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35812
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i

NASA/TP—2010–216437

Documentation of Stainless Steel Lithium 

Circuit Test Section Design

T.J. Godfroy 

Maximum Technology Corporation, Huntsville, Alabama

J.J. Martin

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

With Supplemental Information by

E.T. Stewart

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

N.O. Rhys

Yetispace, Inc., Huntsville, Alabama

August 2010

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812
ii

Acknowledgments

Many thanks to the following for the long hours and excellent work on this project: Roger Harper, Dr. Noah O. Rhys, 

Alok Majumdar, Stan McDonald, Gene Fant, Kenny Webster, Melissa Van Dyke, and Eric Stewart.

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>

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

2. DESIGN CHANGE TO LITHIUM ............................................................................................... 2

2.1 Lithium General Description .................................................................................................. 2

2.2 Stainless Steel and Lithium Incompatibility ........................................................................... 2

2.3 Lithium Melting Temperature ................................................................................................. 3

2.4 Wetting .................................................................................................................................... 3

3. LIQUID METAL CORE DESIGN BASIS .................................................................................... 4

3.1 Core Design—A Los Alamos National Laboratory Design Study.......................................... 4

4. STAINLESS STEEL LITHIUM CIRCUIT LAYOUT .................................................................. 7

4.1 Lithium ................................................................................................................................. 10

4.2 Instrumentation ..................................................................................................................... 10

4.3 Trace Heating and Insulation ................................................................................................ 12

4.4 Tilt Table ............................................................................................................................... 13

4.5 Core ...................................................................................................................................... 13

4.6 Heat Exchanger ..................................................................................................................... 18

4.7 Pump ..................................................................................................................................... 21

4.8 Pump Housing ...................................................................................................................... 23

4.9 Lower Reservoir ................................................................................................................... 25

4.10 Upper Reservoir .................................................................................................................... 28

4.11 Tubing ................................................................................................................................... 30

5. MODELING AND TESTING ....................................................................................................... 37

5.1 Generalized Fluid System Simulation Program ..................................................................... 37

5.2 Lithium Melt/Wetting ............................................................................................................. 39

5.3 Lithium Fill ............................................................................................................................. 40

5.4 Lithium Freeze/Thaw Tests ..................................................................................................... 41

5.5 Lithium Freeze/Thaw Modeling ............................................................................................. 43

6. CONCLUSIONS .......................................................................................................................... 46
iv

TABLE OF CONTENTS (Continued)

(SEE CD INSIDE BACK COVER FOR ALL SUPPLEMENTAL INFORMATION)

REFERENCES .......................................................................................................................... 47
v

LIST OF FIGURES

 1. General core NaK flow loop ............................................................................................... 5

 2. Core end view showing flow passages ............................................................................... 5

 3. End view of core showing inner three fuel pin rings for test hardware fabrication ........... 6

 4. Simplified schematic for SS Li thermal hydraulic flow circuit .......................................... 7

 5. Lithium test circuit as designed .......................................................................................... 8

 6. Lithium test loop as built (HeAr feed lines not shown) ...................................................... 9

 7. Level sensor ........................................................................................................................ 11

 8. Tilt table: (a) During construction and (b) completed ........................................................ 13

 9. Simplified core assembly layout ......................................................................................... 14

10. Thermal simulators (heaters): (a) Drawing and (b) photograph ......................................... 15

11. Heater element layout and heater zone configuration ........................................................ 16

12. Simplified schematic core face sealing system ................................................................... 17

13. Drawing of core face seal ................................................................................................... 18

14. Lithium/HeAr HX unit ....................................................................................................... 18

15. Back view of a HX ............................................................................................................. 19

16. Another back view of a HX ................................................................................................ 20

17. Build view of a HX showing Cu foil wrap ......................................................................... 20

18. Style-VI: (a) ac conduction pump and (b) three-phase autotransformer ............................ 21

19. Style-VI pump flow duct maximum differential pressure. (Exceeding this 

limit will result in circulation of the flat pump duct.) ......................................................... 22
vi

LIST OF FIGURES (Continued)

20. Style-VI ac conduction pump performance curve. (Reproduced 

from MSA Research Corp. Dwg. No. C-510356.) ............................................................. 22

21. Pump housing with pump mounted inside ......................................................................... 23

22. Pump housing viewed from outside ................................................................................... 23

23. Pump enclosure coolant performance curves ..................................................................... 24

24. Lower reservoir instrumentation and trace heater location—front view ............................ 25

25. Lower reservoir instrumentation and trace heater location—rear view ............................. 26

26. As-built view of the LR ...................................................................................................... 27

27. Level sensors to detect Li fill level in the LR ..................................................................... 27

28. Front view of the UR .......................................................................................................... 28

29. Rear view of the UR ........................................................................................................... 29

30. As-built view of the UR ...................................................................................................... 29

31. Photograph of heater failure using tabs to secure to tube ................................................... 31

32. Photograph of successful method of securing heater with Cu foil ..................................... 31

33. Photograph of T1 ................................................................................................................ 32

34. Photograph of T2 ................................................................................................................ 33

35. Photograph of T3 ................................................................................................................ 33

36. Photograph of T4 ................................................................................................................ 34

37. Photograph of T5 ................................................................................................................ 35

38. Photograph of T6 ................................................................................................................ 36

39. GFSSP model of the Li circuit ............................................................................................ 38

40. Stainless steel beaker with freshly sheared Li before melt ................................................. 39
vii

LIST OF FIGURES (Continued)

41. Lithium fill apparatus used to fill the test article ................................................................ 40

42. Freeze/thaw test article ....................................................................................................... 41

43. Freeze/thaw test article with TC locations at various distances ......................................... 42

44. Measured temperature history during Li thawing .............................................................. 42

45. Measured temperature history during Li freezing .............................................................. 43

46. Predicted temperature history of Li thawing ...................................................................... 44

47. Predicted temperature history of Li freezing ...................................................................... 45
viii

LIST OF TABLES

1. System volume breakdown (estimated by CAD models of engineering drawings) ............. 9

2. Heater zone configuration ..................................................................................................... 16
ix

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS

Ar argon

ATM atmosphere

B4C boron carbide

Be beryllium

BHTC bottom heater thermocouple

C carbon

CAD computer-aided design

Cr chromium

Cu copper

EFF–TF Early Flight Fission–Test Facility

EM electromagnetic

GFSSP generalized fluid system simulator program

GHe gaseous helium

GN2 gaseous nitrogen

H2 hydrogen

He helium

HOV hand-operated valve

HX heat exchanger

Li lithium

LR lower reservoir
x

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

LRHA lower reservoir thermocouple

LRHB lower reservoir thermocouple

LRHC lower reservoir thermocouple

LRPG lower reservoir pressure gauge

LRTC lower reservoir thermocouple

MLI multilayer insulation

MSFC Marshall Space Flight Center

N2 nitrogen

Na sodium

NaK sodium-potassium

Ni nickel

NRPCT Naval Reactors Prime Contract Team

O2 oxygen

ROV remote operated valve

SCCM standard cubic centimeters per minute

Si silicon

SS stainless steel

T tube

TC thermocouple

THTC top heater thermocouple

TP Technical Publication
xi

LIST OF ACRONYMS, SYMBOLS, AND ABBREVIATIONS (Continued)

UO2 uranium oxide

UR upper reservoir

VCR vacuum coupling radiation
xii

NOMENCLATURE

Cp specific heat

m mass flow

p pressure

Q	 energy (input heat)

T	 temperature

∆P change in pressure

∆T change in temperature
1

TECHNICAL PUBLICATION

DOCUMENTATION OF STAINLESS STEEL LITHIUM 

CIRCUIT TEST SECTION DESIGN

1. INTRODUCTION

The Early Flight Fission–Test Facility (EFF–TF) was established by the Marshall Space Flight 

Center (MSFC) Propulsion Research Center Nuclear Propulsion Group to perform hardware-directed 

activities relevant to multiple nuclear power reactor concepts using nonnuclear test methodology. This 

includes fabrication and testing at the module/component level and near-prototypic hardware configuration allowing for realistic thermal hydraulic evaluations of systems. In addition, equally important to the 

actual hardware development are: (1) the experience and knowledge gained during fabrication and assembly, (2) the development of documentation and procedures related to integration and operation (includes 

both test hardware and facilities), and (3) the experience gained from early hardware testing with relevant 

materials and environment producing insight and lessons learned for the hardware development program. 

There are a number of reactor and primary heat transport systems applicable to space-based power and 

propulsion. The EFF–TF examined a pumped alkali metal lithium (Li) circuit for the Naval Reactors Prime 

Contract Team (NRPCT). During development of this system, required facility interfaces were leveraged 

off of existing EFF–TF capabilities and test hardware wherever possible. In addition, baseline documentation (including procedures, safety, hazards, etc.) evolved to augment the current EFF–TF operations.

The intent of this work was to gain experience with a Li liquid metal system and to begin to understand the nature of liquid Li, freeze/thaw issues, and operational issues. Specific objectives included:

• Design, fabricate, assemble, and operate a small-scale stainless steel (SS) actively pumped liquid metal 

(Li) flow circuit.

• Gain circuit experience with Li such that the experience can be transferred, where applicable, to followon design and testing of a high-temperature, refractory metal actively pumped Li circuit.

• Investigate the nature of freeze/thaw issues through hardware testing.

• Include, by design, the ability to perform component testing of nominally-sized components that may 

be of interest to NRPCT.

• Document lessons learned for future applications.
2

2. DESIGN CHANGE TO LITHIUM

The Li design for an actively pumped circuit was based on an existing actively pumped sodiumpotassium (NaK) design. 

To establish an active circuit in a short amount of time, NRPCT utilized the existing NaK core, 

heat exchanger (HX), and pump. These components were designed and fabricated or procured for a NaK 

system but would suffice for a Li system with no modifications. However, due to the change to Li, the 

issues of Li and SS incompatibility, the higher melting point of Li, and the unknown wetting temperature 

had to be evaluated.

2.1 Lithium General Description

Lithium is silvery in appearance. It is a solid about half as dense as water at room temperature. The 

melting point is 180.5 °C (356.9 °F) and it has the highest specific heat of any solid element, making it 

well suited for some heat transfer applications. The material texture is very sticky or tacky. A 1-in-diameter bar is more easily cut with pruning shears than a hack saw. A reasonable description of the strength 

of solid Li is that it has the consistency of a ‘Tootsie Roll that has been refrigerated.’1 It must be handled 

in an inert environment, such as argon (Ar), as it has a great affinity for (but not limited to) nitrogen (N2) 

and oxygen (O2) and will immediately tarnish in open air. Care must be taken when storing Li, and proper 

personal protection should be worn at all times. It is corrosive, especially at elevated temperatures (e.g., at 

circuit operating temperature), and special care must be taken to avoid injury to personnel. A brief review 

of Li thermophysical properties can be found in reference 1.

