclear development and testing has gotten continually worse since the 1980’s.  Not only has new development been hindered, but the majority of testing facilities and infrastructure has been overtly phased out within the U.S.  Currently, there is no chance to sustain a nuclear program that consists of design, build, and test iterations (except for the aforementioned nationally mandated program). The irrational fear of nuclear power is now so extreme that following the devastating Japanese earthquake of 2011, the media coverage of the reactor accident for which radiation caused no measurable harm (present or future) was far greater than the coverage of the thousands of lives lost and the crippling economic loss. These unjustified prejudices are what continually makes doing anything nuclear more and more difficult (and if not for the climate change agenda, nuclear energy might be pushed out altogether – despite being technologically one of the most economical energy sources). In general, politicians and bureaucrats in Washington D.C. recognize the benefits that fission power could have on Earth and in space, but the fear of negative publicity, especially if anything might go wrong, makes them unwilling to champion or fund development of nuclear power systems.

In spite of these headwinds, and an increasingly constrained national budget, a path does exist in which space fission systems could be developed affordably today. The window of opportunity is still open to develop an entry-level fission system that can evolve to the technologies needed to significantly expand into the solar system.  The crux of the matter is finding a entry-level system simple enough to be developed affordably, while at the same time providing a capability that is attractive to a near-term mission (unless for some reason the U.S. realizes that developing an entry-level system makes sense purely for the purpose of evolving to future systems, but this kind of foresight is not likely).  The key features of an affordable entry-level system are 1) the use of existing nuclear technologies (e.g. fuels and materials) and 2) the development of simple systems that can be qualified without expensive nuclear-powered system tests.  Unfortunately, these two features eliminate the vast majority of space fission systems that could be useful, including systems that have been previously attempted in the past. In order for a fission power system to be developed today, the reactor module of this system will need to be so “low-tech” from a nuclear perspective that most nuclear engineers will feel it’s a waste of perfectly good uranium. The good news is that while some nuclear technology has been stuck in the 60s, the rest of the technological world has advanced considerably. Advances in modeling capabilities, power conversion, heat rejection, instrumentation and control, and even resistance heaters will allow us to build an attractive space fission system that utilizes a simple, low-tech reactor. 

The second key to an affordable small step is the ability to qualify the system without an expensive nuclear-powered system test or demonstration.  Of course the gut reaction of any good engineer is that the best way to prove a nuclear reactor system is to run that nuclear reactor system. True, but as mentioned above, a conventional nuclear-powered test can be prohibitively expensive (and risky).  Even then, the system you will test is still not going to be fully prototypic of your flight system, and ground safety concerns will prevent the full exercising of the system. The question then becomes whether you design a system that can be qualified without a ground nuclear-powered test (GNT). A blanket answer cannot be given to this question, but the answer is more likely to be yes for a reactor with low thermal power, simple reactor physics, and simple, loose coupling to the power conversion system; this will be the case for first-step or entry-level space fission system.  System dynamics, testing, and how a system could be qualified without a GNT is discussed in the Qualification_Without_GNT reference.

Space Fission Power Post #5:First Step: Affordable, Entry-Level Space Fission Systems

Space fission power has long been identified as a near-term technology that can provide a dramatic leap in our ability to explore and expand into space.  The trap that is sometimes fallen into is that “near-term” and “dramatic leap” are not necessarily consistent with each other.  If a fission system is to be utilized in the near term, a reasonable, measured first step must be taken.  Numerous past space fission power programs have failed because they have tried to do too much, too soon. The ideal development scenario would be a mission where the primary goal is to demonstrate the operation of a space fission system, but this would require a very strong commitment and would likely face many programmatic obstacles.  A more likely scenario is to define an adequate mission, or a mission that has enough utility to retain programmatic support, while keeping technical requirements simple.  Unfortunately, adequate gets increasingly more difficult with time, as solar and radioisotope technology have steadily evolved via small steps, and continue to raise the bar for the first step in space fission technology.  This means that the first step must not only be small, but it should be taken as soon as possible.  Fortunately, recent advances in power conversion, radiator, electronics, spacecraft, and thruster technology allow the development of an adequate fission system with a modest reactor—i.e. relatively low power, low temperature, and reasonable mass.

An affordable, entry-level space fission system should minimize (and hopefully eliminate) the need for research or fundamental technology development, and should be developed within the philosophy below.

• Design with established materials (infrastructure and performance)
• Design for simplicity at all levels (components, integration, reliability, programmatic, etc.)
• Design with large margins (temperatures, stresses, criticality, etc.)
• Design to simplify control (stuck control elements, transient control, etc.)
• Design with an overly robust safety approach that minimizes program safety effort
• Design to minimize the testing required to verify component and system performance          
• Design so that the required test data can be obtained as affordably as possible

The first bullet above is probably most important for a new nuclear system. The easiest way to use established materials is to develop a relatively low temperature system (<1000 K), which for a space reactor is reliant on a “relaxed” mass requirement.  Simplicity is the mantra of any good engineer, and for a new reactor, especially one targeting deployment without a conventional nuclear-powered test, neutronic simplicity is very important.  A compact, fast-spectrum reactor is the simplest reactor from a neutronics perspective, which is discussed in the Fast_Versus_Thermal reference.

The remaining bullets above are best met be keeping the system power requirement low.  Lower power simplifies the entire spacecraft system in terms of development, integration, and thermal management:

Reduced/eliminated material irradiation concerns:  Radiation damage (usually swelling and loss of ductility) to most materials does not occur until a certain neutron fluence threshold is reached.  For most systems, a power level below 400 kWt will keep radiation damage from becoming significant, and keeping power below 100 kWt can eliminate radiation damage altogether.

