Marc A. Gibson and Maxwell H. Briggs

Glenn Research Center, Cleveland, Ohio

James L. Sanzi

Vantage Partners, LLC, Brook Park, Ohio 

Michael H. Brace

Glenn Research Center, Cleveland, Ohio

Heat Pipe Powered Stirling Conversion for the 

Demonstration Using Flattop Fission (DUFF) Test

NASA/TM—2013-216542

December 2013

	

	


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Marc A. Gibson and Maxwell H. Briggs

Glenn Research Center, Cleveland, Ohio

James L. Sanzi

Vantage Partners, LLC, Brook Park, Ohio 

Michael H. Brace

Glenn Research Center, Cleveland, Ohio

Heat Pipe Powered Stirling Conversion for the 

Demonstration Using Flattop Fission (DUFF) Test

NASA/TM—2013-216542

December 2013

	

	

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NASA/TM—2013-216542 1 

Heat Pipe Powered Stirling Conversion for the 

Demonstration Using Flattop Fission (DUFF) Test 

Marc A. Gibson and Maxwell H. Briggs 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

James L. Sanzi 

Vantage Partners, LLC 

Brook Park, Ohio 44142 

Michael H. Brace 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Design studies have shown that heat-pipe-cooled reactors provide a passive, redundant, and lower 

mass option to transfer heat from the reactor fuel to the power conversion system, as opposed to pumped 

loops, typically associated with larger fission power systems (FPSs). These small FPSs, designated as 

“Kilopower,” have been identified as mission-enabling technology capable of providing long-term 

abundant power for higher power science missions and lower power exploration missions. Although 

many research concepts have been examined and a few tested using electrical heating, none have been 

coupled to a real nuclear reactor. The Demonstration Using Flattop Fission, or DUFF, test was planned by 

the Los Alamos National Laboratory (LANL) to use the existing “Flattop” criticality experiment at the 

Nevada National Security Site to provide the nuclear heat source. A team from the NASA Glenn 

Research Center in partnership with the LANL reactor design team designed, built, and tested a heat pipe 

and power conversion system to couple to Flattop with the end goal of demonstrating electrical power 

production using technology applicable to Kilopower concepts. This paper will focus on the research and 

testing performed in preparation for the DUFF test. 

Nomenclature 

DAF Device Assembly Facility 

DUFF Demonstration Using Flattop Fission 

FPS fission power system 

I.D. inner diameter 

kWe kilowatt electric 

LANL Los Alamos National Laboratory 

O.D. outer diameter 

RPS Radioisotope Power System 

SNAP Systems Nuclear Auxiliary Power 

SRG Stirling Radioisotope Generator 

235U uranium isotope 
NASA/TM—2013-216542 2 

Introduction 

NASA’s nuclear power portfolio has been focused on plutonium-fueled radioisotope systems using 

thermoelectric power conversion since the 1960s with many successful exploration and science missions. 

Historically, Radioisotope Power Systems (RPSs) have delivered up to a maximum of 290 W/unit and are 

most practical for deep space science missions where long-term dependable power is needed and solar 

intensity is limited. The first and last U.S. fission power system (FPS) to fly in space, to date, was SNAP

10A (Systems Nuclear Auxiliary Power) in 1965, during a time period when solar cell technology was 

just as precarious as fission power, and the country was uncertain which technology would power larger 

space systems, not suitable for RPSs. Since then, NASA has made significant progress in larger FPS 

developments (>10 kWe) for surface (Ref. 1) and in-space applications but has been reluctant to flight 

qualify such a system, mostly due to development cost and its uncertainty, and a lack of near-term 

mission pull. Today, there exists a portfolio gap between the flight-qualified lower-power RPS and the 

nonflight-qualified higher power FPS. Existing development efforts to fill this gap include a 500-W 

Stirling Radioisotope Generator (SRG5) and an extensible 1- to 10-kWe FPS (Kilopower). The 

development of a new RPS will be fairly well understood using past flight and development programs to 

predict future costs, but the same cannot be said for FPS with the last flight program taking place almost 