2.2 Stainless Steel and Lithium Incompatibility

There are several mechanisms that work to corrode the SS in an actively pumped Li circuit. For 

austenitic steels, the initial stage of high dissolution rates corresponds to the formation of a ferrite layer 

that results from the preferential leaching of nickel (Ni) and, to a lesser extent, chromium (Cr) from the 

surface of the steel.2 These constituents are then observed to deposit downstream in lower temperature 

regions, preferentially coming out of solution based on temperature. As might be expected, the mass transfer of the system also demonstrates a significant temperature dependence, and as such, the overall changes 

in temperature (∆T) of the system must be considered.3 Additionally, high N2 levels in the Li circuit lead 

to general dissolution in austenitic steels and a ‘stoichiometric’ steady-state dissolution behavior may not 

be achievable in a flowing Li environment.

For this circuit, the primary concern associated with these corrosion mechanisms is failure of 

the SS boundary. The rate of corrosion is estimated to be ≈22 μm/yr at 427 °C (800.6 °F) and is slightly 

lower at 482 °C (899.6 °F).2 This rate was used to estimate the system operating time to reach failure even 

though the circuit was anticipated to operate at a slightly higher temperature, ≈500 °C (932 °F). Extensive 

literature searches revealed that the corrosion mechanisms for Li are not as well characterized as for other 

alkali metals, such as sodium (Na), requiring an approximation of this nature. The highest temperature 
3

region will be in the core, where the inner tube has a thickness of 0.089 cm (0.035 in). The potential SS 

corrosion rate in this region would result in failure after ≈40.5 yr of operation; potential failure due to pressure was not considered in this calculation. Considering the anticipated operational lifespan of this circuit 

of ≈1,000 hr and low-pressure operation of ≈103.4 kPa (≈15 psia), this rate of corrosion was considered 

acceptable and posed minimal risk. However, steps were still taken to minimize the ∆T in the system as 

well as to limit the introduction of N2 into the system. This was accomplished by purchasing high-purity 

Li, using Ar cover gas where appropriate, and operating the circuit in a vacuum. There was no planned 

active measurement of the N2 levels in the Li circuit; however, samples of Li were taken initially and 

periodically during testing, if possible, in an attempt to quantify the level of contamination. By design, the 

impurity levels in this circuit were not to be of primary concern. For a more detailed discussion on corrosion mechanisms, see reference 2.

2.3 Lithium Melting Temperature

The melting point of Li is 180.5 °C (356.9 °F). As such, the circuit must be preheated to allow 

introduction of Li to the system. However, because the freeze/thaw behavior in this type of configuration 

is undetermined, it is necessary to drain the system before it is allowed to cool. It should be noted that, 

because Li contracts as it freezes, this process is not the concern; however, as the Li thaws, a condition 

may arise where the expansion of the melted Li exceeds the yield strength of the SS due to unknown/

unpredictable void formation during freeze. Additionally, in the event of power loss, the system must be 

drained quickly before it cools and freezes, thereby resulting in unknown conditions upon thaw. For these 

reasons, the fill/drain reservoir was relocated to the lowest vertical position, and an expansion reservoir 

was added to accommodate the ≈4.5% thermal expansion of the Li at operational temperatures.

2.4 Wetting

Wetting is a concern for any liquid metal. Wetting provides maximum thermal transfer from the 

boundary metal, SS in this application, to the liquid metal. Wetting is a function of temperature and the 

impurity concentrations in the liquid metal and on the boundary metal surface. To achieve wetting, the 

liquid metal must bring into solution the impurities that are present on the surface of the metal to be wet; 

as the type and quantity of impurities increase so does the temperature required to drive them into solution. 

The most cost-effective, time-efficient method to evaluate the temperature at which wetting would occur 

for this circuit was to measure it directly. Relatively impure Li, 98%, in a cleaned but unbaked SS beaker 

was heated over ≈45 min to ≈340 °C (644 °F) in an Ar environment with <0.3 ppm O2 and <1 ppm water 

vapor. The observed wetting temperature was ≈310 °C (590 °F). This was noted by the angle of incidence 

of the Li droplet as it transitioned from acute to obtuse. From this experiment, it was concluded that, with 

higher purity Li and a fully baked-out system, wetting will occur below this temperature. Once the entire 

circuit is brought to ≈325 °C (617 °F), it can be assumed to have properly wet all surfaces. The exception 

to this will be in the standoff regions that lead to the pressure transducers because they are maintained at 

a lower temperature; however, this will not affect the performance of the pressure transducers and a lack 

of wetting may actually be preferred in this area.
4

3. LIQUID METAL CORE DESIGN BASIS

The liquid metal circuit was designed around the core design. While this core design was based on 

a NaK design study, the application of this design to a working Li design still has significant merit. The 

core geometry for this project, based on a 100-kW study performed by the Los Alamos National Laboratory, utilizes annular fuel pins and two-pass coolant flow.4 Section 3.1 details the design with NaK as the 

reactor coolant.

3.1 Core Design—A Los Alamos National Laboratory Design Study

Figure 1 illustrates the general layout of the core assembly with NaK coolant loops and plenums. 

The coolant enters the core through an annular inlet plenum (positioned at the top) that directs the coolant 

into a circumferential flow passage formed between the outer shell and core block. The flow follows this 

perimeter passage, traversing the length of the core and exiting into the lower manifold. This manifold 

distributes the coolant for a return trip to the top of the core via annular gaps formed between the fuel pin 

clad and core block. At the top of the core, an outlet plenum collects the heated NaK. Figure 2 shows an 

end view of this same core design layout, illustrating both the perimeter and annular fuel pin flow paths. 

Design study specifics include the following dimensional parameters:

• Twelve-sided SS core block:

– Outer diameter = 25.75 cm (10.14 in), point to point.

– Length = 68 cm (26.8 in).

• Fuel pin holes (in core block):

– Number = 127.

– Diameter = 1.73 cm (0.680 in).

– Length = through core block.

– Hole pitch = 1.90 cm (0.75 in), center to center of hole.

– Web = 0.18 cm (0.070 in), core block material between holes.

• Fuel pins (clad):

– Outer diameter = 1.4 cm (0.55 in with 0.035-in wall).

– Length = 74 cm (29.13 in), length to clear outer pressure shell.

– Radial gap to core = 0.0064 cm (0.0025 in).

• Outer pressure shell:

– Outer diameter = 26.4 cm (10.4 in with 0.10-in wall).

– Length = 78 cm (30.7 in), sufficient to form upper/lower plenums.

• Thermal simulators (heaters):

– Outer diameter = 1.15 cm (0.453 in leaves a 0.013-in gap).

– Heated length = 60 cm (23.62 in).
5

• Coolant specifics:

– Inlet temperature = 615 °C (1,139 °F).

– Exit temperature = 650 °C (1,202 °F).

– Flow rate = 3.25 kg/s (68.7 g/min).

– Core net ∆P = 6.9 kPa (1.04 psi).

Annular Inlet 

Plenum

Drum 

Reflectors

Lower Manifold

Annular 

Fuel Pins

Outlet Plenum

Figure 1. General core NaK flow loop.

SS

NaK

UO2

Be

B4C

NaK Return 

Annular Passage

NaK Supply 

Perimeter 

Passage

Drum

Figure 2. Core end view showing flow passages.
6

For the sake of simplifying hardware fabrication, integration, and testing while staying within 

overall project funding, the selected approach requires that:

• Both the reflector drums and shields located beyond the outer pressure shell are eliminated from the test 

hardware layout.

• A one-third-power version of the core assembly is employed. This requires that a subset or partial array 

of the core geometry be used; specifically, the geometry encompasses the central three fuel pin rings 

(total of 37 pins), shown in figure 3. All geometric dimensions for the fuel pins and flow passages are 

maintained.

• All other system components are designed as facility elements using standard design practices that err 

on the side of reliable, producing heavier rather than ‘flight like’ components.

The reduction of core size for the purposes of an initial test has a significant impact on overall 

complexity and cost. The reduction in power level eliminates the requirement for two pumps (off-theshelf style 6 units) and flow balancing issues, reduces the size of HXs/flow plumbing, and significantly 

decreases the required number of thermal simulators (heaters).

Target core design goals for the reduced 37-pin system include the following specifics:

• Total fuel pins = 37.

• Input power = 30 kW.

• Coolant flow rate = 1 kg/s (2.21 lb/s).

• Core pressure drop = 6.8 kPa (0.986 psi).

• Inlet temperature = 615 °C (1,139 °F).

• Outlet temperature = 650 °C (1,202 °F).

Figure 3. End view of core showing inner three fuel 

pin rings for test hardware fabrication.
7

4. STAINLESS STEEL LITHIUM CIRCUIT LAYOUT

A general flow diagram, illustrated in figure 4, identifies the key system elements of the SS Li 

flow circuit. The complete system is mounted on a tilt table (making it modular) that is inserted into the 

EFF–TF 9-ft vacuum chamber located in Building 4655. The interfaces/connections with the vacuum 

chamber include instrumentation, pump/heater power, pump cooling, valve control power, and a secondary helium (He) Ar closed-cycle coolant loop (no Li is routed out of the vacuum chamber). The Li circuit 

is an all-welded construction with the exception of a few strategic locations to attach instrumentation such 

as the liquid level sensors, pressure transducers, and test section connection/disconnection points. These 

connections are all made with VCR® (a Swagelok Co. product) fittings. All components and tubing have 

been proof pressure tested to 2,068 kPa (>300 psi) and leak checked using gaseous helium (GHe) to better 

than 10–9 standard cm3 per minute (SCCM) to minimize the potential for leaks. High-temperature valves 

(remote operated where needed) are used at the lower reservoir. Instrumentation measurements are made 

at key locations including the core, HX, reservoir, and pump as well as at necessary points throughout the 

circuit. All components are constructed from SS, primarily 316, with the exception of the level sensors. 

The ceramic portions of the sensors do not come in direct contact with the Li. 

