Maxwell H. Briggs, Marc A. Gibson, and Steven M. Geng

Glenn Research Center, Cleveland, Ohio

Jon Boise Pearson

Marshall Space Flight Center, Huntsville, Alabama

Thomas Godfroy

Maximum Technology Corporation, Huntsville, Alabama 

Development Status of the Fission Power System 

Technology Demonstration Unit

NASA/TM—2012-217717

September 2012

AIAA–2012–3711
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Maxwell H. Briggs, Marc A. Gibson, and Steven M. Geng

Glenn Research Center, Cleveland, Ohio

Jon Boise Pearson

Marshall Space Flight Center, Huntsville, Alabama

Thomas Godfroy

Maximum Technology Corporation, Huntsville, Alabama 

Development Status of the Fission Power System 

Technology Demonstration Unit

NASA/TM—2012-217717

September 2012

AIAA–2012–3711

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Space Administration

Glenn Research Center

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NASA/TM—2012-217717 1 

Development Status of the Fission Power System Technology 

Demonstration Unit 

Maxwell H. Briggs, Marc A. Gibson, and Steven M. Geng 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Jon Boise Pearson 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, Alabama 35812 

Thomas Godfroy 

Maximum Technology Corporation 

Huntsville, Alabama 35814 

Abstract 

This paper summarizes the progress that has been made in the development of the Fission Power 

System Technology Demonstration Unit (TDU). The reactor simulator core and Annular Linear Induction 

Pump have been fabricated and assembled into a test loop at the NASA Marshall Space Flight Center. A 

12 kWe Power Conversion Unit (PCU) is being developed consisting of two 6 kWe free-piston Stirling 

engines. The two 6 kWe engines have been fabricated by Sunpower Inc. and are currently being tested 

separately prior to integration into the PCU. The Facility Cooling System (FCS) used to reject convertor 

waste heat has been assembled and tested at the NASA Glenn Research Center (GRC). The structural 

elements, including a Buildup Assembly Platform (BAP) and Upper Truss Structure (UTS) have been 

fabricated, and will be used to test cold-end components in thermal vacuum prior to TDU testing. Once 

all components have been fully tested at the subsystem level, they will be assembled into an end-to-end 

system and tested in thermal vacuum at GRC. 

Nomenclature 

AFSP Affordable Fission Surface Power 

ALIP Annular Linear Induction Pump 

ATC ALIP Test Circuit 

BAP Buildup Assembly Platform 

BOP Balance of Plant 

FCS Facility Cooling System 

FPS Fission Power System 

GRC NASA Glenn Research Center 

INL Idaho National Laboratory 

LANL Los Alamos National Laboratory 

MSFC Marshall Space Flight Center 

NaK Sodium-Potassium Eutectic 

ORNL Oak Ridge National Laboratory 

PCU Power Conversion Unit 
NASA/TM—2012-217717 2 

RxSim Reactor Simulator NaK Loop 

SNL Sandia National Laboratories 

TDU Technology Demonstration Unit 

TRL Technology Readiness Level 

UTS Upper Truss Structure 

WHX Waste Heat Exchanger 

Introduction 

The Fission Power System (FPS) team is developing technologies for an affordable FPS to support 

future exploration missions. The FPS Initial Concept Definition (Ref. 1) is based on surface power 

applications, using a below-grade reactor and vertical truss to support Balance of Plant (BOP) 

components (Fig. 1). 

This FPS uses a single fast-spectrum uranium dioxide reactor to heat a liquid sodium-potassium 

eutectic (NaK). A pair of fully-redundant Annular Linear Induction Pumps (ALIPs) are used to circulate 

the NaK to a pair of intermediate heat exchangers (IHX). The IHX is a NaK-to-NaK heat exchanger that 

provides a buffer between the reactor and the BOP. Each of the two intermediate NaK loops service two 

PCUs. There are four water heat rejection loops (one for each PCU) that transfers the waste heat to two 

radiator wings (two loops per wing). A more detailed description of the FPS reference concept is given in 

Reference 1. 

