Lee Mason

Glenn Research Center, Cleveland, Ohio

John Casani, John Elliott, Jean-Pierre Fleurial, Duncan MacPherson, and Bill Nesmith

Jet Propulsion Laboratory, Pasadena, California

Michael Houts

Marshall Space Flight Center, Huntsville, Alabama

Ryan Bechtel

Department of Energy, Germantown, Maryland

Jim Werner

Idaho National Laboratory, Idaho Falls, Idaho

Rick Kapernick and David Poston

Los Alamos National Laboratory, Los Alamos, New Mexico

Arthur Lou Qualls

Oak Ridge National Laboratory, Oak Ridge, Tennessee

Ron Lipinski and Ross Radel

Sandia National Laboratories, Albuquerque, New Mexico

Sterling Bailey

Consultant, Grass Valley, California

Abraham Weitzberg

Consultant, Woodland Hills, California

A Small Fission Power System for NASA

Planetary Science Missions

NASA/TM—2011-217099

December 2011

NETS–2011–3318
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientifi c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Offi cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientifi c 

and technical fi ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientifi c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientifi c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question via the Internet to help@

sti.nasa.gov

• Fax your question to the NASA STI Help Desk 

at 443–757–5803

• Telephone the NASA STI Help Desk at

 443–757–5802

• Write to:

 NASA Center for AeroSpace Information (CASI)

 7115 Standard Drive

 Hanover, MD 21076–1320
Lee Mason

Glenn Research Center, Cleveland, Ohio

John Casani, John Elliott, Jean-Pierre Fleurial, Duncan MacPherson, and Bill Nesmith

Jet Propulsion Laboratory, Pasadena, California

Michael Houts

Marshall Space Flight Center, Huntsville, Alabama

Ryan Bechtel

Department of Energy, Germantown, Maryland

Jim Werner

Idaho National Laboratory, Idaho Falls, Idaho

Rick Kapernick and David Poston

Los Alamos National Laboratory, Los Alamos, New Mexico

Arthur Lou Qualls

Oak Ridge National Laboratory, Oak Ridge, Tennessee

Ron Lipinski and Ross Radel

Sandia National Laboratories, Albuquerque, New Mexico

Sterling Bailey

Consultant, Grass Valley, California

Abraham Weitzberg

Consultant, Woodland Hills, California

A Small Fission Power System for NASA

Planetary Science Missions

NASA/TM—2011-217099

December 2011

NETS–2011–3318

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Nuclear and Emerging Technologies for Space (NETS-2011)

cosponsored by the ANS Aerospace Nuclear Science and Technology Division, the ANS Trinity Section,

and the AIAA

Albuquerque, New Mexico, February 7–10, 2011
Acknowledgments

This work was performed for the Decadal Survey Giant Planets Panel (GPP) in coordination with the National Aeronautics and 

Space Administration (NASA) Science Mission Directorate. The authors wish to acknowledge the contributions and support of 

the study sponsors including Heidi Hammel (GPP Chairperson) and Len Dudzinski (Planetary Science Division). The study was 

coordinated through the NASA Glenn Research Center Radioisotope Power System Program Offi ce managed by John Hamley. 

The quality results would not have been possible without the involvement of Scott Harlow from Department of Energy (DOE) 

Headquarters who assembled the DOE team, coordinated their participation, and reviewed the fi nal products.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2011-217099 1 

A Small Fission Power System for NASA Planetary Science Missions 

Lee Mason 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

John Casani, John Elliott, Jean-Pierre Fleurial, Duncan MacPherson, and Bill Nesmith 

National Aeronautics and Space Administration 

Jet Propulsion Laboratory 

Pasadena, California 91109 

Michael Houts 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, Alabama 35812 

Ryan Bechtel 

Department of Energy 

Germantown, Maryland 20874 

Jim Werner 

Idaho National Laboratory 

Idaho Falls, Idaho 83415 

Rick Kapernick and David Poston 

Los Alamos National Laboratory 

Los Alamos, New Mexico 87545 

Arthur Lou Qualls 

Oak Ridge National Laboratory 

Oak Ridge, Tennessee 37831 

Ron Lipinski and Ross Radel 

Sandia National Laboratories 

Albuquerque, New Mexico 87185 

Sterling Bailey 

Consultant 

Grass Valley, California 95945 

Abraham Weitzberg 

Consultant 

Woodland Hills, California 91367 
NASA/TM—2011-217099 2 

Abstract 

In March 2010, the Decadal Survey Giant Planets Panel (GPP) requested a short-turnaround study to 

evaluate the feasibility of a small Fission Power System (FPS) for future unspecified National 

Aeronautics and Space Administration (NASA) science missions. FPS technology was considered a 

potential option for power levels that might not be achievable with radioisotope power systems. A study 

plan was generated and a joint NASA and Department of Energy (DOE) study team was formed. The 

team developed a set of notional requirements that included 1-kW electrical output, 15-year design life, 

and 2020 launch availability. After completing a short round of concept screening studies, the team 

selected a single concept for concentrated study and analysis. The selected concept is a solid block 

uranium-molybdenum reactor core with heat pipe cooling and distributed thermoelectric power converters 

directly coupled to aluminum radiator fins. This paper presents the preliminary configuration, mass 

summary, and proposed development program. 

Nomenclature 

CBE current best estimate 

CD Concept Development 

DOE Department of Energy 

FFTF Fast Flux Test Facility 

FPS Fission Power System 

FSP Fission Surface Power 

GPHS general-purpose heat source 

GPP Giant Planets Panel 

GRC Glenn Research Center 

HEU highly enriched uranium 

HGA high-gain antenna 

INL Idaho National Laboratory 

JPL Jet Propulsion Laboratory 

LA Launch Approval 

LANL Los Alamos National Laboratory 

MSFC Marshall Space Flight Center 

NASA National Aeronautics and Space Administration 

NEPA National Environmental Policy Act 

ORNL Oak Ridge National Laboratory 

RF radiofrequency 

RTG radioisotope thermoelectric generator 

SNL Sandia National Laboratories 

Study Overview 

The small Fission Power System (FPS) study was initiated in March 2010 following a request from 

the Decadal Survey Giant Planets Panel (GPP) and the NASA Science Mission Directorate. A 6-week 

study was commissioned to evaluate the feasibility of a 1-kWe-class FPS for future NASA science 

missions. The study was assigned to Glenn Research Center (GRC) and a study team was formed that 

included participants from the Jet Propulsion Laboratory (JPL); Marshall Space Flight Center (MSFC); 

Department of Energy (DOE) Headquarters; and the DOE National Laboratories at Idaho (INL), Los 
NASA/TM—2011-217099 3 

Alamos (LANL), Oak Ridge (ORNL), and Sandia (SNL). The study scope included a review of 

component technologies and concept options, a down-selection to a single concept for concentrated study, 

and a preliminary assessment of the system performance, configuration, mass, verification strategy, and 

development schedule. The team was also asked to consider the extensibility of the concept to higher 

power levels. The effort culminated in a presentation to the GPP on April 16, 2010, and a narrative report 

completed in mid-May. 

Ground Rules and Assumptions 

At the outset of the study, a set of notional requirements was developed and concurred upon by the 

study sponsors. The requirements were broadly defined to cover a range of potential Giant Planet science 

missions. An over-arching requirement was that the FPS would be safe for launch and operations, 

including assured subcriticality prior to commanded reactor startup. The major parameters that affect 

technology selection are power level, lifetime, and launch date. For this study, these were chosen as 

1-kWe continuous output, 15-year design life, and 2020 launch availability. Several spacecraft integration 

requirements were also adopted. These included the reactor-induced radiation dose 

(1.0×1011 n/cm2

 and 25 krad), payload envelope size (4.5 m diameter), and electrical bus voltage 

(28 Vdc). The radiation limits were selected to allow the use of commercially available electronics at the 

payload dose plane. To better accommodate the radiation dose limits, the payload was assumed to be 

located at a specified separation distance from the reactor within a defined cone angle. This geometry set 

the parameters required to determine the dimensions and mass of the radiation shielding. 