T T

P

P

P

P

L

T

P

P P

T

L

L

T

P

P

P

P

Flowmeter

Thermocouple

Pressure Transducer

Liquid Level Probe

Cool Li Flow

Hot Li Flow

GHe Ar Flow

N2 Flow

Li 

Fill and Drain

Component 

Test Section

Pump Purge 

Enclosure

Highest Point 

in System

High Vacuum

Inert Purge System

Lower Reservoir 

(Lowest Point)

37-Pin Li Core 

(Reactor Simulator)

Li/HeAr

HX

EM

Pump

Upper 

Reservoir

GN2

Facility

To

Atmosphere

Vacuum/Purge System

Inert Pressurant System

Figure 4. Simplified schematic for SS Li thermal hydraulic flow circuit.
8

Based on this schematic, a detailed engineering design evolved, resulting in the overall system 

drawing set included in sections A–H in the supplemental information. The final hardware configuration 

based on these drawings is illustrated in the three-dimensional rendering presented in figure 5. Figure 6 

shows the as-built test circuit. The overall layout of components and tubing was configured to provide 

sufficient flexibility without the use of expansion bellows in the circuit. Bellows were avoided because 

(1) they are a thin-walled component susceptible to mechanical damage, and (2) the convolutes can retain 

residual Li making draining/cleanup more difficult. Additionally, butt welds were used wherever possible 

to eliminate possible cavities in which Li and impurities could be trapped. The flow circuit was designed 

for use on a tilt table, providing a ‘low spot’ in order to utilize gravity to route the Li to the fill/drain vessel (lower reservoir). In the smaller diameter tube sections, such as in the pressure transducer standoffs, 

a purge of high purity Ar was envisioned to overcome the surface tension of the Li and to purge the lines 

of the liquid Li. In general, major flow components are laid out to limit pressure drop (large flow areas) so 

as to maximize flow rate and to minimize pump power consumption. The sections that follow will describe 

the major system components in more detail.

Tube 5

Core

Cart

Tube 3

Tube 2

Tube 4 Tube 1 Upper 

Reservoir

Pump/

Pump Housing

Spill Tray

HX

Lower Reservoir

HeAr

Feed Lines Tube 6

Figure 5. Lithium test circuit as designed.
9

Figure 6. Lithium test loop as built (HeAr feed lines not shown).

The volume of each major component and the overall plumbing are listed in table 1. The total 

system volume is estimated at 7.51 kg (16.56 lb), based on computer-aided design (CAD) models of the 

as-built circuit. Lithium is loaded at ≈250 °C (≈482 °F); at 500 °C (900 °F), the Li in the circuit will expand 

to fill 7.51 kg (16.56 lb) of volume. The tubing leading to the upper reservoir was sized to accommodate 

the volume expansion of the Li; this was necessary to provide sufficient margin for fluid expansion when 

brought from the load temperature to the operating temperature of 500 °C (900 °F). The lower reservoir 

was designed to be of sufficient volume to contain all the Li at the maximum operational temperature, 

500 °C (900 °F) (the maximum Li volume).

Table 1. System volume breakdown (estimated by CAD models 

of engineering drawings).

Volume

Lithium Mass 

at 500 °C (900 °F)

Component (cm3) (in3) (kg) (lb)

Core assembly

HX

Tubing/pump

Upper reservoir

Total

Lower reservoir 

5,424

5,440

3,736

1,049

15,649

16,928

331

332

228

64

955

1,033

2.61

2.61

1.79

0.5

7.51

8.13

5.75

5.75

3.95

1.10

16.56

17.92
10

4.1 Lithium

A list of the predominant suppliers of high-purity Li in the United States is found in reference 5. 

Companies having the capability to provide Li of sufficient purity for the operations planned at the 

EFF–TF and with the capability of providing the larger quantities necessary for the ultimate circulating 

loop (assume ≈15 kg (≈33.07 lb)) are identified. Certain nonmetallic impurities can significantly affect 

the structural integrity of containment materials in Li operations; specifically N2, O2, carbon (C), silicon (Si), and hydrogen (H2) are of interest in this application. It was assumed that the desired purity level 

was <100 ppm of any single nonmetallic impurity, as this purity level matches that of the Li in the cited 

corrosion references.

Lithium is generally shipped using one of two transport methods, either of which is designed to 

prevent contact of the Li metal with the atmosphere. The sample may be packed in a vacuum can with a 

mineral oil coating over the stock material, or it may be packed in a vacuum can with Ar used as a cover 

gas. To maintain the integrity of the Li purity from manufacture to receipt at the EFF–TF, all procured 

material was vacuum packed with an Ar cover gas. Per discussions with product managers from various companies, the maximum purity Li currently available commercially is 99.97%. Given the available 

purity levels and ease of working with the vendor, the top two choices for Li vendors were American 

Elements Corp. and ESPI Metals. All Li for this task was purchased from ESPI. 

4.2 Instrumentation

4.2.1 Temperature

All circuit temperature measurements are performed using type-K thermocouples (TCs) that are 

spot welded directly to the SS. The TC wire, manufactured by Omega (HH–K–24–SLE), is a high temperature, glass-jacketed, 24-gauge wire capable of operation at up to 704 °C (1,299 °F). There is one 

thermowell located at the exit plenum of the core. The TCs are located approximately every 15.24 cm 

(6 in) along the lengths of tubing and in distributed patterns over the remaining components. These diagnostics monitor circuit temperature distribution and help provide system diagnosis. Additional TCs are 

placed on system components to provide backup to other instrumentation systems, such as level measurement or trace heater operational temperature. The TCs are all terminated with miniconnectors that plug 

into a distribution panel inside the vacuum chamber. A feedthrough carries the signal out of the chamber 

to IOTech® data acquisition hardware. This signal is then converted to a temperature measurement in 

degrees Celsius and read into a custom LabVIEWTM (a National Instruments® product) interface where it 

is recorded to file.

4.2.2 Pressure

The pressure transducers selected for this circuit were manufactured by Astro Sensors, model 

number AS9001–SP. Cryogenic units were modified to a pressure range of zero to 172.4 kPa (zero to 

25 psia), an output signal of zero to 30 mV at 3 mV/V, and a compensated range from 20 to 218 °C 

(68 to 424.4 °F). The static accuracy for this unit is 0.1% of full scale, all wetted parts are 316 SS, the diaphragm thickness is > 0.013 cm (0.005 in), and it has a flowthrough design. The flowthrough design provides a method for an inert gas purge that is used to push the liquid Li out of the transducer pressure sense 
11

area. A vacuum coupling radiation- (VCR-) type connection is used to couple the pressure transducers to 

the system.

TCs were placed along the length of the transducer standoff. A calculation was performed to determine the required standoff length to keep the transducer cool enough to operate reliably (sec. I in the 

supplemental information). The results of this study show that if the main flow path has an operating 

temperature of 600 °C (1,112 °F), a standoff of ≈25 cm (≈9.84 in) will be required to allow the Li to cool 

below freezing. An additional 10 cm (3.94 in) were added to this length and a heater was applied in the 

region just before the transducer. This additional length is calculated to be sufficient to allow the Li temperature to drop down into the operational range of the pressure transducer. If the Li temperature becomes 

too low, additional heat can be added by this heater. If problems do arise with the transducer, the VCR 

connection will allow for rapid replacement or capping of the standoff. The signal from the transducer 

is routed to a feedthrough on the chamber and then to a National Instruments SCXI chassis where it is 

converted to a pressure measurement.

4.2.3 Level

Level measurement was needed on the upper reservoir and on the lower reservoir. Figure 7 shows 

a 5-kV, 30-A power feedthrough that is sold by Kurt Lesker Company. The level sensors are high voltage, 

high current electrical feedthroughs terminating in weld lips that are welded to a VCR fitting. This is then 

mated to a VCR fitting on the upper and lower reservoirs. The level sensor works by completing a circuit 

when the level of the Li comes in contact with the SS pin of the feedthrough.

Figure 7. Level sensor.

4.2.4 Flow

Flow measurement is measured in two ways. The first method uses the HX to estimate flow from 

the relationship,

Q = mC& pΔT , (1)

where, in a reasonably well insulated HX, the energy coming in will be equal to that removed:

QLi = QHe . (2)
12

The inlet and outlet temperatures of the gas and the gas flow rate are measured, and the specific 

heat (Cp) is known for the gas. Therefore, the Li mass flow rate can be calculated from equation (3):

m& Li = m& HeCp,HeΔTHe

Cp,LiΔTLi

. (3)

Lithium flow measurement can also be accomplished using a permanent magnet. The magnet is 

used to measure the current induced in a wire, which then can be correlated to the Li flow rate. The most 

difficult part of any flow measurement is the calibration of the device. It is hoped that these two methods 

corroborate each other.

4.3 Trace Heating and Insulation

4.3.1 Trace Heaters

Trace heaters are placed on nearly all circuit components and tubing such that the entire system 

can be heated to ≈525 °C (≈977 °F). The trace heaters used are Watlow brand cable heaters that are SS 

sheathed, 0.318-cm- (0.125-in-) diameter cold formable, 240-V ac resistance heaters capable of continuous operation at temperatures up to 650 °C (1,202 °F) and with intermittent operating periods up to 815 °C 

(1,499 °F). These heaters were cold formed to fit as snuggly as possible along all sections of tubing and 

all major components except the core. An attempt was made to provide two trace heaters for each heated 

section and to make logical sections for control of thermal input into the system.

The first use of the trace heaters was in the conditioning phase to outgas the tubing and other components by performing high-temperature vacuum bake-out prior to the Li loading. During this interval, 

the temperature of all wetted components must be raised in excess of the nominal operating temperature (expected during testing) of 500 °C (932 °F) to release absorbed water and other condensables from 

the metal.

The high vacuum is provided by a turbo pump system with an ion gauge connected to the lower and 

upper reservoirs. During this process, a vacuum level of 10–5 torr (1.93 psi), as read by the ion gauge, is 

targeted when the system is at the target temperature. Once the bake-out temperature is reached and maintained, the pressure will initially increase, then slowly decrease with time; sufficient time should be allowed 

during bake-out for the pressure to return to the low 10–5 torr range. After conditioning, the trace heaters are 

used to provide extra power to the circuit where needed. Because these heaters are operated in a vacuum, 

a type-K TC is embedded in the heater in order to monitor temperature and prevent burnup. The heaters 

were coiled around the tubing in logical division to provide some control over the application of heat. The 

heaters were clamped in place with hose clamps, and several layers of aluminum foil were tightly pressed 

around the heater and tube. The aluminum allows the heat from the portion of the heater not in contact with 

the tube to be dispersed more evenly about the tube. Additionally, a minimum of two layers of multilayer 

insulation (MLI) are stood off via wire standoffs from the tubes. This reduces radiation losses, which are 

the most dominant heat loss mechanism in a vacuum. Each of the components is wrapped in a similar 

manner—the details of which are supplied in sections 4.4–4.11.
13

4.4 Tilt Table

A tilt table fabricated from aluminum structural members and plating has a 2-m (6.56-ft) width 

and 3-m (9.84-ft) length. This table can be tilted from a level, horizontal position to an angle of 5°. The 

support rails are equipped with six rollers, two large hinges, and four intermediate supports (to distribute 

weight). Figure 8(a) shows the table during construction, and figure 8(b) shows it completed. The support 

table holds all components for the circuit. It is also equipped with a SS spill tray (0.25-cm (0.1-in) thick) 

that covers its entire surface. The tray has a 13-cm- (5.12-in-) high lip around its boundary, which is more 

than sufficient to contain all system Li should it leak out while the table is tilted to the maximum angle. 