The FPS Technology Demonstration Unit (TDU) is designed to raise the Technology Readiness Level 

(TRL) of FPS technology by demonstrating end-to-end system performance and robustness in a relevant 

environment. The TDU will be a full-scale demonstration of the primary FPS components (excluding the 

nuclear reactor). The TDU accomplishes this by combining a single NaK loop with a non-nuclear core 

simulator, an ALIP, a 12 kWe PCU, a water heat rejection loop, and a radiator, for an end-to-end system 

test in a thermal vacuum test chamber (Refs. 2 and 3). Figures 2 and 3 show a schematic and an artistic 

rendering of the TDU, respectively. 

The hot-end subsystem, which includes the core simulator, ALIP, volume accumulator, and custom 

instrumentation, is referred to as the RxSim (Reactor Simulator NaK Loop), and will be tested at the 

NASA Marshall Space Flight Center (MSFC). PCU subsystem testing will occur at Sunpower, Inc. 

Cold-end subsystem testing, including the water pump, volume accumulator, facility cooling system, and 

subscale radiators, will be tested at NASA Glenn Research Center (GRC). When these tests have 

concluded, all subsystems will be delivered to GRC and assembled into the TDU where it will be tested 

as an end-to-end system. 

Figure 1.—Affordable FPS configured for surface power applications. 
NASA/TM—2012-217717 3 

Figure 2.—TDU schematic. 

Figure 3.—TDU assembly in GRC thermal vacuum facility. 

TDU Component Development 

This section gives a brief description of the TDU component fabrication status, plans for testing, and 

references to more detailed information. 

Core Simulator 

Core simulators are designed to thermo-hydraulically mimic the behavior of a nuclear core. Nuclear 

fuel rods are replaced with electrically heated thermal simulators with the same dimensions and thermal 

power (Ref. 4). A variety of core simulators have been built and tested by MSFC, including heat-pipecooled, gas-cooled, and liquid-metal- cooled designs. These reactor simulators have been coupled with a 

control system that enables them to respond to thermal transients in a similar fashion to a real reactor, by 

incorporating a software-based reactivity feedback control algorithm (Ref. 5). 

The nuclear reactor core design for FPS was done by the Los Alamos National Laboratory (LANL) 

during the Affordable Fission Surface Power (AFSP) Study (Ref. 6). The 37-pin NaK-cooled design FPS 

design has a nominal power of 200 kW. The core simulator built for the TDU has a nominal thermal 

power of 45 kW, with a peak power capability of 100 kW. The TDU core simulator is FPS sized, but only 

provides the power needed for a single PCU. Thermal simulator rods were custom built by MSFC for use 

in the TDU core (Ref. 7). The control system is being designed to mimic the behavior of the AFSP reactor 

(Ref. 8). The core simulator control system hardware was assembled by Oak Ridge National Laboratory 

(ORNL), data acquisition and control software was written by MSFC, and reactivity feedback control 

software was developed by Sandia National Laboratories (SNL) based on reactor kinetics calculations 

performed by LANL (Ref. 9). Figures 4 and 5 show a three-dimensional rendering of the core simulator 

and the fabricated core simulator installed in the RxSim loop, respectively. 
NASA/TM—2012-217717 4 

Figure 4.—Cutaway view of TDU core simulator. 

Figure 5.—Core simulator installed in 

RxSim at MSFC. 

Annular Linear Induction Pump (ALIP) 

An Annular Linear Induction Pump (ALIP) is an electromagnetic liquid metal pump with no moving 

parts. ALIPs were chosen for the FPS because of their high reliability and long-life operation. Two ALIPs 

were designed and fabricated by the Idaho National Laboratory (INL). One of the ALIPs was designed 

specifically for use in the TDU to deliver full NaK flow to a single PCU. The other pump was a 

prototypic FPS pump, capable of delivering full NaK flow to four PCUs. 