In addition to the assumptions stated above, the study team agreed on several goals for the FPS. 

While no mass or volume metric was set, minimizing these parameters would make the system more 

attractive to potential missions. At the same time, the team judged that low cost and low risk (both 

developmental and operational) were even more important than mass. An additional goal was to minimize 

the system-related accommodations imposed on the spacecraft. These might include the need for control 

actions, operational constraints, additional power sources, or extension booms. Another key FPS goal 

expressed by the team and echoed in the study objectives was the potential for mission extensibility, 

especially as it relates to upward power scalability. 

Technology Options 

Since the advent of the nuclear age, researchers, engineers, and scientists have been exploring and 

testing many forms of nuclear fuels, structural materials, shielding materials, and design configurations. A 

wealth of knowledge and performance capabilities have been established and are available for 

consideration in design options. A listing of the various component technologies and design approaches 

that were considered in developing potential system concepts is provided in Table 1. 

TABLE 1.—POTENTIAL TECHNOLOGY OPTIONS CONSIDERED 

Category Options 

Reactor fuel Oxide (UO2), Metal (UMo, UZr), Nitride (UN), Carbide (UC), Cermet, UZrH 

Fuel form Pins, Plates, Block, Spheres 

Neutron spectrum Thermal, Epithermal, Fast 

Cladding/Structure Stainless Steel, Ni-Based Superalloy, Refractory Alloy 

Control materials Be, BeO (Neutron Reflecting), B4C (Neutron Absorbing) 

Mechanism Rods, Drums, Shutters, Sliders 

Heat transport Conduction, Heat Pipe, Pumped Liquid Metal, Gas 

Reactor heat transfer fluids Sodium, Potassium, NaK, Lithium, HeXe, CO2

Shield materials LiH, B4C, Tungsten, Depleted Uranium, Lead, Water, Polyethylene, Aluminum 

Power conversion Thermoelectric (SiGe, PbTe/TAGS, Skutterudites, Zintl/LaTe/SKD), Stirling, 

Brayton (HeXe, CO2) , Rankine (Organic, Hg, K) 

Heat rejection Direct Radiation, Heat Pipes, Two-Phase Loops, Pumped Liquid Loop 

Radiator fluids Water, Ammonia, Fluorocarbons, Hydrocarbons 

Radiator materials Aluminum, Polymer Composites, Titanium 
NASA/TM—2011-217099 4 

Typical uranium fuel types range from oxides (i.e., UO2), the most commonly used form of fuel in 

terrestrial power reactors, to metal fuels (i.e., UMo and UZr), common to fast reactor technologies, to 

other chemical fuel forms that have specialized performance characteristics (e.g., nitride (UN), carbide 

(UC), cermet, and UZrH). These fuels have been evaluated from a mechanical/physical standpoint and 

performance tested in various forms (e.g., pins, plates, blocks, and microspheres) under expected nuclear 

irradiation conditions. Consequently, a number of fuel types and configurations could be considered for 

this space reactor application. 

The U.S. experience with mixed oxide and metal fuels is substantial, comprising irradiation of over 

50,000 oxide fuel rods and 130,000 metal fuel rods in the Experimental Breeder Reactor-II (EBR–II) and 

the Fast Flux Test Facility (FFTF) alone. Most types of fuel have been demonstrated capable of fuel 

utilization exceeding 10 percent fuel burnup. To varying degrees, life-limiting phenomena for all types 

have been identified and investigated. For this study there are no disqualifying safety-related fuel 

behaviors that would reduce the viability of any of the fuels for use. However, fuel density becomes a 

dominant factor when designing very small reactor systems. Likewise, reactor configurations utilizing 

different neutron spectra for maintaining nuclear criticality have been studied and well established. 

Nuclear reactor designs basically fall into three categories: 1) Thermal: fission neutrons are rapidly 

slowed down to thermal energy ranges before the next fission reaction occurs (e.g., water-moderated 

reactors), 2) Epithermal: neutrons lose most of their energy before next fission (e.g., graphite-moderated 

and gas-cooled reactors), and 3) Fast: neutrons are absorbed with minimal loss of energy prior to the next 

fission (e.g., sodium-cooled reactors). The most “neutron-efficient” reactors are thermal systems; 

however, the smallest, most lightweight reactors have fast spectra. 

Most nuclear fuel cladding and structural materials are made of zirconium alloys and stainless steels. 

Nickel-based superalloys have also been extensively tested in the nuclear industry where higher 

temperatures and operating pressures are desired, and a variety of reactor system applications have been 

pursued. Refractory alloys have been explored and a large amount of data exists regarding performance of 

these materials under high-temperature and high-radiation environments, but their use would require 

additional development and fabrication investments. 

Control of a nuclear reactor system is achieved by utilizing materials that reflect neutrons back into 

the core (e.g., water, graphite, beryllium (Be), or beryllium-oxide (BeO)) and materials that absorb 

neutrons (e.g., boron (B) and boron carbide (B4C)). For small lightweight reactors, Be or BeO are more 

suitable reflector materials, and B4C is more suitable for the neutron absorber. A variety of combinations 

of these two materials have been employed in test reactors as well as previous space designs. Typical 

control options include the use of rods (pins are inserted in the reactor), drums (a section of the drum 

surface has absorbing material and is located outside the reactor core), shutters (reflector or absorbing 

materials are arranged outside the reactor core and their position adjusted to either allow more neutron 

leakage or exposure to neutron absorbing material), or sliders (reflector or absorbing materials are 

arranged outside the reactor core and the materials are made to slide around the core to either allow more 

neutron leakage, reflection, or exposure to neutron absorbing material). 

Reactor heat transport is typically accomplished by flowing a liquid or a gas through the core. 

Coolant options for this type of system include sodium (Na), potassium (K), NaK, lithium (Li), heliumxenon (HeXe), or carbon dioxide (CO2) as the heat transfer fluid. All reactors that transport heat from a 

reactor core using liquid or gas require a circulation pump or rely on gravity to induce circulation by 

convection. A novel approach of using heat pipes for primary heat transport has been investigated over 

the past 30 years and a significant amount of testing has been accomplished in both radiation and 

nonradiation test environments. However, a heat-pipe-cooled reactor has not yet been built or operated. 

A large variety of shielding materials have also been developed and tested over the years. Effective 

shielding requires the use of a specific class of materials to reduce the radiation from neutrons using lowmass elements (e.g., hydrogen (H), Li, and B), and from beta and gamma radiation using higher mass 

elements (e.g., tungsten (W), depleted uranium (DU), and lead (Pb)). The shielding material selections are 

also influenced by other design requirements such as operating temperature, heat load, structural stability, 

and material compatibility. 
NASA/TM—2011-217099 5 

Four potential power conversion technologies were considered for the small-space FPS application: 

dynamic energy conversion using the Brayton, Stirling, or Rankine cycle and static thermoelectric 

conversion. The use of stainless steel or superalloy materials for the hot end limits the maximum 

operating temperature to approximately 1100 K or less. If refractory alloys are utilized, this limit could be 

extended to 1300 or 1400 K. 

