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i

NASA/TP—2012–217453

Development and Analysis of Cold 

Trap for Use in Fission Surface 

Power-Primary Test Circuit

T.M. Wolfe

Department of the Navy, Naval Sea Systems Command, Washington, DC

C.A. Dervan

Georgia Institute of Technology, Atlanta, Georgia

J.B. Pearson and T.J. Godfroy

Marshall Space Flight Center, Huntsville, Alabama

January 2012

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

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

Acknowledgments

The authors from the Department of the Navy and Georgia Institute of Technology would like to thank Boise 

Pearson whose guidance and support has made this experience possible. Also, they would like to thank the following 

people who have been kind enough to take time from their projects and schedules to help them learn and grow, not 

only on this project but as professionals: Tom Godfroy, Bob Reid, Mike Schoenfeld, Anne Garber, and Kevin Berry. 

Finally, we would like to thank Mona Miller, Tina Haymaker, the WV Space Grant Consortium, and all others who 

have made this opportunity available.

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. PROBLEM STATEMENT ................................................................................................. 2

3. RESEARCH ....................................................................................................................... 4

3.1 Cold Trapping ................................................................................................................... 4

3.2 Design Challenges ............................................................................................................. 5

3.3 Initial Design ..................................................................................................................... 5

4. ANALYSIS ......................................................................................................................... 9

4.1 Sodium Potassium Properties ............................................................................................ 9

4.2 Power Analysis .................................................................................................................. 11

4.3 System Cleanup Time Analysis .......................................................................................... 14

4.4 Pressure Drop Analysis ..................................................................................................... 15

5. DESIGN ............................................................................................................................. 18

6. FUTURE WORK ............................................................................................................... 21

7. CONCLUSION .................................................................................................................. 22

REFERENCES ....................................................................................................................... 23
iv

LIST OF FIGURES

 1. FSP-PTC: (a) Corner view and (b) hosing down ....................................................... 3

 2. Enrico Fermi Atomic Power Plant forced circulation cold trap .................................. 4

 3. Cold trap designs: (a) Sodium reactor experiment cold trap with coolant coil 

and (b) design with coolant jacket ............................................................................. 6

 4. Cold trap: (a) Boiling coolant with external economizer and (b) HNPF 

with internal economizer ........................................................................................... 7

 5. FSP-PTC with primary components labeled .............................................................. 9

 6. Cold trap control volume showing temperature and mass flow rate relationships ...... 10

 7. Cold trap control volume showing temperature and mass flow rate 

relationships .......................................................................................................................................... 12

 8. Total power required by core versus percentage of flow rate through cold trap 

at different cold trap locations in FSP-PTC system loop ........................................... 13

 9. Power loss through cold trap versus percentage of flow rate through cold trap 

at varying system operating temperatures .................................................................. 13

10. Schematic of cold trap control volume definition used to derive first order 

oxygen concentration time differentiable equation ..................................................... 14

11. FSP-PTC system purification versus time at various mass flow rates through 

cold trap .................................................................................................................... 15

12. Measured pressure drop across cold trap using varied mesh diameters 

and L/D ratios as a function of percentage flow through the cold trap ...................... 17

13. Schematic of cold trap with components labeled ....................................................... 18

14. Current cold trap design views: (a) Isometric view, (b) side view, 

(c) close-up of top view, and (d) isometric cutaway view ............................................ 20
v

LIST OF TABLES

1. NaK properties ............................................................................................................ 10

2. Oxygen solubility in NaK and Na investigation data ................................................... 10

3. FSP-PTC loop data used in analysis ............................................................................ 11

4. Wick structure permeability ........................................................................................ 16
vi

LIST OF ACRONYMS AND SYMBOLS

EFF-TF Early Flight Fission-Test Facility

EM electromagnetic

FSP-PTC Fission Surface Power-Primary Test Circuit

HNPF Hallam Nuclear Power Facility

HX heat exchanger

K potassium

Na sodium

NaK sodium potassium alloy

NaK-78 alkali metal (78% potassium and 22% sodium by weight)
vii

NOMENCLATURE

Aw open cross-sectional area of the cold trap (m2)

C0 initial concentration of oxygen in the NaK (ppm)

CCT NaK oxygen concentration exiting the cold trap (ppm)

CL NaK oxygen concentration in the loop (ppm)

Cp specific heat capacity of NaK (J kg–1 K–1)

D diameter (in)

L length (in)

Leff effective length over which the ∆P is taken (m)

mL mass of NaK in the loop (kg)

m nominal mass flow rate of NaK (kg s–1) through FSP-PTC

m CT mass flow rate through the cold trap along bypass line

P pressure (psi)

Q power (W)

Q

CT power through the cold trap (W)

T temperature (K, oC)

TCT cold trap operating temperature

Tin inlet temperature

Tout outlet temperature

t time (s)

V total volume of NaK in the FSP-PTC (m3)

Vct cold trap control volume

V volumetric flow rate through the cold trap (m3 s–1)

