i

NASA/TP—2011–216473

Online Oxide Contamination Measurement

and Purification Demonstration

D.E. Bradley

Yetispace, Inc., Huntsville, Alabama

T.J. Godfroy

Maximum Technology Corporation, Huntsville, Alabama

K.L. Webster, A.E. Garber, K.A. Polzin, and D.J. Childers

Marshall Space Flight Center, Huntsville, Alabama

October 2011

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>

Acknowledgments

This work was done with support of the Fission Surface Power program under NASA’s Exploration Technology 

Development Program. Thank you to Mark Black, Roger Harper, Stanley MacDonald, John Bossard, 

and Boise Pearson for their significant contribution to the design and construction of the feasibility test loop.

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official 

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................. 1

1.1 Fission Surface Power Technology Development .......................................................... 1

1.2 Nonnuclear Testing at Marshall Space Flight Center .................................................... 2

2. FEASIBILITY TEST LOOP OVERALL SYSTEM CONFIGURATION ........................ 4

2.1 Interfacing Systems ....................................................................................................... 5

2.2 Feasibility Test Loop Components ............................................................................... 7

3. TEST PROCEDURE AND RESULTS .............................................................................. 28

3.1 Test Matrices ................................................................................................................ 28

3.2 Testing Results .............................................................................................................. 29

4. LESSONS LEARNED ....................................................................................................... 41

4.1 Flow Measurement ..................................................................................................... 41

4.2 Electromagnetic Flowmeters ....................................................................................... 41

4.3 Venturi Flowmeter ...................................................................................................... 41

4.4 Heat Transfer .............................................................................................................. 42

4.5 Loop Design Process .................................................................................................. 42

4.6 Pressure Transducers .................................................................................................. 42

4.7 Valve Positions ............................................................................................................ 43

4.8 Thermocouples ........................................................................................................... 43

4.9 Fluid System Filtration ............................................................................................... 43

4.10 Calibration .................................................................................................................. 43

5. CONCLUSION .................................................................................................................. 44

APPENDIX A—SYSTEM SCHEMATIC .............................................................................. 45

APPENDIX B—MP PUMP PERFORMANCE CURVE ...................................................... 48

APPENDIX C—THERMOCOUPLE LOCATIONS ............................................................. 49

REFERENCES ....................................................................................................................... 51
iv

LIST OF FIGURES

 1. Feasibility test loop overview ................................................................................... 4

 2. Details of parallel flow branches ............................................................................. 5

 3. FTL arrangement in vacuum chamber .................................................................... 6

 4. Argon cover gas arrangement .................................................................................. 7

 5. Cutaway view of heated NaK reservoir ................................................................... 8

 6. Heated NaK reservoir cutaway view showing graphite resistance heater .................. 8

 7. MP Pumps impeller and housing ............................................................................. 9

 8. Mechanical pump assembly ..................................................................................... 10

 9. Mechanical pump cutaway ...................................................................................... 11

10. Mechanical pump magnetic coupling ...................................................................... 11

11. Mechanical pump drive system ................................................................................ 12

12. Electromagnetic flow sensor schematic .................................................................... 12

13. Plugging EM flowmeter internals ............................................................................ 13

14. Plugging flowmeter showing clamping mechanism .................................................. 14

15. Main EM flowmeter configuration changes: (a) old and (b) new ............................ 15

16. As-built main loop flowmeter .................................................................................. 16

17. Plugging flowmeter magnetic flux density model ..................................................... 17

18. Plugging flowmeter predicted signal output ............................................................. 17

19. Preso venturi flowmeter section ............................................................................... 18

20. Venturi flowmeter pressure versus flow rate ............................................................. 18
v

LIST OF FIGURES (Continued)

21. Oxygen solubility in NaK ........................................................................................ 19

22. Plugging indicator .................................................................................................... 20

23. Sample plugging flowmeter output during plugging ................................................. 21

24. Cold trap overview ................................................................................................... 21

25. Cold trap economizer detail ..................................................................................... 22

26. Cold trap branch details ........................................................................................... 23

27. Venturi flowmeter with absolute PTs ........................................................................ 24

28. Pressure transducer cooling water coils .................................................................... 24

29. Thermocouple clamped via cable tie with Cu foil (for 1/8-in (0.32-cm) 

Cu water cooling tubes) ........................................................................................... 25

30. Thermocouple clamped via cable tie with Cu block (for square Cu 

water cooling lines) .................................................................................................. 25

31. Thermocouple clamped via hose clamp (a) with and (b) without Cu foil 

(on SS tubing and/or fittings) ................................................................................... 26

32. Thermocouples clamped as part of thermal well using Swagelok® fittings, 

which are strategically located at major heat inputs and sinks throughout 

the primary NaK loop ............................................................................................. 26

33. Control valve locations ............................................................................................ 27

34. Mechanical pump ∆P versus time ............................................................................ 30

35. Mechanical NaK pump performance curves ............................................................ 30

36. NaK pressures at venturi flowmeter ......................................................................... 32

37. Venturi flowmeter flow rate from ∆P sample calculation from Preso 

Flow Handbook ...................................................................................................... 33

38. Plugging flowmeter signal output data ..................................................................... 34
vi

LIST OF FIGURES (Continued)

39. Prepurification plugging routines ............................................................................. 36

40. Resultant contamination measurement based on prepurification 

plugging routine ...................................................................................................... 36

41. Plots for all plugging events using the same flowmeter calibration ........................... 38

42. Purification routine via cold trapping ...................................................................... 39

43. Postpurification plugging routines compared with prepurification routines ............. 39

44. Resultant contamination measurement based on postpurification plugging 

routine ..................................................................................................................... 40

45. System schematic ..................................................................................................... 46

46. MP Pump performance curve .................................................................................. 48
vii

LIST OF TABLES

1. FTL pressure transducers .......................................................................................... 23

2. Low-temperature (25 oC) test matrix .......................................................................... 28

3. Mid-temperature (25 to 150 oC) test matrix ............................................................... 28

4. High-temperature (25 to 550 oC) test matrix .............................................................. 29

5. Power loss calculations .............................................................................................. 31

6. Summary of plugging events ..................................................................................... 37

7. Cold trapping test data .............................................................................................. 40

8. Thermocouple locations ............................................................................................ 49
viii

LIST OF ACRONYMS AND SYMBOLS

Ar argon

CTMV cold trap motor valve

Cu copper

DAQ data acquisition

DC direct current

EFF-TF Early Flight Fission Test Facility

EM electromagnetic

EMF electromagnetic force

FSP fission surface power

FTL feasibility test loop

GFSSP Generalized Fluid System Simulation Program

GN2 gaseous nitrogen

He helium

HX heat exchanger

ID inside diameter

I/O input/output

MSFC Marshall Space Flight Center

MV motor valve

NaK sodium-potassium

O oxygen
ix

LIST OF ACRONYMS AND SYMBOLS (Continued)

PMV plugging motor valve

PT pressure transducer

RT real time

RTD resistance temperature detector

SS stainless steel

TC thermocouple

TDU Technology Demonstration Unit

TP Technical Publication
x

NOMENCLATURE

B magnetic flux density (Tesla)



E EMF generated 

K constant

l length

m slope

m mass flow rate 

s height; ID of fluid-carrying conduit (cm)

t time

u volumetric flow rate

u(t) time-varying flow rate

V volumetric flow of fluid (cc/s)

v signal voltage generated (mV)

v(t) time-varying voltage

w width

∆P change in pressure

r density
1

TECHNICAL PUBLICATION

ONLINE OXIDE CONTAMINATION MEASUREMENT 

AND PURIFICATION DEMONSTRATION

1. INTRODUCTION

Fission surface power (FSP) systems could be used to provide power on the surface of the 

Moon, Mars, or other planets, asteroids, and moons of our solar system. In addition, fission power 

systems could provide excellent performance at any location, including heavily shaded or permanently shaded regions, and may offer the capability to provide on-demand power at any time, even 

at long distances from the Sun. Fission-based systems also offer the potential for outposts, crew, and 

science instruments to operate in a power-rich environment. NASA has been exploring technologies 

to reduce the cost and technical risk of employing FSP systems. A reference 40-kWe option has been 

devised that is cost competitive with alternatives while providing more power for less mass anywhere 

on the lunar surface. The reference FSP system is also readily extensible for use on Mars or virtually 

any other location in the solar system. On Mars, the system would be capable of operating through 

global dust storms and providing year-round power at any martian latitude. 

The Early Flight Fission Test Facility (EFF-TF) at NASA Marshall Space Flight Center 

(MSFC) provides the capability to perform nonnuclear, hardware-based testing of concepts and 

components. Component design iterations benefit dramatically from low-cost, subscale testing. To 

this end, the FSP team has developed a feasibility test loop (FTL) that provides a subscale, hightemperature liquid metal flow loop for rapid, smaller scale component testing. This Technical Publication (TP) details continuous, long-term efforts to research, design, assemble, and test several 

subscale components—a mechanical coolant pump, a venturi flowmeter, electromagnetic (EM) flowmeters, oxide plugging indicator, and oxide removing cold trap—with the ultimate goal of showing 

flow loop contamination and purification via a plugging meter and cold trap.

1.1 Fission Surface Power Technology Development

Under the NASA Exploration Technology Development program, NASA and the Department of Energy have begun long-lead technology development for potentially supporting future 

integrated FSP systems. The major effort in the FSP technology project has been focused on a reference mission and concept. The reference mission is to provide electrical power to habitats anywhere

on the lunar surface. The requirements derived from this mission are 40 kWe delivered to the habitat, 

and a design life of 8 yr. Although many options exist, NASA’s current reference FSP system uses 

a fast spectrum, pumped liquid sodium-potassium- (NaK-) cooled reactor coupled to a Stirling 

power conversion subsystem. The reference system uses technology with significant terrestrial heritage that can operate at any location on the surface of the Moon, Mars, or elsewhere in the solar 
2

system. Detailed development of the FSP concept and the reference mission are documented in various other reports.1–4

The objectives of the FSP technology project are as follows:5

• Develop FSP concepts that meet expected surface power requirements at reasonable cost with 

added benefits over other options.