Space Brayton converters are a closed-loop derivative of an air-cycle gas turbine in which an inert gas 

working fluid (such as HeXe) is recirculated through a turbine and compressor coupled to a rotary 

alternator. Thermal input is achieved by either direct heating of the gas in the reactor (gas-cooled reactor) 

or through an intermediate heat exchanger. A recuperative heat exchanger improves cycle efficiency 

using the hot turbine exhaust gas to preheat the working fluid before it returns to the heat source. Space 

Rankine converters are derived from conventional steam power plants, in which the working fluid is 

superheated in a boiler, expanded in a turbine, and liquefied in a condenser before being pumped back to 

the boiler. Systems using either an organic (e.g., toluene) or an alkali-metal (e.g., K) working fluid have 

been built and operated in ground tests. Space Stirling converters produce electrical power via a linear 

alternator coupled to a reciprocating power piston and displacer that oscillate in a pressurized cylinder 

containing helium gas. Thermal energy is introduced at the heater head, waste heat is removed from the 

cooler, and a regenerator transfers energy between the heater head and cooler. 

Thermoelectric converters generate a voltage potential by exposing dissimilar semiconductor 

materials to a temperature difference, similar to the operation of a thermocouple. This type of static power 

conversion has been used extensively in radioisotope thermoelectric generators (RTGs) since the early 

1960s with demonstrated lifetimes in excess of 30 years, and it was baselined for the SP–100 Program in 

the 1980s. Thermal energy is provided to the hot end, by either radiative or conductive coupling, and 

waste heat is rejected at the cold end to a radiator. Some common semiconductor materials used in 

thermoelectric devices in the past 50 years include lead-telluride (PbTe) and silicon germanium alloys 

(SiGe). Potentially, more efficient thermoelectric materials are available today in the temperature range of 

interest such as skutterudites based on CoSb3 and refractory rare earth compounds, Yb14MnSb11, Zintl, 

and La3–xTe4. A promising approach is to segment the thermoelectric legs using a material combination of 

Zintl, La3–xTe4, and skutterudites (Zintl/LaTe/SKD). 

There are numerous methods to remove the waste heat from the power conversion. The simplest is to 

directly attach a radiating surface to the cold end of the power converter. However, this becomes more 

difficult as the waste heat load increases and other methods must be employed. Options include singlephase pumped liquid loops, two-phase heat pipes, and two-phase capillary pumped loops. The expected 

range of heat rejection temperatures for this small FPS application is about 350 to 500 K. In this range, 

heat transfer coolants such as water, ammonia, fluorocarbons, and hydrocarbons could be utilized. Above 

500 K, liquid metal coolants such as NaK could be considered. Likely radiator construction materials 

include aluminum (Al), titanium (Ti), or polymer matrix composites. 

Candidate System Concepts 

The various technology options can be combined to produce system concepts. To explore the 

attractiveness of several possible technical approaches aimed at meeting the stated design requirements, 

the study team identified four candidate system concepts with two variants. 

Conduction Reactor With Body-Mounted Thermoelectric Conversion 

As an initial concept, modeled closely after the technical approach used for the very successful 

general-purpose heat source (GPHS)–RTG, the team identified a concept with thermoelectric elements 

mounted directly to the body of a low power reactor. This is the same approach used in the Romashka 

reactor that was developed by the Soviets in the mid-1960’s. (Ref. 1) The approach was also considered 

in the late 1980’s as a low-power variant of the SP–100 concept under an Air Force small reactor study 

(Ref. 2). The team’s approach to this concept uses block UMo fuel, a Be reflector, B4C control rods, a 
NASA/TM—2011-217099 6 

superalloy structure, lithium hydride (LiH)/W shield, SiGe thermoelectric elements on the outside of a 

radial reflector, and a liquid-metal heat pipe radiator, with the entire system forward of shield and within 

the shield cone angle. To minimize the mass of the system, a very compact core was necessary, which 

limited the amount of reactor surface area for mounting the thermoelectrics. Also, because of the nuclear 

radiation from the reactor core, space reactor configurations necessarily have the spacecraft payload 

separated from the reactor by a tapered shield in the shape of a frustum of a cone, with the apex of the 

cone away from the payload. This arrangement severely limits the amount of radiator area that can be 

accommodated in front of the shield. Because of these constraints, it was estimated that the power level of 

this concept would be limited to about 0.5 kWe. Even with the use of alternate reflector configurations, or 

higher temperature core materials, the power level possible from this concept would remain substantially 

below the 1-kWe target. 

Heat Pipe Reactor With Distributed Thermoelectric Conversion 

This concept uses the same reactor and shield technologies as the conduction reactor concept, but 

uses in-core liquid-metal heat pipes to transport heat from the core through the shield to Zintl/LaTe/SKD 

thermoelectric elements distributed along the heat pipes and directly coupled to individual aluminum 

radiator fins at the thermoelectric cold end. Moving the power conversion and heat rejection components 

behind the shield eliminates the volume and area constraints that limited the conduction reactor concept. 

It also increased the radiation margin for the already radiation-hard thermoelectric materials. Although the 

Zintl/LaTe/SKD thermoelectric materials are less mature than the SiGe, their use yields improved 

efficiency at the lower temperature available at the heat pipe condenser. This concept was judged to be a 

good choice for a 1-kWe-class system. 

Heat Pipe Reactor With Compact Thermoelectric Heat Exchangers or 

Stirling Power Conversion 

This concept uses the same reactor, shield, and heat pipe technologies as described for the heat pipe 

reactor above, but replaces the distributed thermoelectric elements with Zintl/LaTe/SKD thermoelectric 

elements integrated into compact heat exchangers. Additionally, secondary water heat pipes are used to 

transfer waste heat from the cold end of the thermoelectrics to an aluminum radiator. This concept is more 

complex than the previous system, but would be scalable up to perhaps 5 kWe. 

A variant of this concept uses the same reactor, shield, and heat pipe technologies, but replaces the 

compact thermoelectric heat exchangers with Stirling converters to increase power output. This concept is 

possibly more complex than the system with compact thermoelectric heat exchangers, but would be 

scalable up to about 10 kWe. 

Pumped Liquid Metal Reactor With Compact Thermoelectric Heat Exchangers or 

Stirling Power Conversion 

This concept uses the well-understood reactor and primary heat transport technologies from terrestrial 

fast reactors and the Fission Surface Power (FSP) concept, now being studied by NASA’s Exploration 

Systems Mission Directorate (Ref. 3). The reactor system utilizes UO2 fuel pins, a Be reflector, rotating 

Be/B4C control drums, stainless steel structure, a LiH/W shield, and a pumped NaK primary loop using a 

thermoelectric electromagnetic pump. The reactor system would provide heat to Zintl/LaTe/SKD 

thermoelectric elements in compact heat exchangers, rejecting heat through pumped NaK secondary loops 

to composite radiators. This concept was assessed to be too heavy for a 1-kWe-class system, but could be 

a viable option for 10 or 20 kWe. 
NASA/TM—2011-217099 7 

A variant of this concept uses all of the technologies of the FSP system including the same reactor 

technologies, but would use an annular linear induction pump to transport the NaK coolant to multiple 

Stirling converters. Waste heat would be rejected via pumped water secondary loops to water heat pipe 

radiators. While too heavy for the 1 kWe class, this concept is well suited for power levels from about 

10 kWe up to perhaps 100 kWe. 

Selected System Concept 

The study team selected the heat pipe reactor with distributed thermoelectric conversion for 

concentrated study and analysis. The main factors leading to this selection were design simplicity, 

reasonable system mass, and low development risk. 