ε effectiveness

κ permeability of mesh (m2)

μl viscosity of NaK (kg m–1 s–1)

ρl density of NaK (kg m–3)
viii
1

TECHNICAL PUBLICATION

DEVELOPMENT AND ANALYSIS OF COLD TRAP FOR USE IN FISSION

SURFACE POWER-PRIMARY TEST CIRCUIT

1. INTRODUCTION

The Early Flight Fission-Test Facility (EFF-TF) is a creation of the NASA Marshall Space 

Flight Center Propulsion Research and Technology Branch Nuclear Propulsion Group. Its purpose 

is to recreate and simulate nuclear reactor capabilities using non-nuclear methodologies. One focus 

of the EFF-TF is the testing of a pumped alkali metal (NaK-78) flow circuit, the Fission Surface 

Power-Primary Test Circuit (FSP-PTC).1–3 The sodium potassium alloy (NaK) in the FSP-PTC has 

on occasion become contaminated with oxygen, which can cause corrosion and plugging of the test 

circuit. This Technical Publication centers around the design and analysis of a forced circulation cold 

trap to remove the oxygen from eutectic NaK-78. 
2

2. PROBLEM STATEMENT

Eutectic NaK-78, which is an alkali metal alloy consisting of 78% potassium (K) and 22% 

sodium (Na) by weight, has often been used as a coolant in nuclear reactors.1 Because of its low melting point and high thermal conductivity, NaK acts as an excellent working fluid to use in the FSPPTC, shown in figure 1. However, oxygen impurities can be found in NaK that may cause increased 

corrosion and possible blockage of flow passages via oxide precipitation that can cause operating 

problems within a pumped NaK system.4 Considering the effects these oxygen impurities can have, 

it was decided to incorporate a method to remove and contain the majority of oxygen present in the 

FSP-PTC by means of a cold trap. 

A cold trap works by running the NaK flow through a cold part of the system. The cold trap 

temperature is lower than the system temperature to cause oxides to precipitate out and be trapped 

within the cold trap, not allowing oxide circulation through the system.5 This process acts as an efficient means of removing oxygen from the bulk NaK flow. 

During the cleanup/take-down phase of the FSP-PTC when all NaK is removed from the 

system, impurities were found in the NaK mixture. These impurities, mainly various forms of Na 

and K oxides, were a result of oxygen being trapped within the closed system. Impurities in the NaK 

mixture, even at concentrations <30 ppm, can cause significant corrosion and reduced performance.6

In addition, oxygen impurities can cause blockages of flow via oxide precipitation, which can further enhance operating problems within the NaK system.4 To prevent these negative effects on the 

system, a method for purification had to be developed that could be used in the system to purify the 

NaK. It was decided to design a cold trap for use in the FSP-PTC to isolate oxides present in the 

NaK loop. The goal of the cold trap is to have the capability to clean the NaK with a theoretical 

oxygen concentration of between 13 and 1,000 ppm.
3

(a) 

(b) 

Figure 1. FSP-PTC: (a) Corner view and (b) hosing down.
4

3. RESEARCH

Cold trapping is one of the oldest forms of alkali metal purification. A cold trap works by 

running the NaK flow through a system that induces a cold point lower than the system temperature 

where oxides precipitate and are trapped, inhibiting oxide circulation through the system. The cold 

trapping process can be performed either by allowing the contaminants to diffuse to the cold point, 

or by circulating the fluid through a cold region.5

3.1 Cold Trapping

There are two types of cold traps—natural circulation and forced circulation. Forced circulation cold traps operate along a bypass line as opposed to the main line of the system and work by 

circulating the flow through a cold region to reach a specified oxidation temperature. Natural circulation cold traps usually operate in-line and work by running the main flow through a cold point where 

a specified oxidation temperature can be reached. Natural circulation cold traps are often used for 

smaller laboratory-scale experiments, whereas forced circulation cold traps are for larger systems.5

Due to these differences, a forced circulation cold trap was selected as the method of choice for the 

FSP-PTC. A forced circulation cold trap is shown in figure 2.

Purified

Sodium Out

Contaminated

Sodium In

Thermocouple

Tube

Sodium

Level

Gas

Space

NaK Out

7 ft 6 in

4 ft 2 in

NaK In

6 ft

4 ft 8 in

Stainless

Steel Mesh

12 lb/ft3

Figure 2. Enrico Fermi Atomic Power Plant forced circulation cold trap.
5

Forced circulation cold traps are generally used in large heat transfer systems. This particular 

method of cold trapping operates along a bypass line as opposed to the main line in the system. This 

allows drawing of a small portion of the main flow into the cold trap without considerably affecting 

the rest of the system. 

Most forced circulation cold traps consist of an economizer, which acts as a regenerative 

heat exchanger (HX), a crystalline tank for collecting the oxide precipitates, and a coolant system. 