• Establish a nonnuclear, hardware-based technical foundation for FSP design concepts to reduce 

overall development risk.

• Reduce the cost uncertainties for FSP and establish greater credibility for flight system cost 

estimates.

• Generate key nonnuclear products to allow Agency decision makers to consider FSP as a viable 

option for potential future flight development.

The current FSP concept draws heavily on existing component technologies to reduce cost and 

risk. The greatest technical challenge is to demonstrate that the components can function together 

as an integrated system. Making meaningful progress in FSP technology also requires hardware 

development and testing. In order to simultaneously pursue hardware advancement and address the 

concept’s major technical challenge, the Technology Demonstration Unit (TDU) is currently under 

development. The TDU will contain a reactor simulator, power conversion unit, power management and distribution, two liquid metal pumps, and a heat rejection system. The TDU will address 

integrated system performance and control, liquid metal heat transport, electric power generation, 

and waste heat removal. It will not address issues involving the generation and control of nuclear 

heat, operational lifetime, and launch/landing survivability. During testing, the system will be subjected to realistic operating conditions; data will be gathered to better understand performance sensitivity, control sensitivity, and response characteristics. Upon completion of this testing, FSP will 

achieve a nonnuclear technology readiness level of 6: System Demonstration in an Operational 

Environment.

1.2 Nonnuclear Testing at Marshall Space Flight Center

The EFF-TF was established by MSFC to perform hardware-directed activities to support 

multiple in-space nuclear reactor concepts by using a nonnuclear test methodology.6–8 This includes 

fabrication and testing at both the module/component level and near prototypic reactor components 

and configurations, allowing for realistic thermal-hydraulic evaluations of systems. The FTL provides the means for evaluating preliminary component design and analysis in a hardware test bed. 

It uses pumped liquid metal NaK to simulate a primary reactor cooling system.

For this test, the FTL employed an oxide measurement and purification system intended 

to demonstrate online coolant purification. A mechanical centrifugal pump provided NaK circulation. Electromagnetic flowmeters combined with a reference venturi flowmeter provided volumetric flow rate data. Calibration of EM flowmeters was performed in the past by an independent 
3

contractor. The integration of a reference venturi flowmeter allowed in-loop calibration of EM flowmeters. Finally, a graphite resistance heater was installed to provide heat input to the system. This 

TP describes the design rationale, testing, and resulting performance characterization of the above 

components, as well as lessons learned in test that can be used for the benefit of the TDU. 
4

2. FEASIBILITY TEST LOOP OVERALL SYSTEM CONFIGURATION

While the overall objective of testing was to show contamination and purification in the FTL, 

several component iterations were devised and tested before the ultimate goal could be achieved. To 

that end, this configuration of the FTL served as a proof-of-concept testing platform for several new 

candidate component designs. 

The test article consists of an all stainless steel (SS) primary flow loop using 0.5-in- (1.27-

cm-) diameter tubing to connect the following major components: a graphite resistance heating element (heat input), a gaseous nitrogen (GN2) counterflow heat exchanger (HX) (simulated heat load), 

a mechanical centrifugal NaK pump (coolant pump), direct current (DC) EM flowmeters and 

a venturi flowmeter (flow rate measurement), and a combined coolant contamination measuring/

purification system. Figure 1 shows the major components of the FTL.

Venturi

Flowmeter

DC EM

Flowmeters

Mechanical

Pump

Heat Input

Graphite Heater in

Flow-Through

Reservoir

Contamination

Measurement

and Purification

Heat Load

Counterflow

Heat Exchanger

Figure 1. Feasibility test loop overview.

In addition to the primary flow loop, the current configuration has two alternative, parallel 

flow branches available for conducting real-time NaK contamination measurement and purification. These branches are known as the plugging branch and the cold trap branch, and are user 

selectable via remotely operated motor-controlled valves. The primary loop is also provided with 
5

a motor-operated valve so that any combination of flow through the three parallel branches is available during test. The parallel flow branches are highlighted in figure 2.

Plugging Branch

Cold Trap Branch

MoterControlled

Valves

Figure 2. Details of parallel flow branches.

2.1 Interfacing Systems

Since 2008 the FTL has been used to evaluate designs for EM flowmeters, plugging/

oxide precipitation measurement devices, EM pumps, resistance and radio frequency heaters 

and bonded strain gauge pressure transducers. The results of these tests are documented in reference 8. The FTL is filled with the eutectic sodium-potassium alloy (NaK-78). NaK is liquid at room temperature even though sodium and potassium, as stand-alone elements, are 

solid at room temperature. NaK-78 has the lowest melting point of all NaK alloys. While NaK 

has excellent thermal conductivity properties, it is highly reactive in the presence of oxygen 

(O), particularly at elevated temperatures. For this reason, all testing of the apparatus is conducted within a 3-ft (~1-m) vacuum chamber. Vacuum levels reach the range of 10–3 torr. Should 

a leak develop while the FTL is hot, no O is present to burn. A second, equally important reason to 

operate under vacuum is to reduce thermal losses during hot tests. Figure 3 shows the test loop as 

installed inside the vacuum chamber.
6

Vacuum Chamber Mechanical Pump

Pump Drive Motor

Flow-Through

Reservoir

Heater Power

Feed-Through

Figure 3. FTL arrangement in vacuum chamber.

Argon (Ar) is used as the cover gas for the liquid metal in the circuit. When applied through 

the reservoir, it serves as head pressure for the mechanical NaK pump. A separate volume of Ar is 

used to prevent NaK from migrating into the top coupling of the mechanical pump. It is continuously monitored for temperature and pressure during testing, as shown in figure 4.

A three-line GN2 system interfaces with the FTL to provide cooling in three locations: in the 

liquid metal-to-gas HX, the plugging valve, and the cold trap. The GN2 lines all vent to atmosphere 

outside the high bay where testing takes place.

In addition to the primary system graphite-heating element, several small resistance-heating 

elements are individually controlled to provide heating at discrete positions throughout the loop. The 

graphite heater element provides bulk heating of the NaK and is controlled via DC power supply. 

The smaller elements are located to counter heat sinks throughout the system and provide isothermal 

performance at steady-state conditions. These smaller elements operate on 110 VAC.

A complete system schematic is provided in appendix A.
7

Argon

0.25-in-Diameter Tube

Pressure Gauge

Pressure Gauge

Figure 4. Argon cover gas arrangement.

2.2 Feasibility Test Loop Components

2.2.1 Heated Flow-Through Reservoir

Electrical heat is input to the FTL through the loop reservoir, simulating the heat of fission. 

While this design is not prototypical of a flight system, it fulfills three important roles for testing in 

a relatively small vacuum chamber: (1) A reservoir for application of a blanket gas to preclude pump 

cavitation, (2) a reservoir to allow for expansion of heated NaK, and (3) reduction of the overall 

footprint by integration of heaters. The reservoir has an internal volume of ≈8 L and is equipped 

with two all-welded horizontal ports sized to receive approximately 0.63-in- (1.6-cm-) diameter 

by 16-in- (40.64-cm-) long graphite resistance heater elements capable of providing 3 kW each (see 

fig. 5). For this experiment, only one heater was used. 

The loop operates with the reservoir half full of NaK. The horizontal ports are located such 

that they are fully submerged in NaK at all times during operation (see fig. 6). Heating is accomplished via radiation between the heating element and the SS port, which then heats the NaK via 

forced convection as the NaK circulates through the loop and into the reservoir where it is forced to 

flow around the graphite heater. 

In the EFF-TF, graphite heaters are thermally coupled to their housings—in this case, the 

horizontal ports in the reservoir—by back-filling the housing spaces with helium (He). However, it 

was determined that the heater power levels required for this test series were sufficiently low to render 

the use of He unnecessary. Thermocouples (TCs) were placed to measure the graphite heater port 

wall temperatures in order to maintain safe operation.
8

Forced Convection

Flow Configuration

Reservior With

Ar Blanket

Graphite Resistance Heater

Figure 5. Cutaway view of heated NaK reservoir.

Figure 6. Heated NaK reservoir cutaway view showing graphite resistance heater.

The reservoir is also equipped with four 1-in (2.54-cm) vertical access ports in addition to the 

two 0.5-in (1.27-cm) horizontal flow-through ports for NaK passage. Two level sensors are installed 
9

in two of these vertical ports. The other two vertical ports provide for the Ar cover gas and a purge 

line to the cold trap to facilitate draining.

2.2.2 Mechanical Centrifugal Sodium-Potassium Pump

The NaK pump design must provide sufficient volumetric flow to facilitate component testing. 

Analysis of previous FTL testing data identified low volumetric flow to be a considerable obstacle to 

component testing. The previous FTL configuration employed an EM pump whose design flow rate 

was a maximum of 42 cc/s at nominal temperature with the actual hardware ultimately producing 

a maximum flow rate of 5 cc/s. With an estimated overall loop transit length of ≈3 m, this flow rate 

produced a loop transit time of roughly 1 min. This required a significant amount of time to achieve 

high operating temperatures, and resulted in difficulties in maintaining steady-state temperatures due 

to the high variability of NaK temperature around the loop. While the mechanical pump was not 

designed for a specific flow rate, it was expected to produce a minimum improvement of an order of 

magnitude, or a minimum of ≈55 cc/s.