The selected concept accomplishes the goals of minimizing the complexity of fission systems and 

making them competitive with radioisotope power systems on a mass basis. The compact nature of the 

block UMo core and BeO radial reflector allows the reactor diameter to be as small as practical while still 

meeting the neutronic and thermal power demands. This directly translates to lower shield mass since the 

reactor diameter dictates the footprint of the radiation shield. The use of LiH and W provides a massefficient material combination for meeting the radiation requirements at the spacecraft. The reactor 

control is simplified through the use of a single, core-centered control rod whose operation is only 

required during reactor startup. The use of heat pipes offers a straightforward primary heat transport 

approach using proven liquid-metal heat pipe technology. Further, the elimination of complex heat 

transport components, both at the reactor side and radiator side, contributes to reducing the total partcount 

and lowering system mass. The thermoelectric technology, albeit using relatively new materials, has 

heritage tracing to a long history of successful and long-lived space power systems. The direct integration 

of the thermoelectric converters with the heat pipes combined with the direct coupling of the 

thermoelectric converters to the radiator makes this concept very simple from a heat transfer perspective. 

The simple thermal interfaces serve to minimize the temperature drop that would otherwise degrade 

system performance and necessitate a bigger reactor and a larger radiator. 

Low development risk results from the use of available technology. The selected concept uses a 

reactor fuel form based on metal fuels that are currently in production by DOE (Ref. 4). The structural 

materials are conventional nickel-based superalloys that have been thoroughly tested at the temperatures 

and radiation fluences required for this system. Liquid metal heat pipe technology is also well established, 

with several vendors capable of designing and delivering the required heat pipes. The thermoelectric 

configuration is based on past designs that have been successfully operated, although the new materials 

introduce some risk. Therefore, a long-term development and demonstration program is planned to verify 

the thermoelectric materials, including their performance in the space and reactor environment where they 

would be operated. The only significant development issue for the aluminum radiator is thermal 

integration with the thermoelectric cold end. 

It should be noted that a heat-pipe-cooled reactor has never been built and operated. It would require 

rigorous analysis and testing to verify the design and establish a statistical database of reliability and 

performance. Fission power reactors (of any type) at this scale are not common. However, the small size, 

low fuel burnup, and low irradiation fluences offer a significant advantage because robust margins can be 

incorporated into this design relative to those assumed for larger power reactors. 

The relatively small scale of the system and the availability of test facilities to validate the design also 

help to reduce development risk. The reactor core is the size of a large coffee can. The entire reactor and 

shield assembly is about the height of an average adolescent child. An individual heat pipe along with the 

distributed thermoelectric converters and radiator panel can be tested as a modular assembly to validate 

performance. At this scale, numerous test facilities exist within the NASA community to conduct thermalvacuum performance and mechanical vibration testing. The reactor fuel can be fabricated in existing 

production facilities, and the reactor neutronics can be verified in existing nuclear criticality test facilities 

within the DOE complex. 
NASA/TM—2011-217099 8 

Reactor Design 

The selected reactor concept utilizes a metallic fuel that conducts fission-generated heat to in-core 

heat pipes, which then transfer the power through the shield to the thermoelectric converters. The reactor 

is controlled by a single in-core control rod. An annular safety collar is used to satisfy expected prestartup 

launch safety requirements. The reactor and shield layout is shown in Figure 1 and the reactor radial cross 

section is depicted in Figure 2. The reactor was designed using standard engineering and reactor physics 

design practices. Limited structural and nuclear analyses were performed to show that physical integrity 

and nuclear performance objectives including safety could be achieved. Sufficient excess reactivity was 

included to ensure that the reactor would operate well beyond its 15-year mission requirement. Also, 

because the only significant radiological risk from a fission reactor would be the possibility of inadvertent 

criticality when personnel are in close proximity, the design was confirmed to remain safely subcritical 

throughout ground processing and during potential launch accident scenarios. A number of launch 

accident configurations were evaluated as part of this study, and the reactor design was shown to be 

subcritical with sufficient margin for all cases analyzed (k-eff < 0.985, where k-eff of 1.00 is a selfsustaining fission reaction). The reactor analyses also confirmed that the temperature coefficients of 

reactivity are strongly negative, which ensure a safe thermal power response to adverse temperature 

transients. The key reactor design features are described in the proceeding paragraphs. 

Figure 1.—Reactor and shield layout. 

Figure 2.—Reactor radial cross section. 
NASA/TM—2011-217099 9 

Fuel: The fuel selected for the FPS reactor concept is 93 percent enriched U10Mo (uranium with 

10 wt% molybdenum). This fuel was selected for its high uranium density, high thermal conductivity, and 

excellent neutronic characteristics for this application (very low neutron capture at fast energies, modest 

capture at moderated energies, and inherent gamma shielding). One concern with U10Mo is material 

swelling at relatively low fuel burnup (~1 percent); however the burnup in this application is so low (~0.1 

percent) that swelling is not a significant technical risk. The anticipated form of the U10Mo fuel is 

cylindrical plates, with the thickness defined by manufacturing and assembly considerations. These plates 

would contain holes for a central control rod and in-core heat pipes and would be stacked to form a 

monolithic block. The fueled region is 12.9 cm in diameter and 30 cm tall. For this study, a maximum 

fuel temperature of 1200 K was imposed, which significantly influences the core and heat pipe geometry. 

Core internal structure: One of the key features of the selected concept is that the fuel can provide its 

own structural support. The fuel assembly would be contained in a can or a liner to preclude material 

interactions and prevent fission gas release (which may be negligible due to the low burnup), but this 

envelope is not needed to maintain physical fuel integrity. The low fuel burnup minimizes possible 

mechanical changes in the fuel that would necessitate structural containment. The advantages of this 

approach are that it minimizes the amount of neutron-absorbing structural material in the core (keeping 

the core compact), and it reduces the heat conduction losses through the fuel structure. Molybdenum is a 

good candidate material for the liner. If some minimal structural material is required, a nickel-based 

superalloy (e.g., Hastelloy-X) could be used. 

Heat pipes: The FPS reactor is cooled by liquid metal heat pipes. The configuration uses 18 heat 

pipes: 6 in an inner ring and 12 in an outer ring. A ring geometry provides symmetry and simplifies heat 

pipe routing through the shield and radiator integration. The heat pipe arrangement minimizes the heat 

transfer distance within the fuel; which keeps the fuel temperature below 1200 K, and the temperature 

gradients and thermal stresses relatively low. The heat pipe vapor temperature is 1100 K, which led to the 

selection of a sodium working fluid and superalloy wick and shell. Each heat pipe is 1.11 cm (7/16 in.) in 

diameter and approximately 4 m long. The heat pipe design includes a factor of two throughput margin, 

given the nominal heat pipe power of about 750 W, and the peak axial and radial heat fluxes are also well 

within the established limits. Similar heat pipes have also proven to be reliable at neutron fluences greater 

than one order of magnitude higher than produced within the FPS core. There are several possible options 

for thermal integration of the heat pipes with the fuel, including brazing, hot isostatic pressing, or a liquidmetal or gas bonding. As the design evolves, each option would have to be evaluated for technical risk, 

cost, and reliability. 