Reduced/eliminated nuclear fuel issues:  As nuclear fuels are irradiated (“burned”), they generally swell, lose structural integrity, and release fission gas into the fuel pin volume.  In general, for 10-year lifetimes, swelling and integrity will not be a problem at powers less than 400 kWt, and fission gas buildup should be manageable.  At powers less that 100 kWt, fission gas buildup also becomes negligible.

Reduced material strength concerns:  At lower powers, there is considerably less stress on structural materials because of smaller temperature gradients and less fission gas buildup.  In addition, other things being equal, peak temperatures will be lower, so materials will retain more of their strength.

Simplified electrical testing:  Lower power reactors are more amenable to realistic non-nuclear testing.  At higher power levels, it is harder to provide the required power density with electrical heaters, because of physical size limitations for leads and connections, and more difficult heater technology.  Lower power should also allow more existing facilities to be available for electrical testing.

Simplified nuclear testing:  Lower neutron fluence could provide more flexibility in performing component irradiation testing (more potential facilities and more flux chambers within those facilities), and/or reduce the time required for in-pile tests.  More so, if reactor power is low enough, then this type of testing may not be required at all.  Lower power would also make a nuclear powered ground test much easier, both from a technical and regulatory perspective, although this type of test is only technically necessary if the gain in reliability is worth the cost and programmatic risk of performing the test.

Simplified qualification process:  At lower powers (less neutron fluence) there is a more extensive experimental database, which facilitates design qualification with minimal testing.  Also, there is less material property change due to irradiation, so that common/existing analytical tools can be used, minimizing the need for more specialized tools that might have greater uncertainty, less history, and demand significant benchmarking. Most importantly, at lower powers, there is a higher probability that a system can be qualified with non-nuclear testing.  There is a point where, at extremely low-power levels, a nuclear-powered ground test effectively becomes a low-cost “zero-power critical” test).

Simplified safety process:  Lower power cores require less volume for heat transfer.  This allows a more compact system, with smaller voids that could be filled by water in an accident.  These factors make accidents easier to mitigate, and can provide more safety margin and flexibility to meet changing safety requirements.  This can greatly simplify and expedite the launch approval process.  Lower power systems will also have less burnup reactivity loss, so it will be easier to maintain the required criticality safety margin.  In addition, lower power systems will have lower total mass, which allows a higher altitude orbit insertion, and potentially an Earth escape trajectory.  This should simplify the launch approval process as well.

Simplified handling and deployment: A lower power system will be physically smaller than a higher power system.  This could make transport easier by giving more options for handling and transport (cranes, trusses, transport containers, etc.), and more flexibility for launch shroud configuration.

Simplified reactor system component design:  The design of several reactor components can be made simpler by lower power and reduced neutron fluence.  In higher power systems, increased power deposition in the reflector and shield can dictate more complex designs, less favorable material choices and/or require additional design features to accommodate cooling.  In addition, higher fluences can cause degradation of control element bearings and reactor instrumentation.

Simplified reactor control:  Lower power reactors will experience less distortion due to temperature gradients.  This could significantly reduce the time and effort put into understanding reactor dynamics, or designing more elaborate core structures to control distortion.  In addition, compact reactors have numerous control technology options and more potential control element worth.  This could make it easier to tailor the control elements to allow a simpler, more robust control system.  Lower power reactors will also have less chance of control elements bowing/sticking/failing because of lower fluence and lower thermal distortion and stress.  This could eliminate a layer of complexity and/or redundancy in the control system design.  Finally, lower power reactors have less burnup reactivity swing, which makes overall control issues simpler.

Simplified decay heat removal:  At low power, decay heat removal becomes much easier.  Lower power systems (~400 kWt) are more likely to utilize passive decay heat removal.  Lowest power systems (~50 kWt) may not even require any special design considerations or complexity to remove decay heat.

Simplified system integration:  At higher powers, spacecraft system components will more likely be pushing the envelope of performance, and have less flexibility to meet changing demands set by other components.  Reduced flexibility translates to longer development time and lower probability of success.  Potential changes in interface requirements could significantly impact the reactor development time and risk.  Lower power may also simplify steady-state and transient thermal issues for the entire power system and spacecraft, and reduce the impact of thermal cycles.  This may be one the most complex engineering issues of an early-flight system, and lower power will make it easier to model, predict, and test.

Reduced size and complexity of mission:  One of the biggest risks of attempting to develop a higher power reactor is that a spacecraft and mission must be designed to utilize it.  Higher power adds cost and risk to almost every spacecraft component, including the power conversion, management, distribution, and rejection systems.  Also, a mission that can utilize the benefit of increased power is likely to use a more costly and risky payload.  In the near term, there is a higher probability of finding missions to utilize lower-power systems. 

There are numerous reasons presented why a lower-power fission system could require significantly less development time than a higher-power system.  Many of the issues individually may not have significant impact, but as a whole they present a compelling argument.  Some of the issues presented could turn out to be non-issues upon further examination, but further examination in itself can increase development time.  In general, the reason to develop a low-power system is to reduce the probability that some of the issues above will significantly impact program progress and success.

Upon the investigation of stainless-steel, UO2 entry-level fission systems, it was found that there were numerous knees-in-the-curve that could complicate system development at higher powers. Fortunately, it was found that at a power level of ~200 kWt and a lifetime of 10 years, there was a sweet spot prior to where most of the knees occurred. A discussion of these issues and limits is in the Entry_Level_Sweet_Spot reference.