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.

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

Fission Surface Power (FSP) systems are well suited to be the workhorse of human exploration infrastructure on the Moon, Mars, or other potential destinations (e.g. Titan, large asteroid).  Some potential surface power electrical loads include habitats, in-situ resource utilization plants, rechargeable rovers, construction equipment, and science experiments.  The power output of a single workhorse surface system might be in the range of 20 to 50 kWe, with a lifetime of ~5 to 10 years. In addition, the relatively low-power and “high” mass allowance of a surface reactor might make it the easiest space fission system to develop.  For a human surface exploration mission, fission power has so many advantages over other power alternatives that the mass requirements will not be as stringent.  This allows “mundane” reactor technologies to be used (i.e. stainless-steel, UO₂) at very benign power/flux levels, which makes development simple.  Plus, as part of a “heavy” mission architecture, there should be less programmatic pressure to meet mass targets, or worse, decrease mass during development—a contributor to the downfall of the SP-100 program. 

There are many possible reactor and power conversion technologies available for an FSP system.  Stirling power conversion is generally considered the best option for systems in the 20 to 50 kWe range, with Brayton power conversion systems generally preferred at higher power.  Also, it is a general consensus that a fast reactor using UO₂ fuel and stainless-steel cladding and structure provide the lowest cost and risk system.  Heat transport for this class of Stirling system is via heat pipes or pumped liquid metal.  For a higher-power Brayton system, a gas-cooled reactor should also be strongly considered. These technologies could all be developed affordably; each having specific strengths and weaknesses. The liquid metal systems have issues with pump technology and system reliability, while heat pipe systems have some fabrication and integration issues, and gas-cooled systems are generally heavy.  For more information see FSP_Reactor_Compare and FSP_Cooling_Pros_and_Cons references.

In 2008, NASA and DOE developed a 40 kWe Stirling-based FSP concept that utilized a pumped liquid metal (NaK-78) reactor with a stainless steel and UO₂ fuel system.   This concept was designed with all of the attributes required for an entry-level space fission system.  The power level and lifetime fall comfortably below the knees-in-the curve that could affect development (e.g. irradiation damage, reflector cooling, fuel burnup, passive safety).  For more information on the reactor module see FSP_Reference_Reactor and for the complete system see FSP_Reference_System.  NASA is expected to complete an electrically-heated Technology Demonstration Unit (TDU) in 2013. A successful demonstration will place the system at TRL 6 and ready for flight development. The TDU will also be the first step in a series of non-nuclear tests that, when combined with component irradiation testing, enable a zero-power critical test to be used to qualify the flight system without a nuclear-powered system test.  For more info see the FSP_TDU reference.

Surface power radiation shielding is quite different from a space power mission, where a shadow shield can be used. This is because of scattering from the regolith and spacecraft components, so the shield design usually needs to surround the entire reactor. As a result, surface reactor shields can be very heavy, and can dominate the system mass. Fortunately there are many potential options for using in-situ resources as shielding materials, which can provide substantial mass savings. Regolith could be used in several ways: berming, sandbagging, digging/burying, using natural topography (e.g. craters), or any combination of these. In-situ resources could also be used to fill permanent structures or cans as a fixed part of the reactor structure (e.g. sand could be scooped or vacuumed), and in the most optimistic scenarios in-situ water could fill shield tanks or perhaps concrete could be made. All shielding options that require in-situ resources will depend on significant robotic ability (or existing robotic/human infrastructure) to successfully complete the mission. This could add significantly to mission risk, but the mass savings potential is so large that in-situ options warrant serious consideration. Any option that can pre-deploy and verify in-situ shielding prior to human mission departure would have a significant advantage. A comparison of deployment and shielding architectures can be found in the FSP_Deployment_Architectures and FSP_Shielding_Options references.

The reference FSP concept was designed for use on the Moon, but with the idea of also being used on Mars. There are a few differences between the applications, which are discussed in the FSP_Moon_to_Mars reference. A comparison of the estimated mass of solar and fission lunar/Mars power systems is shown below.