50 years ago, during a much different nuclear era. The most successful FPS developments since the 

SNAP program typically have an end goal of a ground test demonstration using an electrically heated 

reactor core to simulate the performance of the uranium fuel. This is a practical test philosophy and 

addresses many key technology concerns but does not address the certainty of the reactor design and 

control, fuel manufacturing, and nuclear ground testing that make up a significant portion of the cost and 

development time to achieve a flight-qualified FPS. Careful consideration is needed to effectively 

determine which tests, both nuclear and nonnuclear, will bring the FPS technology to an advanced state 

where it will be converted from a technology demonstration into a flight program as mission pull 

increases. 

A new emergence of small FPS is on the forefront with designers envisioning power-rich spacecraft 

that will enable exploration of new frontiers and pave the way for more advanced fission systems. The 1- 

to 10-kWe power gap can be filled with Kilopower and allow the plutonium-fueled RPS to continue their 

legacy for smaller science payloads. The current nuclear systems team, which includes NASA and the 

Department of Energy (DOE), has recognized the opportunity for the Kilopower FPS and initiated plans 

to address the development of these systems. During initial planning, the Los Alamos National 

Laboratory (LANL) reactor design team investigated using existing nuclear facilities to support 

development of the Kilopower concepts with the idea that proof-of-concept nuclear testing could be 

affordable and achievable. The LANL team identified a criticality experiment named “Flattop,” located 

in the Device Assembly Facility (DAF) at the Nevada National Security Site, that had specific 

characteristics to facilitate a substantial proof-of-concept test. The Demonstration Using Flattop 

Fission, or DUFF, test was initiated with the goal of producing electrical power from the Flattop reactor 

(Figure 1). The Kilopower system concept designs all stemmed around a heat-pipe-cooled reactor coupled 

to either thermoelectric or Stirling power conversion systems (Ref. 2). Continuing with this principle, a 

heat pipe would be inserted into Flattop’s 235U (uranium isotope) fuel with the purpose of transporting the 

heat out to a pair of Stirling engines where conversion from heat to electricity could be made. A portion 

of the electricity produced would power a light to signify the proof of concept was complete, in addition 

to other engineering conclusions. 
NASA/TM—2013-216542 3 

Figure 1.—Flattop reactor showing reflector safety blocks, fuel 

and pedestal, hemispherical reflector, and assorted 

components. 

DUFF System Requirements 

The Kilopower proof-of-concept test using the Flattop reactor was initiated in May 2012 with goals of 

completing the testing by summer’s end. The system design would be driven by Flattop requirements 

concerning hardware integration, thermal characteristics, and safety processes. The higher temperature 

Kilopower flight concepts and associated design requirements were understood, in part, to be different 

than the DUFF requirements, but much was to be learned in their similarities for this proof-of-concept 

demonstration. 

Mechanical Integration 

Hardware integration required the heat pipe and thermal interfaces to conform to the Flattop geometry 

built in 1951. Flattop had a specific purpose concerning reactor criticality experiments and had never been 

configured to accept a heat pipe for the purpose of extracting thermal power. The uniqueness of the DUFF 

experiment required both the LANL and Glenn Research Center teams to investigate numerous designs to 

ensure that the experiment would be successful. Figure 2 shows the Flattop geometry and provides 

dimensional details for the assembly. 
NASA/TM—2013-216542 4 

Figure 2.—Flattop reactor diagrams depicting internal structure and glory 

hole used for heat pipe placement. 
NASA/TM—2013-216542 5 

Figure 3.—Test configuration model showing 19-in. reflector sphere, heat pipe, convertors, 

and test fixturing. 

The heat pipe would be inserted through the 19-in.-diameter stationary reflector and into the 5.0-in.-

diameter uranium core, both having a 0.505-in. hole. The evaporator would have, at most, 4.75 in. of heat 

transfer length but concerns about excess reactivity could potentially decrease the available space with the 

addition of fuel plugs. Special requirements were established between the heat pipe and uranium core to 

ensure that the fuel would not get damaged or that the heat pipe would not become lodged in the fuel. 