Design drawings for the support table are located in section J of the supplemental information. 

(a) (b)

Figure 8. Tilt table: (a) During construction and (b) completed.

4.5 Core

The core assembly is oriented horizontally to simplify heater installation and to allow sufficient 

table tilt for complete drainage. Figure 9 provides a simplified schematic of the core block assembly, 

which is constructed of SS 316 (all-welded configuration) and inspected by a complete radiographic survey and He leak detection (to a level of 1×10–9 SCCM). The complete core assembly consists of six 

primary components:

(1) The core block is a solid, six-sided SS structure with 37 fuel pin holes gun-drilled through 

the full length of the core block. The top end is flanged to provide a support for the inlet plenum, and the 

backside is grooved to accept the outlet plenum cap.

(2) The outer pressure shell is a large tubular jacket that surrounds the core block (forming the 

perimeter flow passage). This shell is flared at the top end (using a conical reducer) to provide the inlet 

plenum structure. The shell length extends below the bottom of the core block, forming the outer wall of 

the lower manifold.
14

(3) The inlet plenum is an annular manifold that is part of the outer pressure shell assemblies. It is 

equipped with a large feed line to minimize system pressure drop.

(4) The outlet plenum is a cap structure located on the top portion of the core block assembly. This 

plenum is equipped with a large drain line to minimize system pressure drop.

(5) The lower manifold is formed between the lower segment of the outer pressure shell and the 

end of the core block. A SS faceplate (drilled to match the core block fuel pin pattern) seals the end of the 

manifold. This manifold receives inlet flow (from the perimeter passage) and directs it back up around the 

fuel pins.

(6) The fuel pin clad is a tube that is seal welded to the lower manifold and extends along the 

core block to the outlet plenum. (The plenum end of the tube is sealed.) Heater elements are inserted into 

the open tube extending beyond the lower manifold faceplate. The outer edges of the fuel-pin clad are 

equipped with standoffs at the center and outlet plenum end, set at 120° intervals, in order to maintain the 

annular flow path dimension between the core block and clad.

The complete engineering design drawings for the core assembly are located in section C of the 

supplemental information. The core block is held in place by a set of SS support collars that allow for 

thermal contraction/expansion during test operations. To minimize core heat losses during operation, the 

core is insulated with a set of slip-on SS MLI shields (very effective if operated in vacuum conditions). 

Core surface temperature measurements are made by spot welding TCs directly to the surface. There are 

no provisions for placing thermowells into the core block or the lower manifold because they significantly 

disrupt the Li flow. One thermowell is placed in the outlet plenum. Pressure transducers were purchased 

and are planned for placement on the inlet/outlet plenum lines and in the outer pressure shell just upstream 

of the lower plenum.

Lower Manifold

T

Core Block Fuel Pin Clad Outlet Plenum

Thermal Simulators Outer Pressure Shell Inlet Plenum

T

P T T T

T P

T

T

T

P

Figure 9. Simplified core assembly layout.
15

4.5.1 Thermal Simulators

The heater elements selected for this system were based on a graphite design that has been successfully used on a number of EFF–TF projects. The graphite heater element has a diameter of 0.775 cm 

(0.305 in) and an overall length of 59.7 cm (23.5 in). The heater element is equipped with alumina insulating rings that measure 1.27 cm (0.5 in) in length and have an outside diameter of 1.067 cm (0.42 in). The 

elements are positioned ≈4.5 cm (1.8 in) into the core as measured from the outer surface of the lower 

manifold faceplate. Each heater has a resistance of ≈0.9 Ω at room temperature and 0.5 Ω at an operating 

temperature of 700 °C (1,292 °F). Figure 10(a) is a drawing of one of these heaters, and figure 10(b) is a 

photograph of the heaters.

Alumina Insulator Rings (×3)

(Cemented to Graphite)

Graphite Heater Element 

(Split Rod)

Alumina Insulator Shims (×2)

(Cemented to Graphite)

Molybdenum Power Leads

(Cemented Into Graphite)

(a)

(b)

Figure 10. Thermal simulators (heaters): (a) Drawing and (b) photograph.

4.5.2 Power Zones

The baseline approach requires that heaters be assembled in zones that are connected to individual 

power supplies (one power supply per zone). Each power supply is rated for 15 kW of direct current power 

delivered via 150 V at 100 A maximum. The 37-pin core assembly is divided into three control zones, 

as depicted in figure 11. The grouping of heaters in these zones (series/parallel) is laid out such that the 

equivalent resistance attempts to maximize the power supply output capability. 
16

Zone 1 – 7 Heater Elements

Zone 2 – 12 Heater Elements

Zone 3 – 18 Heater Elements

Figure 11. Heater element layout and heater zone configuration.

Table 2 lists the estimated equivalent resistance per zone and the maximum power that can be 

transferred by the power supplies with the heaters at room temperature and at nominal operating temperature. The central zone could be split into two parallel circuits producing an equivalent resistance of 1.2 Ω 

and resulting in a total power of 11.66 kW at 700 °C (1,292 °F). The total available input power would 

then be 41.66 kW at 700 °C (1,292 °F). If additional power is required (to simulate transients), more power 

supplies could be added to create additional zones. To match the power supply load for maximum output, 

a load of 1.5 Ω is desired; therefore, a maximum of 12 zones could be created (approximately three heaters 

per zone) resulting in a total available input power of 180 kW.

Table 2. Heater zone configuration. 

Power Supply Parameters

Heater 

Ring

Number 

Heaters

Res/Heater 

@ 25 °C 

(77 °F) 

(Ω)

Res/Heater 

@ 700 °C 

(1,292 °F) 

(Ω)

Number 

Parallel 

Heater 

Strings

Number 

Heater Per 

String

Load

@ 25 °C 

(77 °F)

(Ω)

Load

@ 700 °C 

(1,292 °F)

(Ω)

Max Power 

@ 25 °C 

(77 °F) 

(W)

Max Power 

@ 700 °C 

(1,292 °F) 

(W)

1 7 0.9 0.5 1 7 6.3 3.5 3,571 6,429

2 12 0.9 0.5 2 6 2.7 1.5 8,333 15,000

3 18 0.9 0.5 3 9 2.7 1.5 8,333 15,000

Total 20,238 36,429

Note: Assumes one 15-kW power supply per zone (150 V (max), 100 A (max)).

4.5.3 Face Seal

To improve the thermal performance of the 37-pin Li system there is a core face sealing assembly. 

This assembly, which is attached to the power inlet end of the core, allows for a He environment to be established between the fuel cladding and the heater, significantly improving the overall thermal conductivity 

(coupling) between the heaters and core clad. This better simulates the thermal conditions in the actual core 

in which GHe within the fuel pin provides excellent heat transfer across the pin. Figure 12 illustrates this 

concept. The core face sealing assembly is connected to the main core body by a 16.24-cm- (6-in-) diameter ConFlat® (a Varian, Inc. product) flange (shown as a weld in fig. 13) and a second 16.24-cm- (6-in-) 

diameter ConFlat flange to rear access flange. These flanges use a two ConFlat type sealing surface and 
17

a Ni seal, allowing for operation up to 500 °C (932 °F). A series of vacuum-rated power feedthroughs are 

seal welded to the perimeter of the seal assembly and connected to the heater elements through the rear 

access flange. One flange provides the rear access and the other provides the connection to the core. To 

reduce the amount of heat conducted out of the face seal assembly, an area of the assembly barrel is turned 

down to a thickness of 0.05 cm (0.02 in) or less, which still provides sufficient mechanical strength, yet 

reduces the cross-sectional area for heat transfer. In addition, a series of perforated stainless foil shields 

(three to five) are mounted over the heater leads (using alumina spacers for insulation) to provide a thermal 

radiation barrier. The outer diameter of the assembly matches that of the core outer pressure shell, while its 

length 20 to 30 cm (7.87 to 11.81 in) allows sufficient standoff length from the core so that the rear access 

flange temperature is maintained well below 500 °C (932 °F). 

Multilayer Foil Heat Shields 

With Alumina Insulators

Thinned Material to Create 

a Low Conduction Path

Heater Elements

Lower Manifold

Vacuum Seal (Conflat Type) With 

Ni Seal High-Temperature 

Compatible 500 °C (932 °F)

Rear Access Flange

He Supply

Face Seal Assembly

Three Pairs of Power 

Feedthroughs 

(One Set per Heater Zone)

Core Block

Raised Ring Attached to Core 

Lower Face Plate Mounting Location 

for Core Face Seal Assembly

Fuel Pin 

Clad

Outer Pressure 

Shell

Figure 12. Simplified schematic core face sealing system.
18

Figure 13. Drawing of core face seal.

