Cheryl L. Bowman

Glenn Research Center, Cleveland, Ohio

E. Eugene Shin

Ohio Aerospace Institute, Brook Park, Ohio

Omar R. Mireles

Marshall Space Flight Center

Ross F. Radel

Phoenix Nuclear Labs, Middleton, Wisconsin

A. Louis Qualls

Department of Energy, Oak Ridge National Laboratory, Knoxville, Tennessee

Radiation Specifi cations for Fission Power 

Conversion Component Materials

NASA/TM—2011-216996

June 2011

NETS –2011–3505
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Cheryl L. Bowman

Glenn Research Center, Cleveland, Ohio

E. Eugene Shin

Ohio Aerospace Institute, Brook Park, Ohio

Omar R. Mireles

Marshall Space Flight Center

Ross F. Radel

Phoenix Nuclear Labs, Middleton, Wisconsin

A. Louis Qualls

Department of Energy, Oak Ridge National Laboratory, Knoxville, Tennessee

Radiation Specifi cations for Fission Power 

Conversion Component Materials

NASA/TM—2011-216996

June 2011

NETS –2011–3505

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Nuclear and Emerging Technologies for Space (NETS 2011)

cosponsored by the ANS Aerospace Nuclear Science and Technology Division, the ANS Trinity Section, 

and the AIAA

Albuquerque, New Mexico, February 7–10, 2011
Acknowledgments

This work was facilitated by the efforts of numerous individuals at the associated laboratories. The authors would like 

specifically thank Cliff Fralick at SunPower Inc.; Christopher Blasio, Michael Brace, Peggy Cornell, Howard Eakin, 

James Gaier, Lee Mason, Terry McCue, Salvitore Oriti, Robert Pelaez, Derek Quade, Daniel Scheiman, Tim Ubienski, 

and Alan Van Dyke, at NASA Glenn Research Center; Keith Leonard, Eric Manneschmidt, Robert Anthony Joseph III, 

Larry Merryman, Randy Hobbs, and Dwight Clayton, at DOE Oak Ridge National Laboratory; Shannon Bragg-Sitton, 

Latha Vasudevan, Jerry Newhouse, and Albert Tijerina of the Nuclear Science Center at Texas A&M University; and 

Don Hanson, DOE Sandia National Laboratories GIF supervisor.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

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

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-216996 1 

Radiation Specifications for Fission Power Conversion 

Component Materials 

Cheryl L. Bowman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

E. Eugene Shin 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Omar R. Mireles 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, Alabama 35812 

Ross F. Radel 

Phoenix Nuclear Labs 

Middleton, Wisconsin 53562 

A. Louis Qualls 

Department of Energy 

Oak Ridge National Laboratory 

Knoxville, Tennessee 37831 

Abstract 

NASA has been supporting design studies and technology development that could provide power to 

an outpost on the moon, Mars, or an asteroid. One power-generation system that is independent of 

sunlight or power-storage limitations is a fission-based power plant. There is a wealth of terrestrial system 

heritage that can be transferred to the design and fabrication of a fission power system for space missions, 

but there are certain design aspects that require qualification. The radiation tolerance of the power 

conversion system requires scrutiny because the compact nature of a space power plant restricts the dose 

reduction methodologies compared to those used in terrestrial systems. An integrated research program 

has been conducted to establish the radiation tolerance of power conversion system-component materials. 

The radiation limit specifications proposed for a Fission Power System power convertor is 10 Mrad 

ionizing dose and 5×1014 neutron/cm2

 fluence for a convertor operating at 150 °C. Specific component 

materials and their radiation tolerances are discussed. This assessment is for the power convertor 

hardware; electronic components are not covered here. 

