Maxwell Briggs, Marc Gibson, and David Ellis

Glenn Research Center, Cleveland, Ohio

James Sanzi

Vantage Partners, LLC, Brook Park, Ohio 

Cold-End Subsystem Testing for the Fission Power 

System Technology Demonstration Unit

NASA/TM—2013-216545

August 2013

NETS–2013–6728
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Maxwell Briggs, Marc Gibson, and David Ellis

Glenn Research Center, Cleveland, Ohio

James Sanzi

Vantage Partners, LLC, Brook Park, Ohio 

Cold-End Subsystem Testing for the Fission Power 

System Technology Demonstration Unit

NASA/TM—2013-216545

August 2013

NETS–2013–6728

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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cosponsored by the American Nuclear Society and Aerojet

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

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-216545 1

Cold-End Subsystem Testing for the Fission Power System 

Technology Demonstration Unit

Maxwell Briggs, Marc Gibson, and David Ellis

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

James Sanzi

Vantage Partners, LLC

Brook Park, Ohio 44142

Abstract

The Fission Power System (FPS) Technology Demonstration Unit (TDU) consists of a pumped 

sodium-potassium (NaK) loop that provides heat to a Stirling Power Conversion Unit (PCU), which 

converts some of that heat into electricity and rejects the waste heat to a pumped water loop. Each of the 

TDU subsystems is being tested independently prior to full system testing at the NASA Glenn Research 

Center. The pumped NaK loop is being tested at NASA Marshall Space Flight Center; the Stirling PCU 

and electrical controller are being tested by Sunpower Inc.; and the pumped water loop is being tested at 

Glenn. This paper describes cold-end subsystem setup and testing at Glenn. The TDU cold end has been 

assembled in Vacuum Facility 6 (VF 6) at Glenn, the same chamber that will be used for TDU testing.

Cold-end testing in VF 6 will demonstrate functionality; validated cold-end fill, drain, and emergency 

backup systems; and generated pump performance and system pressure drop data used to validate models.

In addition, a low-cost proof-of concept radiator has been built and tested at Glenn, validating the design 

and demonstrating the feasibility of using low-cost metal radiators as an alternative to high-cost 

composite radiators in an end-to-end TDU test.

Introduction

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

future exploration missions. The FPS Initial Concept Definition (Fission Surface Power Team (2011)) is 

based on surface power applications, using a below-grade reactor and vertical truss to support balance-ofplant (BOP) components. The FPS uses a single fast-spectrum uranium dioxide reactor to heat a liquid 

sodium-potassium (NaK) eutectic. Two 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 Power Conversion Units (PCUs). There are four water heat rejection loops (one for 

each PCU) that transfer the waste heat to two radiator wings (two loops per wing) (Figure 1). A more 

detailed description of the FPS reference concept is given in Briggs (2012a).

The FPS Technology Demonstration Unit (TDU) was 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 original TDU design included prototypic versions of all major FPS components, 

excluding the nuclear reactor as shown in Figure 2 (Mason (2006)). In the original design, an electrically 

heated core simulator provided heat to the primary liquid metal loop, which was pumped using an ALIP. 

The primary liquid metal loop flowed through an IHX to a secondary liquid metal loop, which transferred 

the heat to the Stirling hot-end heat exchanger. The PCU converted some of the heat into electricity and 

rejected waste heat to a pumped water loop. This water loop exchanged heat with six full-scale FPS 

radiator panels, which radiated the waste heat to the thermal vacuum environment. 
NASA/TM—2013-216545 2

Figure 1.—Original fission power system design schematic.

Figure 2.—Original Technology Demonstration Unit design schematic.

Budget reductions have driven many design changes in the TDU over the past few years, reducing the 

scope of the TDU and preventing the TDU from achieving the originally intended goal of being an endto-end system-level demonstration (Mason et al. (2011) and Briggs (2012b)). Radiators and the 

intermediate liquid metal loop were removed completely from the TDU (Figure 3). In lieu of radiators, 

the current TDU design rejects waste heat from the Stirling convertors by flowing cooling water outside 

of the vacuum chamber to a liquid-to-forced air heat exchanger called the Waste Heat Exchanger (WHX). 

The response of the forced air heat exchanger is fundamentally different from the response of a radiator, 

so temperature feedback and thermal transients in the new TDU design will be substantially different 

from those expected in a flight-like FPS. 
NASA/TM—2013-216545 3

Figure 3.—Current Technology Demonstration Unit design schematic.