Both pumps have been fabricated and tested in the ALIP Test Circuit (ATC) at MSFC (Refs. 11 and 

12). These tests defined the operating envelope and measured steady-state performance and transient 

response of the pumps. The TDU ALIP is currently being tested with the core simulator and other hot-end 

components in the RxSim loop at MSFC. Figure 6 shows the TDU ALIP installed in the ATC. 

Stirling Power Conversion Unit 

The 12-kWe Stirling PCU and electronic controller are being developed by Sunpower, Inc., of 

Athens, Ohio, under contract to NASA GRC. The 12-kWe PCU is composed of two 6-kWe free-piston 

Stirling engines that are joined (thermodynamically coupled) at their hot ends as shown in Figure 7. This 

dual-opposed configuration results in a dynamically balanced mechanical system. In this configuration, 

the two engines share a common expansion space and are rigidly connected in the hot region. The rigid 
NASA/TM—2012-217717 5 

Figure 6.—TDU ALIP installed in ATC at MSFC. 

connection minimizes forces that are transmitted to external attachments such as the plumbing. This 

configuration also results in a smaller footprint and a simplified heater head design. The unit is designed 

to produce 12 kWe at the operating conditions of a 570 C hot-end temperature, a 111 C cold-end 

temperature, and a mean working space pressure of 6.2 MPa. The design frequency is 60 Hz and the 

working fluid is helium. The unit is about 1.1 m in length and 0.3 m in diameter and weighs about 190 kg. 

The PCU features a unique NaK heat exchanger heater head. This heater head allows the PCU to be 

integrated with a pumped NaK loop, which is the method chosen in the FPS reference concept for 

transporting thermal energy from the heat source to the Stirling PCU. The heat exchanger is designed 

assuming NaK-78 (78 percent K and 22 percent Na) at a supply temperature of 577 C, pressure of 

150 kPa, and flow rate of 1.75 kg/s. The use of NaK for heat input and the use of the thermodynamically 

coupled configuration have both been tested and proven with subscale component tests (Refs. 13 to 15). 

A PCU electronic controller has been designed and built that features the key characteristics required 

for the FPS application. The controller is capable of (1) synchronizing the piston amplitudes of the two 

engines, (2) constant command frequency operation, (3) controlling the engine piston amplitudes thereby 

modulating power output from zero to full power, (4) dissipating any unused power (i.e., power not 

consumed by the user load), and (5) converting the output voltage to 120 V dc. The controller (excluding 

the DC-DC convertor) has been successfully tested on a pair of 1-kWe convertors that are similar in 

design to the 6-kWe PCU convertors. The controller will be tested further after the PCU has been 

assembled. A more complete description of the PCU and controller can be found in Reference 16. 

Figure 7.—12-kWe Stirling PCU. 

NaK Heat Exchanger 

Engine #1 Engine #2 
NASA/TM—2012-217717 6 

Structural Components 

The TDU structural components can be broken into two main pieces, the Buildup Assembly Platform 

(BAP) and the Upper Truss Structure (UTS) (Fig. 8). The BAP is a facility integration structure that 

provides support for all systems and personnel. The BAP will use the existing facility rail system to move 

in and out of the chamber to facilitate modifications and additions to test components throughout the test 

program. The BAP was designed as a mating adapter between the power conversion systems on the upper 

half and the reactor simulator system on the lower half. During TDU preparations, this will allow parallel 

fabrication and buildup as systems from several team centers are combined. 

Attached to the top of the BAP is the UTS, which will support all the power conversion and heat 

rejection components. The UTS was designed to emulate a full-scale 40-kWe fission surface power truss 

structure (Fig. 1) that would allow all the components to be mounted in a representative arrangement. 

This design will allow a higher fidelity TDU test as well as accommodate future additions of multiple 

PCUs, deployable radiator arrays, PMAD systems, and other additions that support future development 

without the cost of rebuilding. 

