James L. Sanzi

Sest, Inc., Middleburg Heights, Ohio

Donald A. Jaworske and Debra A. Goodenow

Glenn Research Center, Cleveland, Ohio

Titanium-Water Thermosyphon Gamma

Radiation Exposure and Results

NASA/TM—2012-217732

September 2012

AIAA–2012–4078
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James L. Sanzi

Sest, Inc., Middleburg Heights, Ohio

Donald A. Jaworske and Debra A. Goodenow

Glenn Research Center, Cleveland, Ohio

Titanium-Water Thermosyphon Gamma

Radiation Exposure and Results

NASA/TM—2012-217732

September 2012

AIAA–2012–4078

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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NASA/TM—2012-217732 1 

Titanium-Water Thermosyphon Gamma 

Radiation Exposure and Results 

James L. Sanzi 

Sest, Inc. 

Middleburg Heights, Ohio 44130 

Donald A. Jaworske and Debra A. Goodenow 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Titanium-water thermosyphons are being considered for use in heat rejection systems for fission 

power systems. Their proximity to the nuclear reactor will result in some gamma irradiation. 

Noncondensable gas formation from radiation-induced breakdown of water over time may render portions 

of the thermosyphon condenser inoperable. A series of developmental thermosyphons were operated at 

nominal operating temperature under accelerated gamma irradiation, with exposures on the same order of 

magnitude as that expected in 8 years of heat rejection system operation. Temperature data were obtained 

during exposure at three locations on each thermosyphon: evaporator, condenser, and condenser end cap. 

Some noncondensable gas was evident; however, thermosyphon performance was not affected because 

the noncondensable gas was compressed into the fill tube region at the top of the thermosyphon, away 

from the heat rejecting fin. The trend appeared to be an increasing amount of noncondensable gas 

formation with increasing gamma irradiation dose. Hydrogen is thought to be the most likely candidate 

for the noncondensable gas and hydrogen is known to diffuse through grain boundaries. Post-exposure 

evaluation of one thermosyphon in a vacuum chamber and at temperature revealed that the noncondensable gas diffused out of the thermosyphon over a relatively short period of time. Further research shows a 

number of experimental and theoretical examples of radiolysis occurring through gamma radiation alone 

in pure water. 

Nomenclature 

B total number of atoms diffusing 

C(t) concentration at time t

D diffusivity 

GIF Gamma Irradiation Facility 

hrs hours 

MRad (Si) million radiation absorbed dose to silicon 

TLD thermoluminescent detector 

t time 

SNL Sandia National Laboratories 

Introduction 

Fission power systems are contemplated for the human exploration of the moon and Mars, and an 

artist’s concept is shown in Figure 1. Waste heat will be rejected to the environment utilizing a heat 

rejection system consisting of heat pipes embedded in a radiator panel (Ref. 1). Titanium-water heat 

pipes, operating as thermosyphons, will provide the means of spreading heat across the expanse of the 
NASA/TM—2012-217732 2 

Figure 1.—Artist’s concept of a fission power system on Mars. 

panel. Though the reactor will be shielded, the thermosyphons in the radiator panel will reside in a 

gamma radiation environment expected to vary across the span of the panel. Noncondensable gas 

formation from radiation-induced breakdown of water over time may render portions of the 

thermosyphon inoperable. Information on radiolysis gleaned from the literature offers differing points of 

view (Refs. 2 to 6). Radiolysis is guided by energy deposition over a given path length. Hydrogen and 

hydroxyl radicals are created followed by either the formation of numerous hydrogen and oxygen species, 

or prompt recombination to form water. One school of thought suggests that the amount of water in the 

thermosyphons is small resulting in a small path length and prompt recombination, yielding a minimal 

amount of gas formation (Ref. 3). Another thought suggests that the distribution of fluid and vapor within 

the operating thermosyphons inhibits recombination, making gas formation from the working fluid in the 

thermosyphons likely (Ref. 2). The latter was found to be the case here. 

Titanium-water heat pipes operating as thermosyphons are being considered for use in heat rejection 

systems for fission power systems. Their proximity to the nuclear reactor may result in some exposure to 

gamma radiation. Noncondensable gas formation from radiation-induced breakdown of water over time 

may render portions of the thermosyphon condenser inoperable. In this test, thermosyphons were operated 

at temperature under accelerated gamma irradiation, with exposures on the same order of magnitude as 

that expected in 8 years of actual heat rejection system operation, though this testing was considerably 

accelerated, occurring in hours. Temperature data were obtained during exposure at three locations on 

each thermosyphon: evaporator, condenser, and fill tube. Under ideal conditions, the thermosyphon 

should operate isothermally. A decrease in temperature near the fill tube compared to the evaporator 

serves as an initial indication of noncondensable gas formation. A second indication is a decrease in 