 

Power System	Specific Power (W/kg)
40 kWe Solar + Storage on Lunar Surface (equator)	0.6 W/kg (with 100% power at night) 1.3 W/kg (with 50% power at night)
40 kWe Solar + Storage on Mars Surface (clean surface and atmosphere)	5 W/kg (with 100% power at night) 8 W/kg (with 50% power at night)
40 kWe Solar + Storage on Mars (dust storm, effective insolation 100 W/m2??)	1.8 W/kg (with 100% power at night) 2.8 W/kg (with 50% power at night)
40 kWe FSP system: SS/UO2, NaK-cooled, Stirling conversion.	8 W/kg (with shaped or regolith assisted shield)

These solar power system specific masses are based on 25% efficient, sun-tracking photovoltaic arrays with 50% round-trip efficient regenerative fuel cells.  Despite being closer to the sun, the Moon is in many ways more challenging for solar power than Mars because of the cold 14 days of darkness. The fission system is dramatically better than solar on the lunar equator, and also substantially better at the poles (not shown in the table above).  On Mars, the solar system that provides only 50% power at night is about the same mass, but this does not include the risk of major, prolonged dust storms.  Also, in both cases when higher power systems are utilized, the specific power of the fission system becomes much more attractive.  Ultimately, second generation technologies can provide fission systems with substantially higher specific power.

In summary, the FSP’s combination of mission need and simple development makes it an ideal entry-level space fission system.  As such, this system has seen the majority of work over recent decades. Please refer to the documents referenced within for more detail on the system and the development approach.

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

A space fission system that utilizes a low power space reactor (LPSR) meets the two primary criteria of an entry-level space fission system: 1) a near-term space exploration need and 2) an affordable development approach. The need for a LPSR system is driven by NASA’s need to continue and expand its robotic exploration of the solar system. NASA has dependably relied on radioisotope systems to power science missions for many decades; however, there is no current supply of Pu-238 to meet NASA’s near-term exploration goals.  Because of this, NASA prompted DOE to restart production of Pu-238, although the present effort will produce only a small fraction of NASA’s anticipated needs, and not for several years.  Even if an option existed that could produce all the Pu-238 that NASA would like, the integrated cost of producing the power sources would inhibit the number/level of missions that NASA could pursue. To mitigate the Pu-238 supply/cost issue, NASA has invested in Stirling conversion technology, which can provide ~4 times the electrical output per gram of Pu-238 as compared to thermoelectrics.  This approach appears to be on a successful track, but does not solve the underlying problem.  Even with Stirling engines, a long-term supply of Pu-238 to meet NASA’s exploration needs will require substantial investment, and the program would be threatened continuously by shifting political and programmatic forces.  Furthermore, all of this effort would simply maintain current exploration capabilities, whereas NASA’s historical charter has been to expand our ability to explore space.

As an alternative, a simple fission system based on existing technology could provide robust, long-lived power at 0.5 kWe, 1 kWe, or whatever might be optimal for a specific exploration goal.  The mass of an LPSR system would be similar to a radioisotope system, but more importantly it would likely be of lower cost.  The fission system would be fueled with enriched U-235, which currently exists in substantial quantity and if needed can be made relatively inexpensively. In addition to solving the Pu-238 cost issue, the fission system would also provide NASA with a technology that could grow into a much more ambitious space program; e.g. orders of magnitude more science/communications, outer planet orbiters, Mars surface power, nuclear propulsion, etc.  Fission systems have other advantages, in that they do not have to be cooled during all pre-deployment operations (fabrication, assembly, transport, storage, on the launch pad, etc.), they can load follow power demand, and the lack of radioactivity prior to deployment eliminates the need to mitigate the risk of transport/launch dispersal accidents.

Since the 1970s, there have been several failed US space reactor programs, primarily due to the high cost, risk, and time required for nuclear development and testing. The thermal power of a ~1 kWe system is so low that it can introduce three major advantages.  First, existing technology can be used for all components (and the irradiation environment will be benign).  Second, the burnup of fuel is so small that the loss of reactivity is negligible even over a 10+ year mission, which means that no reactor control is required after initial startup (this improves reliability and decreases development cost).  Even without reactor control thermal power will passively follow the demands of the power conversion system.  Third, the thermal power is low enough to allow a unique testing approach that is not afforded by higher power systems – a low-cost nuclear-powered system test.   This last advantage has the potential to reduce the cost of demonstrating nuclear-powered operation of a space fission power system from ~$1B to <<$100M.  Zero-power critical testing is considered an essential element of all envisioned space reactor programs, as it provides valuable nuclear data at relatively low cost and risk.  The costs of these tests are generally 2 orders of magnitude lower than those estimated nuclear-powered tests of typical space power reactors. At the low thermal power of a LPFS, nuclear-powered system operation (e.g. system startup to full power) might fall within the scope of existing critical test facilities and procedures.