The NaK pump employed in this testing was based on an off-the-shelf, all 316 SS water pump 

housing and impeller by MP Pumps, Fraser, Michigan, shown in figure 7. The pump design was not 

intended to be flight-like, but rather to provide adequate performance to successfully achieve testing 

goals.

Figure 7. MP Pumps impeller and housing.
10

With the exception of these two components, the pump was fabricated in-house as a low-cost 

demonstration unit (fig. 8). With the manufacturer’s pump performance curve and an analytical 

model of the loop using the Generalized Fluid System Simulation Program (GFSSP), the impeller/

involute design was predicted to provide 3.5 gpm at 13 ft (3.96 m) of head. A full pump performance 

curve, as provided by the pump manufacturer, is included in appendix B.

Figure 8. Mechanical pump assembly.

Two opposed, tapered roller bearing assemblies support the pump shaft. They are submerged 

in NaK for lubrication (fig. 9). The separation of the pump drive system and the pump body is 

accomplished using a magnetic coupling that transmits torque through the pump housing without 

any direct mechanical connection. A pump body cooling system consisting of clamped on watercooling tubes, is provided to limit heat transfer to the drive magnets.

The magnetic coupling consists of neodymium rare earth magnets in two separate drive and 

driven connectors as shown in figure 10. The driven element is fully encased in SS. Argon is used as 

a blanket gas in the pump body to prevent NaK from contacting the magnetic coupling.

To employ a conventional DC motor outside the vacuum chamber, the drive connector is 

coupled to a DC motor via a support bearing, flexible drive shaft, and vacuum-rated rotary feedthrough (fig. 11).

2.2.3 Flow Sensor Theory of Operation

A DC EM flow sensor was implemented into the plugging loop to monitor the change in flow 

rate as oxides precipitate out of the NaK solution and block the plugging meter holes. An idealized 

schematic of a DC EM flow sensor is presented in figure 12. The liquid metal flows though a channel 

of width (w), length (l), and height (s). The magnetic flux density (B) and time-varying flow rate (u(t)) 

are perpendicular both to each other and the direction across which the voltage (v(t)) is measured. 
11

All Stainless Steel

Water Pump Body

(modified)

NaK Liquid Level

During Operation

Common Tapered

Roller Bearings

Mild Steel Pump Shaft

Sealed Magnetic Coupling

Argon Blanket Gas

Port/Thermocouple Well

(Two Separate Parts)

Inlet

Outlet

Figure 9. Mechanical pump cutaway.

Driven Connector

Drive Connector

Coupling Sealing Shroud.

Sealed With Weld to Pump

Body, Thus Isolating NaK

From Vacuum Chamber

All wetted parts are 316 SS

Figure 10. Mechanical pump magnetic coupling.

When an electrically conductive medium (a conducting liquid in this case) moves transverse to an 

applied field, a back-electromagnetic force (EMF) is induced according to the vector equation: 



E = −u × B . (1)
12

24V DC Motor

Ferrofluid

Vacuum-Rated

Rotary Feed-Through

Vacuum-Rated

Greased Ball Bearing

Vacuum Grease

Lubricated Bushings

NaK Lubricated Tapered

Roller Bearings (2)

Flexible

Drive Shaft

Spider Shaft Coupling

Figure 11. Mechanical pump drive system.

B

s

w

l

u(t)

v(t)

Figure 12. Electromagnetic flow sensor schematic.

Integrating the electric field over the width (w) (assuming spatially uniform values of u and 

B over the indicated faces), the voltage induced across the channel for a given mass flow rate, or the 

corresponding volumetric flow rate, is equal to 

v = mB

ρs =V B

s . (2)

The voltage output by the flow sensor is linearly dependent on the ratio B/s, making even an absolute 

measurement relatively simple once the meter has been calibrated. 
13

2.2.4 Electromagnetic Flowmeters

The FTL is equipped with two separate EM flowmeters. Electromagnetic flowmeters take 

advantage of fluid conductivity to generate an electrical EMF by vector cross product of fluid flow 

with an orthogonal magnetic field as shown in figure 13. 

NaK Flow

Magnetic Field

EMF

Figure 13. Plugging EM flowmeter internals.

Because an EM flowmeter is a DC generator inducing an EMF, the electrical field strength is 

proportional to the following factors:

v = KV B

s , (3)

where

v = signal voltage generated (mV)

K = constant

V = volumetric flow of fluid (cc/s)

B = magnetic flux density (T)

s = ID of fluid-carrying conduit (cm).
14

The constant (K) provides for losses in the measured voltage associated with current shunting through the conductive walls of the NaK flow tube rather than through the NaK itself. An ideal 

flowmeter would have the entire current passing through the NaK or a factor of K=1.

Flowmeter signals are generally very small (fractions of a millivolt), and are difficult to predict accurately from hardware geometry. Their chief advantage is output signal linearity with volumetric fluid flow rate. Electromagnetic flowmeters are generally calibrated using measured volumes 

of fluid captured in finite time periods (capture and count). The EM flowmeters use nickel-plated 

neodymium rare earth magnets with a magnetically permeable bale to complete the magnetic flow 

circuit. The configuration of the bale focuses the magnetic flux in the vicinity of the fluid flow tube 

in order to maximize signal output as shown in figure 13.

The flowmeters are equipped with water-cooled copper (Cu) blocks, which conduct heat away 

from the magnets. The cooling system protects the magnets but is primarily intended to maintain 

isothermal conditions so that consistent magnetic field strength is available at all temperatures.

Previous flowmeter configurations experienced signal variations due to expansion of loop 

tubing and consequent movement of the flowmeter NaK channel within the magnetic field structure. 

To ensure that the NaK channels remain stationary within their magnetic field structure, each flowmeter has a clamping mechanism provided to index all magnetic field structure relative to NaK flow 

tubing (fig. 14).

NaK Flow

Signal Electrode

Signal Electrode

Clamping Mechanism

Water-Cooled

Cu Blocks

Figure 14. Plugging flowmeter showing clamping mechanism.

2.2.4.1 Main Loop Electromagnetic Flowmeter. The main loop flowmeter records the total 

volume flow of NaK produced by the pump. The main loop flowmeter uses a design carried over 

from a previous FTL configuration with minor modifications. 
15

The previous design used a NaK flow channel that transitioned from a full 0.5-in- (1.27-cm-) 

diameter tube to a flattened channel back to full diameter as shown in figure 15. The purpose of 

the flattened section was to allow the magnets in the magnetic field structure to have a minimal gap 

between them, and therefore the highest magnetic flux in the region of the NaK flow channel. 

(a) (b)

Figure15. Main EM flowmeter configuration changes: (a) old and (b) new.

The transition represents a cross-sectional flow area restriction of nearly 50%. The resulting 

hydraulic resistance trades poorly with the relative increase in magnetic field strength. As a result, 

the flowmeter design was modified to incorporate a full 0.5-in- (1.27-cm-) diameter tube throughout 

the magnetic field structure, resulting in a larger gap between the magnets. Simulations indicated that 

although the magnetic flux is reduced overall, the signal-to-noise ratio of the new design would be 

more than sufficient to record. The final, as-built main loop flowmeter can be seen in figure 16. 

2.2.4.2 Plugging Branch Electromagnetic Flowmeter. The plugging branch flowmeter records 

only flow through the plugging branch of the loop. The plugging flowmeter is a complete redesign 

based on a desire to remove the magnets from the immediate vicinity of the heated NaK flow 

channel.

The smaller plugging branch flowmeter is designed to measure very low volumetric flow 

rates on the order of zero to 15 cc/s. A magnetic field modeling software analysis was performed to 

obtain magnetic field strength predictions as shown in figure 17. The field strength predicted by the 

model was 0.9 T in the vicinity of the NaK flow tube. Given that the flow tube is a nominal 0.25-in 

(0.64-cm) tube with 0.035-in (0.09-cm) wall thickness, and assuming an ideal flowmeter (K=1), then

K = 1

B = 0.9 T

s = (0.25 in– 2(0.035 in)× 2.54 cm/in=0.4572 cm

V =(1)(v)(0.9)/(0.4572)=0.197*v(cc/s).
16

Figure 16. As-built main loop flowmeter.

The resulting flowmeter predicted output is shown in figure 18.

2.2.5 Venturi Flowmeter

The venturi flowmeter was purchased from Preso Meters, Racine, Wisconsin. It consists of 

a flow section with a precisely defined profile equipped with two pressure taps, one upstream of the 

meter and one at the smallest restriction (fig. 19). By measuring the difference in the fluid pressures, 

the flow rate can be readily determined.

Given the wide variation of temperatures expected for operation, postprocessing of pressure 

readings is necessary to account for changes in fluid density and viscosity. The venturi flowmeter is 

implemented as a redundant and alternative flow measurement to EM flowmeters with the potential to act as a calibration reference. Pressure drop across a venturi meter has a quadratic relationship with volumetric flow rate (fig. 20).

Measurement of NaK flow is important as it ultimately represents an energy rate or power 

level when combined with fluid temperature. Electromagnetic flowmeters, which are relatively new 

technology, rely on principles of electrical induction to generate a measurable signal. The venturi 

flowmeter, on the other hand, relies on long-standing fluid flow principles in and around restrictions that have been proven and documented extensively. The venturi flowmeter therefore provides 
17

1.76

1.54

1.32

1.10

0.88

0.66

0.44

0.22

0

Figure 17. Plugging flowmeter magnetic flux density model.

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

Voltage Output (mV)

Mass Flow Rate (cc/s)

Analytical Model

y=0.1969x

0 1 2 3 4 5 6 7 8

Figure 18. Plugging flowmeter predicted signal output.
18

3.889±0.063 (98.8±1.6)

0.302 (7.7)

0.1.25 (31.8)

0.402 (10.2)

0.563 (14.3)

(2)

0.531 (13.5)

(2)

Flow

H L

0.188 Through

0.281 0.250

(2)

2

1

Tack Weld

Both Sides

Figure19. Preso venturi flowmeter section.