temperature at the top of the condenser compared to the evaporator and serves to indicate that the 

noncondensable gas is beginning to infringe on the working fluid at the condenser. The objective for this 

testing was to gather temperature data from these three locations on thermosyphons operating at 400 K in 

a gamma irradiation exposure environment to reveal the presence and magnitude of noncondensable gas 

formation and its impact on condenser operation. The testing revealed the formation of a small amount of 

noncondensable gas in all heat pipes, with those having the greatest dose yielding the greatest amount of 

noncondensable gas. Though noncondensable gas was evident, thermosyphon performance was not 

affected because the noncondensable gas was compressed into the fill tube region at the top of the 

thermosyphon, away from the heat rejecting fins. As an aside, one thermosyphon was heated in vacuum 

after gamma irradiation in order to drive off the noncondensable gas thereby returning thermosyphon 

operation to normal. 
NASA/TM—2012-217732 3 

Radiolysis 

Radiolysis, in the context of this paper, is the disassociation of water into its constituents of hydrogen 

and hydroxyl through energy being imparted by incident radiation, commonly neutron or gamma. In this 

case, the thermosyphons were irradiated only in a gamma radiation field. The gamma fluence values were 

based on calculations provided by Sandia National Laboratories (SNL). This fluence is based on a 

projected 8-year lifetime for the fission power system installation (Ref. 1). 

Initial research concluded that the amount of water in the thermosyphons would lead to negligible 

radiolysis as discussed (Ref. 3). It was thought that the gamma radiation would not produce a significant 

amount of noncondensable gas through radiolysis or that free radicals created would recombine as soon as 

they were produced. When the thermosyphons were irradiated, however, noncondensable gas formed. The 

experimental results showed a change in the temperature distribution of the thermosyphons that was not 

noted during the nonradiation testing, and indicated a buildup of noncondensable gas, presumably 

hydrogen. The amount of gas created was small but noticeable based on the temperature distribution. 

Further research also showed a number of documented experimental and theoretical examples of 

radiolysis occurring only through gamma irradiation of pure water (Refs. 4 and 5). 

The amount of radiolysis observed was measured by the delta T across the length of the 

thermosyphon, not by volume of gas generated or efficiency degradation of the thermosyphons. Also 

subsequent testing at NASA Glenn Research Center in a vacuum chamber indicated the hydrogen diffuses 

out through the titanium and by the end of the test the thermosyphon tested was operating at near preirradiation temperature gradients. The likely reason for the higher-than-expected radiolysis in the 

thermosyphons could be the fact that much of the water inventory was arranged in a thin film around the 

inside of the thermosyphon, where the film of water is best modeled as a two-dimensional feature limiting 

free radical recombination and precluding the bulk of the water from self shielding. 

Methods and Materials 

Details of the experiment hardware are reported elsewhere (Ref. 7). In short, the experiment consisted 

of an electrically heated heater block holding six thermosyphons at a time, and a total of 10 identical 

titanium-water thermosyphons. Master Bond, Inc., EP30HT–LO epoxy doped with silver powder was used 

to bond each thermosyphon to Poco graphite foam saddles and graphite fiber—RS–3 isocyanate resin 

polymer matrix composite fins, installed for cooling. Multiple exposures were obtained through sequential 

removal and replacement of thermosyphon pairs. The thermosyphons were arranged in a hexagonal pattern, 

as shown in Figure 2. Radiation-resistant type K thermocouples were attached to the thermosyphon via band 

clamps. Three thermocouples were attached to each thermosyphon at the evaporator, condenser, and fill 

tube. Thermal performance was characterized in air at the operating temperature of 400 K, before, during, 

and after gamma irradiation exposure. The temperature difference between evaporator and fill tube and 

between evaporator and condenser revealed the presence or absence of noncondensable gas. A decrease in 

temperature near the condenser end cap compared to the evaporator serves to indicate that the noncondensable gas is beginning to infringe on the working fluid at the condenser. 

Gamma Irradiation Exposure 

Exposure to gamma radiation was achieved at the SNL Gamma Irradiation Facility (GIF). The 

sequential exposures were as follows: one thermoluminescent detector (TLD) run of 10 min, three 

sequential exposures of 48, 143, and 286 min, and a second TLD run of 10 min. The first TLD run was 

for facility documentation and the second TLD run was for calibration. Both of these short exposures 

were considered negligible in the subsequent total dose calculations. These three exposure times were 

selected to bracket the expected doses anticipated in lunar operation, though the dose rate was greatly 

accelerated. At the conclusion of irradiation, all 10 thermosyphons were returned to NASA Glenn 

Research Center. 
NASA/TM—2012-217732 4 

Figure 2.—Thermosyphon test setup. 