A design approach was developed for LPSR concepts based on anticipated requirements and uses, which is referenced in LPSR_Design_Approach. Science mission power reactors will likely be coupled to either thermoelectric or Stirling power conversion systems. The choice of power conversion might be driven by the “usual” parameters – mass, heritage, reliability, etc., but in this case there might be a strong desire to use Stirling engines in order to keep the thermal power within the scope of critical testing. A concept study of a 1-kWe thermoelectric LPFS was conducted in 2009 by NASA/DOE. The system uses a solid core of U-MO fuel that conducts fission power to Na heat pipes.  The heat pipes travel through the shield to thermoelectric generators. The identical reactor core could also be attached to Stirling engines to produce 3 to 5 kWe. This concept is referenced in LPSR_Reference_System and LPSR_Reference_reactor.

An even lower power reactor concept that provides 500 We via Stirling engines has been developed because a <2 kWt reactor is almost certain to fall within the test envelope of existing facilities. This system directly meets the power needs for near-term NASA space exploration missions, and may be the simplest and most affordable. This concept, referred to as the very low power fission system (VLPFS) is discussed in the LPSR_VLPSR reference.

The only factor that could complicate the development of a LPFS would be a requirement to reduce mass below what could be obtained with existing technology.  In addition to keeping the power system mass low, there are likely to be given stringent dose requirements, which will drive the design to low-mass shielding materials, tight shadow-shielded geometries, and large payload separation distances.  The table below shows the mass of two Pu-238 systems along with 3 possible first generation LPSR systems; the mass in parenthesis includes a shield that reduces dose to allow the use of off-the-shelf electronics (25 kRad).

Engineered System	Alpha (kg/kWe)
120 We Pu-238 thermoelectric (MMRTG)	410
140 We Pu-238 Stirling (ASRG)	180
500 We 1st generation LPSR-Stirling	tbd
1 kWe 1st generation LPSR-Thermoelectric	~420 (770 with COTS electronics shield)
10 kWe 1st generation LPSR-Stirling	~100 (170 with COTS electronics shield)

In conclusion, if the thermal power level can be kept low, and the mass requirement allows existing technology to be used, then a LPSR system offers the lowest cost, shortest path to a nuclear-powered system demonstration (realistically <3 years for <$100M).  A LPSR system also has the nearest term potential usage by NASA, with missions identified that could use the system as soon as it is available.

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

The previous set blog posts were focused on the need for space fission power (SFP) systems and the entry-level systems that could establish their use in space.  There are numerous potential applications of SFP systems that could be performed by second generation systems; i.e. systems that are hard to develop affordably from scratch, but rather easy to develop after the technology, infrastructure, and programmatic precedent has been established for an entry-level system. One of the main anticipated uses of second generation systems are high power science mission systems that produce 10 kWe to 200 kWe at specific powers of 25 to 100 W/kg. This class of reactor might be referred to as “traditional” space reactors, which has been the focus of two significant US space reactor efforts: SP-100 and JIMO.  These SFP concepts were deemed too expensive to continue development generally because of material issues associated with high temperatures and the need for a nuclear-powered ground test.

First and second generation SFP systems have many potential uses, some of which are included below.

• Ambitious space science and exploration.

-        Mars/lunar/asteroid surface power/propulsion (robotic and manned).

-        Outer-planet missions: Jupiter/Saturn moon orbiters/landers, Pluto orbiter, etc.

-        Interstellar precursor or near-Sun missions.

• Enhanced national and planetary defense.

-        High power and enhanced mobility for defense applications.

-        Potential use for comet/asteroid defense.

-        Synergy with advanced terrestrial and airborne defense systems.

• Significant commercial value.

-        Satellite power, mobility, maintenance, retrieval.

-        Space junk sweeper.

-        Space tourism (orbital, lunar, ?).

-        Eventually, resource extraction and delivery from Moon or asteroids.

The list of candidate reactor and power conversion technologies for second generation SFP systems is very broad, and attempting to summarize these options in a short blog post is impractical. In addition, the best technologies for specific missions are dictated by requirements, and it is hard to discuss/compare second generation technology options in the absence of a defined application.  Finally, it is best to keep our focus on entry-level systems until we establish the use of fission power in space; then we can turn our attention to utilizing the vast potential of space fission power.

Space Fission Power Post #9:Nuclear Thermal Propulsion (NTP)

Solid-core nuclear thermal rockets (NTRs) have the potential for specific impulse (ISP) a factor of 2 better than the best chemical propulsion can offer (NTRs can potentially offer up to 1000 seconds ISP).  This added efficiency can potentially be used to decrease trip times, carry larger payloads, and/or decrease the number of heavy launches from Earth. A significantly shorter trip time is not practical for near-term Mars missions, so the benefit is generally accepted as additional (~2x) payload delivered per kg launched into Earth orbit.  Therefore, the near-term benefit of an NTR is directly correlated with the cost per kg of Earth-to-orbit launches; the higher the cost of launching from Earth, the greater the attractiveness of NTP, and vice-versa.