4.6 Heat Exchanger

Once the Li coolant is heated by the core assembly to a temperature of ≈500 °C (932 °F), it passes 

into a HX to remove up to 40 kW. The baseline for this HX was a ‘battleship’ or facility-style design that 

provides significant robustness in this initial flow circuit (see fig. 14). It is a 0.6-m- (1.97-ft-) long counter 

flow design with Li confined by the outer jacket (14.1-cm (5.56-in) outer diameter) and the secondary 

coolant flow (a gas, either HeAr or N2, in this application), confined by 107 tubes (0.8-cm (0.32-in) outside 

diameter) that pass through the Li flow pool. Engineering drawings for this HX and its high temperature gas 

feed lines are presented in sections A and D of the supplemental information. The exchanger is equipped 

with temperature and pressure measurements to monitor material and fluid conditions. Two 165.1-cm- 

(65-in-) long, 240-V, 675-W Watlow cable heaters were positioned on the HX to provide heat during a thaw

T

T P

T

P

T T T T

T

T

P

T

P

Secondary Coolant 

Outlet

Secondary Coolant 

Inlet

Li Inlet

Li Outlet Supports

Figure 14. Lithium/HeAr HX unit.
19

startup condition, or as needed during filling or transient operations. It is not anticipated that these trace 

heaters would be required during steady-state operations. One trace heater was positioned on the top and 

one on the bottom of the HX (see fig. 15). Each heater has two TCs positioned between the trace heater 

and the HX body to provide feedback temperatures for automatic control of the system. Figures 15 and 16 

illustrate the position of the top heater TCs (THTCs), which are identified as THTC1 and THTC2, and the 

bottom heater TCs (BHTCs), BHTC1 and BHTC2. Additional type-K TCs, TC1 through TC8, are spot 

welded at various locations around the body, as indicated in the figures, to provide temperature information. The affixed trace heaters and TCs are then wrapped with 0.076-mm (0.003-in) copper foil to aid in 

thermal conduction from the trace heater to the body (see fig. 17). Any gaps between the trace heater and 

the core body would result in localized heating of the trace heater and this excess temperature could result 

in trace heater failure. Additionally, the copper wrap allows the trace heater to grow and contract as needed 

due to the differing rates of expansion caused by the temperature difference between the trace heaters and 

the HX. The two options for the secondary coolant flow on this heat exchange include:

• A closed-loop He system that can provide flow rates up to 0.2 kg/s (7.06 oz/s), operating pressure up to 

1.4 MPa (203.1 psi), and inlet temperatures up to 490 °C (914 °F). Maximum loop pressure is 2.4 MPa 

(348.1 psi); however, system temperature must be reduced to maintain sufficient material strength 

margins.

• A single-pass, gaseous N2 (GN2) configuration that can provide 1.4 MPa (203.1 psi), at flow rates in 

excess of 0.4 kg/s (14.11 oz/s), and inlet temperatures of 490 °C (914 °F). The heated exhaust gas is 

vented to the atmosphere.

Section 5 discusses the network flow model developed to calculate the required flow rates. 

TC2

THTC2

TC8

BHTC2

TC7

TC1

Figure 15. Back view of a HX.
20

TC1

TC3

TC5

TC6 BHTC1

THTC1

TC2

TC4

Figure 16. Another back view of a HX. 

Figure 17. Build view of a HX showing Cu foil wrap.

During the HX design, an analysis was performed to assess its performance over 0.6- and 1.2-m 

lengths, for various secondary coolant gases (He, HeAr, and N2) and at various flow rates. The description 

and results of this analysis are provided in section K of the supplemental information. To minimize heat 

losses from the HX, the next step (not shown) was to cover the unit with MLI radiation shielding (layers 

of aluminum foil) and to use alumina isolation standoffs from the support structure. 
21

4.7 Pump

An electromagnetic (EM) pump was selected as the baseline in order to maintain the integrity of 

the all-welded flow loop with no moving parts and to build experience with a pump type that is applicable to a space power system. A general product search and open procurement in the commercial market 

resulted in a single pump submission that met the flow, temperature, and pressure requirements. The pump 

is a Style-VI, two-stage ac conduction unit with a SS 316 duct originally marketed by Mine Safety Appliance and currently offered by Creative Engineers, Inc. (The design dates back to the 1950s.) This pump 

makes use of commercial windings. Figure 18(a) shows the pump system (protective shielding removed) 

before integration into the pump housing. Figure 18(b) shows the three-phase auto transformer. This pump 

provides continuous operation at temperatures up to 816 °C (1,501 °F), with flow control from 10% to 

100%. Control is provided by a three-phase, motor-driven variable transformer. 

(a)

Pump

Inlet

Pump

Outlet

1st Stage

2nd Stage

Magnetic 

Pole

Magnetic 

Electrical 

Pole

Closeup of Pump Stage Closeup of Pump Stage

B

J

F

(b)

Electrical 

Pole

Figure 18. Style-VI: (a) ac conduction pump and (b) three-phase autotransformer. 

The Style-VI pump makes use of a flattened flow duct in the pumping stages; therefore, the pressure differential across the piping is limited to prevent circularization of the duct and pump failure. The 

maximum allowable pressure differential as a function of temperature is shown in figure 19 (data supplied 

by manufacturer). Nominal operation of the 37-pin system will require a pressure differential (relative to 

vacuum) of ≈20.3 psi (≈140 kPa). This pressure drop is within the pump’s limits. 
22

500,000

450,000

400,000

350,000

300,000

250,000

200,000

150,000

100,000

50,000

0

0 100 200 300 400 500 600 700 800 900

Temperature (°C)

Maximum Flow Duct Differential Pressure (N/m2)

Estimated Nominal Operating 

Condition for 37-pin NaK System 

(Relative to Vacuum)

Figure 19. Style-VI pump flow duct maximum differential pressure. (Exceeding this 

limit will result in circularization of the flat pump duct.)

The Style-IV pump performance for NaK-56 alloy is shown in figure 20. The expected nominal 

flow and pressure differential required to operate the 37-pin NaK flow loop is shown by the data point in 

the figure. Currently there are no pump performance curves for Li, but this figure provides a basis for making an educated guess as to the pump performance with Li. The manufacturer of the pump has expressed 

a desire to gather data from this testing. 

 413.7

344.7

275.8

206.8

137.9

68.95

0

60

50

40

30

20

10

0

0 5 10 15 20 25 30 35 40 45 50

22 A/270 V

Design Point

15 A/170 V=2.55 kW

18 A/200 V

12 A/140 V

Minimum Recommended Flow Rate

Developed Pressure (kPa)

Developed Pressure (psig)

Flow Rate (gpm)

1 kg/s (2.2 lb/s) 2 kg/s (4.4 lb/s)

Fluid=NaK (56%)

Temperature=571°C (1,060°F)

Figure 20. Style-VI ac conduction pump performance curve. (Reproduced 

from MSA Research Corp. Dwg. No. C-510356.)
23

4.8 Pump Housing

Because of the heat generated by the pump and the temperature limitation of the commercial magnet coils, a SS pump enclosure has been designed so that a GN2 cooling purge can be provided to the unit 

during high-temperature operation (fig. 21).

Pump Housing

Pump

Figure 21. Pump housing with pump mounted inside. 

Flow 

Direction

Flow 

Direction

Power/Instrumentation 

Feedthroughs

Power/Instrumentation 

Feedthroughs

Li Flow Path Inlet Li Flow Path Inlet

Coolant Gas 

Exhaust

Coolant Gas 

Exhaust

Cooling Gas 

Inlet

Cooling Gas 

Inlet

Figure 22. Pump housing viewed from outside.
24

The pump enclosure is necessary because the entire system will be placed in a vacuum chamber 

with pressure conditions of 10–5 torr or lower, eliminating natural convective cooling of the pump. The 

pump enclosure was purged with N2, providing necessary cooling for the pump. The pressure vessel has 

a maximum pressure rating of 290 kPa (42 psia). Failure of the pressure vessel could result in catastrophic 

failure of the liquid metal system because the pressure vessel is welded directly to the Li flow lines via 

expansion bellows. To ensure that this maximum pressure cannot be exceeded, pressure relief valves were 

set at 241 kPa (35 psia) such that system operating pressure can never exceed 241 kPa. Nominal operating 

pressure was set at 172 kPa (25 psia); however, because this is an open cycle system venting to atmosphere 

(100 kPa (14.5 psia)), this leaves an ≈69 kPa (≈10 psi) pressure differential available between the pump 

housing pressure and atmospheric pressure (outside the building). Calculations, detailed in section L of the 

supplemental information, indicated that 5.08-cm- (2-in-) diameter exit lines would be required to allow 

a minimal pressure drop to satisfy the pressure drop constraint (fig. 22). The drawings for this enclosure 

are provided in section E of the supplemental information. An initial estimate of the N2 flow rates for 

various heat loads was performed; results are shown in figure 23. 

1,000

100

10

Total Power to Dissipate

Nominal Operation

– 4,000 W to Dissipate

– 0.1 kg/s Mass Flow

– 60 °C Outlet

2,500 W

4,000 W

5,500 W

0.001 0.01 0.1 1

Nitrogen Flow Rate (kg/s)

Nitrogen Outlet Temperature (°C)

Facility

Limitation

Figure 23. Pump enclosure coolant performance curves.

The total power to dissipate includes the pump excess (≈2,500 W) that is lost through the pump 

duct insulation. There is ≈1 m (≈3.28 ft) of pump duct within the enclosure, transferring ≈1,500 W (duct 

assumed covered with an Insulfrax® blanket (a Unifrax Corp. product)). The resulting load on the cooling system is ≈4,000 W. The facility N2 supply temperature varies from 25 °C (77 °F) at low flow rates to 

0 °C (32 °F) at high flow rates (flow cools as it expands through regulators; line pressure is reduced from 

10.34 to 0.14 MPa (1,500 to 20 psi)). The selected operating point provides a coolant mass flow of 0.1 kg/s 

(3.53 oz/s), an inlet temperature of 25 °C (77 °F), and an outlet temperature of 70 °C (158 °F).
25

4.9 Lower Reservoir

A lower reservoir (LR) is placed at the lowest elevation of the system to serve as a storage vessel for both the fill and drain operations. The LR also serves as the storage tank for the Li between tests. 

After the system has been baked out, the Li will be loaded molten from the fill pot in the tube shown in 

figure 24. Once loading from the fill pot tube is completed, this tube would be connected to a pressurization system that would supply low pressure Ar, measured with an Astro sensor pressure gauge located at 

LRPG1. During fill operations, applying pressure to the top of the liquid metal will force the liquid up and 

into the evacuated tubes of the circuit via the to/from circuit tube that extends to the bottom of the LR. The 

fill/drain valve (a Swagelok SS–12UW series valve using a series 8 actuator, not shown) located between 

the LR and the circuit on the to/from circuit tube (fig. 24) was specified for remote operation. However, 

procurement of this valve and actuator was stopped before delivery. Section M in the supplemental information contains the specifications for this remote operated valve (ROV) kit. 

From Fill 

Pot

To/From Circuit

LRHA1

LRHB1

LRTC17

LRTC10

LRPG1

LRTC1 LRTC2

LRTC3

LRTC4

LRTC5

LRTC7 LRTC6

LRTC8

LRTC9

LRTC13

LRTC14

VAC/PRESS

LRTC11 LS1

LRTC12

LS3 LS5 LS7

LS6

LS2 LS4

LRTC15

LRHC1 LRTC19 LRHC1

LRTC23

LRTC18

Figure 24. Lower reservoir instrumentation and trace heater location—front view.