Neutron reflectors: The neutron reflector material is very important to the FPS concept design. A 

reflector with a high-reactivity worth is needed not only to keep system size small, but also to make 

launch safety accidents relatively easy to accommodate. The reflector material specified for most space 

reactors is Be or BeO: no other candidate material has sufficient reactivity worth to meet the currently 

assumed launch accident criticality requirements. For the FPS, a compact geometry is highly desirable; 

thus BeO was chosen because it is a denser and higher worth material per unit thickness than Be. The FPS 

concept assumes the BeO in brick form within a stainless steel (or superalloy) container. The neutron 

fluence is expected to be low enough that BeO swelling and cracking should not be a significant technical 

issue. If later, more detailed design studies indicate that material degradation is an issue, then BeO 

powder may be substituted; however, the lower density of the powder would necessitate a thicker 

reflector. The radial reflector surrounding the core is 7.7 cm thick. The upper axial reflector is 5 cm thick 

and the lower axial reflector is 7 cm thick. The lower reflector optimizes to the thicker geometry because 

it also provides shielding benefits. 

Control rod: Nominal reactor control is performed with a central boron carbide (B4C) control rod that 

is clad with Hastelloy-X. The rod would be moved by a drive mechanism on the backside of the shield, 

with a driveshaft penetrating straight through the shield. The control rod contains the required reactivity 

worth to ensure the reactor can be started from a cold subcritical condition and progress to fulltemperature critical operation. One of the significant advantages of the FPS concept is that further reactor 

control is not required after startup. In eliminating post-startup control rod adjustments, the reactor 
NASA/TM—2011-217099 10 

temperature will slowly degrade over time, by approximately 3 K per year or 45 K over the entire 15-year 

mission. The hot end temperature drop would cause a small decrease in reactor thermal power, although 

the drop in effective heat sink temperature in the outer solar system may counteract this effect, allowing 

full power throughout the mission. A possible alternative approach would be to design the system for 

control rod adjustments throughout the mission lifetime, to counteract this small temperature drop and 

possibly respond to other unanticipated issues. 

Safety collar: The simplest design approach to meet the assumed launch accident safety requirements 

is to place an annular B4C collar between the reactor and the radial reflector. This collar would be 

removed from the core once the probability of Earth return became negligible. The collar mechanism 

design would ensure that the safety collar remains in place during launch, but extracted prior to reactor 

startup. 

Radiation shield: The FPS shield utilizes LiH (in stainless steel) as the neutron shield material and W 

as the gamma shield material. The LiH is enriched in (Ref. 6) Li to reduce the gamma source from 

neutron capture in the stainless steel and tungsten, and because (Ref. 6) Li is naturally lower mass than 

(Ref. 7) Li. Tungsten and other high-atomic-mass materials provide excellent gamma attenuation because 

they reduce the thickness and therefore, the geometric expansion of the shield diameter into the shield 

cone. The shield utilizes three layers of LiH and W, with each layer of LiH being placed in an individual 

stainless steel envelope. The shield contains full penetrations for the heat pipes and the control rod shaft. 

A gap is provided around each heat pipe in which multifoil insulation would be placed to prevent shield 

heating and parasitic power loss. Shielding calculations showed that the streaming dose through these 

penetrations accounted for approximately 50 percent of the payload dose. Consequently, a secondary plug 

shield (see Fig. 1) was added behind the main shield to reduce this streaming dose, with a relatively small 

mass addition. 

Balance-of-Plant Design 

To accommodate the targeted electrical power output of 1 kWe and the planned reactor heat pipe 

operating temperature of 1100 K, the selected concept uses a distributed thermoelectric converter 

approach integrated with a passive heat rejection radiator structure. The concept is very similar to 

approaches developed for prior RTGs in that it uses radiative thermal coupling of thermoelectric devices 

along the length of each of the 18 heat pipes and rejects the heat directly through a finned radiator surface. 

The key power system performance characteristics are summarized in Table 2. 

TABLE 2.—SMALL FPS PERFORMANCE SUMMARY 

Heat from fission reactor 13,000 W 

Heat pipe temperature 1100 K 

Number of heat pipes 18 

Heat per pipe 722 W 

Heat loss 10% 

Temperature drop from heat pipe to TE couple 50 K 

TE hot end temperature 1050 K 

TE cold end temperature 525 K 

Converter efficiency 9.83% 

Electrical wiring losses 10% 

Temperature drop from TE couple to radiator 50 K 

Effective radiator temperature 475 K 

Radiator size required 5.035 m2

Radiator panel width (1 panel per heat pipe) 11.7 cm 

Radiator length required 240 cm 

System net electrical power output 1035 W 

Overall system efficiency 7.96% 
NASA/TM—2011-217099 11 

 

Figure 3.—Thermoelectric converter concept. 

The power conversion assembly consists of 18 panels, each about 12 by 250 cm for a total radiator 

surface area of 5 m2

. Each panel is composed of 21 thermoelectric converters with attached radiator fin as 

shown in Figure 3. Each converter is coupled to the reactor heat pipe by means of a highly efficient but 

mechanically compliant heat pipe saddle and radiation coupler that would accommodate thermal 

expansion of the heat pipe. This type of radiation coupler approach has been successfully used in other 

space systems, including the International Space Station. The materials selected for the thermoelectric 

couples include n-type conductivity La3–xTe4/double-filled CoSb3 skutterudite with p-type conductivity 

Yb14MnSb11 Zintl/filled CeFe3.5Co0.5Sb12 skutterudite (Ref. 5). These materials are stacked in a segmented 

device configuration similar to that used in proven heritage RTGs (such as Si0.8Ge0.2/Si0.63Ge0.37

materials). The rare earth compounds are currently baselined for operation in next-generation RTGs at 

temperatures up to 1275 K, well in excess of the requirement for the small FPS application. All of these 

materials are produced through metallurgy techniques in large batches, and skutterudites are currently 

targeted for large-scale waste heat-recovery power systems. Efficiencies of approximately 9 to 13 percent 

have been demonstrated in devices based on these materials. 

Taking into account the thermal resistances on both the hot and cold ends of the thermoelectric 

device, the couples would operate with a 525 K temperature differential (1050 minus 525 K) with a 

projected efficiency close to 10 percent. The predicted overall system efficiency would be about 

8 percent, with a net power output of 1035 W. The thermoelectric devices would be connected in 

series/parallel strings to match the spacecraft bus voltage of 28 Vdc. 

System Configuration and Mass 

The basic power system layout geometry and spacecraft interfaces are shown in Figure 4. The reactor 

is mounted to a truncated-cone radiation shield designed to meet the radiation dose limits assumed in this 

study (1.0×1011 n/cm2

 and 25 krad) at a dose plane 10 m from the reactor/shield interface as shown. The 

shield provides a conical shadow of radiation protection with a half angle dictated by the separation 

distance and the diameter of the desired area to be protected at the dose plane, assumed for this study to 

be 4.5 m. The power conversion and radiators are located behind the shield within the cone angle. A 

support structure would provide mounting for the power conversion and radiator panels as well as provide 

the load path from the reactor to the spacecraft. The FPS would interface with the spacecraft through an 

adapter flange at the base of this support structure. The extension boom that connects the power system to 

the spacecraft is not included in the FPS mass estimate. 

The FPS design configuration is shown in Figure 5. The power system that resulted from this design 

study has a total length of about 4 m, including the 2.5-m-long radiator panels. The reactor heat pipes 

extend from the core straight through the shield (shown as partially transparent) and then flare outward to 

where the radiator panels begin. The secondary plug shield discussed earlier is not shown, but would be 

inserted behind the main shield to attenuate heat pipe radiation streaming down the straight section of heat 

pipes. The aluminum radiator panels are rectangular, but are angled to form a conical structure to improve 

structural rigidity and radiator view factor. A tubular aluminum truss structure supports the radiator 

panels and extends to the backside of the radiation shield. The truss structure and radiators are fixed and 

do not require any in-space deployment. The 1.7-m-diameter end skirt would include an adaptor flange to 

permit spacecraft attachment. 
NASA/TM—2011-217099 12 

Figure 4.—Concept layout and spacecraft interfaces. 