2.5

2

1.5

1

0.5

0

Pressure Drop (psid)

Flow Rate (gpm)

0 1 2 3 4 5 6

Figure 20. Venturi flowmeter pressure versus flow rate.

a reference flow measurement that may be compared to the signal output of the EM flowmeter for 

performance confirmation and calibration when appropriate.

It should be noted, however, that venturi flowmeters are most accurate when designed to 

perform at very specific fluid flow conditions and suffer significant loss of accuracy when deviating 

from those conditions. The venturi flowmeter measurement relies on recording a pressure drop that 

is enabled by use of one or more pressure transducers (PTs). A low pressure drop is desirable so as to 

present minimal parasitic loss to system flow. A low pressure drop is difficult to record, however, so 

a performance tradeoff exists for a venturi flowmeter. If the venturi flowmeter in conjunction with 

its associated PT(s) is sized for best accuracy at the nominal system flow rate, it will generally suffer 

from signal inaccuracy at much reduced flow rates.
19

The EM flowmeter output signal, according to theory, should be zero at zero flow rate, rising 

linearly with increasing flow rate. The EM flowmeter signal may therefore be estimated by a straight 

line, y=mx+b intersecting the y axis at (x,y)=(0,0). Any nonzero flow condition will provide a point 

on that line (x1,y1) from which the slope, m, may be determined.

Assuming the venturi flowmeter is located in series with a given EM flowmeter so that all 

flow passing through it also passes through the EM flowmeter, the venturi flowmeter can be used to 

provide an operational data point for use in calibrating the EM flowmeter. The EM flowmeter thus 

calibrated can then be used at a full range of flow rates, which fall outside the accuracy regime of the 

venturi flowmeter.

2.2.6 Plugging Indicator for Contamination Measurement

When working with most higher temperature alkali metal systems, corrosion is a major concern. The primary contaminant of interest in a pumped NaK cooling system is O. Oxygen dissolves 

into NaK and forms sodium and potassium oxides. The saturation concentration of O dissolved in 

NaK versus NaK temperature is well known. See figure 21.6

10,000

1,000

100

10

0

Oxygen (ppm)

Temperature (°C)

0 200 400 600 800

Data

5th Order Poly Fit

Exponential Fit

Figure 21. Oxygen solubility in NaK.

Several methods of contamination measurement have been proposed.6 For the purposes of 

the current FTL configuration, it was desired to characterize a plugging indicator as this was within 

both the technical and cost scope of the FTL.
20

The mechanism of plugging assumes O is present in the system. As the temperature of the 

NaK in the loop increases, O dissolves into the NaK solution. This process continues until all available O has been dissolved into solution, or the saturation concentration of oxides at the NaK temperature has been reached and no further dissolution of oxides is possible. As the NaK cools, the 

dissolved oxides precipitate out of solution.

A plugging indicator is the combination of an adjustable flow orifice, a mechanism for cooling the NaK solution, and a flow measuring device. The FTL plugging indicator consists of a flowmetering valve, a GN2 cooling block, and the plugging EM flowmeter (fig. 22).

Metering Valve

Gas Cooling

Block

NaK Flow

Plugging EM Flowmeter

Valve Control Stepper Motor

Figure 22. Plugging indicator.

At a steady rate of flow, the NaK solution is slowly cooled, causing oxides to precipitate in 

the adjustable orifice. During this cooling process, the flow rate is observed. The ‘plugging’ event is 

identified when flow rate through the device drops dramatically. See figure 23.

Comparing the solubility curve for O in NaK with the measured temperature of the NaK 

solution at the time of the plugging provides an oxide contamination concentration. By discontinuing cooling and adjusting the orifice to increase its cross-sectional flow area, the precipitated oxides 

may dissolve back into the heated NaK solution, permitting reuse of the plugging indicator.

2.2.7 Cold Trap for Online Sodium-Potassium Purification

The mechanism for purification is similar to that of the plugging indicator. As NaK is cooled, 

any dissolved oxides precipitate out of solution. This mechanism is used to force precipitation to 

occur in a containment filter known as a cold trap (fig. 24). The oxides then can be captured by placing a material with a large surface area in contact with the NaK solution and providing cooling in 

the same region.
21

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

Flow Rate (g/s)

Time (s)

0 100 200 300 400 500 600 700 800 900 1,000

Figure 23. Sample plugging flowmeter output during plugging.

GN2 Out

GN2 In

Drain Port

NaK Out

NaK In

Stainless Steel

Wool

Thermocouple

Well

Figure 24. Cold trap overview.

The cold trap is cooled by flowing GN2 around the outer surface of an annular, counterflow 

NaK economizer. Within the economizer are two SS wool filters of different densities that provide 

maximum surface area for solid precipitation. See details in figures 24 and 25.

The available surface area in the cold trap defines the total oxide quantity that can be removed. 

The rate of O removal is dependent on cooling capacity and NaK latency within the trap. The longer
22

GN2 Out

GN2 In

3

1

4 2

5

(1) Hot NaK enters trap.

(2) NaK flows down outer 

 annulus and is cooled 

 by GN2 in jacket and 

 cooled NaK flowing 

 inside opposite.

(3) Coolest NaK.

(4) NaK passes through 

 stainless steel wool 

 depositing oxides.

(5) NaK exits trap.

NaK Out NaK In

Figure 25. Cold trap economizer detail.

the NaK solution is in contact with the cool material, the greater the amount of O that is removed 

(in the form of precipitating oxides). A flow control valve is provided to regulate NaK latency within 

the trap.

Note that in order to restrict the precipitation of oxides to the cold trap (and nowhere else in 

the NaK flow path), it is important to tightly control the temperature of the solution such that it is 

the same at the inlet and exit of the trap, but drops to a significantly lower level within the cold trap. 

The economizer is included to reheat the NaK as much as possible as it exits the cold trap. Furthermore, a resistance heater is located downstream of the trap to minimize further oxide precipitation 

outside of the trap. See figure 26.

2.2.8 Pressure Transducers

There are five separate PTs in use in the FTL (table 1).

Pressure transducers PT-02 and PT-03 are associated with flow rate measurement by the venturi meter (fig. 27). Cooling water coils were applied to these transducers to combat thermal drift. 

Thermal drift has been identified as an issue during longer duration testing where PTs are exposed 

to radiation heat transfer from the NaK loop (fig. 28). This effort is similar to that of cooling the 

magnets in the EM flowmeters.

2.2.9 Temperature Measurements

Temperature measurements are made throughout the system with type-K, ungrounded TCs. 

A complete summary of TC locations is provided in appendix C. The TCs are clamped in place using 

several different methods as necessary to obtain useful measurements (figs. 29–32).
23

NaK Flow Direction

Drain Port

Cold Trap

Resistance Heater

Branch Flow Control Valve

Figure 26. Cold trap branch details.

Table 1. FTL pressure transducers.

ID Type Location Fluid Range Vendor

PT-01 Absolute Reservoir Ar gas blanket pressure Ar 0–100 psia MKS

PT-02 Absolute Venturi upstream pressure NaK 0–25 psia Delta Metrics

PT-03 Absolute Venturi throat pressure NaK 0–25 psia Delta Metrics

PT-04 Absolute Mechanical NaK pump Ar gas pressure Ar/NaK 0–50 psia Delta Metrics

DP-01 Differential Mechanical NaK pump inlet/outlet NaK +7 psid Delta Metrics
24

Figure 27. Venturi flowmeter with absolute PTs.

Figure 28. Pressure transducer cooling water coils.
25

Figure 29. Thermocouple clamped via cable tie with Cu foil (for 1/8-in (0.32-cm) 

Cu water cooling tubes).

Figure 30. Thermocouple clamped via cable tie with Cu block (for square Cu 

water cooling lines).

2.2.10 Motor-Controlled Valves

NaK flow in the loop is regulated by a combination of three motor-controlled valves. The 

control motors are vacuum-rated stepper motors. The three valves allow flow diversion through any 

combination of three paths: the main loop, the plugging loop, or the cold trap (fig. 33). NaK nominally flows through the main loop bypass branch controlled by the motor valve (MV). NaK may be 

allowed to flow through the cold trap motor valve (CTMV) or the plugging motor valve (PMV).

In a simulated operational environment, the NaK flows nominally through the MV and is 

warmed via the heated flow-through reservoir. At a given steady operational temperature, the plugging branch may be opened to perform an oxide measurement or the cold trap branch may be opened 

to remove oxides from the NaK.
26

(a) (b)

Figure 31. Thermocouple clamped via hose clamp (a) with and (b) without Cu foil 

(on SS tubing and/or fittings).

Thermocouple

Weld

Note: Thermocouple proximity to flowing NaK

NaK Flow

Figure 32. Thermocouples clamped as part of thermal well using Swagelok® fittings, 

which are strategically located at major heat inputs and sinks throughout 

the primary NaK loop.

2.2.11 Data Acquisition and Control Platform

The data acquisition (DAQ) and control system is based on National Instruments hardware 

and software. The system consists of a Windows XP-based PC for user interaction and data recording, a PXI-1042 chassis for DAQ and control, and an SCXI-1001 chassis for signal conditioning.