The entire reservoir was wrapped with copper foil to aid in thermal conduction and to avoid trace 

heater burnup; however, this will tend to flatten the thermal gradients produced from the trace heaters and 

may retard some of the thermal control of the freeze/thaw process. 

High-temperature valves, used to drain the circuit reservoir, were placed at the lowest elevation 

(fig. 5). The selected valves are all SS bellows suitable for a maximum operating temperature of 650 °C 

(1,202 °F).

This reservoir is equipped with two blocking valves, level sensors, TCs, trace heaters, and a purge 

line to assist in fill and drain operations. A Li melt pot (with its own pressurization system) is coupled to 

the LR for the initial fill.
26

4.9.1 Trace Heaters

Three 2.36-m- (93-in-) long, 240-V, 1,000-W Watlow cable-type trace heaters (LRHC1, LRHB1, 

LRHA1) were positioned on the LR (figs. 24 and 25) to provide the heat that will be required to melt the 

Li before introduction into the system for a test. The trace heater locations were selected in an attempt to 

provide some control over the heating profile. Void formation in the LR will occur at the top of the reservoir due to gravity; therefore, controlled freeze/thaw is desirable and can be accomplished using the trace 

heaters in a specified configuration.

LS7

LRTC22

LS5

LS3 LS1 LS6 LS4 LS2

LRTC21 LRTC20

LRHA2

LRHB2

LRHC2

Figure 25. Lower reservoir instrumentation and trace heater location—rear view.

4.9.2 Thermocouples

There are 23 TCs (LRTC1–LRTC23) spot welded to the LR. In addition to providing bulk temperature data, LRTC1–LRTC6 and LRTC10–LRTC15 were placed to provide a second method of level 

sensing. In earlier experimentation with Li at elevated temperatures, it was observed that the skin temperature of the vessel quickly normalizes when Li is present on the opposing side.

4.9.3 Level Sensors

The level sensors, described in section 4.2.3, were installed in the LR as shown in figure 26. This 

sensor uses a single SS pin; only the sensor pin comes in contact with the liquid metal. The ceramic insulating material used in the feedthrough is several inches above the height of the liquid metal, such that it 

should never come in contact with the Li. Sensor LS7 was intended as a high-level indicator from which 

a shutdown procedure could be initiated. The level sensor lengths were sized to provide readings of the Li 

at the levels indicated in figure 27. The system functions by using one sensor, LS1, as the common ground;
27

Figure 26. As-built view of the LR.

LS1 LS2 LS3 LS4 LS5 LS6 LS7

12.7 cm (5 in)

10.79 cm (4.25 in)

8.89 cm (3.50 in)

5.08 cm (2 in)

3.18 cm (1.25 in)

1.27 cm (0.5 in) 0.64 cm 

(0.25 in) Reservoir Bottom

Estimated Level 

at Fill Temperature

Estimated 

Operational 

Level

Figure 27. Level sensors to detect Li fill level in the LR.

the balance of the sensors complete a circuit to indicate fill level in the presence of the Li. Calculations 

indicate that the Li will rise to a height between LS6 and LS7 at the fill temperature, ≈250 °C (482 °F).
28

4.10 Upper Reservoir

Because the fill/drain reservoir was relocated to the lowest height in the circuit, additional volume, 

the upper reservoir (UR), was required to accommodate the difference in thermal expansion of the Li between 

the fill temperature, ≈250 °C (≈482 °F), and the operational temperature, ≈500 °C (≈932 °F). This required 

volume is located at the highest point in the system. During operation, the top of the UR would be pressurized 

with ultra-high-purity Ar (at relatively low pressure) to maintain a pressure head on the system to prevent 

the EM pump from oscillating rather than circulating. This same pressurization system was also intended 

for use in drain procedures to drive the Li out of the UR. Figures 28 and 29 show the placement of trace 

heaters and instrumentation on the UR. The entire UR was wrapped with copper foil to aid in thermal 

conduction and to avoid heater burnout, as shown in figure 30.

To VAC/PRES 

System

Upper 

Reservoir 

Heater 

(URH1)

LS10

URTC1

H1TC1

LS9

LS8

URTC2

URTC3

Tube 2

Top Heater (HT)

Figure 28. Front view of the UR.
29

LS8

LS9

To VAC/PRES 

System

Upper Reservoir 

Heater

LS10

H1TC2

Tube 2 

Bottom 

Heater (HB)

Figure 29. Rear view of the UR. 

LS10 LS9

URTC3

URTC2

LS8

URTC1

Figure 30. As-built view of the UR.
30

4.10.1 Trace Heaters

One 1.19-m- (47-in-) long, 240-V, 345-W Watlow cable type trace heater (H1) was placed on the 

UR to replace the thermal power that is lost to the environment and to provide control during freeze and 

thaw experiments.

4.10.2 Thermocouples

There are three UR TCs (URTC1–URTC3) spot welded to the UR, providing gross temperature 

data for monitoring or debugging. Two TCs are mounted directly underneath the trace heater, H1TC1 and 

H1TC2.

4.10.3 Level Sensors

The level sensors, described in section 4.2.3, were installed in the UR locations as shown in figure 29. Sensor LS8 was intended as a high-level indicator, LS9 the low-level indicator, and LS10 as the 

common ground. See figures 28–30.

4.11 Tubing

The circuit tubing is formed from seamless 316 SS tubing with a 0.24-cm (0.095-in) wall. The 

tube is formed per the engineering drawings, providing the system with the required flexibility; however, 

most of the manufacturing, fabrication, and assembly tolerances are taken up in the final fit assembly. This 

tubing connects all the components of the circuit; 1-in VCR fittings are used to connect and disconnect 

tube 4 (fig. 5) from the system. These connections define the connection points for a test section where test 

articles could be attached and would be available to mate any components to the circuit in the component 

test bypass area. Thermocouples were attached every 15.24 cm (6 in) along the length of each tube. Two 

heaters were attached to each tube, placing one on the top surface and one on the bottom surface to allow 

for controlled freeze/thaw, forcing the void space to the top. However, since the copper foil was required 

to disperse the heat from the heaters to avoid burnup and to allow the heaters to expand and contract at 

different rates than the underlying tube, the ability to control the input thermal gradient to the tube was 

reduced due to the superior thermal conductivity of Cu.

Heater failure occurred in early tests conducted in a vacuum environment (fig. 31). This failure 

was attributed to the welding tabs used to affix the trace heaters to the tubes. The intent of these tabs was 

to maintain good contact between the tube and the heater. If the heater did not come in contact with the 

tube, it would have limited paths to give off its energy and would quickly exceed its design temperatures 

in a vacuum. These tabs solved the thermal conduction problem but introduced the problem of differing 

thermal expansion rates. The heater heats and cools on a much shorter time scale than the underlying 

tube. By firmly affixing the heater to the tube, these differing rates of thermal expansion and contraction 

resulted in the heater separating from the tube, most likely on cooldown. 
31

Heater Heater Failure 

Point

TC

Tabs Tube

Figure 31. Photograph of heater failure using tabs to secure to tube.

These results indicated that the heater must be allowed to ‘slip’ yet still transfer its thermal energy 

to the tube. This was accomplished by using Cu foil wrapped around the tube, the trace heater, and the 

TCs. Figure 32 shows the test specimen with some of the foil removed.

Cu Foil Heater

TCs Tube

Figure 32. Photograph of successful method of securing heater with Cu foil.

The 0.008-cm- (0.003-in-) thick, 5.08-cm- (2-in-) wide Cu foil allowed the heater to slip and grow. 

When the heater separated from the tube due to expansion or contraction, it would still conduct the thermal 

energy from the heater to the tube. This system worked very well; however, it limits the ability to apply 

a strong, directed thermal gradient to the tube or other component, thus limiting the ability to control the 

freeze or thaw void placement. Despite this drawback, this was determined to be an acceptable solution 

for this flow circuit.
32

4.11.1 Tube 1

Tube 1 (T1) was instrumented with 16 TCs (T1TC1–T1TC16) that record the Li temperatures as it 

moves from the core to the HX. Pressure measurements are made at P1 and P2. T1H1A, T1H1B, T1H2A, 

and T1H2B denote TCs that are located underneath heaters T1H1 and T1H2. Heater T1H1 traces the top 

of the tube while T1H2 traces the bottom of the tube (fig. 33).

P1 T1TC1

T1TC2

T1TC3

T1TC4 T1TC5 T1H1A

T1TC8 T1TC9

T1TC11

T1H2B

T1TC12

T1TC13

T1TC15

T1TC16

P6

T1TC14

T1TC7

T1TC3

P1 T1TC1

T1TC2 T1H1A

T1TC8 T1TC9

T1TC11

T1H2B

T1TC12

T1TC13

T1TC15

T1TC16

P6

T1TC14

T1TC7

T1TC6

T1TC10

T1TC6

T1TC10

Figure 33. Photograph of T1.

4.11.2 Tube 2

Tube 2 (T2) was instrumented with 14 TCs (T2TC1–T2TC14) that record the temperatures of the 

stagnant Li from the main loop to the expansion tank or the upper reservoir. The Li is not flowing in this 

section of T2, but the TCs can be used as level indicators. The successful use of these TCs as level indicators remained to be determined. T2HT1, T2HB1, T2HB2, and T2HT2 denote TCs that are located underneath heaters T2HT and T2HB. Heater T2HT traces the top of the tube while T2HB traces the bottom of 

the tube (fig. 34).
33

T2TC11 T2TC12

T2TC4 T2TC3

T2TC2

T2TC1

T2TC13

T2HT2 T2TC10

T2TC5

T2HT1

T2TC9

T2TC8

T2HB2

T2TC7

T2TC6

T2HB1

T2TC14

T2HT1 T2TC4 T2TC3

T2TC2

T2TC13

T2TC1

Figure 34. Photograph of T2.

4.11.3 Tube 3

Tube 3 (T3) was instrumented with 16 TCs (T3TC1–T3TC16) that record the Li temperatures as 

it moves from the HX to the pump. Pressure measurements are made at P1 and P2 (not shown). T3H1A, 

T3H1B, T3H2A, and T3H2B denote TCs that are located underneath heaters T3H1 and T3H2. Heater 

T3H1 traces the top of the tube while T3H2 traces the bottom of the tube (fig. 35).