Water Pump for Thermal Vacuum 

The TDU water pump must pump high-pressure, high-temperature water and operate in vacuum, 

without convective cooling of the motor. This requires the use of high-temperature motors, the ability to 

cool the motor with the process fluid, and hermetic sealing of the process fluid. A commercial pump from 

the Nikisso Company met these requirements and is shown schematically in Figure 9. This pump has 

been delivered to GRC and will be tested with other cold-end components in thermal vacuum at GRC 

prior to TDU assembly. 

Figure 8.—UTS mounted to BAP at GRC. 

Figure 9.—Hermetically sealed self-cooling water pump. 
NASA/TM—2012-217717 7 

Waste Heat Exchanger (WHX) 

The WHX is a water-to-forced-air heat exchanger that is used to reject waste heat from the PCU 

during TDU testing. The WHX is a commercially available unit designed to reject 52 kWt at the TDU 

nominal conditions of 0.375 kg/s water flow at 400 K. The heat rejection can be tuned to a specific value 

by controlling the forced convection on the fin-side of the heat exchanger. The WHX was assembled into 

a test loop and both performance and control were verified at GRC. Figure 10 shows the WHX outside of 

the vacuum chamber at GRC. 

Affordable Radiator Proof of Concept 

The original TDU test plan called for two phases of testing. In the first phase, PCU cooling would be 

provided by a pumped water loop coupled to the WHX, which rejects heat to ambient air outside of the 

vacuum chamber. This phase of testing is not a true end-to-end system because the radiators, a major FPS 

component, are not present. During the second phase of testing, the WHX was to be replaced with 

prototypic radiators (Ref. 17) allowing for a true end-to-end system test. However, budget reductions 

have eliminated the second phase of TDU testing. Without an end-to-end system test it is impossible to 

advance FPS technology beyond TRL 5, which reduces the scope of TDU testing. 

In response, the FPS team has developed an affordable radiator design, focused on cost reduction and 

maintaining radiator performance (Ref. 18). This radiator could provide an affordable option for restoring 

the original scope of the TDU. The affordable radiator design reduces cost by eliminating high-cost 

materials, reducing the overall number of components, and simplifying processing and assembly. The 

original design called for two carbon-carbon facesheets to be bonded to heat pipes through a POCO foam 

saddle. The new design uses a single aluminum facesheet bonded directly to each heat pipe, resulting in a 

dramatic reduction in both cost and weight. The difference in these panel layouts is shown in Figure 11. 

The FPS team has not tested single facesheet designs as thoroughly as dual facesheet designs, so 

single facesheet designs are considered a higher technical risk. In order to reduce this technical risk, a 

proof-of-concept radiator, based on the new design, has been fabricated and will be tested in thermal 

vacuum to verify functionality and performance. 

Figure 10.—WHX outside of vacuum 

chamber at GRC. 

 (a) (b)

Figure 11.—Comparison of (a) dual facesheet and (b) single facesheet panel layouts. 

Dual

Facesheet

Saddle

Titanium - Water 

Heat Pipe

Titanium 

Heat Pipe

Single 

Facesheet

Direct Bond
NASA/TM—2012-217717 8 

Subsystem Testing Status 

Reactor Simulator Loop 

The RxSim consists of the pumped NaK loop and all of its components except for the PCU, and its 

purpose is to deliver hot NaK to the PCU (Ref. 19). The RxSim consists of the core simulator, ALIP, 

elctromagnetic flow meter, cold trap, and volume accumulator. It also includes the control system, lower 

reservoir, gas-to-NaK heat exchanger (in place of the PCU), and all associated instrumentation. The 

RxSim has been assembled and is currently being tested at MSFC, prior to shipment to GRC, where the 

PCU and heat rejection will be integrated, and the full TDU tested in a thermal-vacuum environment. The 

RxSim test will allow all its components to be checked out as an integrated system. Figure 12 shows the 

RxSim loop outside of the vacuum facility at MSFC. 