Although it is not required, stainless steel wire or mesh, called packing, is often used in cold traps to 

increase the surface area onto which crystallization of the oxide can occur.5

3.2 Design Challenges

Several factors have to be considered to design a cold trap to effectively remove oxide contaminants from the NaK fluid. The first challenge is to design a trap that prevents plugging. Plugging 

makes the cold trap inoperable by blocking the flow of NaK into the trap. 

In addition, the cooling load imposed by the cold trap must take into consideration the maximum power capacity of the simulated core within the system, so that the core is not overloaded. The 

maximum capacity of the core used in the FSP-PTC is 30 kW.

Another concern that must be resolved is the pressure drop that is created across the cold 

trap. Large pressure drops are problematic, as they can cause a decrease in flow across the cold trap

or could potentially create a ‘back flow’ in which the bypass flow travels in the opposite direction of 

the main flow. The change in flow would then cause further problems, such as affecting the cleanup 

time necessary for the cold trap and the required power needed to operate it.

To ensure that the maximum amount of oxide is being filtered out of the NaK as it flows 

through the cold trap, a sufficient residence time was determined. Previous work has shown that 

increasing the residence time to 5 min greatly increased the effectiveness of the cold trap to remove 

oxygen; however, the effectiveness was not sufficiently affected by an increase from 5 to 10 min.7

Research has shown that a 5-min residence time is generally accepted for cold trap designs.

3.3 Initial Design

A design was developed to remove oxygen from the FSP-PTC. This design has been developed to prevent and resolve the various design challenges from oxide plugging to pressure drops.

3.3.1 Coolant System

To reach a specified cold trap temperature, a coolant system must be incorporated into the 

cold trap design. The coolant design is based largely on the type of coolant used to achieve the cold 

trap’s cooling region temperature. Both liquid and gaseous cooling mediums were considered for 

internal cooling. External cooling is also possible through the implementation of cooling fins, but 

was deemed impractical in a vacuum environment like the one the FSP-PTC will be operated in during normal operation. In the past, many cold traps used liquid coolants such as tetralin, toluene, and 
6

DowthermTM. Although these coolants exhibit excellent heat transfer qualities, they are less safe than 

other coolant methods and are not compatible in direct contact with NaK because they are organic 

compounds. Although the coolant is not intended to come in direct contact with the NaK used in 

the FSP-PTC, the system is experimental and failures can occur with test equipment. Nitrogen was 

ultimately selected as the coolant for use in the FSP-PTC cold trap. This was due to the availability 

of nitrogen in the EFF-TF facility, compatibility with NaK, and success in the past with other cold 

trap designs such as the Hallam Nuclear Power Facility (HNPF) cold trap.7 Figure 3(a) and (b)show 

a cooling coil and a cooling jacket, respectively.

Vent Line

Stainless

Steel Mesh

Bypass Tube

Spacer

Tetrolin Jacket

Tetrolin Outlet

Tetrolin Inlet

Sodium In Sodium Out

Freeze Trap

(Sodium With

Impurities) Sodium

(Purified) 26

25 Economizer

28 27

24 17

19 Mesh

10 Cold Trap Container

18 Inner Cylinder

13

20 Annulus

14 Polyphase Stator

21 Baffle

34 Nucleation at Walls

Cooling

Air In

Air Out

Polyphase

AC Power

Source

15

Tc 22

31

32

30

29

33

11

12

23

(a) (b)

Figure 3. Cold trap designs: (a) Sodium reactor experiment cold trap with coolant coil7

and (b) design with coolant jacket.8

With the coolant selection in mind, it was determined that a cooling jacket placed around the 

main cold trap assembly would be the best cooling method. This jacket would include an inlet at the 

bottom of the cold trap to ensure a cooler region at the bottom of the cold trap, acting in counterflow to the NaK entering the cold trap. Two methods were determined to generate the proper cold 

trap temperature through the use of the nitrogen coolant—control of the flow rate of nitrogen and 

control of the incoming nitrogen temperature through the use of an electric immersion heater element. Finally, a thermocouple port included in the cold trap design will allow the monitoring of the 

cold trap operating temperature. 
7

3.3.2 Economizer

One issue with the implementation of a cold trap in a system loop is the large drop in temperature of the bypass working fluid, and its effect on the heat balance of the circuit. To reduce the 

overall impact of the cold trap in the FSP-PTC, a decision was made to include an economizer to the 

cold trap design. Economizers help to aid cooling upon entry and heating upon exit of the cold trap, 

recover enthalpy, and improve efficiency. Previously used external and internal economizer designs 

were researched and are shown in figure 4(a) and (b). External economizers are placed outside of 

the main cold trap and are often arranged in a counterflow heat exchanger configuration. However, 

the main problem with an external economizer is its tendency to plug. This is caused by early oxidation of the working fluid in the economizer, causing oxidation buildup and eventually plugging.7 An 

internal economizer does not have this problem because the flow does not have to be contained as in 

an external economizer. The basic design of an internal economizer for use in a cold trap consists of 

a coil used on the outflow running out of the top of the cold trap while the working fluid runs freely 

outside of the coil. Considering the internal economizers free flow design, this type of economizer 

was selected for incorporation into the cold trap design for the FSP-PTC. 