This requirement allowed a graphite lubricant to be used between the heat pipe and fuel but would not 

allow any type of thermal grease or other products that could potentially react with the uranium metal. 

The temperature drops associated with the lubricant and radial gap would need to be tested for expected 

thermal performance during Glenn laboratory testing. 

Mechanically coupled to the backside of the core and pedestal was the hemispherical reflector that 

housed a 1.0-in. outer diameter (O.D.) by 0.5-in. inner diameter (I.D.) uranium sleeve. In order for the 

heat pipe vapor to reach the Stirling convertors, it was necessary that this 7.0-in. section of heat pipe be as 

adiabatic as possible, minimizing the heat transfer to the reflector assembly. To meet this requirement, the 

uranium sleeve would need to be replaced with a new alumina or stainless steel sleeve with a larger I.D. 

The new sleeve would provide an air gap between the heat pipe and reflector thus minimizing large 

amounts of heat transfer from leaving the heat pipe assembly. Both alumina and stainless steel sleeves 

were made and either could be used depending on reactivity affects. 

Once outside the reflector, the heat pipe would have the freedom to attach to the Stirling convertors as 

long as the engine assembly and test fixturing did not interfere with the reflector and was confined inside 

the edges of the table. The Glenn convertor test assembly would lie on the Flattop table and adjustments 

to align the centerlines of the heat pipe, reflector sleeve, and glory hole would be modified through 

adjustments of leveling feet. A generic model of the DUFF test assembly can be seen in Figure 3. 

Thermal Requirements 

Flattop is typically configured for two temperature measurements of the uranium core: one at the 

outer surface positioned in the core pedestal, and the other attached to a probe positioned in the glory 
NASA/TM—2013-216542 6 

hole. The DUFF heat pipe would replace the glory hole probe, leaving only the outside surface 

measurement of the fuel at the pedestal mount. The configuration changes with the internal uranium 

plugs, reflector sleeve, and heat pipe insertion had never been tested and would leave reactivity levels to 

be modeled computationally. The LANL team modeled a number of Flattop free run scenarios and 

compared against Flattop test data to verify model predictions. Once the Flattop model had been verified 

it was modified to incorporate the DUFF test configuration and used to predict expected performance. 

The Glenn team would eventually use the DUFF model predictions as inputs for the electrically simulated 

testing, and provide the heat pipe and conversion data back to LANL for model improvements. The initial 

DUFF model estimated that a 30 cent insertion could deliver up to 700 W of thermal power to the 

evaporator, ramping the glory hole temperature from 23 °C up to 300 °C in 2 min. The time period 

between reaching the peak fuel temperature and reactor scram was not specified but was estimated to be 

approximately 10 min. This scenario was defined by the Glenn team as the maximum transient condition 

that the heat pipe and conversion system would have to endure. A steady-state scenario was also 

examined using a 10 min ramp from room temperature up to a steady-state glory hole temperature of 

200 °C that would be held for 10 min. The heat pipe and conversion system would have to make power in 

both scenarios. 

Heat Pipe Design 

The heat pipe design was driven by the transient and steady-state thermal requirements described 

above as well as the horizontal orientation while in Flattop. The major concern with the initial design was 

to minimize thermal resistance through the heat pipe knowing that reactor run times and temperatures 

were not well defined. The expected temperatures were somewhat troublesome as few heat pipe fluids are 

known to perform well in the range between 200 and 300 °C. Dowtherm A and water were tested to 

determine which fluid would ultimately provide the best performance for the DUFF heat pipe. Water is 

typically used up to 200 °C but extending the operation to 300 °C would require additional analysis with 

pressures exceeding 1200 psi. Dowtherm A would operate at a much lower pressure (35 psi at 300 °C), 

allowing a thinner wall section, but had inferior heat transfer properties to water. The horizontal heat pipe 

orientation, which is often used for zero-gravity simulations for space flight articles, would require the 

heat pipe to have significant pumping capacity in the wick structure to handle the high heat flux transient 

requirements. These constraints would require the heat pipe wick to have superior qualities in all major 

categories; capillary pumping, thermal conductivity, and permeability. A Glenn-designed heat pipe model 

was used to predict the performance of several wick designs using both water and Dowtherm as the 

working fluid. Figure 4 shows the expected performance of the heat pipe using the final design 

configuration and the two working fluids. 