Figure 5.—Small FPS design configuration. 

TABLE 3.—SMALL FPS MASS ESTIMATE 

 Current Best 

Estimate (CBE), 

kg 

CBE + Margina

, 

kg 

Core assembly 133 159 

Heat pipe assembly 19 25 

Shield assembly 271 353 

Thermoelectric assembly 89 115 

Truss assembly 60 78 

Inst. and control assembly 32 42 

Total small FPS 604 

(1.7 W/kg) 

772 

(1.3 W/kg) a

Margin is assumed as 30% for all components except reactor fuel (5%). 

The system mass summary is presented in Table 3. The total current best estimate (CBE) mass is 

604 kg, or about 1.7 W/kg. The shield assembly has the greatest individual mass at 271 kg, approximately 

45 percent of the total. This could be reduced if the radiation limits are relaxed or the separation distance 

is increased. A 30 percent mass margin was applied to all components with the exception of the reactor 

fuel. The fuel mass was calculated specifically to meet the required criticality and safety conditions, and a 

mass margin of 5 percent (which translates to a large reactivity margin) was deemed appropriate for the 

56 kg of UMo fuel. The total FPS mass with margin is 772 kg, or about 1.3 W/kg. 
NASA/TM—2011-217099 13 

Since the shield represents a significant fraction of the total system mass, the team performed a mass 

sensitivity analysis relative to reactor separation distance. As expected, the analysis showed that greater 

separation distances result in lower shield mass. In theory, an optimum separation distance results when 

shield and boom mass are properly balanced. In practice, a larger boom length introduces numerous 

integration issues including greater difficulty in packaging the spacecraft within the launch vehicle 

shroud, the possible need for post-launch deployments, and the potential for unstable science-orbit 

dynamics. Based on the sensitivity analysis performed and the possible spacecraft integration issues that 

arise with longer booms, it appears that the 10 m separation represents a reasonable design choice. 

Spacecraft Integration 

The FPS design approach results in a conical configuration similar to all space reactors flown thus far. 

This packaging lends itself to a relatively straightforward spacecraft integration; the base of the FPS 

radiator array structure is simply attached to the upper portion of the spacecraft structure. Separation 

distance of the dose plane from the reactor is most easily achieved through a simple extension of the 

spacecraft body. This extended structure would provide ample room for propulsion tanks and other 

equipment that are less susceptible to radiation damage and thus can be closer to the reactor. An example 

spacecraft configuration, based on the Europa Orbiter mission (Ref. 6), is shown in Figure 6. 

The configuration is representative of a typical orbiter design, incorporating a large bipropellant 

propulsion system for deep space maneuvers and orbit injection. Radiation-sensitive avionics are centrally 

located in the avionics bus, which begins at the defined dose plane distance of 10 m from the back of the 

reactor. Science instruments are grouped around the aft end of the spacecraft, which would be nadirpointed when operating in science orbit. This design lends itself to gravity gradient stabilization and this 

is easily accommodated by the configuration. The high-gain antenna (HGA) is located on a fixed mount, 

sufficiently separated from the spacecraft body to allow full articulation while remaining within the 

reactor shield cone angle. A feature of this design concept is that the higher power available from the FPS 

can increase telecommunications radiofrequency (RF) power output, facilitating high data return from a 

relatively small HGA. This feature has a number of benefits, including eliminating the need for antenna 

booms and deployment, and greatly reducing the tight spacecraft pointing and stability requirements 

imposed by larger HGAs typically used for such missions. 

Figure 6.—Notional FPS-based Europa Orbiter configuration. 
NASA/TM—2011-217099 14 

Concept-of-Operations 

The reactor is launched at ambient temperature and at zero power. The reactor core is not a 

radiological hazard during launch processing and can be approached by personnel without concern of 

radiation effects. The reactor is maintained subcritical with essentially no fission activity because it has a 

neutron-absorbing control rod inserted in the core, and a neutron-absorbing sleeve between the reactor 

fuel and the reflector. The sleeve must be removed and the rod must be withdrawn to initiate reactor 

operation. Once the spacecraft is on the appropriate trajectory, the sleeve is removed from its position 

between the fuel and the reflector using a fault-tolerant ejector mechanism. To start the fission process, 

the control rod is then gradually withdrawn from the core using a redundant motor drive. A sustained 

fission reaction would initiate spontaneously once a critical configuration is achieved. 

The position of the control rod is adjusted within the core to control the reactor thermal power level. 

The reactor can sustain a critical reaction at very low power levels. The control rod is adjusted slowly 

until a statistically significant response is recorded on system neutron detectors. To increase the thermal 

power level, the control rod is further withdrawn while neutron flux is monitored, until enough power is 

generated to produce the required thermal response. 

Due to the nature of the reactor design, the thermal power output is self regulating once the fuel 

temperature begins to rise. This negative temperature feedback effect causes the reactor power level to 

automatically decrease if the reactor temperature increases and to increase if the reactor temperature 

decreases (in the absence of a control rod adjustment). Once the reactor fuel temperature begins to 

increase, the temperature feedback mechanisms begin to function and the reactor is then brought to full 

power and temperature. An increase to full reactor power is expected to occur over a period of 

approximately 8 hr, but it could be done faster if required. 

Heat removal from the core to the power conversion system is the only significant source of heat loss; 

thus, the reactor power would inherently and correctly respond to changes in the system power demand 

(intended or otherwise). Therefore, once the reactor is started additional reactivity control is not required 

for short-term power changes. The reactor power density is low enough that the fuel can be cooled with a 

combination of passive conduction and heat pipe operation. Therefore, no active control systems related 

to core cooling are required. Another beneficial feature of low-power operation is that very little 

fissionable material is consumed. This fact, coupled with the negative temperature coefficient of 

reactivity allows the reactor to operate at essentially full power and temperature over extended periods 

without a reactivity adjustment. Current estimates suggest that the electrical power output of the power 

system would decrease by only 5 percent during a 15-year mission without reactivity adjustment after full 

power has been achieved. 

Reactor startup would be initiated using battery power within a few hours after launch. Once the heat 

pipes begin operation, the thermoelectric converters would begin producing electrical power to meet 

spacecraft power needs. The reactor would operate under steady-state full-power conditions throughout 

the mission and any electrical power not needed by the spacecraft would be diverted to a parasitic radiator 

and rejected to space. While the study requirement called for continuous power output from the FPS, 

reactor power adjustments could be accommodated if desired. A possible benefit of reducing thermal 

power during the mission is the corresponding reduction in reactor-induced radiation at the spacecraft 

electronics bus. 

Extensibility 

The design of the small FPS concept is optimized to provide 1 kWe for approximately 15 years. The 

system is extensible to lower power levels; however, there would be a relatively small mass difference 

between a 0.5-kWe system and a 1-kWe system because the reactor is already near the minimum size 

needed to sustain a fission reaction. Extensibility to higher power levels can be divided into two 

categories—use of the same reactor (e.g., identical core and reflector) and use of the same reactor 

technology and approach but with some modifications. 
NASA/TM—2011-217099 15 

In the first category, the same reactor could be used with longer heat pipes and additional distributed 

thermoelectrics to produce power up to about 1.5 kWe. The use of a more advanced heat pipe wick would 

allow an additional power increase up to about 2 kWe. A 2-kWe system could also be developed by using 

thermoelectrics in a compact heat exchanger with secondary heat pipes. The power limit using the same 

reactor but different power conversion appears to be about 5 kWe; the principal change would be the use 

of high-efficiency Stirling power conversion instead of thermoelectric conversion (Ref. 7). 