The three major TDU subsystems are the hot end, the PCU, and the cold end. Prior to assembly and 

testing of the full TDU at Glenn, each of the subsystems is undergoing independent testing. The hot end, 

referred to as the RxSim, has already been tested at Marshall. The Stirling PCU, controller, and PMAD 

are being tested at Sunpower, Inc. The cold-end Heat Rejection System (HRS) is being tested at Glenn

and is the subject of this paper. HRS testing will verify functionality, validate operating procedures, and 

document component performance. Additional component level testing has been completed on an 

affordable, proof-of-concept radiator design, which could potentially provide a cost-effective option for 

restoring realistic thermal transients and thermal feedback to the TDU cold end. 

Heat Rejection System

Heat Rejection System Test Design

The HRS consists of a commercial-off-the-shelf hermetically sealed water pump, a volume 

accumulator used to accommodate thermal expansion of the water and control loop pressure, and a WHX

(Figure 3). During TDU testing, the PCU would transfer 36 kWt of heat to the water through the Stirling 

rejector. This heat would then be rejected through the WHX. During HRS subsystem testing at Glenn, the 

PCU will not be in the loop, so the heat load on the water comes only from pump inefficiency and fluid 

friction through the loop, estimated to be less than 1 kWt. The WHX rejects more than 1 kWt even with 

the fan turned off, which would prevent the water loop from reaching the nominal operating temperature. 

To reach the nominal operating temperature, the WHX has been replaced with a temperature control loop 

for HRS subsystem testing. The temperature control loop consisted of an oil heater/cooler coupled to the 

water loop through a plate-fin heat exchanger. Since the WHX has already been successfully tested at the 

component level removing it from HRS testing was not a concern.

∆P +28 kPa

at 1.75 kg/s

(0.5-2.5 kg/s)

NaK

850 K

∆P +190 kPa

at 0.375 kg/s

(0.2-0.6 kg/s)

160 kPa 440 kPa

H2O

~375 K

(925 K and

275 kPa max) 

(450 K and

690 kPa max)

Core Sim

48 kWe nom

(90 kWe max)

Waste Heat 

Exchanger

Pwr Conv

12 kWe nom

(14.4 kWe max)

PVA RVA

PP RP

∆P ≤14 kPa

∆P ≤7 kPa

∆P ≤15 kPa

Vacuum

≤1x10-5 torr

PCU Cntl

12 kWe nom

6-13.2 kWe reg.

ELS

≥10 kWe

400 Vac

η ≥10% η ≥25%

η ≥90%

120+/-6 Vdc

Waste heat 

rejected to 

ambient air 
NASA/TM—2013-216545 4

The HRS test will verify functionality and performance of the major TDU components over a wide 

range of conditions. All support systems, including the fill and drain system, emergency backup water 

system, nitrogen cover gas regulation, and instrumentation/data acquisition system will be tested.

Procedural tests, such as fill/drain and emergency cooling will be performed at atmospheric temperature 

and pressure. All performance data will be collected at a pressure <10‒6 torr. Finally, the entire loop will 

be run through a range of operating conditions in a thermal vacuum environment (<10‒6 torr < 100 K cold 

wall temperature). 

The HRS components have been mounted to the Upper Truss Structure (UTS), which is the structure 

that will house all of the TDU components during the full system-level test. Figure 4 shows the HRS 

components mounted on the UTS and assembled in VF 6 at NASA Glenn. A nitrogen line plumbed to the 

top of the volume accumulator provides a cover pressure to prevent flashing and cavitation. Water leaving 

the HRS pump flows to the temperature control loop, which has been assembled outside VF 6 (Figure 5) 

before being returned to the pump inlet. The pump will be operated in a vacuum environment across a 

range of flow rates (by varying pump speed) and pressure drops (by adjusting a throttling valve), and 

water temperatures to document pump performance through the entire range of expected operating 

conditions.

Figure 4.—Heat Rejection System and Upper Truss 

Structure Vacuum Facility 6.
NASA/TM—2013-216545 5

Figure 5.—Temperature control loop.

Affordable Radiator

Radiator Design and Fabrication

Several different combinations of component materials and layouts were considered for the affordable 

radiator design, including high-conductivity composite facesheets, low-conductivity composite 

facesheets, aluminum facesheets, graphite foam saddles, and aluminum saddles. Both single facesheet and 

dual facesheet configurations were evaluated. Panel geometry and the number of required heat pipes were 

chosen using an optimization routine to minimize mass for each configuration, allowing for radiator 

surface area margin. Table I compares the results of the optimized designs.