TABLE 1.—EXPOSURE MATRIX WITH RUN LOCATION AND CALCULATED TOTAL DOSE 

Time TS 5, 14 TS 4, 12 TS 2, 9 TS 3, 8 TS 15, 6 

10 min 

(TLD) 

X X X 

48 min X X X 

143 min X X X 

286 min X X X 

10 min 

(TLD) 

X X X 

Total (min) 48 

(0.3 Mrad dose) 

191 

(1.3 Mrad dose) 

286 

(2 Mrad dose) 

429 

(3.5 Mrad dose) 

577 

(4.1 Mrad dose) 

Thermoluminescent Detector Calculations 

Total dose was calculated based on location, time, and the counts obtained from the processing of 

TLDs. In brief, the TLDs were supplied in packs of four, placed at three heights along each 

thermosyphon, at all six thermosyphon locations, thus supplying a total of 72 TLD readings per 

calibration run. The average dose per thermosyphon location was found and the average dose per location 

was multiplied by the time that each thermosyphon was at that location to identify the specific 

thermosyphon dose. For each titanium-water thermosyphon exposed, the counts for all three heights were 

averaged and a single dose was calculated and expressed with respect to silicon. Actual values were 

slightly less than predicted values, with the actual dose (Si) falling in the range of 0.3 to 4.1 MRad. 

Selected Post-Gamma Irradiation Exposure 

One post-irradiation thermosyphon was placed in a vacuum facility at NASA Glenn Research Center 

to understand the nature of the noncondensable gas, the idea being that the noncondensable gas 

composition was primarily hydrogen and that hydrogen is known to diffuse through titanium at elevated 

temperatures. Chemical recombination of hydrogen and oxygen was not considered. The vacuum facility 

selected was Vacuum Facility 17 and the thermocouple temperatures were monitored over time. 

Approximately 1000 hrs were added to the operation of thermosyphon 9. As before, the temperature 

difference between evaporator and fill tube revealed the retention or loss of noncondensable gas. 
NASA/TM—2012-217732 5 

Results and Discussion 

Temperature difference (delta T) was used as a measure of noncondensable gas formation in the 

thermosyphons before, during, and after gamma radiation exposure. A delta T greater than 2 K served to 

suggest the presence of noncondensable gas at that thermocouple location. Figure 3 summarizes each 

delta T for 9 of 10 thermosyphons and all five exposures, with the temperature difference between 

evaporator and fill tube (“Evap to Fill”) and the temperature difference between evaporator and condenser 

(“Evap to Cond”) identified. Data from one thermosyphon was spurious due to thermocouple error and 

was omitted. Small amounts of noncondensable gas that collect only at the top of the thermosyphon do 

not interfere with condenser operation, being compressed there by the working fluid of the pipe. Larger 

amounts of noncondensable gas that start to infringe on the finned region of the condenser begin to 

interfere with thermosyphon heat rejection capacity. 

Most thermosyphons exhibited some noncondensable gas formation with the indicator being an 

evaporator to fill tube temperature difference greater than 2 K, though not enough to begin infringing on 

condenser operation. The trend appears to be an increasing amount of noncondensable gas formation with 

increasing gamma irradiation dose. The evaporator to condenser data indicate all of the thermosyphons 

have fully functional condensers with the fins being unaffected by the noncondensable gas, as all have an 

evaporator to condenser temperature difference less than or equal to 2 K. 

Post-exposure evaluation of one thermosyphon operating continuously under vacuum at 400 K 

revealed that the noncondensable gas diffused out of the thermosyphon over a matter of several hundred 

hours. Fick’s second law of diffusion was used as a model to approximate the diffusion of a finite number 

of atoms from a fixed enclosure. 

Equation (1) summarizes the relevant function from a finite source of atoms, that is, the interior of the 

thermosyphon: 

   

0.5 C t  B / Dt (1) 

where concentration in the source, C, at a given time (t) is a function of the total number of atoms 

diffusing, B, the diffusivity of the atoms, D, and time, t. Though the number of atoms is not specifically 

known, a plot of the value of delta T from the thermocouples measuring evaporator and fill tube 

temperatures, and the inverse of the square root of time should provide a linear function, passing through 

zero at infinite time. The data from thermosyphon 9 are presented in this fashion in Figure 4. 

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

#2, Evap to Fill

#2, Evap to Cond

#3, Evap to Fill

#3, Evap to Cond

#4, Evap to Fill

#4, Evap to Cond

#5, Evap to Fill

#5, Evap to Cond

#6, Evap to Fill

#6, Evap to Cond

#8, Evap to Fill

#8, Evap to Cond

#9, Evap to Fill

#9, Evap to Cond

#12, Evap to Fill

#12, Evap to Cond

#14, Evap to Fill

#14, Evap to Cond

delta T, K

Dose, MRad

Figure 3.—Delta T versus dose (Si). 
NASA/TM—2012-217732 6 

0

2

4

6

8

0 0.05 0.1

delta T, K

1/(time, hrs)0.5

Figure 4.—Delta T versus an inverse function of time, under vacuum at 400 K. 