For power levels above 5 kWe, several small reactor design changes would be needed. In this second 

category of extensibility, increasing power to 10 kWe would require a slight change in core volume and 

the number (or size) of heat pipes. Increasing power beyond 10 kWe would require at least two changes: 

additional and larger diameter heat pipes, and possibly lower density UMo fuel to accommodate increased 

uranium burnup. These two changes would result in an increase in reactor size and system mass. Other 

options for increasing power beyond 10 kWe could include changing to a higher temperature fuel block 

(e.g., W/UN or Mo/UN cermet) or changing to pin-type fuel. 

At power levels above 10 or 20 kWe, the use of superalloy heat pipes to provide primary core cooling 

may become limiting. For these power levels, a different reactor cooling methodology could be utilized. 

Pumped liquid metals, refractory metal heat pipes, or direct gas cooling with Brayton power conversion are 

all possible options. More advanced, higher temperature fuels, heat transport systems, and power conversion 

technologies would allow fission systems to scale to considerably higher power levels. Although these highpower systems would use different technologies, they would still greatly benefit from the knowledge and 

experience gained from designing, developing, testing, and flying the 1-kWe FPS. 

System Development 

Development and verification of the system design and the flight hardware would follow the “test as 

you fly, fly as you test” maxim to the extent practical. Specifically, the components would be subjected to 

mechanical, thermal, vacuum, vibration, launch load, material compatibility, radiation, nuclear, 

functional, electrical, instrumentation, and controls testing. In order to achieve the required test results 

and provide the most cost-effective approach for system verification, the system testing would be split 

into two categories: “zero-power” ground nuclear tests and “electrically heated” system tests. Groundbased nuclear testing would confirm the nuclear design. Tests that are of particular interest include 1) 

verify the amount of subcriticality margin at launch for nominal and accident conditions (e.g., surrounded 

by water), 2) verify the negative thermal feedback coefficient for the nuclear power as the reactor 

temperature rises, and 3) verify the control rod position for initial criticality and the rod margin for 

accommodating heat-up. The first is needed for the launch safety analysis. The second and third are 

needed for operation and design verification. None of these tests require full-power reactor operation (i.e., 

13,000 W-thermal); they can be done with zero-power critical tests using external electrical heaters to 

achieve the desired temperatures and temperature gradients. In a zero-power critical test, the reactor is in 

a critical state and is producing a self-sustaining nuclear reaction, but the amount of nuclear power is kept 

at a very low level (nominally 1 to 10 W-thermal) so that the buildup of radioactive fission products is 

negligible. This greatly reduces the safety concerns and the cost of the experiment and permits repeated 

access to the hardware during testing. 

The first zero-power critical test can be performed early in the program with a mockup to verify the 

nuclear material cross sections, the neutron spectrum, and the leakage fraction of the FPS reactor concept. 

The second set of criticality experiments would be done with a higher fidelity reactor prototype that 

includes the shield and heat pipes. Electrical heaters wrapped around the core, and heaters and cooling 

systems on the heat pipes themselves, would allow test engineers to alternately drive heat into the core 

and remove heat from the core in a manner that could exercise the reactor over a full range of 

temperatures and temperature gradients that might be encountered with nuclear heating. The nuclear 

feedback and the nuclear controls could be fully evaluated under these conditions. A zero-power critical 

acceptance test would also be performed on the flight reactor to verify the control rod position required to 

achieve criticality. 
NASA/TM—2011-217099 16 

In the electrically heated tests, the full system would be evaluated with the nuclear heat of the core 

represented by electrical resistance heaters. Electrically heated testing would include startup, full power 

operation, and a wide range of operational transients. This approach has been developed over the past 

decade as a complement to nuclear testing for space FPSs. The simulated core, possibly made using 

depleted UMo, would have embedded electrical heaters to generate the heat. The design would mimic the 

heat transfer conditions anticipated at the heat pipes, with similar thermal inertia and materials 

interactions as expected in the actual nuclear core. The main differences relative to the real core are the 

radiation environment and the nuclear feedback behavior. However, the heaters can be programmed to 

simulate the reactor behavior based on analytical models for reactivity feedback. Testing would be used to 

demonstrate adequate performance margin over an operational range that includes all significant 

reactivity feedback coefficients, the potential range of values for those reactivity feedback coefficients, 

and all potential effective delayed neutron fractions. Lifetime testing would also be performed on 

components to confirm long-term performance and degradation characteristics, using accelerated test 

methods as practical. 

The electrically heated system testing would allow thorough evaluation of the heat transport, power 

conversion, and heat rejection subsystems in a thermal-vacuum environment while demonstrating control 

algorithms for both transient and steady-state operations. For the engineering unit, it would lead to design 

verification that would help ensure the success of the flight system. For the flight unit, it would ensure 

proper functionality prior to launch. Nuclear feedback effects on the flight unit would be shown to be 

within acceptable margins by coupling measured reactivity feedback coefficients with full-power 

electrically heated tests. 

This two-pronged approach of zero-power nuclear testing and electrically heated system testing 

provides a robust verification strategy for the FPS. The testing would provide all the required data to 

confirm the flight design prior to launch. The key elements would be fully characterized under expected 

nuclear, thermal, mechanical, and electrical conditions. The combination of this test approach, the low 

thermal power and energy density of the reactor, the extensive suite of available analytical tools, and the 

broad nuclear experience base provides confidence that the flight system would meet the intended mission 

requirements. 

Test Facilities 

The FPS development effort would employ a number of existing DOE and NASA facilities and test 

capabilities to support the nuclear design and verification. Existing NASA facilities would permit the 

non-nuclear mechanical, thermal, vacuum, vibration, launch loads, material compatibility, electrical, 

instrumentation, and controls testing of individual components or the entire system. 

DOE facilities would also be utilized to support the design, development, and fabrication of the 

nuclear fuels, conduct nuclear irradiations testing, and perform the zero-power critical testing. DOE 

laboratories and fuel manufacturing capabilities at INL and ORNL would be utilized to develop the fuel 

manufacturing process and FPS reactor core assembly. Because the flight unit would be fueled with 

highly enriched uranium (HEU), specialized and dedicated security facilities would be needed. A number 

of nuclear irradiation facilities are available to perform irradiation experiments on various components, 

instruments, and materials using neutron or gamma irradiation capabilities. The SNL Gamma Irradiation 

and the fuel fabrication equipment at the INL are examples of such existing capabilities within the DOE 

complex. In addition, DOE has recently established capabilities at the Nevada Test Site to perform zeropower critical testing of nuclear reactor cores. Because of the size and type of materials selected for the 

FPS design, and because other DOE projects have established relevant fuel fabrication, testing, and 

experimental facilities, it is anticipated that no new facility-related infrastructure would be required for 

this system. Some facility and process fabrication equipment would need to be installed or modified 

within the existing facilities to support the FPS fabrication and testing requirements. The project would 

also explore available university or industrial test capabilities to reduce costs or improve schedule 

considerations. 
NASA/TM—2011-217099 17 

Figure 7.—Small FPS development program. 

Development Program 

The FPS development program is summarized in Figure 7. The 10-year program is organized into 

three phases: Development (3 years), Engineering (3 years), and Flight Qualification (4 years). A basic set 

of program-related milestones is shown on the “Program” line. Within each phase, a listing of the key 

system engineering and testing activities is presented along with the expected National Environmental 

Policy Act (NEPA) and Launch Approval (LA) products. 