TABLE I.—COMPARISON OF OPTIMIZED RADIATOR DESIGNS

Description Fin 

thickness,

mm

Fin

length,

mm

Two-sided 

area, 

m2

Total 

mass,

kg

Estimated 

relative 

cost

Dual—High-conductivity composite facesheet—poco saddles 0.5 200 45.2 90.7 1

Single—High-conductivity composite facesheet 0.5 150 50.0 47 0.35

Single—Low-conductivity composite facesheet 0.5 100 48.4 53.9 0.33

Single—Aluminum facesheet 0.5 100 48.4 65.3 0.18

Dual—Low-conductivity composite facesheet—poco saddles 0.5 200 47.9 95.9 0.88

Dual—Low-conductivity composite facesheet—aluminum saddles 1 200 44.3 265 0.63

Dual—Aluminum facesheet—aluminum saddles 0.5 200 47.9 266 0.30

The clear cost winner in this trade study was the single aluminum facesheet design. Single aluminum 

facesheet designs force all of the heat through a single lower conductivity facesheet, resulting in steeper 

temperature gradients in the fin than those seen in dual composite facesheet designs. Consequently, the 

optimization routine converged on a design with reduced fin length, more required heat pipes, and a larger 

overall surface area to reject 36 kWt. Although the increase in the number of heat pipes and overall 

surface area negatively affected radiator mass, the elimination of saddles and one facesheet compensates 

for this. This trade study and preliminary design showed that single aluminum facesheet designs have the 

potential to reduce material costs by a factor of five and decrease overall radiator mass compared to the 

original high-conductivity composite dual facesheet design (Briggs (2012a)).

This conceptual design for the affordable TDU radiator was used as the basis for a subscale proof-ofconcept design, focused on process simplification, cost reduction, and extensibility to full-size TDU

panels. The proof-of-concept radiator (Figure 6) consisted of three titanium-water heat pipes coupled to
NASA/TM—2013-216545 6

two redundant water loops through a conductive aluminum manifold block. The heat pipes were bonded

directly to four 12- by 18-in. fins, with fin thickness and fin length determined by the affordable radiator 

conceptual design (Table I).

The Second Generation (2nd Gen) Radiator Demonstration Unit (RDU) (Ellis et al. (2011)) was a 

full-scale prototype radiator built by Material Innovation, Inc., and tested at Glenn. The 2nd Gen RDU 

manifold design flowed water directly over vertical heat pipe evaporators to maximize thermal 

performance. This method required that each heat pipe be welded to the manifold and required 

complicated and heavy tubing runs (Figure 7) to compensate for CTE mismatch between the manifold 

and the facesheet. The proof-of-concept radiator design sacrifices optimal thermal performance for 

simplicity, as shown in Figure 8. The heat pipes’ evaporators are horizontal so that straight tubing runs 

could be used in the manifold. The water lines and heat pipe evaporators are conductively coupled 

through an aluminum manifold block, eliminating the need for welding. The design relies upon clamping 

rather than adhesive bonding or brazing. Thermal grease minimizes the thermal resistance across the 

interfaces and allows slippage so that thermal expansion in the manifold does not induce stress in the 

facesheet. The conductively coupled manifold design has the additional benefit of allowing all heat pipes 

to remain active even if one water line was lost.

Figure 6.—Proof-of-concept radiator.

 

Figure 7.—Second Generation Radiator Demonstration Unit manifold.
NASA/TM—2013-216545 7

Figure 8.—Proof-of-concept radiator manifold.

Both adhesive bonding and brazing were considered for joining aluminum facesheets to the titanium 

heat pipes. Brazing was the clear winner from a performance standpoint as it resulted in a stronger and 

more highly conductive bond. However, pressurized heat pipes cannot be put into a brazing oven, which 

complicates radiator assembly. In addition, finding a brazing furnace capable of brazing full-scale TDU 

radiator panels could be cost prohibitive, so adhesive bonding was chosen. Several adhesives were 

considered, including two-part epoxies, some with silver additions for improved thermal conductivity, 

and structural film adhesives. Each of the epoxies and film adhesives that were considered had cure 

temperatures below the operating temperature of the heat pipe. This is important because the adhesive can 

be cured by running the heat pipe, eliminating the need for large-scale ovens. In addition, curing near the 

operating temperature reduces the thermally induced stress during operation where the adhesives are 

weakest, and raises the thermally induced stress at room temperature where the adhesives are strongest.