P3

P4

T3TC1

T3TC2

T3TC3

T3TC4

T3H1A

T3TC5

T3TC6

T3H2A

T3H1B

T3TC7

T3TC8 T3H2B

T3TC9

T3TC10

T3TC11

T3TC12

T3TC16

T3TC15

T3TC14

T3TC13

T3H2A

T3TC1

T3TC2

T3TC3

T3TC4

T3H1A

T3TC5

T3TC6

T3H1B

T3TC7

T3TC8 T3H2B

T3TC9

T3TC10

T3TC11

T3TC12

T3TC16

T3TC15

T3TC14

T3TC13

Figure 35. Photograph of T3.
34

4.11.4 Tube 4

Tube 4 (T4) was instrumented with six TCs (T4TC1–T4TC6) that record the Li temperatures as 

it moves from the pump to tube 5 (T5) and ultimately to the core. A pressure measurement is made at P5. 

Thermocouple T4H1 is located underneath heater T4H1. Only one heater, T4H1, spirals around the tube 

section (fig. 36).

T4TC6

T4TC5

T4TC4

P5

T4TC3

T4TC2

T4TC1

T4H1

P5

T4TC3

T4TC2

T4H1

T4TC1

T4TC5

Figure 36. Photograph of T4.

4.11.5 Tube 5

Tube 5 (T5) was instrumented with five TCs (T5TC1–T5TC5) that record the Li temperatures as it 

moves from T4 to the core, as shown in figure 37. A pressure measurement is made at P6. TCs T5HA and 

T5HB are located underneath heater T5H1. Only one heater, T5H1, spirals around the tube section.
35

T5HB

T5TC1

P1

T5TC2

T5HA

T5TC3

T5TC4

P6

T5TC2

T5HA

T5TC3

T5TC4

T5TC5 T5TC5

T5HB

P6

T5TC1

P1

Figure 37. Photograph of T5.

4.11.6 Tube 6

Tube 6 (T6) was instrumented with four TCs (T6TC1– T6TC4) that record the Li temperatures 

as it moves to and from the LR into the system, as shown in figure 38. A hand-operated valve- (HOV-) 

V4 was to be converted to an ROV as discussed in section 4.9. Only one heater, T6H1, spirals around the 

tube section.
36

T6TC2

T6TC3

T6TC1

T6TC4

HOV-V4 T6TC2

T6TC3

T6TC1

T6TC4

HOV-V4

Figure 38. Photograph of T6.
37

5. MODELING AND TESTING

Initial experiments were performed to gain experience with handling Li in both solid and liquid 

form at elevated temperatures in order to understand the behavior of void formation in Li and to gather 

data that could be used to calibrate upgrades to a generalized fluid system simulation program (GFSSP) 

model. These upgrades would enable GFSSP to model the freeze/thaw of Li in the system to ultimately 

and hopefully produce a model that could be used to predict the freeze/thaw of a complete system. This 

ability would then aid in further designs of liquid Li systems.

5.1 Generalized Fluid System Simulation Program

The Li circuit was modeled using GFSSP. This is a general-purpose computer program for analyzing steady-state and time-dependent flow rates, pressures, temperatures, and concentrations in a complex 

flow network. The program is capable of modeling phase changes, compressibility, mixture thermodynamics, and external body forces such as gravity and centrifugal forces.

The program contains subroutines for computing ‘real fluid’ thermodynamic and thermophysical 

properties for 12 fluids. The program also provides the option for using any incompressible fluid with 

constant density and viscosity.

Seventeen different resistance/source options are provided for modeling momentum sources or 

sinks in the branches. These options include: (1) pipe flow, (2) flowthrough restriction, (3) noncircular 

duct, (4) pipe flow with entrance and/or exit losses, (5) thin, sharp orifice, (6) thick orifice, (7) square edge 

reduction, (8) square edge expansion, (9) rotating annular duct, (10) rotating radial duct, (11) labyrinth 

seal, (12) parallel plates, (13) common fittings and valves, (14) pump characteristics, (15) pump power, 

(16) valve with a given loss coefficient, and (17) a Joule-Thompson device.

GFSSP employs a finite volume formation of mass, momentum, and energy conservation equations in conjunction with the thermodynamic equations of state for real fluids. Mass, energy, and species 

conservation equations are solved at the nodes and the momentum conservation equations are solved 

in the branches. The system of equations describing the fluid network is solved by a hybrid numerical 

method that is a combination of the Newton-Raphson and successive substitution methods.

The computer program is composed of three major parts. The first part consists of the preprocessor subroutines that allow the user to interactively create the flow network model consisting of nodes and 

branches. All of the input specifications, including the boundary conditions, are specified through the preprocessor. The second major part of the program consists of the subroutines that provide the initial conditions and then develop and solve all of the conservation equations in the flow network. The third part of 

the program consists of the thermodynamic property programs that provide the necessary thermodynamic 

and thermophysical property data to solve the resulting system of equations.
38

The specific objective of the Li circuit analysis using GFSSP was to calculate pressure and temperature distribution in a Li-cooled reactor system in order to assess the location and constraints of pressure 

transducers necessary for circuit instrumentation. The model includes the pump, reactor core, HX, He line, 

and feed lines. The model calculates the pressure and temperature of each component during a steady-state 

operation. Figure 39 shows the GFSSP model graphical representation of the analysis.

Pump Input = 170 V

Li Flow Rate = 1.17 kg/s

He Flow Rate = 0.045 kg/s

Total System Pressure Drop = 8.36 psi (57.64 kPa)

Reactor Inlet Temperature = 488.1 °C

Reactor Outlet Temperature = 501.0 °C

18 17 16 15 14 13 12

19

20

21

22

1

2K

2K

2K

2K

2 3 4 5 6 7

8

P=100 psia 9

T=247 °C

P=13.73 psia

T=488.1 °C

P=14.04 psia

T=488.1 °C

P=14.28 psia

T=488.1 °C

P=14.59 psia

T=488.1 °C

P=15.55 psia

T=488.1 °C

P=7.19 psia

T=488.1 °C

P=7.97 psia

T=488.1 °C

P=8.57 psia

T=488.1 °C

P=9.27 psia

T=488.1 °C

P=9.27 psia

T=488.1 °C

P=9.37 psia

T=501.0 °C

P=9.92 psia

T=501.0 °C

P=10.39 psia

T=501.0 °C

P=10.39 psia

T=501.0 °C

P=11.04 psia

T=501.0 °C

P=11.80 psia

T=501.0 °C

P=11.87 psia

T=501.0 °C

P=11.80 psia

T=501.0 °C

P=11.80 psia

T=488.1 °C

P=11.87 psia

T=488.1 °C M=2.57 lbm/s

P=14.7 psia

T=15 °C

35

34

30 33

31

30

10

11

Reactor 

Outlet

Reactor 

Inlet Heat 

Exchanger 

Inlet

Heat Exchanger 

Pump Outlet

Helium 

Line

R

R

Figure 39. GFSSP model of the Li circuit.

The GFSSP analysis results provide numerical predictions of pressure and temperature at various 

locations in the flow circuit. Highlights of these results are:

• Electromagnetic pump is anticipated to raise the pressure from 49.64 kPa (7.2 psia) to 107.2 kPa 

(15.55 psia) at a flow rate of 1.17 kg/s (2.57 lb/s).

• Total pressure drop in the reactor is 24.48 kPa (3.55 psi).

• Total pressure drop in the HX is 13.44 kPa (1.95 psi).

• Total pressure drop in the feedline between the pump and reactor inlet is 6.62 kPa (0.96 psi).

• Total pressure drop in the feedline between the reactor outlet and HX inlet is 7.72 kPa (1.12 psi).
39

• Total pressure drop in the feedline between the HX outlet and pump is 5.399 kPa (0.783 psi).

• Total pressure drop in the system is 57.64 kPa (8.36 psi), which is in balance with the pressure rise 

in the pump.

In the reactor, temperature rises from 488 °C (910 °F) to 501 °C (934 °F). Temperature drops 

by the same amount in the HX. This confirms that the model correctly accounts for the cyclic boundary 

condition.

The temperature level of the Li system was determined by He inlet temperature and He flow rate. 

In the present model, He inlet temperature and He flow rate were set to 277 °C (477 °F) and 0.045 kg/s 

(0.1 lb/s), respectively.

From this model, the expected system pressures were used to specify the pressure transducers as 

well as to provide locations for placement.

5.2 Lithium Melt/Wetting

One of the first tests conducted was to visually identify the approximate wetting temperature of Li. 

These tests, while basic, were intended to give experience in three areas: (1) General physical manipulation of Li, (2) visual indication of wetting, and (3) experience with handling of Li at elevated temperatures, 

≈300 °C (572 °F).

Figure 40. Stainless steel beaker with freshly sheared Li before melt.
40

A SS beaker was wrapped with heater tape and outfitted with TCs spot welded to the bottom surface (fig. 40). The beaker and the solid Li sample, packaged in foil wrap with Ar cover gas, were passed 

into a glove box. This glove box was designed to maintain an atmosphere having <1 ppm O2, <1 ppm N2, 

and <1 ppm water vapor, suitable for handling alkali metals such as Na, NaK, and Li. After trial and error, 

it was determined that the best method to cut the 1.27-cm- (0.5-in-) diameter, 15.24-cm- (6-in-) long piece 

of solid Li was using pruning shears. Sawing with a hacksaw quickly proved ineffective due to the gummy 

nature of the Li. A small piece was sheared off and placed in the beaker with three TCs that were connected 

to a data acquisition system programmed in LabVIEW. The beaker heater was turned on and the wetting 

temperature of the Li was observed. A cleaned but unbaked SS beaker in an Ar atmosphere (as opposed to 

actual conditions of a vacuum baking and vacuum loading) was considered a good condition in which to 

establish the upper limit wet temperature when combined with 98% pure Li (as opposed to the more pure 

99.7% purchased for the circuit). The observed wetting temperature was 300 to 310 °C (572 to 590 °F).

5.3 Lithium Fill

Prior to filling the entire Li circuit with Li, a small test article was filled. This test article was used 

to gain further experience with Li and to gather data for the freeze/thaw tests; however, an equal amount 

of experience and information was gained from the fill process. Figure 41 shows the fill apparatus in 

a horizontal orientation. The actual orientation was vertical to allow gravity to help the Li move into the 

test article. Not shown are the tape heaters wrapped around the test article, transfer valve, and melt pot.

Valve to Glove Box ATM

2.75-in Conflat

Transfer Valve Test Article

TCs Melt Pot

Valve to 

VAC System

Figure 41. Lithium fill apparatus used to fill the test article.

To complete the test article fill, the melt pot was opened at the 6.99-cm (2.75-in) Conflat flanges. 