Vent Line to

Freeze Trap

Tetralin In

Tetralin Out

Condenser

Toluene

Reservoir

Stainless Steel

Mesh

Toluene Jacket

Drain Line

Sodium In

Sodium Out

Economizer

Nitrogen

Outlet

Sodium

Inlet

Sodium

Outlet

Slip

Connection

Insulated

Nitrogen Duct

Thermocouple Tube

Perforated Upper Plate

Longitudinal Fin

Tie Rod

6-in Return Tube

Knitted-Wire Mesh

Sodium Buffer Zone

Nitrogen Inlet Plenum

Labyrinth Air Seal

Nitrogen Inlet

Disk-Doughnut Baffles

Economizer Coil

Support

(a) (b)

Figure 4. Cold trap: (a) Boiling coolant with external economizer 

and (b) HNPF with internal economizer.
8

3.3.3 Packing

Although it is not absolutely necessary for cold trap operation, stainless steel mesh packing 

was included in the design of the cold trap. Packing creates increased surface area for the precipitating oxide to cling to in addition to the walls of the crystalline tank. It also creates a more uniform 

deposition of oxides, which could help to minimize pressure drops across the cold trap.9 Packless 

cold traps have been tested and proven to work, but present operational problems presumed to be 

caused by plugging from the inability of a packless trap to trap the oxide.7 To avoid plugging of the 

cold trap, packing was used for the initial design of the cold trap used in the FSP-PTC. There are 

several types of packed trap filters that have been used including knitted steel wire, steel wire mesh, 

screen, and Raschig rings. From previous studies, stainless steel wire mesh was determined to provide 

best results for oxide deposition and has been chosen for the FSP-PTC cold trap design.7

3.3.4 Baffles

Disk-doughnut baffles are often used in the design of cold traps due to their effectiveness at 

aiding in the filtration of oxides. Baffles prevent straight-line channeling, forcing the NaK to flow 

through a larger portion of the mesh. In essence, baffles help to disrupt the natural flow of NaK 

through the cold trap. By doing this, they allow for a more uniform cooling of the flow, which helps 

to precipitate out more oxide than would otherwise be achievable without the use of baffles.7

The implementation of baffles into a cold trap has also been found to reduce the need 

for larger length (L)-to-diameter (D) ratios. It has recently been proposed that baffles could efficiently increase the residence time of the NaK within the cold trap. This could prove beneficial 

for pumped-flow systems that have space limitations and cannot achieve the desired residence time 

of 5 min with their current designs.
9

4. ANALYSIS

The FSP-PTC consists of several main components including a heater core, HX, electromagnetic (EM) pump, and a flow meter (fig. 5). To effectively design the cold trap, an analysis of the 

impact the cold trap would have on the overall system was completed. 

EM Pump

Core

Heat Exchanger

Figure 5. FSP-PTC with primary components labeled.

4.1 Sodium Potassium Properties

To determine the cold trap operating temperature, oxygen solubility versus temperature data 

was analyzed for both NaK and Na curves as seen in figure 6. Table 1 shows the NaK properties.
10

(B) NaK-78—Lindemar et al. (1981)

(E) Na—Noden (1973)

(H) Na—Eichelberger (1968)

(C) NaK-78—Cassidy (1970)

(F) Na—Smith & Lee (1772)

(D) NaK—Kashjgjhgaf et al. (1989)

(A) NaK-78–Lyon (1952)

(D) Na—Takeda et al. (1975)

(G) Na—Rutkauskas (1968)

(I) Na—Claxton (1965)

200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500

70

60

50

40

30

20

10

0

Amount of 

O2 in Solution (ppm)

Temperature (K)

C

A

I

F

D

H B E

G

Figure 6. Cold trap control volume showing temperature and mass flow rate relationships.4,10,11

Table 1. NaK properties.10

Viscosity 1.599×10–4 kg m–1 s–1

Density 738.78 kg m–3

Specific heat 874.07 J kg–1 K–1

Table 2 shows the investigator, equation, valid temperature range, and number of data points 

in the curve fitting for the investigation of oxygen solubility in NaK and Na.

Table 2. Oxygen solubility in NaK and Na investigation data.

Liquid 

Metal Investigator

Number 

of Data

Temperature 

Range (K) Equation

NaK-78 Lindemar et al. (1981) 39 363~483 log10S=7.09–2795/T

Lyon (1952) 9 370~800 S=1.4013exp(T/134.5)

NaK Cassidy (1970) N/A N/A N/A

Na Takeda et al. (1975) 37 423~323 log10S=6.118-2383/T

Rutkauskas (1968) 52 398~573 log10S=8.25-3499/T

Noden (1973) 217 387~828 log10S=6.1579-2384.2/T

Smith & Lee (1972) 51 398~518 log10S=7.023-2828/T

Eichelberger (1968) 107 398~828 log10S=6.239-2447/T

Claxton (1965) 88 383~828 log10S=5.21-1777/T
11

4.2 Power Analysis

An analysis of the optimal cold trap location within the FSP-PTC was performed to determine the impact of the cold trap on the overall system and to determine inlet boundary conditions 

for the cold trap that would have an impact on the specific design of the cold trap. The first analysis 

consisted of examining temperature data around the FSP-PTC loop at normal operation with a flow 

rate of 13.18 gpm (table 3).2 Three locations were considered for placement of the cold trap, although 

the conditions for placement can be applied to other locations in the loop. The three locations considered are in parallel with the core, in parallel with the EM pump, and in parallel with the HX.