When compared to Nuclear Electric Propulsion (NEP), the performance advantage of an NTR is higher thrust-to-weight, although with a much lower ISP. The technical advantage of an NTR over an NEP system is that the balance of plant is much simpler for an NTR.  This feature also presents a major disadvantage, because all of the technical risk and development is within the reactor (i.e. nuclear in nature), as opposed to non-nuclear.  Thus, NTR systems cannot utilize the development approach of using a simple reactor (with low nuclear technical risk), and transferring the development risk to non-nuclear technologies and system demonstrations. 

In a solid core NTR, propellant/coolant flows through the reactor core, and thrust is created by expanding the propellant through a nozzle. Hydrogen is usually used as the propellant because the low atomic mass causes the greatest exhaust velocity (i.e. ISP); however, hydrogen is not a preferred reactor coolant because it is highly corrosive, dangerous to handle, and it requires cryogenic storage.  The greatest challenge of an NTR is the extremely high temperature core to provide hydrogen exit temperatures of >2500 K. Materials must be found that have sufficient mechanical properties to support thermal and vibrational stresses at these temperatures while also withstanding possible erosion/corrosion with fast flowing hydrogen. There are numerous fuel forms that can be considered for NTRs, the most extensively studies and tested are the graphite-based fuels of the ROVER/NERVA programs.  In more recent decades, there has been a good deal of research in cermet-based cores and pebble-bed cores.

The component technologies developed during the ROVER/NERVA programs were very impressive and fairly mature, and the existence of the previous work certainly makes the development of NTRs much more feasible than if the program had never taken place, but this does not mean that a modern day development program will be easy.  The fact is that back in the NERVA program they were still testing and developing; and they were in some ways backtracking from system testing to special effects testing of fuel – because fuel erosion was still a major issue.  There was no flight-system ready to fly (certainly not one that would be approved to fly in today’s environment); they were close, but the ability to get back to that point, and proceed from there would be a difficult task.  There are 3 things missing from the old NERVA program that eliminate the potential for affordably developing a NERVA-based NTR. 

1) Physical Infrastructure.  The manufacturing and testing infrastructure that existed at the end of the NERVA program makes any nuclear development capability that exists within DOE or NASA today look miniscule. 

2) Knowledge and Brainpower.  There were hundreds of design, manufacturing, fabrication, testing and system experts working on NERVA in the ‘70s.  Not only are most of these people gone, there has been relatively little knowledge transfer since this was before the era of digital record keeping, and use of the technology has not been continuous.   Back then, nuclear development was more art and elbow-grease, as compared to science and CPUs; they built, tested and tweaked things and used their ingenuity to make things work.  There are many examples of people trying to reproduce products (materials, parts, systems) developed 50 years ago that fail, likely because there were some steps in the process that workers did not record, or simply took for granted (but they would not be intuitive to someone starting from scratch today).

3) Political Environment.  This issue is more than who is in power in Washington, D.C., at any particular moment.  Unfortunately, an anti-nuclear bias is firmly entrenched in the US and the world.  The bias is generally based on an uninformed fear of the dangers of radiation, which unfortunately is perpetuated by the media because exploiting this fear gets ratings (e.g. Fukushima, where there were no deaths and likely no future deaths caused by radiation, but the nuclear disaster got more press and caused more panic than the killing of ~20,000 people by the quake/tsunami).  These fears have made the cost and risk of getting approval to run a nuclear-powered reactor test at least an order of magnitude higher than back in the 1970s.  A program that relies on a nuclear-powered demonstration might have a chance in today’s environment (i.e., a nuclear test that you are confident will work), but one that requires iterative nuclear tests is generally unthinkable without a huge long term cost commitment (on the order of billions of dollars per year for a decade).  In addition to development, launch approval is another factor where politics, public perception can kill a program.  Nuclear material safeguards are a significant issue for all space reactors (except for a small class of low-performance reactors that could reduce the safeguards category).  The difficulty in creating an acceptable nuclear and launch accident safety case is a function of system power and the drive for low mass, and an NTR is much more difficult than a modest power reactor in this respect. Another final programmatic factor that makes NTR development extremely difficult is time - a realistic program will take more than a decade (at least 2x the development time of a simple power system).  Given the realities that occur in Washington D.C., a program that takes over 5 years is risky and a program that takes over 10 years is essentially a non-starter (even the incredibly ambitious Apollo program went from conception to landing a man on the moon is 9 years).