Several candidate adhesives were tested to identify viable candidates. Each adhesive was cured, 

thermal cycled in a rough vacuum, and single lap shear tested at Glenn to identify viable candidates for 

this application. The adhesive processing and material testing are discussed in more detail in Ellis et al.

(2013). These tests showed that the two- part epoxies typically failed in the 700 to 3500 kPa range, well 

below the manufacturer’s published shear stress. The structural film adhesives had shear strengths in 

excess of 10 000 kPa. The shear strength of these bonds was likely higher, but substrate yielded, making

it impossible to measure higher lap shear strengths. Since the thickness of the substrate is an integral part 

of the thermal stress calculation, it was not possible to increase the substrate thickness to prevent yielding

and measure the true shear strength of the adhesive. The lack of failure of the bond at the yield load of the 

substrate showed that there will be strain relief, in the form of facesheet or heat pipe yielding, prior to 

failure of the adhesive. This makes structural adhesives a viable option, even though they are relatively 

poor thermal performers. The structural adhesives were also easier to apply and cure, which improved 

manufacturability, the other key issue in reducing cost.

Affordable Radiator Test Results

The proof-of-concept radiator was designed to be simpler, lighter, and more affordable than the 2nd

Gen RDU. However, testing was required to demonstrate performance and determine if the structural 

adhesive could handle the CTE mismatch between the titanium heat pipe and the aluminum facesheet.

The panel was tested at water temperatures ranging from 330 to 400 K and a sink temperature of 190 K.

At the nominal TDU water inlet temperature of 400 K and flow rate of 0.375 kg/s the 1.1 m2 panel (twosided area) rejected 620 W of heat, for a heat rejection per unit area of 570 W/m2

. For comparison 

purposes the full-scale 2nd Gen RDU panel rejected 760 W/m2 when operating at similar conditions (Ellis 

et al. (2011)). The lower heat rejection per unit area of the proof-of-concept radiator is due to larger

temperature drops through the conductively coupled manifolds, and steeper temperature gradients along 

the aluminum fin. This decrease was expected (Briggs (2012a)) and deemed acceptable given the 

estimated reductions in cost and mass. When these radiators are compared on a mass basis, the 2.0-kg 
NASA/TM—2013-216545 8

affordable radiator rejected 300 W/kg compared to 240 W/kg for the 30-kg 2nd Gen RDU. Additional 

heat pipes could be added to the affordable radiator design (decreased fin length) in order to reduce area 

at the expense of mass as needed to meet TDU requirements.

Conclusion

The Fission Power System team at NASA Glenn Research Center is near the end of the buildup for 

the Heat Rejection System (HRS) subsystem. All components have been mounted to the Technology 

Demonstration Unit structural elements and have been assembled in the Vacuum Facility 6 test chamber 

at Glenn. HRS testing will include performance testing of the pump in a vacuum environment and 

functionally testing the entire subsystem in a thermal vacuum environment. Additional testing has been 

completed on a proof-of-concept radiator that could offer an affordable alternative to dual highconductivity composite facesheet designs, which have become cost prohibitive due to budget reductions.

The affordable design was shown to increase heat rejection per unit mass compared to the Second 

Generation Radiator Demonstration Unit at a fraction of the cost.

References

Briggs, M. (2012a): Conceptual Design of an Affordable Radiator for the Fission Power System 

Technology Demonstration Unit. NETS 2012, Paper 3031.

Briggs, M., et al. (2012b): Development Status of the Fission Power System Technology Demonstration 

Unit. NASA/TM—2012-217717, AIAA Paper 2012‒3711.

Fission Surface Power Team (2010): Fission Surface Power System Initial Concept Definition.

NASA/TM—2010-216772.

Ellis, D.; Briggs, M.; and McGowan, R. (2013): Lap Shear Testing of Candidate Radiator Panel 

Adhesives. NETS 2013.

Ellis, D.; Calder, J.; and Siamidis, J. (2011): Summary of the Manufacture, Testing and Model Validation 

of a Full-Scale Radiator for Fission Surface Power Applications. NETS 2011, Paper 3181.

Mason, L., et al. (2011): Design and Test Plans for a Non-Nuclear Fission Power System Technology 

Demonstration Unit. NASA/TM—2011-217100, NETS–2011–3327.

Mason, L. (2006): A Practical Approach to Starting Fission Surface Power Development. NASA/TM—

2006-214366.