Conclusions 

The heat rejection system is an important part of a fission surface power system envisioned for use on 

the moon or Mars. To generate 40 kwe, approximately 140 kWt of waste heat will need to be rejected, 

according to one reference concept. Spreading the heat across the expanse of a radiator panel is 

envisioned to occur through the use of titanium-water heat pipes operating as thermosyphons. Even with 

shielding around the reactor, there may be some gamma radiation impinging on the water in the 

thermosyphons serving as the working fluid. A number of thermosyphons operating at temperature were 

exposed to a gamma radiation source at the Gamma Irradiation Facility at Sandia National Laboratories to 

understand the impact of gamma irradiation on thermosyphon operation. Though a small amount of 

hydrogen was generated from the radiolysis of the water, increasing with increasing dose, results from 

subsequent vacuum testing on the diffusion of hydrogen through the titanium vessel were modeled 

utilizing Fick’s second law of diffusion and it was found that diffusion at temperature was rapid compared 

to the generation of hydrogen over the anticipated lifetime of the system. Hence, accumulation of 

hydrogen inside the operating thermosyphons is not expected. 

References 

1. Mason, L., “Recent Advances in Power Conversion and Heat Rejection Technology for Fission 

Surface Power,” Proceedings of Nuclear and Emerging Technologies for Space 2009, American 

Nuclear Society, Atlanta, GA, 2009, paper 204298. 

2. Vinson, D.W., Deible, R.W., and Sindelar, R.L., “Evaluation of Hydrogen Generation from 

Radiolysis from Breached Spent Fuel,” Westinghouse Savannah River Company, WSRC–MS–2002–

00728, Savannah River Site, Aiken, SC, 2002. 

3. Hart, E.J., McDonell, W.R., and Gordon, S., “The Decomposition of Light and Heavy Water Boric 

Acid Solutions by Nuclear Reactor Radiations,” Proceedings of the International Conference on the 

Peaceful Uses of Atomic Energy, Geneva 1955, P/839, Vol. 7, United Nations, New York, NY, 1956, 

pp. 593–598. 

4. Bjergbakke, E., Draganic, Z.D., Sehested, K., and Draganic, I.G., “Radiolytic Products in Waters Part 

I: Computer Simulation of Some Radiolytic Processes in the Laboratory,” Radiochemica Acta, Vol. 

48, 1989, pp. 65–71. 
NASA/TM—2012-217732 7 

5. Loyd, J.A., Eller, P.G., and Hyder, L., “Literature Search on Hydrogen/Oxygen Recombination and 

Generation in Plutonium Storage Environments,” Los Alamos National Laboratory, LA–UR–98–

4557, Los Alamos, NM, 1998. 

6. LaVerne, J.A., “OH Radicals and Oxidizing Products in the Gamma Radiolysis of Water,” Radiation 

Research Society, Vol. 153, No. 2, Feb. 2000, pp. 196–200. 

7. Sanzi, J.L., and Jaworske, D.A., “Titanium-Water Thermosyphon Gamma Radiation Effects,” 

Nuclear and Emerging Technologies for Space 2012, American Nuclear Society, The Woodlands, 

TX, 2012, paper 3055. 
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14. ABSTRACT

Titanium-water thermosyphons are being considered for use in heat rejection systems for fission power systems. Their proximity to the 

nuclear reactor will result in some gamma irradiation. Noncondensable gas formation from radiation-induced breakdown of water over time 

may render portions of the thermosyphon condenser inoperable. A series of developmental thermosyphons were operated at nominal 

operating temperature under accelerated gamma irradiation, with exposures on the same order of magnitude as that expected in 8 years of 

heat rejection system operation. Temperature data were obtained during exposure at three locations on each thermosyphon: evaporator, 

condenser, and condenser end cap. Some noncondensable gas was evident; however, thermosyphon performance was not affected because 

the noncondensable gas was compressed into the fill tube region at the top of the thermosyphon, away from the heat rejecting fin. The trend 

appeared to be an increasing amount of noncondensable gas formation with increasing gamma irradiation dose. Hydrogen is thought to be 

the most likely candidate for the noncondensable gas and hydrogen is known to diffuse through grain boundaries. Post-exposure evaluation 

of one thermosyphon in a vacuum chamber and at temperature revealed that the noncondensable gas diffused out of the thermosyphon over 

a relatively short period of time. Further research shows a number of experimental and theoretical examples of radiolysis occurring through 

gamma radiation alone in pure water. 

15. SUBJECT TERMS

Heat pipes; Thermosyphons; Gamma radiation; Radiators 

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