Table 3. FSP-PTC loop data used in analysis.2

Applied pump power 1,124 W

Average pump temperature 334.2 °C

HX inlet temperature, NaK 345 °C

Core inlet temperature 332.9 °C

NaK flow rate 13.18 gpm

System volume 30 lb

One driving decision in cold trap location is the amount of power required by the core for the 

addition of the cold trap at each location. For each location, the power required by the cold trap for 

operation was determined using equation (1):

Q = mC p ΔT , (1)

where

Q = power through each analyzed component of the FSP-PTC

m = total system mass flow rate through each component

Cp = specific heat of NaK-78

= difference in temperature at the outlet and inlet of each component.

By varying the percentage of mass flow rate, the power through the cold trap could be simulated for different bypass mass flow rates ranging from 1% to 100% of the total system mass flow rate 

at each location in the loop. Because the cold trap operates by reducing the temperature, an internal 

coil economizer is used to reduce the impact of the total system temperature drop. The temperature 

and mass flow rate relationship of the control volume is shown in figure 7. By examining data from 

the HNPF cold trap that uses a similar style economizer, a curve fit of the economizer effectiveness 

was generated at varying mass flow rates and the corresponding effectiveness for each mass flow rate 

could then be interpolated.9 With the effectiveness determined, the outlet temperature could be estimated from the following relationship:

ε = Tout −TCT

Tin −TCT

, (2)

ΔT
12

where

ε = economizer effectiveness

Tin = cold trap inlet temperature

Tout = cold trap outlet temperature

TCT = cold trap operating temperature. 

Cold Trap

Control

Volume m, Tout

. m, Tin.

TCT

Figure 7. Cold trap control volume showing temperature

and mass flow rate relationships.

The corresponding cold trap operating temperature to achieve 12.5 ppm oxygen solubility 

was estimated to be around 370 K. With these data, the outlet temperature could be determined 

using equation (2). By incorporating the economizer effectiveness and temperature drop through the 

cold trap, the cold trap power could be determined using the following relationship:

Q

CT = m bypass

Cp ΔTε , (3)

where

Q

CT = power through the cold trap

m bypass = mass flow rate through the cold trap along the bypass line as a percentage 

 of the total system mass flow rate 

ΔT = difference in temperature at the outlet and inlet of the cold trap.

In this relationship, the cold trap power is determined at varying percentages of total system 

mass flow rate. Finally, with all FSP-PTC component power determined at varying percentages of 

total system mass flow rate through the cold trap, the system could be looked at as a whole and the 

required core power for different cold trap locations could be determined as seen in figure 8. 

As shown, when the cold trap is around the HX, the required core power is at its minimum. 

Because the core can only operate at a maximum of 30 kW, the percentage of mass flow rate through 

the cold trap could not exceed 95%. It was requested that the required core power not exceed 10 kW, 

which narrowed the cold trap bypass percentage to under 25%. 
13

0 10 20 30 40 50 60 70 80 90 100

Total System Mass Flow Rate Through CT (% )

40

35

30

25

20

15

10

5

0

CT Around HX

CT Around Core/After EM Pump

CT Around EM Pump/After HX

Power Input Required by Core (kW)

Figure 8. Total power required by core versus percentage of flow rate through

cold trap at different cold trap locations in FSP-PTC system loop.

The next design step was to choose a location for the cold trap and test different system operating temperatures including the average operating temperature of the location selected, a minimum 

operating temperature of 473 K, and a maximum operating temperature of 805 K. Using these conditions, a closer look at the power through the cold trap was examined as seen in figure 9. Although 

the HX provided the lowest required core power, lower mass flow rates showed that the difference in 

required power to be similar. With this in mind, it was decided to place the cold trap around the EM 

pump which provides the cold trap with a large D and provides the coolest location in the system 

loop, which will consequently require less cooling of the NaK in the cold trap. 

0

0

1

2

3

4

5

6

7

8

9

10

5 10 15 20 25

Total System Mass Flow Rate Through CT (%)

Power Loss Through Cold Trap (kW)

Tmin = 473 K

Tavg = 605.9 K

Tmax = 805 K

Figure 9. Power loss through cold trap versus percentage of flow rate through

cold trap at varying system operating temperatures.
14

From figure 9, the decision to keep the percentage of total mass flow rate through the cold 

trap under 5% was made due to the strong rise in cold trap power as the percentage of total system 

mass flow rate increased.