The PC runs an in-house-developed LabVIEWTM application that communicates via Transmission Control Protocol/Internet Protocol with the real-time LabVIEW application running on
27

MV

MV

CT

CTMV

CTMV

PMV

PMV

PFM

PHX

Figure 33. Control valve locations.

the controller in the PXI-1042 chassis. The PC application receives data and status information for 

presentation and recording and transmits user-initiated commands to the controller in order to operate the experiment. The application provides a user interface for operating the heaters, valves, HXs 

and data recorder. It also presents the data acquired from the system and provides visual feedback 

on the status of the system. The PXI-1042 chassis includes a PXI-8110 Real-Time (RT) embedded 

controller, a PXI-6281 DAQ module, a PXI-6704 analog output module, a PXI-6528 digital input/

output (I/O) module, and a PXI-6521 industrial digital I/O module. The PXI-8110 has a 2.26-GHz 

quad-core processor running the LabVIEW RT operating system. The controller runs an in-housedeveloped LabVIEW RT application that acquires data through the PXI-6281 DAQ module, scales 

the data into engineering units, monitors the data for alarm conditions, and transmits the data to 

the PC application for recording and display. The RT application also controls the operation of 

theexperiment in direct response to commands from the PC application and provides appropriate 

feedback status to the PC application. Any closed-loop control is handled by the RT application. 

The PXI-6281 DAQ module is used to scan all available channels from the SCXI-1001 chassis, which 

handles all signal conditioning.

The SCXI-1001 chassis includes SCXI-1102 analog input modules, an SCXI-1503 resistance 

temperature detector (RTD) analog input module, and an SCXI-1520 universal strain gauge input 

module. The SCXI-1102 module is used to condition analog voltage signals from various instrumentation transducers, including TCs and PTs. The SCXI-1503 RTD module is used to condition 

readings from an RTD, which measures the temperature of a TC cold junction located on the test 

rig inside the vacuum chamber. The SCXI-1520 is used to condition the signals from full-bridge 

strain gauge-based PTs. The NaK PTs originally featured signal conditioning amplifiers, which were 

located near the transducers inside the chamber. During testing it was determined that the amplifiers 

were overheating. The amplifiers inside the chamber were removed and replaced with the SCXI-1520 

module outside the chamber.
28

3. TEST PROCEDURE AND RESULTS

While the ultimate goal of testing was to show flow loop contamination and purification 

via a plugging meter and cold trap, continuous, long-term efforts were required to research, design, 

assemble, and test several subscale components in order to successfully complete testing. The FTL 

test procedure began with preliminary individual subsystem tests of each of the major components. 

These were proof-of-concept tests for both new designs and component design iterations. Upon 

successful testing of individual components, increasing levels of control, integration, and operational temperature were imposed to verify or revise operational procedures. Once these procedures 

were verified, contamination and purification testing began. This required a minimum of two contaminant measurements from a higher to a lower level of contamination performed before and after 

a minimum of one online purification operation. 

3.1 Test Matrices 

The following tables list test descriptions of the various subsystems at three different temperature ranges (tables 2–4). 

Table 2. Low-temperature (25 oC) test matrix.

Component or Subsystem Test Description

Mechanical pump Demonstrate flow of NaK through main loop. 

Demonstrate speed control and repeatability.

Venturi flowmeter Demonstrate ∆P correlation with varied flow rate.

Motor-controlled valves Demonstrate ability to remotely control NaK flow through all flow branches 

while under chamber vacuum. Demonstrate repeatability of valve setting.

Table 3. Mid-temperature (25 to 150 oC) test matrix.

Component or Subsystem Test Description

In-reservoir heater Demonstrate ability to regulate power input. 

Demonstrate ability to heat entire loop via pumped circulation.

EM flowmeters Demonstrate ‘wetting’ of flowmeters using 100 °C (212 °F) NaK flow 

(observe and record signal change).

Flow measurement calibration Calibrate EM flowmeters using venturi flowmeter. Confirm flow rates via 

advance of temperature front in primary NaK loop.

It is the nature of research hardware that both physical and operational characteristics of the 

hardware are established during operation and are not strictly dependent on intended design. Testing 

therefore proceeds from low technical risk and advances to increasing complexity based on operating 

criteria established during testing. 
29

Table 4. High-temperature (25 to 550 oC) test matrix.

Component or Subsystem Test Description

Plugging indicator Demonstrate flow-through plugging branch.

Record plugging event with main loop at steady-state temperature.

Clear plugging indicator for reuse.

Cold trap Operate primary loop at temperature.

Operate cold trap branch to remove oxides.

Mechanical pump Pump performance and endurance at increasing temperatures of operation.

Low-temperature testing was conducted for major mechanical components as proof of concept and to establish operating procedures.

Mid-temperature testing established control procedures for the loop heating system and identified low-temperature performance dependence on temperature for flowmeters.

High-temperature testing represented prototypical, full-scale system operational characteristics simulating a NaK-cooled fission reactor. Testing provided proof of concept for oxide measurement and coolant purification via plugging and cold-trapping.

3.2 Testing Results

3.2.1 Mechanical Pump

The mechanical pump successfully provided NaK flow rates as high as 1.8 gpm or ≈115 cc/s. 

By comparison, previous FTL testing using an EM NaK pump achieved maximum flow rates of 

≈5 cc/s.

A sample of performance data is provided below. Flow of NaK through the primary flow 

loop by record of a change in pressure rise across the pump as measured by DP-01 is shown in figure 34. Two separate runs from full stop to full speed indicated the same pressure rise, confirming 

speed control repeatability.

Using the resulting pressures from the venturi flowmeter, the calculated flow rate produced 

by the pump was 1.51 gpm at 4.4 psid (11.7-ft (3.57-m) head). This is in contrast to the pump performance curve that predicts 3.5 gpm at 13 ft (3.96 m). This difference reflects substantial deviation of 

the as-built flow system from the predicted GFSSP model. The actual system provided substantially 

more flow resistance than originally modeled. In addition, modifications to the pump for service 

in NaK (instead of water) may have altered the impeller/involute efficiency (fig. 35). Physical performance loss is most likely due to undesired flow between the high- and low-pressure sides of the 

impeller. During the pump modification process, the impeller and involute clearances were both 

altered and a mechanical seal was removed. This introduced an alternative flow path for accelerated 

fluids within the pump.

The mechanical pump design successfully achieved performance objectives and provided 

robust operation for subsequent successes in testing other system components.
30

5

4

3

2

1

0

–1

Pressure (psid)

Time (s)

5,750 5,800 5,850 5,900 5,950 6,000 6,050 6,100

Figure 34. Mechanical pump ∆P versus time.

18

16

14

12

10

8

6

4

2

0

Head (ft)

Flow Rate (gpm)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

As-Built Test Data

y=–0.9326x+15.404

Manufacturing Data

y=–0.8926x+16.467

Figure 35. Mechanical NaK pump performance curves.

As a result of this successful testing and due to lower-than-expected efficiencies in larger scale 

system EM NaK pumps, there was considerable interest in the overall efficiency of the mechanical 

NaK pump.

While the mechanical pump successfully increased the flow rate of the old EM pump, it is 

interesting to note that overall pump efficiency was very poor. Data analysis was performed to determine overall pump efficiency as built. A typical nominal operating point of 1.3 gpm at 14.1 ft (4.3 m) 
31

was selected for the calculation. It should be noted that some of the power losses calculated in 

table 5 were not easily verified by test and should be used as estimates only.

Table 5. Power loss calculations.

Power measured at DC motor input 134.8 W

Efficiency of DC motor (from manufacturing data) 81%

Motor power delivered to pump drive train 134.8W×0.81=109.2 W

Power loss due to friction torque in single-row, greased ball bearing 

(estimated calculation)

–7.7 W

Power loss due to friction torque in bronze bushings (estimated calculation) –22.4 W

Power loss due to flex shaft (obtained by test) –16.7 W

Power loss due to friction torque at vacuum rotary feedthrough 

(from mfg, verified by test)

–18.5 W

Total power delivered by drive train to pump impeller shaft 43.7 W

Total fluid power as measured from NaK flow and pressure rise data 3 W

Net efficiency of pump as tested 3 W/43.7 W=6.9%

According to the MP Pumps performance specifications, the pump would be expected to have 

an efficiency of ≈9.6% at the operational point considered above. Manufacturer’s efficiency data are 

based on their testing of an unmodified pump as would typically be delivered. The pump used in 

this FTL testing required some modification for use in NaK. The resulting difference in as-delivered 

versus as-tested efficiency is therefore due to these modifications or the estimates used in the calculations as noted above. The demonstration unit utilized in this testing used the same pump impeller 

and involute design as delivered and was selected purely on the basis of material and inlet/outlet port 

size compatibility. As a result, the pump was poorly matched to the test loop and therefore operated 

at low efficiency. 

For reference, the pump was equipped with a six-vane, 1.6-in- (4.06-cm-) diameter impeller. 

It was ultimately intended to operate at a design point of (8 gpm at 9.6 ft (2.93 m) at 3,600 rpm). At 

the operating point described above (1.3 gpm at 14.1 ft (4.3 m) at 3,600 rpm) the ideal, most efficient 

impeller design would be a three-vane design with a diameter of approximately 2 in (5.08 cm).9 Given 

a close-coupled motor without extensive drive train components and basic engineering rigor in the 

design of the pump impeller, efficiencies between 40% and 50% may be expected.

3.2.2 Venturi Flowmeter

Pressures recorded from the venturi flowmeter correlated well with the pump pressure rise. 

See figure 36. Calculations made using the net pressure difference between the upstream and throat 

pressure (estimated at 0.18 psid) indicate a flow rate of 1.51 gpm (fig. 37).

The venturi flowmeter was sized for an estimated maximum flow of 5 gpm as predicted from 

early system models. The as-built system, which deviated from that model substantially due to modifications resulting from early testing, represented a significantly higher flow resistance. The resulting reduction in overall flow rate did not negatively affect the overall testing goals but did limit the 
32

21

20.5

20

19.5

19

18.5

18

17.5

17

16.5

16

Pressure (psia)

Time (s)

5,750 5,800 5,850 5,900 5,950 6,000 6,050 6,100

Figure 36. NaK pressures at venturi flowmeter.

usefulness of the venturi flowmeter since its signal is decidedly nonlinear at lower flow rates and is 

also subject to greater PT line noise from mechanical vibration. The data provided by the venturi 

flowmeter provided a welcome confirmation of flow to provide redundancy to EM flowmeters and 

should be considered for all flow loops when properly sized for expected flow rates.