Power Conversion Testing 

Since the PCU consists of two Stirling engines, Sunpower decided to build, test, and verify the 

engines individually prior to integration into the PCU. Electrically heated test heads were fabricated to 

allow for single-engine operation. In this configuration, the engines are bolted to a large mass (4000 lb) to 

minimize housing motion. 

Fabrication of engine #1 was completed in the Fall of 2011. Checkout testing began in October and 

full-power operation (6.1 kWe) was demonstrated in December 2011. Figure 13 shows engine #1 on test 

at Sunpower with the heater head insulation removed. A series of thirty 1-kWe cartridge heaters (allows 

for insulation losses) were used to supply thermal energy to the heater head. Fabrication of engine #2 was 

completed in the Spring of 2012, and check-out testing began in May of 2012. 

After satisfactory performance is achieved for each engine, the two engines will be tested in a dualopposed, thermodynamically coupled configuration using an electrically heated dual engine test head. After 

successful operation using the electrically heated head, the PCU will be reassembled with the NaK heat 

exchanger heater head in preparation for delivery to NASA GRC. Delivery is expected by August 2013. The 

PCU will then be integrated into the TDU at NASA GRC for full-scale system demonstration testing. 

Figure 12.—RxSim loop at MSFC. 
NASA/TM—2012-217717 9 

Figure 13.—Engine #1 on test at Sunpower. 

Cold-End Testing 

Cold-end testing consists of subsystem testing and radiator proof-of-concept testing. During 

subsystem testing, the water pump and volume accumulator will be mounted on the UTS and tested in 

vacuum at GRC. This test will be used to validate fill, drain, and emergency cooling procedures to verify 

system functionality and to document pump performance. 

Radiator proof-of-concept testing will be used to verify functionality, measure radiator performance, 

and validate detailed radiator models. The results of this test will be used to finalize the TDU radiator 

design. Once the design is finalized, the team will have more accurate cost estimates, which could be used 

to assess the feasibility of building radiators for TDU, returning it to an end-to-end system demonstration. 

Conclusion 

The Fission Power System (FPS) project is a hardware-focused project that strives to demonstrate 

component and system technologies through rigorous testing. The focus of this project is the Technology 

Demonstration Unit (TDU), an end-to-end system demonstration of an FPS, which is currently scheduled 

to begin testing in early 2014. Subscale and full-scale component testing and demonstration have been 

completed. Full-scale TDU components have been fabricated and are being assembled into subsystems to 

be tested at Glenn Research Center, Sunpower, Inc., and Marshall Space Flight Center. These subsystem 

tests are either being conducted or are in the buildup phase and are on schedule for the start of TDU 

testing in early 2014. The FPS team, consisting of many NASA, Department of Energy, and industry 

partners, has showed clear progress toward achieving the goal of a non-nuclear system-level 

demonstration of an FPS. 
NASA/TM—2012-217717 10 

References 

1. Fission Surface Power Team, “Fission Surface Power System Initial Concept Definition,” 

NASA/TM—2010-216772, 2010. 

2. Mason, L., et al., “Design and Test Plans for a Non-Nuclear Fission Power System Technology 

Demonstration Unit,” NASA/TM—2011-217100, 2011. 

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

Update,” NASA/TM—2011-216976, 2011. 

4. Bragg-Sitton, S.M., et al., “Heater Development, Fabrication, and Testing: Analysis of Fabricated 

Heaters,” NASA/TM—2008-215466, 2008. 

5. Bragg-Sitton, S., Webster, K., “Application of Simulated Reactivity Feedback in Non-Nuclear 

Testing of a Direct Drive Gas Cooled Reactor,” Internal MSFC report ER11-05-WI4c4-001, 2005. 

6. Mason, L., Poston, D., Qualls, L., “System Concepts for Affordable Fission Surface Power,” 

NASA/TM—2008-215166, 2008. 

7. Garber, A., Pearson, J.B., “Design and Test of Advanced Thermal Simulators for an Alkali MetalCooled Reactor Simulator,” Nuclear and Emerging Technologies for Space, 2011. 