If all 3 of the above issues could be reset back to where they were in the early 1970s then an iterative nuclear test program could be feasible within a realistic NASA program.  However, even if #1 and #2 were still available, #3 makes the costs and programmatic risks unacceptable to any program other than a national-mandate program like Apollo.  The fact is that an adequate human exploration program can be performed with chemical propulsion, so it is unlikely that a nationally mandated NTR program would occur in the foreseeable future.

In conclusion, a nuclear thermal rocket would have significant benefit to NASA (albeit not as much as other fission systems), but it does not warrant more than a small fraction of near-term funding until an entry-level fission system is established in space.

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

A high performance NEP reactor offers more potential benefit to long-term human space exploration than any other space reactor application. The reactor for a high-performance NEP systems would power a multi-megawatt (MMW) system (>>1 MWe) with a very low specific mass (<< 10 kg/kWe). Whereas NTRs offer a factor of 2 performance benefit over chemical rocket technology, a higher performance NEP system can offer orders of magnitude better performance (ISP) – a truly enabling and paradigm shifting technology.  Advances are continuously being made in all of the non-nuclear technologies needed for an NEP system, most importantly power conversion, heat rejection, and thrusters.  Significant advances are being made in ion thrusters (HiPEP) and plasma thrusters (e.g. VASIMR), such that the deployment of high power EP systems appear possible within a decade or two.  Conversely, a reactor for this kind of NEP system is essentially no closer to deployment than it was 50 years ago; which is another testament that non-nuclear system development is vastly easier than nuclear system development.  The difficulty in this reactor is not achieving high power, but delivering power with a very low mass system, which implies very high temperature operation.

A high-performance MMW reactor has some of the same issues as an NTR, most importantly the fuel.  An MMW system will need a very high temperature fuel with a high uranium density (to allow low mass).  Additionally, this fuel will have to go to high nuclear burnup, because every additional U-235 atom burned increases the energy density achieved by the system.  A high temperature structural material that can withstand significant irradiation damage must be found, which is also compatible with the fuel form.  Once these two major reactor issues are resolved, a high temperature, high efficiency power conversion system must be developed that can integrate well with the reactor.  On the plus side, the balance of the system can be developed in a non-nuclear fashion, to some extent utilizing the lessons learned and infrastructure from the earlier development of entry-level and second generation systems.  Even then, the high power density of the core might make resistance-heated system testing impractical, so that a nuclear-powered ground test will be needed; unless several smaller reactors had been previously qualified and operated, and the data from those systems could be used to qualify the MMW system.

There are many potential technologies that have been proposed for low-mass, high temperature MMW space reactors.  Refractory-metal gas-cooled reactors have been seriously considered in previous programs, with high temperature fuel in a particle/pellet configuration.  This reactor could then be coupled to a refractory Brayton power conversion system.  High-temperature liquid-metal cooled reactors (e.g. Li) have also been considered; coupled to potential liquid-metal Rankine systems or perhaps a refractory Brayton system.  Both these options might offer hot-end temperatures between 1300 K and 1600 K, which is high enough to enable a high efficiency thermodynamic cycle while utilizing a high temperature radiator.  For an MMW reactor, radiator size is one of the most important issues, to keep mass low and minimize issues such as deployment, shielding (scatter), and maneuverability. Thermionic systems are a possible candidate, in that they offer very high temperatures with reasonable efficiencies. Molten salt reactors have many attractive attributes (potential for low-mass, long-lived reactors) but a salt would need to be found that can reliably operate at temperatures high enough to make an attractive system. Finally, on paper, MHD power conversion systems could provide very attractive ultra-high temperature performance. Ultimately, this space fission system class (MMW NEP) has several sub-classes within it, and hopefully would evolve to higher and higher performance systems as they evolve.

It is debatable which propulsion system would be easier and more affordable to develop: NTP or MMW NEP. Starting from where we are today, an NTP system might be easier to develop; although both systems are well beyond the bounds of what could be an affordable system. If an evolutionary approach is undertaken for space fission power reactors, then eventually the MMW NEP system would be much easier to develop. Solar electric propulsion (SEP) systems could also serve as precursor NEP systems.  What is not debatable is that the MMW NEP system increases our ability to explore and expand into space much more than an NTP system.