3.2.3 Electromagnetic Flowmeters

Both EM flowmeters provided ample signal-to-noise ratios and were subject to minimal temperature deviations due to their water-cooled magnets. Voltage signals could be calibrated to correlate with engineering units of gallons per minute and grams per second as required by testing. The 

use of full flow tubes did not observably affect the ability to measure flow and was therefore an excellent trade versus the loss of signal strength due to increased magnet separation. Water-cooling, while 

excellent for maintaining isothermal magnets and therefore constant magnetic field strength, had 

an undesirable effect as-built. The heat absorption provided by the water caused a net drop of NaK 

temperature as it passed through the flowmeter. The impact was particularly observed in the plugging flowmeter. During plugging tests, additional heaters were therefore provided along the plugging 

branch to compensate for this heat sink. In future testing, this issue could be eliminated by thermally 

isolating the water cooling from the NaK tubing or physically removing the magnets from the near 

vicinity of the heated flow tubes.

Sample data for the plugging flowmeter showing signal strength versus volumetric flow rate 

are provided below and include a comparison to predicted performance (fig. 38).

It should be noted that the plugging flowmeter output signal recorded during test was ≈60% 

of that predicted. The analytical model assumes an ideal flowmeter where all signal current passes 

only through the NaK in the flow tube. Given the similar conductivities of the 316 SS flow tube and
33

FLOW RATE VS. DIFFERENTIAL PRESSURE

(REFERENCING PRESO FLOW HANDBOOK)

5/31/11

25 T, fluid temperature, °C

866.7 ρ, fluid density, kg/m3 (see sheet “NaK Density”)

0.8667 SGf, specific gravity of fluid (NaK) (water at 25°C = 1.0)

0.628 μcp, fluid viscosity, Cp (see sheet “NaK Viscosity”) (water at 25°C = 0.89 cP)

0.1800 ∆P, differential pressure, psid

4.9878 Hw, differential pressure, inches of H2O at 68°F

5.666 N, units conversion factor (5.666 for GPM)

0.75 β, Venturi throat to tube inner diameter ratio

0.6701

K, Flow Coefficient

SSL-1: 0.1215

SSL-2: 0.2436

SSL-3: 0.4259

SSL-4: 0.6701

0.402 Di, tube inner diameter, inches

0.3015 Dv, venturi throat diameter, inches

0.9996 Fa, thermal expansion correction factor (see sheet “Fa”)

16348 Re, Reynolds Number

1.0250

Fre, Reynolds Number Factor (for flow in pipe)

Fre=1.0000 for 100,000 < Re < 1,000,000

Fre=1.0250 for Re < 100,000

Fre=0.9985 for Re > 1,000,000

1.0742 Fsgf, Flowing Specific Gravity Factor

0.6753 C1, coefficient

0.9850 CD, discharge coefficient

1.508 Q, flow rate, GPM

0.182 mdot, mass flow rate, lbm/sec

SSM / SSL VENTURIS (ALL SIZES)

MODEL BETA RATIO K FACTOR

SSM/SSL-1 .35 .1215

SSM/SSL-2 .49 .2436

SSM/SSL-3 .63 .4259

SSM/SSL-4 .75 .6701

 κ = CDβ2

1− β4 CD ( Discharge Coefficient ) = .985

Q = C1 Hw

Hw = (Q / C1)

2

C1 = N i K iDi 2 iFa iFre iFsgf

Re = 3160iQiSG f

Dii μcp

Fsgf = 1

SG f

m = Q

1 min

60 sec

⎛

⎝

⎜

⎞

⎠

⎟

0.1336806 ft

3

1Gal

⎛

⎝

⎜

⎞

⎠

⎟ ρ H2OSG f

Figure 37. Venturi flowmeter flow rate from ∆P sample calculation from Preso Flow Handbook.

the NaK, current shunts partially through the SS wall. The 40% loss of predicted signal strength may 

be reasonably attributed to this effect.

3.2.4 Contamination Measurement and Purification

Online O contamination measurement and purification were successfully demonstrated. For 

the purposes of testing, a plugging routine was defined as a complete evolution performed at steady 

main loop temperatures whereby the plugging indicator was slowly cooled and dropoff of flow rate 

observed from a baseline flow rate to zero flow. Several separate plugging routines were performed 

without any external modifications that could affect O content in the loop.

A cold-trapping routine was defined as a deliberate cooling of main loop NaK in the specially 

designed cold trap branch. Main loop temperature was maintained at a steady-state temperature 

above the identified prepurification plugging temperature to ensure oxides in the main loop were

fully dissolved. The cold trap was maintained at a steady temperature delta from the main loop of 
34

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

Voltage Output (mV)

Mass Flow Rate (cc/s)

0 1 2 3 4 5 6 7 8

Analytical Model

y=0.1969x

Test Data

Test Data

y=0.1163

Figure 38. Plugging flowmeter signal output data.

approximately 40–50 °C (104–122 °F). Flow was controlled such that ≈10% of the main loop flow 

passed through the cold trap. The above conditions were maintained for a full 30 min. For absolute 

purification at this temperature, an infinite amount of time would theoretically be required. The temperature delta and time duration were selected to reasonably demonstrate purification.

A definitive coolant purification cycle consists of the following:

• Prepurification O measurement via plugging routine.

• Purification via cold trap routine.

• Postpurification O measurement via plugging routine.

3.2.5 Plugging Operations

Several distinct plugging operations were performed. The test hardware allowed for real-time 

adjustment of operational parameters including:

• Cross-sectional flow area at plugging valve (and therefore initial NaK flow rate).

• Rate of cooling of plugging valve orifice via gas cooling combined with electric heating.

It was decided that the best opportunity to observe a plugging event and identify a precise 

plugging temperature would require a very shallow, steady temperature drop at the plugging valve, 

allowing for the slow and steady deposit of precipitates in the orifice.

The standard procedure employed for performing a plugging operation is as follows:
35

(1) Open main loop bypass valve completely. Open plugging valve completely.

(2) Preheat the plugging valve and as much of the plugging branch as possible. (At all times 

when flowing leading up to a plugging event, the plugging branch should be maintained at or above 

the main loop temperature so that plugging does not occur.)

(3) Run main loop while heating with graphite heater core, maintaining plugging branch 

temperature above that of the main loop.

(4) Once desired main loop temperature is reached, heaters are adjusted to create a steadystate temperature.

(5) The plugging flowmeter signal strength is baselined by running the plugging valve from 

fully open to fully closed and returning to fully open. Finally, a valve position is selected to achieve 

a desired flow ratio usually achieved by observing the ability to balance heat inputs and outputs.

(6) Temperature of the plugging flowmeter is regulated carefully to achieve a steady temperature drop, linear with time. Heaters applied downstream of the valve serve to prevent plugging from 

occurring outside of the plugging valve orifice.

(7) In order to ensure that oxide precipitation takes place only in the valve orifice, heaters are 

applied along the balance of the plugging branch plumbing and maximum heat applied. The maximum heating rate is a function of the incoming NaK flow rate combined with the valve cooling rate 

and is therefore continually adjusted.

(8) As the temperature falls, the plugging flow meter signal is observed continuously with any 

major change in trend noted as onset of plugging.

(9) At the onset of plugging, the dynamic balance of heating and cooling requires less cooling to maintain a linear temperature drop with time. Ultimately, there is a tipping point where the 

action of plugging drives a reduction in flow rate and therefore decreases the heat input into the plugging valve. At this point, cooling is no longer required and plugging runs until flow stops.

(10) When flow is reduced to zero, the flowmeter signal hits a floor and conditions are maintained for 5 full minutes.

(11) Heat is applied to all of the plugging branch and the valve is opened fully to allow hot 

NaK to flow through. The combination of incoming hot NaK and external heating serves to drive 

precipitated oxides back into solution, restoring full flow.

Figure 39 plots plugging flowmeter output versus plugging flow temperature during several 

plugging routines. Step changes in flow signal were attributed to larger solids in the flow stream becoming lodged and/or dislodged in the plugging valve orifice. An approximation of 313 °C (595.4 °F) for 

onset of plugging is visible in all three traces.
36

3

2.5

2

1.5

1

0.5

0

–0.5

Plugging Branch Mass Flow Rate (g/s)

Temperature Immediately Downstream of Plugging Valve, T-38 (°C)

400 380 360 340 320 300 280 260 240 220 200

Plugging Event

Before Cold Trapping

≈313 °C

Figure 39. Prepurification plugging routines.

Figure 40 is a plot of O solubility in NaK with an oxide concentration level highlighted at 

313 °C (595.4 °F). This temperature coincides with an oxide saturation concentration of 110 ppm.

10,000

1,000

100

10

0

Oxygen (ppm)

Temperature (°C)

0 200 400 600 800

Data

5th Order Poly Fit

Exponential Fit

(313 °C, 110 ppm)

Figure 40. Resultant contamination measurement based on prepurification plugging routine.
37

A summary of plugging events is provided in table 6. As shown in table 6, there was a range 

of flow percentage allowable through the plugging branch in order to achieve a successful plugging 

event. This represents the system hardware’s ability to be adjusted to provide more or less cooling as necessary to achieve steady temperature reduction. In general, the following limitations are 

identified:

Table 6. Summary of plugging events.