8. Poston, D., “System Modeling Comparisons of the Fission Surface Power (FSP) System and the FSP 

Technology Demonstration Unit,” Nuclear and Emerging Technologies for Space, 2011. 

9. Dughie, D., Dixon, D., Poston, D., “Calculation of Kinetics Parameters for the Affordable Fission 

Surface Power Reference Design,” Nuclear and Emerging Technologies for Space, 2011. 

10. Polzon, K., et al., “Performance Testing of a Prototypic Annular Linear Induction Pump for Fission 

Surface Power,” NASA/TP—2010-216430, 2010. 

11. Adkins, H., Werner, J.E., “Analysis of the Fission Surface Power Annular Linear Induction Pump 

Design Tools Based on Performance Test Results,” Idaho National Laboratory Technical Report 

INL/EXT–10–18211, Idaho Falls, ID, 2010. 

12. Polzin, K., “Performance Testing of an Annular Linear Induction Pump for Incorporation into a 

Fission Power System Technology Demonstration Unit” (To be published) 

13. Geng, S., Briggs, M., Hervol, D., “Performance of a Kilowatt-Class Stirling Power Conversion 

System in a Thermodynamically Coupled Configuration,” Nuclear and Emerging Technologies for 

Space, Paper 3269, 2011. 

14. Briggs, M., Geng, S., Pearson, J., Godfroy, T., “Summary of Test Results from a 1 kWe-Class FreePiston Stirling Power Convertor Integrated with a Pumped NaK Loop,” International Energy 

Conversion Engineering Conference, Paper 7173, 2010.

15. Briggs, M., “Dynamic Behavior of Kilowatt Class Stirling Convertors with Coupled Expansion 

Spaces,” Nuclear and Emerging Technologies for Space, Paper 3032, 2012. 

16. Wood, G., Buffalino, A., Holliday, E., Penswick, B., Gedeon, G., “Free-Piston Stirling Power 

Conversion Unit for Fission Surface Power, Phase I Final Report,” NASA/CR—2010-216750, 2010. 

17. Ellis, D., Calder, J., Siamidis, J., “Summary of the Manufacture, Testing and Model Validation of a 

Full-Scale Radiator for Fission Surface Power Applications,” Nuclear and Emerging Technologies 

for Space, Paper 3181, 2011. 

18. Briggs, M., “Conceptual Design of an Affordable Radiator for the Fission Power System 

Technology Demonstration Unit,” Nuclear and Emerging Technologies for Space, Paper 3031, 

2012. 

19. Godfroy, T., Pearson, B., Webster, K., “Build Status of Reactor Simulator for Fission Surface Power 

Technology Demonstration Unit,” Nuclear and Emerging Technologies for Space, Paper 3092, 

2012. 

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14. ABSTRACT

This paper summarizes the progress that has been made in the development of the Fission Power System Technology Demonstration Unit 

(TDU). The reactor simulator core and Annular Linear Induction Pump have been fabricated and assembled into a test loop at the NASA 

Marshall Space Flight Center. A 12 kWe Power Conversion Unit (PCU) is being developed consisting of two 6 kWe free-piston Stirling 

engines. The two 6 kWe engines have been fabricated by Sunpower Inc. and are currently being tested separately prior to integration into the 

PCU. The Facility Cooling System (FCS) used to reject convertor waste heat has been assembled and tested at the NASA Glenn Research 

Center (GRC). The structural elements, including a Buildup Assembly Platform (BAP) and Upper Truss Structure (UTS) have been 

fabricated, and will be used to test cold-end components in thermal vacuum prior to TDU testing. Once all components have been fully 

tested at the subsystem level, they will be assembled into an end-to-end system and tested in thermal vacuum at GRC. 

15. SUBJECT TERMS

Stirling power conversion; Linear alternator; Power conversion; Heat rejection; Nuclear power systems; Dynamic power conversion; 

Power; Electromagnetic pump 

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