After a detailed analysis was completed, it was determined that a nickel sintered wick with integral 

arteries would provide the best performance given the design requirements and short schedule. Three 

sizes of nickel particles, 100-, 200-, and 325-mesh, were prepared for the wick structure to establish a 

range of capillary performance. Stainless steel (316L) was chosen as the heat pipe material to allow 

sintering of the nickel wick and provide strength at the operating temperatures. The fact that these heat 

pipes were only intended for the DUFF test, as opposed to a flight design, relaxed the weight constraints, 

allowing use of the heavier stainless and nickel materials. Grade 2 titanium was also considered, but 

lacked the strength needed to contain the water pressures during the higher temperature operations as well 

as the uncertainty of its compatibility with Dowtherm. Details regarding the heat pipe design can be seen 

in Figure 5. 
NASA/TM—2013-216542 7 

 

Figure 4.—Predicted heat pipe performance using water, left, and Dowtherm A, right. 
NASA/TM—2013-216542 8 

Figure 5.—DUFF heat pipe schematic, dimensions in inches. 

Three DUFF heat pipes were fabricated for initial Glenn checkout testing, each with a different 

particle size. The 325-mesh heat pipe was tested first with Dowtherm A as the working fluid. The initial 

testing quickly showed that the heat pipe was not performing as predicted and the temperature drops were 

more significant than anticipated. It was quickly determined that the Dowtherm fluid was not going to 

function as expected for the DUFF testing. It is presumed that insufficient wetting between the fluid and 

wick material affected the pumping capacity and artery priming ability. This translated into a decreased 

capacity in the thermal power and would not suffice the DUFF requirements. 

The second 200-mesh heat pipe was charged with water as the working fluid and tested over a wide 

range of temperatures and power throughputs. The water heat pipe worked as predicted and was able to 

carry the expected power to the Stirling convertors over numerous transient and steady-state test 

scenarios. The water also provided a much earlier startup temperature (50 °C) than the Dowtherm, which 

gave the heat pipe more time to heat up the Stirling convertors as the fuel temperature was rising. The 

only downfall of the water was the pressures that were produced at 300 °C which required additional 

testing and analysis. To ensure that the stainless tube would not rupture during testing, the heat pipe 

assembly was hydrostatically tested to 4000 psi as well as operated at above nominal conditions up to 

350 °C. Performance characteristics of the DUFF heat pipe and convertors can be seen in the testing 

section of this paper. 

Stirling Power Conversion 

Fabrication of an ASC-like convertor with a heat pipe thermal interface was outside the budget and 

time constraints of this project. Therefore, the DUFF convertor had to be chosen from the existing stock

of RPS-style convertors. Since neither the Flattop reactor nor any of the candidate convertors were 

designed to transfer heat through a heat pipe, temperature drops across the thermal interfaces were 

uncertain and expected to be rather large. On the reactor side, this was caused by the 5-mil radial gap and 

the inability to use thermal interface materials. On the convertor side, this was caused by the heat 

collector design of all candidate convertors, which required low surface area in the heat pipe condenser 

section and relatively large temperature drops. Allowing for margin on both the maximum fuel 

temperature and the temperature drop across the suboptimal thermal interfaces, the convertor had to 

produce power at a worst-case hot-end temperature of 150 °C. Most of the Stirling convertors that have 

been designed for space applications (both RPS and FPS) operate at a nominal hot-end temperature 

between 550 to 850 °C, and do not produce power below 200 °C. The one exception was an early 

predecessor of the ASC, called the Buzz convertors which had been modified for low-temperature 

operation (Figure 6). 
NASA/TM—2013-216542 9 

Figure 6.—Buzz convertors mounted being built up at Glenn. 