4.3 System Cleanup Time Analysis

The time required for system cleanup was calculated to ensure the cold trap could clean the 

NaK in the FSP-PTC in a reasonable amount of time (preferably within a few hours). The cleanup 

time was determined by treating the loop as a control volume and deriving a first order differential 

equation with the given boundary conditions. 

The NaK loop is represented by a control volume (fig. 10) with a branch flow through the cold 

trap. The loop has a total NaK mass of mL, and an instantaneous oxygen concentration of CL(t). 

Exiting the control volume is the flow rate through the cold trap, mCT at the oxygen concentration 

of the loop. Returning to the control volume is the mass flow rate through the cold trap at the oxygen 

concentration of the cold trap, CCT. This leads to the following relationship:

m

dC

dt L mC mC L =   CT CT CT L − . (4)

Loop

Control

Volume

mL

CL(t)

m! CT ,CL m! (t) CT ,CCT

Figure 10. Schematic of cold trap control volume definition used to derive 

first order oxygen concentration time differentiable equation.

This relationship can be rearranged, integrated with using the initial time of zero and the 

initial loop concentration of C0:

m

dC

C C L m dt L

CT L C

C

CT

L t

− = ∫ ∫ 0 0  (5)

and solved for CL:

C Ce C e L

m

m

t

CT

m

m

t CT

L

CT

L = + −

⎛

⎝

⎜

⎜

⎞

⎠

⎟

⎟

− −

0 1

 

. (6)
15

With this equation, the oxygen concentration of the loop can be plotted versus time at various 

mass flow rates through the cold trap. As the concentration approaches CCT asymptotically over 

time, it is convenient to plot CL–CCT on a log scale.

As a bounding case, equation (6) is used to examine the loop at 805 K, with the oxygen 

concentration of the loop at saturation (556 ppm), and a cold trap temperature of 300 K, with 

oxygen concentration leaving the cold trap of 13 ppm. These results are plotted at various mCT in 

figure 11.

0 1

0.1

1

10

100

1,000

2 3

0.006

0.01

0.03

0.06

4 5 6

CL–CCT (ppm)

Time (hr)

Figure 11. FSP-PTC system purification versus time at various mass 

flow rates through cold trap.

The nominal mass flow rate of the FSP-PTC is 0.61 kg/s, so the cold trap mass flow rates in 

figure 11 represent a range of 1% to 10% of the total mass flow rate diverted through the cold trap. In 

the preceding section, a limit was set of 5% of the nominal flow rate, or 0.03 kg/s. A system cleanup 

time of 1–4 hr is possible, if the goal is to achieve CL–CCT=1.

4.4 Pressure Drop Analysis

As stated earlier, one of the key design challenges in the creation of a cold trap is to implement a method or technique to effectively reduce the pressure drop. A large pressure drop can cause 

one of two effects: it can either stop the flow of NaK through the cold trap, or it can create a ‘back 

flow’ that would cause the flow through the cold trap to travel in the opposite direction of the main 
16

flow. The use of packing in the design of the cold trap helps to decrease the pressure drop by allowing 

for a more uniform deposition of oxide.

In addition to the use of packing, the L/D ratio of the cold trap also impacts the pressure 

drop. According to research, previous cold trap designs have typically been limited to a 5:1 L/D

ratio. Although there is no set standard L/D ratio for cold traps, proper sizing for optimum performance is necessary. A small L/D ratio does result in lower pressure drops across the cold trap, 

however it also causes negative effects on performance. As the L/D ratio decreases, the ability of 

the coolant jacket to effectively cool the NaK decreases. This results in less oxide precipitating out of 

the fluid, which in turn requires the purification time to increase.

To effectively quantify the pressure drop for varying L/D relationships, Darcy’s law is used. 

Darcy’s law is represented in equation (7):

ΔP = μlLeff mCT

κρlAw

, (7)

where

μl = viscosity of NaK

Leff = length over which the DP is taken

m CT = mass flow rate of NaK flowing through the cold trap

κ = permeability of mesh

ρl = density of the NaK fluid

Aw = open cross-sectional area of the cold trap. 

Table 4 shows the wick structure permeability.12

Table 4. Wick structure permeability.

Stainless Steel Fibers Permeability (m2)

33 μm diameter 2× 10–10

75 μm diameter 1.16× 10–9

Two separate L/D ratios were used to analyze the effects that they had on the pressure drop—

ratios 3:1 and 5:1 with a set diameter of 1.5 and 2.5 in, respectively. However, to do the calculations, 

the permeability of the mesh needed to be known. Two different thicknesses of mesh were chosen 

with diameters of 33 and 75 μm. Figure 12 represents the data calculated from the two different L/D

ratios for the two types of mesh.
17

2 2.5 3 3.5 4 4.5 5

0

0.5

1

1.5

2

2.5

3

75 μm 3:1

75 μm 5:1

33 μm 3:1

33 μm 5:1

Total System Mass Flow Rate Through Cold Trap (%)

∆P (kPa)

Figure 12. Measured pressure drop across cold trap using varied mesh diameters

and L/D ratios as a function of percentage flow through the cold trap.