Prior to beginning the Development phase, a Concept Development (CD) activity would be 

conducted to generate a FPS conceptual design and address some of the possible technology risks. The 

major areas identified for technology risk reduction during this period are 1) fabricate a 4-m sodium heat 

pipe and demonstrate power throughput at FPS operating temperatures and irradiation levels, 2) fabricate 

a Zintl/LaTe/SKD thermoelectric module with integral radiator fin and demonstrate power output and 

efficiency at FPS operating temperatures, and 3) fabricate UMo fuel coupons and characterize mechanical 

properties and evaluate core fabrication methods. These activities would be conducted in parallel and can 

be completed within 6 to 8 months with a minimal technology investment. Starting this effort in 2011 

would enhance the ability to meet a 2020 flight readiness opportunity. 

The FPS Development phase would focus on concept validation. Design requirements would be fully 

defined and analytical models would be developed. Testing would be performed on components and 

subassemblies to evaluate performance. An electrically heated power generation test would be completed 

in thermal-vacuum using a single heat pipe and thermoelectric panel assembly. Material properties and 

design standards would be formulated and manufacturing processes would be established. The 

Development phase would culminate with a zero-power critical test of a reactor mockup. 

The Engineering phase would verify FPS form, fit, and function. Detailed design drawings would be 

completed and analytical models would be fully validated with test data. Thermal-vacuum performance 

testing would be conducted on a full-size, 18-panel engineering model using an electrically heated reactor 

simulator and spacecraft mockup. Comprehensive vibration and radiation testing of the panel would also 

be performed. The reactor instrumentation and control elements would be tested to verify performance in 

an operational environment and the flight software would be validated. A full-size reactor prototype 

would be fabricated and characterized in a series of zero-power critical tests. 

The Flight Qualification phase would deliver the flight system to the launch site. It would include an 

electrically heated qualification test unit and the actual flight system. The flight system would be fueled 

and assembled at a DOE facility and delivered to the launch site ready for installation on the spacecraft. 

Prior to spacecraft integration, a final zero-power critical acceptance test would be performed on the flight 

reactor. A 1-year spacecraft and launch vehicle integration period was assumed as part of the 10-year 

development program. 
NASA/TM—2011-217099 18 

Conclusions 

The team successfully developed a viable, low-risk Fission Power System (FPS) concept that can be 

delivered for launch by 2020 and meet the 1-kWe power output and 15-year lifetime requirements defined 

for the study. The power system concept consists of a UMo-fueled, heat-pipe-cooled reactor with 

distributed thermoelectric converters integrated with an aluminum radiator. The system mass is 772 kg 

(including margin); and the development program can be completed in 10 years. The basic concept can be 

readily scaled to several kilowatts with thermoelectric conversion or up to 10 kWe using Stirling power 

conversion with minimal changes to the reactor design. This power system would provide an enabling 

capability for potential future space science missions that may otherwise be power limited. The 

technology is highly adaptable to provide significantly greater power using alternative, but similarly 

mature design approaches. 

Although the team completed this study in only 6 weeks, the conclusions are based on a solid 

foundation of more than 50 years of nuclear engineering and technology development by the Department 

of Energy (DOE), the National Aeronautics and Space Administration (NASA), the Navy, the Air Force, 

and private industry. Hundreds of commercial, military, and university reactors are operating safely in the 

United States today, and hundreds more worldwide, as a result of the Nation’s investment in nuclear 

technology. The materials, components, and test facilities needed to develop the selected FPS are readily 

available within the existing U.S. nuclear infrastructure. 

Space reactors are not new or particularly complicated. The United States successfully built and 

launched a space reactor in 1965, and the Russians launched over 30 systems in the years between 1976 

and 1988. This type of development has been done before and does not present a major challenge beyond 

what is routinely done in other space flight systems. For missions requiring 1 kWe or more, fission 

systems offer the potential for a reliable, low-cost power source that can be reproduced to meet the 

demands of outer planetary science spacecraft for the next century. 

References 

1. Bennett, G.L., “A Look at the Soviet Space Nuclear Power Program,” Proceedings of the 24th 

Intersociety Energy Conversion Engineering Conference, IECEC–89, Washington, D.C., 6-11 

August 1989. 

2. “Space Reactor Power Systems for 5 to 40 Kilowatts,” A Report of the Evaluation Panel for Small 

Space Reactor Systems, Prepared for the Air Force Technology Center and the Department of 

Energy, March 1988. 

3. Palac, D.T., Mason, L.S., Houts, M.G., and Harlow, S., “Fission Surface Power Technology 

Development Update,” AIAA–2010–8711, AIAA SPACE 2010 Conference and Exposition, 

Anaheim, California, 30 Aug.–2 Sep. 2010. 

4. Robinson, A.B., Chang, G.S., Keiser, D.D., Jr., Wachs, D.M., and Porter, D.L., “Irradiation 

Performance of U-Mo Alloy Based ‘Monolithic’ Plate-Type Fuel - Design Selection,” INL/EXT–09–

16807, Idaho National Laboratory, August 2009. 

5. Fleurial, J.P., Johnson, K., Mondt, J., Sakamoto, J., Snyder, J., Chen-Kuo Huang, Blair, R., Stapfer, 

G., Caillat, T., Frye, P., Determan, W., Heshmatpour, B., Brooks, M., and Tuttle, K., “Development 

of Segmented Thermoelectric Multicouple Converter Technology,” 2006 IEEE Aerospace 

Conference, Big Sky, Montana, 4–11 March 2006. 

6. Clark, K., Tan-Wang, G., Boldt, J., Greeley, R., Jun, I., Lock, R., Ludwinski, J., Pappalardo, R., Van 

Houten, T., and Yan, T., “Return to Europa: Overview of the Jupiter Europa Orbiter Mission,” 2009 

IEEE Aerospace Conference, Big Sky, Montana, 7–14 March 2009. 

7. Mason, L., and Carmichael, C, “A Small Fission Power System With Stirling Power Conversion for 

NASA Science Missions,” Proceedings of Nuclear and Emerging Technologies for Space, NETS–

2011, Paper 3326, Albuquerque, New Mexico, 7-10 February 2011.
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-12-2011 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

A Small Fission Power System for NASA Planetary Science Missions 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Mason, Lee; Casani, John; Elliott, John; Fleurial, Jean-Pierre; MacPherson, Duncan; Nesmith, 

Bill; Houts, Michael; Bechtel, Ryan; Werner, Jim; Kapernick, Rick; Poston, David; Qualls, 

Arthur, Lou; Lipinski, Ron; Radel, Ross; Bailey, Sterling; Weitzberg, Abraham 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 463169.04.03.01.02 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17606 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2011-217099 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Category: 20 

Available electronically at http://www.sti.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

In March 2010, the Decadal Survey Giant Planets Panel (GPP) requested a short-turnaround study to evaluate the feasibility of a small 

Fission Power System (FPS) for future unspecified National Aeronautics and Space Administration (NASA) science missions. FPS 

technology was considered a potential option for power levels that might not be achievable with radioisotope power systems. A study plan 

was generated and a joint NASA and Department of Energy (DOE) study team was formed. The team developed a set of notional 

requirements that included 1-kW electrical output, 15-year design life, and 2020 launch availability. After completing a short round of 

concept screening studies, the team selected a single concept for concentrated study and analysis. The selected concept is a solid block 

uranium-molybdenum reactor core with heat pipe cooling and distributed thermoelectric power converters directly coupled to aluminum 

radiator fins. This paper presents the preliminary configuration, mass summary, and proposed development program. 

15. SUBJECT TERMS

Space power; Fission power systems; Space reactors 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

24 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18