Pre-measured and weighed slugs of Li were placed in the melt pot. The entire vessel was baked out until 

the vacuum remained stable at 10–5 torr prior to introduction of the Li and again after the introduction of 

the Li. (The glove box atmosphere valve was closed and the vacuum valve was connected to a vacuum 

system with ion gauges.) Following melt pot bake-out, a vacuum was trapped in the test article by closing the transfer valve while under vacuum. The Li was melted to ≈300 °C (≈572 °F) under vacuum and 

the melt pot was gently tapped with a wrench until all of the gasses trapped in the melted Li evolved, as 

indicated on the ion gauges. The Li was introduced into the test article at 300 °C (572 °F) by isolating 

the vacuum system and opening the glove box atmosphere valve. When the transfer valve was opened, 

the transfer of the Li to the test article was assisted by the pressure difference (vacuum in the test article 
41

and atmospheric pressure pushing on the Li in the melt pot) and gravity to overcome any surface tension 

or friction. This transfer was monitored and verified by recording TC data along the test article, the valve 

body, and the melt pot. The Li in the test article was then raised to 375 °C (707 °F) and the transfer valve 

was closed. When the Li cooled, a void was left in the test article and was used in follow-on testing in an 

attempt to understand void formation during freeze. Some of the most important lessons learned were:

• Solid Li outgasses when melted.

• TCs on the outside of the tubes normalize to the Li temperature.

• Wetting can be accomplished (as predicted) at the test temperatures.

• The technique for introducing Li was verified.

These lessons learned were applied to the design and operation of the circuit.

5.4 Lithium Freeze/Thaw Tests

Freeze and thaw tests were performed to provide hands-on experience: (1) With a freeze/thaw front 

and power heat loss values from the test section (article) to the environment, (2) to investigate void formation, and (3) to provide actual data for comparison to GFFSP module results. These tests were conducted 

in a vacuum chamber using a cable heater to supply heat to the valve end of the test article (fig. 42). 

Transfer Valve MLI Test Article

Stand

TCs

Standoffs 

for MLI

Heater 

Leads

Figure 42. Freeze/thaw test article.
42

The test article was mounted on a stand at the valve end with the heat load applied at the other end 

via a cable heater. MLI consisting of loose wraps of aluminum foil was wrapped around the test article. 

(The test section is shown unwrapped in fig. 42.) The TCs were located at measured distances and used to 

gather the temperature data (fig. 43). 0 inIsolation Valve

2.54 cm (1 in)5.08 cm (2 in)7.62 cm (3 in)8.89 cm (3.5 in)10.16 cm (4 in) 11.43 cm (4.5 in)12.7 cm (5 in)13.97 cm (5.5 in) 15.24 cm (6 in) 16.51 cm (6.5 in) 17.78 cm (7 in) 19.05 cm (7.5 in)20.32 cm (8 in) 21.59 cm (8.5 in) 22.86 cm (9 in)25.4 cm (10 in)27.94 cm (11 in)30.48 cm (12 in)35.56 cm (14 in)40.64 cm (16 in)

Figure 43. Freeze/thaw test article with TC locations at various distances. 

Figure 44 shows a graph of data from a thaw cycle, and figure 45 shows data from a freeze cycle. 

400

350

300

250

200

150

100

50

0

 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000

Temperature (°C)

Time (s)

X= 2.54 cm

X= 5.08 cm

X= 10.16 cm

X = 15.24 cm

X= 20.32 cm

X= 30.48 cm

X= 35.56 cm

X= 40.64 cm

Figure 44. Measured temperature history during Li thawing. 
43

400

350

300

250

200

150

100

50

0

 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

Temperature (°C)

Time (s)

X= 2.54 cm

X= 5.08 cm

X= 10.16 cm

X = 15.24 cm

X= 20.32 cm

X= 30.48 cm

X= 35.56 cm

X= 40.64 cm

Figure 45. Measured temperature history during Li freezing.

5.5 Lithium Freeze/Thaw Modeling

Actual data taken during freeze/thaw testing were used to help develop and to calibrate a freeze/

thaw node in GFSSP. The conjugate heat transfer option in GFSSP allows for modeling of heat conduction 

in static solid nodes that may be connected to fluid nodes. For this case, the modeling capability of heat 

conduction through solid nodes was upgraded to model phase change (thaw and freeze). Further details 

are provided in reference 6.

5.5.1 Lithium Thaw Model

The measured temperature history at eight axial locations during Li thaw is shown in figure 44. 

The temperature history shows a point of inflection at the melting temperature. This is due to the change in 

thermophysical properties between solid and liquid Li. After 10,000 s, the temperature distribution began 

to exhibit steady-state behavior. The point of inflection is, therefore, less prominent at locations close to 

the free end of the rod.

The predicted temperature history for the same locations is shown in figure 46. Similar behavior 

is present in the predicted temperature history in which a point of inflection is observed at the melting 

temperature and steady-state characteristics are shown after 10,000 s. The slightly unsteady temperature 

history observed near the melting point can be attributed to the discontinuity in thermophysical properties 

during the phase change.
44

400

350

300

250

200

150

100

50

0

 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000

Temperature (°C)

Time (s)

X= 2.54 cm

X= 5.08 cm

X= 10.16 cm

X = 15.24 cm

X= 20.32 cm

X= 30.48 cm

X= 35.56 cm

X= 40.64 cm

Figure 46. Predicted temperature history of Li thawing.

Overall, good correlation was observed between prediction and measurement. A discrepancy, however, is observed near the free end of the rod. This discrepancy may be attributed to approximations made 

in calculating the heat loss to the wall and through the support structure.

5.5.2 Lithium Freeze Model

The freeze model used the same nodal structure as the thaw model. However, the initial and boundary conditions were different. After 16,116 s, the heater was turned off and the test assembly was allowed 

to cool. The temperature measured at that instant was assigned to all 19 GFSSP nodes.

Figure 45 shows the measured temperature history during Li freeze at the locations where thaw 

results were shown previously. Unlike during the thaw cycle, the freeze line propagates from the free end 

to the valve end. It took a little over 7,000 s to freeze the entire rod. The predicted temperature history 

is shown in figure 47. The predicted behavior of the freezing process resembles the pattern observed in 

the test.
45

400

350

300

250

200

150

100

50

0

 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

Temperature (°C)

Time (s)

X= 2.54 cm

X= 5.08 cm

X= 10.16 cm

X = 15.24 cm

X= 20.32 cm

X= 30.48 cm

X= 35.56 cm

X= 40.64 cm

Figure 47. Predicted temperature history of Li freezing.

As with the thaw model, there is a good agreement between predictions and measurements. The 

fact that temperature history during thaw is very different from the freeze process is predicted accurately 

by the numerical model. The observed disagreement near the free end is due to approximation in the heat 

loss calculation.

Modeling of Li phase change due to heat conduction was done using GFSSP. It shows that the 

numerical predictions compare well with test data for both thaw and freeze processes.
46

6. CONCLUSIONS

The SS, Li circuit test section design project was cancelled due to a shift in program direction with 

the selection of a gas-cooled reactor concept to support proposed nuclear electric propulsion. As a result, 

all activities have been closed out using the most logical approach to terminate procurements in an effort 

to recoup cost without total loss. The circuit was removed from the vacuum test chamber and has been 

dressed for storage. This Technical Publication has summarized the design of the pumped liquid metal Li 

flow circuit as of May 1, 2005.
47

REFERENCES

1. Rhys, N.O., Ph.D.: “A Brief Review of Thermophysical Properties of Lithium,” NASA Marshall Space 

Flight Center Internal Document, ER11-04-WI4A-002-1, February 11, 2005.

2. Chopra, O.K.; and Smith, D.L.: “Corrosion of Ferrous Alloys in a Flowing Lithium Environment,” 

J. Nucl. Mater., Vols. 133–134, pp. 861–866, August 1985.

3. Tortorelli, P.F.; and Chopra, O.K.: “Corrosion and Compatibility Considerations of Liquid Metals for 

Fusion Reactor Applications,” J. Nucl. Mater., Vol. 103, pp. 621–632, 1981.

4. Poston, D.; Kapernak, R.; Willcutt, G.; and Guffee, R.: “100 kWt SS NaK Cooled Reactor Concept,” 

Los Alamos National Laboratory Design Study Presentation, Los Alamos, NM, 2003.

5. Bragg-Sitton, S.M.: “High Purity Lithium Suppliers in the United States,” ER11-04-WI4A-005, NASA 

Marshall Space Flight Center, AL, October 12, 2004.

6. Majumdar, A.K.: “Verification of Numerical Modeling of Lithium Thaw and Freeze,” ER11-04-

WI4A-008, NASA Marshall Space Flight Center, AL, March 8, 2005.
48

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Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18

Documentation of Stainless Steel Lithium Circuit Test Section Design

T.J. Godfroy* and J.J. Martin

with Supplemental Information by E.T. Stewart and N.O. Rhys**

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 29

Availability: NASA CASI (443–757–5802)

Prepared by the Propulsion Systems Department, Engineering Directorate

 * Maximum Technology Corporation, Huntsville, AL 35816

** Yetispace, Inc., Huntsville, AL 35802

M–1286

Technical Publication

NASA/TP—2010–216437

space reactor, alkali metal, liquid metal, lithium circuit, pumped loop

01–08–2010

UU 60

STI Help Desk at email: help@sti.nasa.gov

NASA

STI Help Desk at: 443–757–5802

U U U

The Early Flight Fission–Test Facilities (EFF–TF) team was tasked by Naval Reactors Prime Contract Team (NRPCT) to design, fabricate, and test an 

actively pumped lithium (Li) flow circuit. This Li circuit takes advantage of work in progress at the EFF–TF on a stainless steel sodium/potassium (NaK) 

circuit. The effort involved modifying the original stainless steel NaK circuit such that it could be operated with Li in place of NaK. This new design 

considered freeze/thaw issues and required the addition of an expansion tank and expansion/extrusion volumes in the circuit plumbing. Instrumentation 

has been specified for Li and circuit heaters have been placed throughout the design to ensure adequate operational temperatures and no uncontrolled 

freezing of the Li. All major components have been designed and fabricated prior to circuit redesign for Li and were not modified. Basic circuit components include: reactor segment, Li to gas heat exchanger, electromagnetic liquid metal pump, load/drain reservoir, expansion reservoir, instrumentation, 

and trace heaters. The reactor segment, based on a Los Alamos National Laboratory 100-kW design study with 120 fuel pins, is the only prototypic 

component in the circuit. However, due to earlier funding constraints, a 37-pin partial-array of the core, including the central three rings of fuel pins (pin 

and flow path dimensions are the same as those in the full design), was selected for fabrication and test. This Technical Publication summarizes the design 

and integration of the pumped liquid metal Li flow circuit as of May 1, 2005.
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