When these convertors were run at the minimum coolant temperature of 50 °C, the convertor 

pressure vessel gas temperature reached 0 °C. For an expected hot-end temperature between 150 and 

250 °C, the temperature ratio ranges between 1.55 and 1.92, which was enough for the convertors to 

produce power. Although the Buzz convertors do not represent the state of the art in Stirling design and 

performance, they were affordable, available, and compatible with the DUFF test constraints, making 

them the best choice for this proof-of-concept test. 

Final Preparation and Testing 

Several steady-state and transient tests were run at Glenn using electrical heaters prior to nuclear 

testing at DAF. Steady-state performance maps were used to verify temperature drop estimates across 

thermal interfaces and verify that the Buzz convertors produced power at the required temperatures. An 

electrically heated core simulator was manufactured to match the hole dimensions of the Flattop fuel so 

that measured temperature drops and heat fluxes accurately reflected those that would be experienced 

during nuclear testing. Figure 7 shows temperature drop data taken from electrically heated testing at 

Glenn. The data showed that the fuel to hot-end temperature drop ranged from 50 to 100 °C over the 

expected convertor operating range, depending on the heat draw from the convertors. Approximately half 

of this temperature drop occurred across the thermal interface between the fuel simulator and the heat 

pipe. The other half is attributed to thermal resistance through the copper adapter interfaces, and along the 

axial length of the Stirling heat collector. The convertor produced power at each of these conditions, 

including the worst-case fuel temperature condition of 200 °C, corresponding to a convertor hot-end 

temperature of 150 °C. 

Transient tests were run to verify operation and performance under the most strenuous conditions and 

to validate modeling efforts. Figure 8 shows a maximum power startup transient, in which the heater 

delivered its maximum power of 750 W for just under 3 min and was then set to lower power operation to 

maintain the fuel simulator temperature. The initial transient represented the highest heat flux condition 

for the heat pipe, and thus carried the highest risk of evaporator dryout. However, this test demonstrated 

that the heat pipe was able to handle the heat load and successfully transfer heat to the Stirling hot end. 

This test was also used to estimate the minimum required startup time for DUFF testing and to validate 

the balance of plant portion of the nuclear system model developed by LANL. 
NASA/TM—2013-216542 10 

Figure 7.—Steady-state temperature data taken during electrically heated 

testing at Glenn. 

Figure 8.—Electrically heated simulation of a Flattop 30 cent insertion. 

Conclusion 

Small fission power systems (FPSs) are capable of providing new mission-enabling options for 

power-rich science missions and human exploration precursor missions in the 1- to 10-kWe range. 

Currently, this power level is absent from NASA’s nuclear power portfolio with a low-power (<300 W) 

flight-qualified Radioisotope Power System (RPS) on the low end and numerous nonflight-qualified 

higher power (>10-kWe) FPS on the high end. Since the Systems Nuclear Auxiliary Power (SNAP) 

program, FPS have made significant nonnuclear technology advancements but have been unsuccessful in 

achieving flight status due, in part, to the flight development cost and its uncertainty. The systems are at a 

point where nuclear testing is essential to maturing the technology beyond its current state, both 

technically and programmatically. The Demonstration Using Flattop Fission (DUFF) test was a first step 

in addressing nuclear testing of FPS and maturing the technology beyond electrically simulated testing. In 
NASA/TM—2013-216542 11 

a few short months, the Glenn team performed the electrically simulated system test using a stainless steel 

water heat pipe and a pair of Stirling convertors, which were delivered to LANL for a successful DUFF 

test, proving that heat-pipe-cooled reactors are a viable option for Kilopower FPS. 

References 

1. Mason, Lee; Poston, David; and Qualls, Louis: System Concepts for Affordable Fission Surface 

Power. NASA/TM—2008-215166, 2008. 

2. Mason, Lee, et al.: A Small Fission Power System for NASA Planetary Science Missions. 

Proceedings of Nuclear and Emerging Technologies for Space 2011, Albuquerque, NM, 2011. 

Bibliography 

Mason, Lee S.; Palac, Donald T.; and Harlow, Scott: Fission Surface Power Technology Development 

Status. NASA/TM—2010-216249, 2009.