Chart

Main Loop 

Bypass Valve 

Setting 

(Turns)

Plugging 

Valve Setting 

(Turns)

Main Loop 

Flow Rate 

(cc/s)

Main Loop 

Temperature 

(°C)

Plugging 

Branch Flow 

Rate 

(cc/s)

Plugging 

Orifice 

Temperature 

(°C)

Plugging Flow 

to Main Flow 

Ratio 

(%)

Plugging 

Operation 

Cooling Rate 

(°C/min)

*1 1 2 76.0 304 20.1 NA (285) 26.4 –5.1

*2 1 3 76.7 310 18.9 NA (289) 24.7 –6.0

*3 1.7 1.3 100.8 419 18.8 306 18.7 –2.5

A 1.6 2.5 103.2 383 10.1 323 9.8 –0.9

B 1.6 2.5 102.7 387 10.0 304 9.8 –1.2

C 1.6 2.5, 2.4 102.1 371 11.7 306 11.4 –1.5

D 1.6 1.4 102.1 317 6.1 245 6.0 –3.1

E 1.6 1.4 102.8 340 4.8 266 4.7 –1.3

*For charts 1 and 2, plugging commenced immediately, indicating that the temperature started too low. Chart 3’s plugging flowmeter was not properly calibrated 

 and the chart is not shown in figure 44.

(1) Higher flow rates and therefore higher plugging branch to main loop flow ratios require 

greater time to produce a plugging ‘event’ with the risk of having insufficient cooling at the plugging 

valve to produce a plugging event.

(2) Lower flow rates and therefore lower plugging branch to main loop flow ratios cool more 

rapidly, making identification of precise plugging temperature difficult.

The plugging apparatus may therefore be used quickly to obtain a general idea of contamination level (a range of saturation temperatures) for a system with unknown contaminant content by 

use of small plugging branch flow rate and high rate of cooling. Following such a plugging event, the 

plugging apparatus is ‘cleaned’ by passing heated NaK back through while opening the adjustable 

orifice (plugging valve) completely. A second plugging operation may then be conducted at a slower 

rate of cooling close to the temperature range identified above, allowing a more precise identification 

of saturation temperature. 

Figure 41 shows plots for all plugging events using the same flowmeter calibration. Plots A, B, 

and C are plugging events occurring prior to a purification operation via cold trapping. Plots D and 

E are plugging events occurring after purification. Note the net reduction in temperature at plugging 

(major change in slope) between the groups representing the change in NaK saturation temperature.
38

0.00016

0.00014

0.00012

0.0001

0.00008

0.00006

0.00004

0.00002

0

–0.00002

Plugging Flowmeter Output Signal (V)

Plugging Orifice Tempereature (°C)

450 400 350 300 250 200 150

A

B

C

D

E

Figure 41. Plots for all plugging events using the same flowmeter calibration.

3.2.6 Cold Trapping 

A single cold-trapping operation was demonstrated during testing. Given the difficulty and 

time requirement for running plugging operations, cold trapping was initially postponed until several 

plugging routines were performed and shown to indicate reasonable repeatability. Figure 42 plots 

the temperatures of the primary NaK flow loop and the cold trap temperature during a cold-trap 

routine. Note the temperature delta of 45–70 °C (113–158 °F).

The single cold-trapping operation set out to demonstrate purification by a clear margin, 

i.e., a clearly visible change in oxide saturation temperature of NaK. Cold trapping took place at 

two distinct regimes as shown in figure 43 and table 7. Figure 43 plots both pre-cold and post-cold 

trapping plugging routines for comparison. There is clear indication that the cold-trap routine has 

changed the parameters of the subsequent plugging routine.

The total mass of NaK loaded into the FTL is ≈3.9 Kg. The trap draws a percentage of total 

loop volume continuously, purifies it, and then returns it to the main loop. The rate of purification 

is therefore not linear with time, but will be a maximum at the outset, working towards an O saturation level at the temperature of purification asymptotically. For the purposes of this demonstration, 

the goal was to demonstrate purification without necessarily quantifying precise rates of decontamination. Figure 44 highlights the oxide saturation level at the temperature measured during the postpurification cold-trapping, successfully indicating a new, lower contaminant concentration of 

70 ppm.
39

350

330

310

290

270

250

230

Temperature (°C)

Time (s)

9,200 9,500 9,800 10,100 10,400 10,700 11,000 11,300 11,600 11,900 12,200

Main Loop Temps

Cold Trap Temp ≈265 °C

Figure 42. Purification routine via cold trapping.

3

2.5

2

1.5

1

0.5

0

–0.5

Plugging Branch Mass Flow Rate (g/s)

Temperature Immediately Downstream of Plugging Valve, T-38 (°C)

9,200 9,500 9,800 10,100 10,400 10,700 11,000 11,300 11,600 11,900 12,200

Plugging Event

Before Cold Trapping

Plugging Event

After Cold Trapping

≈244 °C

Figure 43. Postpurification plugging routines compared with prepurification routines.
40

Table 7. Cold trapping test data.

Main Loop 

Temperature 

(°C)

Main Loop 

Flow 

(cc/s)

Cold-Trap 

Flow* 

(cc/s)

Cold Trap 

to Main Loop 

Flow Ratio 

(%)

Cold-Trap 

Temperature 

(°C)

Duration 

of Trapping 

(s)

Total Volume 

Through Trap 

(cc)

Total Mass 

Through Trap 

(Kg)

334.7 115.6 13.572 13 291.6 843 11,441.2 9.1

328.8 111.8 9.796 10 265.8 2,790 27,330.8 21.7

*This flow rate is estimated from system models and other flow testing since no flowmeter was installed on the cold trap branch.

10,000

1,000

100

10

0

Oxygen (ppm)

Temperature (°C)

0 200 400 600 800

Data

5th Order Poly Fit

Exponential Fit

(244 °C, 70 ppm)

Figure 44. Resultant contamination measurement based on postpurification 

plugging routine.
41

4. LESSONS LEARNED

Valuable lessons learned during development and testing of the hardware used in this successful testing are documented in this section. While not specifically covered in testing objectives 

as defined at the outset of this research and development work, these were identified during testing 

either as ‘a-ha’ moments or as repeated observations with obvious enabling value for future work.

4.1 Flow Measurement

Some mechanism of flow measurement should be provided on all branches of a fluid flow 

system. Electromagnetic flowmeters are an excellent, noninvasive means of providing a flow measurement provided they are calibrated using a well-known reference. A venturi flowmeter provides 

a starting reference and subsequently represents a minimal pressure loss to the system.

4.2 Electromagnetic Flowmeters

A common observation at the beginning of EM flowmeter operation is the need for high 

temperature operation to ‘wet’ the flowmeter, i.e., force the relatively high surface tension NaK to 

relax and intermingle with its surrounding flow channel. (This can be likened to a drop of water 

either beading up on a surface (nonwetted) or spreading freely (wetting)). During this testing, it was 

observed that EM flowmeter signal strength was not subject to this limitation. It is hypothesized that 

the turbulent flow of the higher flow rates achieved in this current hardware configuration produced 

a similar result.

4.3 Venturi Flowmeter

Venturi flowmeters provide a relatively simple and well-documented method for measuring 

flow rate in a fluid flow system. The accuracy of a venturi depends wholly on matching the design to 

the expected flow conditions and ensuring accurate pressure readings are taken.

Careful design of the flow loop and study of the resulting system curve provides the only reliable means for identifying and properly sizing a venturi flowmeter.

Low-pressure drops are usually desired to reduce the overall parasitic losses in a system. The 

lower the pressure drop allowable, the more difficult to obtain a precise pressure reading. In the case 

of a venturi flowmeter, a ∆P PT should be employed. This ∆P transducer will be very sensitive and 

should therefore be isolated from any system test and checkout where potentially large pressure transients are expected.
42

4.4 Heat Transfer

Forced convection heat transfer is far more effective than natural convection due to temperature and therefore density changes in a fluid. In fact, natural convection may be identified as 

a negligible mechanism except when coupled with an appropriate geometry (high vertical aspect 

ratio such as a vertically oriented reservoir). Initial designs for the heated NaK reservoir employed 

in the FTL relied on natural convection to mix the NaK flowing through the loop. The final design 

provided internal flow channels in the flow reservoir to force incoming (cool) NaK to come in direct 

contact with the graphite heater port in the reservoir, thus allowing maximum absorption of applied 

heat. This is important to note as heating transients require long durations when heat transfer is not 

facilitated with effective design, thus forcing much longer testing durations.

It has been repeatably observed that heat-loss paths are insufficiently estimated, resulting in 

higher net heating requirements and cooling occurring in critical areas where isothermal behavior is 

the assumption.

4.5 Loop Design Process

Since initial configurations of the FTL were first tested, it has been apparent that there are 

flow rate/heating/cooling combinations that fall in fairly narrow bands to achieve balanced, steadystate operation for the tubing sizes employed (0.5-in (1.27-cm) and 0.25-in (0.64-cm) nominal tube). 

Without well-known heat transfer coefficients for natural and forced convection between SS and 

NaK fluid, effective configurations have been achieved through iterative trial and error.

The trial-and-error process generally began with a preliminary estimate is prepared for 

expected loop performance. The hardware is then built precisely to carefully match predicted configurations and conditions as much as possible. This configuration is rigorously tested to obtain sufficient data for anchoring the original estimates or identifying areas where estimates are obviously 

incorrect. Subsequent design work addresses areas where increased design fidelity is required.

4.6 Pressure Transducers

Pressure transducers must be carefully selected so that their measurement range will always 

exceed any exposure they will be subjected to. In the case of fluid flow loops, it is generally a simple 

process to size a transducer to measure expected pressure drops, which are well known. An additional subtlety of a hermetically-isolated, heated fluid flow loop is the increase in line pressure due 

to a temperature increase of any blanketing gasses. Care should be taken to combine expected fluid 

pressures with these changes of gas pressure to arrive at the correct pressure range to be measured.