Figure 12 provides valuable insight into the effects of mesh sizing and L/D ratios on the 

pressure drop across the cold trap. As expected, the 5:1 L/D ratio creates a larger pressure drop as 

opposed to the 3:1 ratio. The 33-μm mesh also provides a larger DP, as expected, because its open 

area is less than that of the 75 μm. Proper mesh sizing and overall sizing of the cold trap are necessary to eliminate the effects of the baffles on the DP. Because the baffles help to disrupt the natural 

flow of NaK through the cold trap, they also tend to cause an increase in the pressure drop. Altogether, it has been determined that the maximum estimatedDP across the cold trap is ≈0.45 psi.
18

5. DESIGN

The current configuration of the cold trap design consists of the following components: 

a nitrogen cooling jacket, the outer shell of the cold trap, an inlet port, an economizer coil, a stainless 

steel mesh, baffles, an outlet line running to the economizer, and a drain line and valve. The details 

of this design can be seen in figure 13.

Economizer

N2 Outlet

N2 Inlet

Baffles

Valve

Drain

Mesh

Jacket

Outlet Inlet

NaK

Figure 13. Schematic of cold trap with components labeled.

After completing analysis on the cold trap, a range of 2% to 5% of the total system mass 

flow rate was determined to provide sufficient cold trap performance as it relates to power consumption and system cleanup time. The current sizing of the overall cold trap was based on the smallest 

possible size that would yield between a 3:1 and 5:1 L/D ratio: a height of ≈11 in and a diameter of 

2.875 in. This design has ≈1 L of total volume. The maximum volume of the system should not 

exceed 2 L to avoid resizing of the FSP-PTC NaK reservoirs. The total mass of NaK in the FSP-PTC 

is 30 lb, which corresponds to ≈7 mL of total oxide that will be trapped at one time in the cold trap. 

With the total cold trap volume of nearly 1 L, this allows many runs of the FSP-PTC before the cold 
19

trap will need to be cleaned or replaced. The current method for cleaning is to heat the oxide to a liquid state and drain through the bottom of the cold trap. Next, steam cleaning the cold trap can cause 

a reaction of the NaK within the cold trap. This method, however, is not foolproof and may require 

additional efforts to ensure total cold trap cleanup or total replacement of the cold trap system.

The actual construction of the cold trap will consist mainly of stainless steel material, including a 2.5-in pipe with American Standard of Mechanical Engineers flanged and dished heads or 

pipe caps on the ends for the actual cold trap body, quarter-inch line for the inlet, drain, outlet, 

and economizer, stainless steel mesh and baffles, and a nitrogen jacket, which has not yet been fully 

designed. Connections are to be made using butt-welded fittings to ensure NaK cannot be entrapped 

in fittings. Figure 14(a)–(d) shows three-dimensional renderings of the current cold trap design.
20

(a)

(c)

(d)

(b)

Figure 14. Current cold trap design views: (a) Isometric view, (b) side view, 

(c) close-up of top view, and (d) isometric cutaway view.
21

6. FUTURE WORK

For the FSP-PTC, more details need to be examined before the design is finalized. The current 

cold trap design needs to be increased in overall size to allow a larger residence time and increased 

economizer coil size from 1⁄4- to 3/8-in tubing. By increasing the size of the economizer, the possibility of blockage is reduced. Additional solubility data for NaK at lower temperatures (approximately 

150–500 K) also need to be researched or determined experimentally to more accurately determine 

the optimal cold trap operating temperature and to refine system cleanup time calculations based on 

these data. These data can have an effect on required power and system cleanup time calculations. 

These data could be achieved with the addition of a plugging line in parallel with the cold trap, which 

can also act to determine if the cold trap is not operating effectively.13

Packing for the cold trap also needs to be examined in further detail. If a stainless steel mesh 

is selected, the size of the mesh must still be determined. It may also be possible to use other forms 

of packing such as a honeycomb filter. 

Another component to consider is the nitrogen jacket used for cooling. Sizing of the inlet/

outlet ports and spacing between the cold trap wall and the inner jacket wall needs to be completed. 

A thermocouple port needs to be incorporated into the cold trap design to determine the operating 

temperature of the cold trap so that the nitrogen flow rate or temperature can be varied using the 

nitrogen valve or in-line heater element yet to be chosen for final design. 

Heat transfer calculations must be performed on the economizer and coolant jacket to determine how to optimize and properly design the coolant jacket and determine the theoretical efficiency 

of the economizer more accurately. Performing heat transfer calculations of the nitrogen’s ability 

to cool the NaK through the cold trap wall and internal wire mesh is vital in the overall sizing of 

the nitrogen jacket and to determine the temperature the nitrogen will operate. Calculations of the 

effects on the temperature of the NaK internal to the economizer from the external hot flow are 

needed to determine how effective the economizer will be at reheating the outlet flow.