Care must be taken to calibrate PTs in such a way as to not exceed the measuring range provided by the manufacturers as this range is limited by elastic deformation of a thin metal diaphragm. 

Exceeding the range will bias future readings and be subject to hysteresis. It is therefore critical that 

all PTs be specified so that all possible measured pressures fall within the recommended pressure 

range. It is not recommended to attempt recalibration after testing has commenced as this effectively 

renders test-to-test comparison of results unusable.
43

One special case that requires extra attention is the use of ∆P PTs. The more precise a measurement range, the finer and more fragile the metal diaphragm employed in the transducer and 

therefore the more susceptible to plastic deformation. It is critical to design calibration and testing 

procedures to eliminate off-design pressure exposure that effectively render transducers unreliable 

and unable to produce repeatable results.

4.7 Valve Positions

Use of remotely operated valves in a flow system provide excellent flexibility in adjusting 

areas of study in a flow loop. Some means for recording valve positions in the time-stamped data 

stream should be provided to allow subsequent correlation. In addition, care should be exercised 

to ensure that valve positions are repeatable and regularly confirmed by test prior to major testing 

events.

4.8 Thermocouples

For critical temperature measurements, redundancy is essential so that operations may proceed or be safely terminated if a temperature measurement fails. Provide multiple TCs as necessary 

at critical measuring points.

After identifying critical safety measurements, identify desired measurement locations 

required for the purposes of studying and carrying out research goals. Only after the above are 

adequately in place should additional, nonessential measurements be populated.

4.9 Fluid System Filtration

A mechanical filter screen is desirable in a heated NaK flow loop. Repeated temperature 

transients in such a system will transport O from a very distributed initial concentration to discrete deposits of oxides. These deposits may subsequently become suspended in flowing NaK where 

the rate at which they dissolve back into solution is extremely limited. These discrete particles may 

become lodged in such a way as to be effective mechanical obstructions. A filter screen located in the 

loop and subjected to high-velocity, forced fluid flow provides the greatest likelihood that the particles may be dissolved back into solution due to fluid impingement.

4.10 Calibration 

Calibration should be a distinct part of pretest and checkout only. Once testing commences, 

any adjustments to signals should be performed by postprocessing on collected data so that all test 

data has the same baseline. Without this restriction, inter-test comparison of test data is null and 

void.
44

5. CONCLUSION

Testing successfully demonstrated flow loop contamination and purification via plugging 

meter and cold trap. NaK contamination was measured via a plugging meter consisting of a gascooled, motor-controlled metering valve and a water-cooled EM flow meter. NaK purification was 

demonstrated via use of a gas-cooled cold trap with integrated economizer and subsequent contamination measurement via plugging meter as above. 

In addition, a nonflight-like mechanical NaK pump provided a robust, high flow rate solution for system testing employing readily available component parts. Full-flow EM flowmeters with 

unmodified flow channels were shown to have sufficient signal-to-noise ratio to obviate the previous 

FTL design’s need for flow channel modification and subsequent potential flow restriction. A venturi 

flowmeter was a relatively simple addition to the flow loop and provided calibration reference for 

EM flowmeters. A reservoir heated via a graphite heater provided effective, rapid NaK heating with 

a relatively small footprint. Finally, motor-controlled valves provided high-fidelity remote operation 

during online flow tests. 
45

APPENDIX A—SYSTEM SCHEMATIC

Figure 45 shows the system schematic.
46
47

Figure 45. System schematic.
48

APPENDIX B—MP PUMP PERFORMANCE CURVE

The MP Pump performance curve is shown in figure 46.

5.7

5.4

5.1

4.8

4.5

4.2

3.9

3.6

3.3

3

18

16

14

12

10

8

6

4

2

0

Head (ft)

Amps

Flow (gpm)

0 2 4 6 8 10 12 14 16

Figure 46. MP Pump performance curve.
49

APPENDIX C—THERMOCOUPLE LOCATIONS

Table 8 shows the 36 TC locations.

Table 8. Thermocouple locations.

No. Description

T-01 ∆P PT, DP-01

T-02 Absolute PT, PT-01, venturi upstream

T-03 Absolute PT, PT-02, venturi throat

T-04 Absolute PT, PT-04, NaK pump Ar blanket

T-05 NaK temperature via thermal well, immediately downstream of reservoir

T-06 NaK temperature via thermal well, immediately downstream of main EM flowmeter

T-07 NaK temperature via thermal well, immediately downstream of main GN2 HX

T-08 NaK temperature via clamped on TC, immediately upstream of venturi

T-09 NaK temperature via thermal well, at plugging branch tee

T-10 NaK temperature via clamped-on TC, immediately upstream of bypass valve

T-11 NaK temperature via clamped-on TC, main loop between cold trap and plugging branch exit tees

T-12 NaK temperature via thermal well, immediately upstream of reservoir

T-13 Water temperature, mechanical pump cooling inlet

T-14 Water temperature, mechanical pump cooling outlet

T-15 Water temperature, main EM flowmeter inlet

T-16 Water temperature, main EM flowmeter outlet

T-17 Water temperature, plugging EM flowmeter inlet

T-18 Water temperature, plugging EM flowmeter outlet

T-19 GN2 temperature, main HX inlet

T-20 GN2 temperature, main HX outlet

T-21 GN2 temperature, plugging HX inlet

T-22 GN2 temperature, plugging HX outlet

T-23 GN2 temperature, cold trap inlet

T-24 GN2 temperature, cold trap outlet

T-25 NaK temperature via clamped-on TC, immediately downstream of plugging valve

T-26 NaK temperature via thermal well, downstream of plugging valve, upstream of plugging branch heater

T-27 NaK temperature via thermal well, downstream of plugging branch heater and plugging EM flowmeter

T-28 NaK temperature via clamped-on TC, cold trap inlet

T-29 NaK temperature via thermal well, coolest region of cold trap

T-30 NaK temperature via clamped-on TC, cold trap outlet

T-31 Cartridge heater core temperature, plugging branch heater

T-32 Cartridge heater core temperature, cold trap branch heater

T-33 SS reservoir temperature at graphite heater port

T-34 Ar blanket temperature at mechanical pump

T-35 SS pump body temperature

T-36 SS reservoir temperature at outer wall
50
51

REFERENCES

1. Nainiger, J.: “Affordable Fission Surface Power System Study Final Report,” NASA Exploration 

Systems Mission Directorate Report, October 2007.

2. Mason, L.S.: “A Comparison of Fission Power System Options for Lunar and Mars Surface 

Applications,” NASA/TM—2006–214120, Glenn Research Center, Cleveland, OH, February 

2006.

3. Mason, L.S.: “A Practical Approach to Starting Fission Surface Power Development,” NASA/

TM—2006–214366, Glenn Research Center, Cleveland, OH, July 2006.

4. Mason, L.S.; Poston, D.; and Qualls, L.: “System Concepts for Affordable Fission Surface Power,” 

NASA/TM—2008–215166, Glenn Research Center, Cleveland, OH, January 2008.

5. Palac, D.; Mason, L.; and Harlow, S.: “Fission Surface Power Technology Development Status,” 

NASA/TM—2009–215602, Glenn Research Center, Cleveland, OH, March 2009.

6. Mausteller, J.W.; Tepper, F.; and Rodgers, S.J.: Alkali Metal Handling and Systems Operating 

Techniques, Gordon and Breach, New York, 1967.

7. Garber, A.E.; Webster, K.L.; Godfroy, T.J; et al.: “Interim Report: Performance Testing of a 

7-Pin Bundle in the Fission Surface Power—Primary Test Circuit,” Marshall Space Flight Center, 

Huntsville, AL, July 2010.

8. Godfroy, T.J.; Bradley, D.E.; Pearson, J.B.; et al.: “Hardware-Based Assessment of Subscale 

Sodium-Potassium Cooled Fission Power Reactor Simuators and Related Component Designs,” 

NASA/TP—2011–xxxxx, Marshall Space Flight Center, Huntsville, AL, June 2011.

9. Stepanoff, A.J.: Centrifugal and Axial Flow Pumps: Theory, Design, and Application, Wiley, New 

York, 1957. 
52

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Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18

Online Oxide Contamination Measurement 

and Purification Demonstration

D.E. Bradley,* T.J. Godfroy,** K.L. Webster, A.E. Garber, 

K.A. Polzin, and D.J. Childers

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 20

Availability: NASA CASI (443–757–5802)

Prepared by the Propulsion Systems Department, Engineering Directorate

*Yetispace, Inc., Huntsville, AL **Maximum Technology Corporation, Huntsville, AL

M–1322

Technical Publication

NASA/TP—2011–216473

NaK, mechanical NaK pump, electromagnetic flowmeter, NaK plugging meter, NaK cold trap, online oxygen 

measurement in NaK, hardware, test

01–10–2011

UU 62

NASA

U U U

Liquid metal sodium-potassium (NaK) has advantageous thermodynamic properties indicating its use as a fission reactor coolant for a surface (lunar, martian) power system. A major area of concern for fission reactor cooling sysgtems is system corrosion 

due to oxygen contaminants at the high operating temperatures experienced. A small-scale, ≈4-L capacity, simulated fission reactor cooling system employing NaK as a coolant was fabricated and tested with the goal of demonstrating a noninvasive oxygen 

detection and purification system. In order to generate prototypical conditions in the simulated cooling system, several system 

components were designed, fabricated, and tested. These major components were a fully-sealed, magnetically-coupled mechanical NaK pump, a graphite element heated reservoir, a plugging indicator system, and a cold trap. All system components were 

successfully demonstrated at a maximum system flow rate of ≈150 cc/s at temperatures up to 550 oC. Coolant purification was 

accomplished using a cold trap before and after plugging operations which showed a relative reduction in oxygen content. 

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