Finally, a suitable method for cleanup needs to be decided upon. Previous cold trap designs 

have had problems cleaning the NaK and oxide from the cold trap completely, even after reheating, 

draining, and steam cleaning, which is the current preferred cleanup method. One possible option is 

to use a flange that could be used to take apart the cold trap, although it would most likely have to 

contain a custom-made stainless steel seal that could be a difficult and potentially hazardous option. 
22

7. CONCLUSION

The primary objective of this project was to design and implement a purification technique, 

more specifically, a forced circulated cold trap, into the FSP-PTC to effectively remove oxygen from 

the NaK fluid. Several key components of the cold trap were analyzed and researched to best satisfy 

the demands of the Propulsion Research and Technology Branch. Although future design analysis is 

necessary to further understand the heat transfer capabilities of the cold trap, an initial design that 

removes oxygen from the FSP-PTC has been achieved.
23

REFERENCES

 1. Garber, A.E.; and Godfroy, T.J.: “Assembly, Analysis and Fill of the Stainless Steel 

NaK-Cooled Circuit (SNaKC),” NASA Marshall Space Flight Center Internal Document, 

ER24-06-SNAKC, 2006.

 2. Garber, A.E.: “Fission Surface Power Primary Test Circuit (FSP-PTC): Initial Testing 

and EM Pump Performance,” NASA Marshall Space Flight Center Internal Document, 

ER24-07-WBS1.2-001, 2007.

 3. Garber, A.E.; Polzin, K.A.; Reid, R.S.; and Pearson, J.B.: “Results from ALIP Tests 

in the Fission Surface Power Primary Test Circuit (FSP-PTC),” NASA Marshall Space Flight

Center Internal Document, ER24-08-WBS1.2-002, 2008.

 4. Sakai, T.; Gunji, M.; Matsumoto, S.; and Ishii, Y.: “Determination of Oxygen Solubility 

in NaK-78 by Vacuum Distillation Method,” Journal of Nuclear Science and Technology, 

Vol. 20, No. 12, pp. 1032–1038, December 1983.

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

Techniques, Gordon and Breach, Science Publishers, Inc., 1967.

 6. Zimmerman, C.A.: “Corrosion of the 316 Stainless Steel in NaK Service – A Literature 

Survey,” AEC Research and Development Report, TID-4500, 37th ed., IDO-14651, 

February 1965.

 7. Foust, O.J.: Sodium-NaK Engineering Handbook, Volume V, Gordon and Breach, Science 

Publishers, Inc., 1979.

 8. Pohl, L.E.: “Cold Trap,” U.S. Patent No. 3,618,770, 1971.

 9. Cygan, R.: “HNPF Cold Trap Evaluation,” AEC Report NAA-SR-4382, December 1959.

10. Lyon, R.N.: Liquid-Metals Handbook, Atomic Energy Commission, Oak Ridge, 1952.

11. Cassidy, F.H.: “Oxides in NaK,” NASA–CR–72963, AGC-TM-4923:70-614, February 1970.

12. Mills, A.F.: Basic Heat and Mass Transfer, 2nd ed., Prentice Hall, Inc., 1999. 

13. Pearson, J.B.; Godfroy, T.J.; Reid, R.S.; and Polzin, K.A.: “NaK Plugging Meter Design 

for the Feasibility Test Loops,” Proceedings of ICAPP ‘08, Paper No. 8288, Anaheim, CA, 

June, 8–12, 2008. 
24

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Development and Analysis of Cold Trap for Use in Fission

Surface Power-Primary Test Circuit

T.M. Wolfe,* C.A. Dervan,** J.B. Pearson, and T.J. Godfroy

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 20

Availability: NASA CASI (443–757–5802)

Prepared by the Propulsion Systems Department, Engineering Directorate

*Department of the Navy, Naval Sea Systems Command, Washington, DC **Georgia Institute of Technology, Atlanta, GA

M–1329

Technical Publication

NASA/TP—2012–217453

NaK, purification, alkali metal, nuclear reactor, fission

01–01–2012

UU 36

NASA

U U U

The design and analysis of a cold trap proposed for use in the purification of circulated eutectic sodium potassium (NaK78) loops is presented. The cold trap is designed to be incorporated into the Fission Surface Power-Primary Test Circuit 

(FSP-PTC), which incorporates a pumped NaK loop to simulate in-space nuclear reactor-based technology using nonnuclear test methodology as developed by the Early Flight Fission-Test Facility. The FSP-PTC provides a test circuit for the 

development of fission surface power technology. This system operates at temperatures that would be similar to those found in 

a reactor (500–800 K). By dropping the operating temperature of a specified percentage of NaK flow through a bypass containing 

a forced circulation cold trap, the NaK purity level can be increased by precipitating oxides from the NaK and capturing them 

within the cold trap. This would prevent recirculation of these oxides back through the system, which may help prevent corrosion.

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