Background 

A fission reactor combined with a Stirling engine power convertor is one technology option being 

explored for generating space-mission power. Fission power offers robust, environment-independent 

flexibility and adaptability for a broad range of space mission power requirements. A detailed lunar/Mars 

design concept has been developed previously by a joint NASA and DOE team (Ref. 1), and serves as a 

foundation for system-level technology development that can be readily adapted to a broad range of 

missions. The Fission Surface Power System (FSPS) concept developed for the moon and Mars would 
NASA/TM—2011-216996 2 

provide 40 kWe (kilowatt electric) power for an eight-year service life. The power conversion system 

would utilize multiple, dual-opposed, free-piston Stirling engines with linear alternators providing 

nominally 6 kWe power output per alternator. The system was designed with convertor engines assuming 

heater head and heat rejection temperatures of approximately 525 and 125 °C, respectively. This 

reference design suggested a radiation shield between the reactor system and power conversion system 

that would limit radiation to 5 Mrad (gamma dose) and 2.5×1014 neutron/cm2

 (neutron fluence) at the 

power convertor components (Ref. 1). One risk reduction task for this program was to examine the power 

system component materials and establish more rigorous radiation-limit specifications for the power 

conversion system. 

The study of space-mission radiation effects on materials is not new. Guidelines for determining 

radiation tolerance have been presented previously for space vehicle design (Ref. 2). The recommended 

three-step approach for assessing spacecraft radiation protection is to first define the environment, then to 

review the anticipated radiation response on representative component materials, and finally to determine 

that each design subsystem will perform as required in the predicted environment. The FSPS definition 

document (Ref. 1) provided the baseline mission and environment definition. Previous reports identified 

candidate materials-of-construction that may be sensitive to the radiation environment resulting from a 

fast-spectrum fission reactor (Refs. 3 and 4). The Stirling engine-based power conversion system 

structure is primarily fabricated from metallic alloys. Metallic alloys experience inconsequential changes 

in mechanical properties for neutron fluences of less than 1017 neutron/cm2

 based on established 

guidelines and are not structurally affected by ionizing radiation (Refs. 2 and 5). However, there are 

components within the power conversion system that rely on polymeric and magnetic materials and these 

materials require radiation hardness scrutiny. 

Polymeric Components 

Polymeric materials are employed for a number of applications in Stirling power convertors including 

insulation, bonding, sealing, and surface treatment. In terrestrial Stirling convertors, polymer selection is 

driven primarily by the upper-temperature limit. Polymer stability can be characterized in a number of 

ways including mechanical properties, outgassing, and physical properties such as the glass-transition 

temperature, Tg. The mechanical properties of interest depend on the specific component application. 

Outgassing refers to the loss of volatile species from the polymers. The Tg refers to the transition from a 

low-temperature glassy state to a high-temperature rubbery state and is a common gauge of both the 

chemical stability and the changes in mechanical properties. 

Polymeric materials can be used in a variety of bonding applications in a free-piston convertor/ 

alternator system including lamination and permanent magnet attachment, and sensor attachment. 

Similarly there are potting applications such as coil fixation and feed-through reinforcement. Durability is 

key to the bonding material functionality in the alternator. The bonding materials must retain thermal and 

dimensional stability, as well as maintain tensile and/or shear strength. Bonding materials must obtain a 

highly-cured state at temperatures compatible with the components being bonded. This is a particularly 

relative concern for adhesively-bonded permanent magnets for which the bonding-material cure 

temperatures cannot induce demagnetization. In potting applications, dimensional and thermal stability is 

crucial while the tensile or shear strength is less important. Low resin viscosity also may be required to 

obtain adequate coverage. Due to the need to keep the assembly temperatures below the demagnetization 

temperatures, epoxies are the baseline choice for bonding and potting. 

Although metallic or close-clearance seals dominate Stirling designs, there are instances were 

polymeric seals can provide additional functionality. Dimensional stability, extrusion resistance, and 

resistance to embrittlement are required in these sealing applications. Rubbery elastomers, such as silicon, 

are needed for these seals. 

Electrical insulation is required for various wires and electrical connections, especially in the 

alternator section of a power convertor. Although dielectric strength is the primary requirement of an 

electrical insulation, the insulation coating must be flexible enough at the beginning of life to 
NASA/TM—2011-216996 3 

accommodate assembly and launch loads. Over the long-term operation of a FSPS mission, impact and 

vibration loads are minimal, but severe embrittlement and spallation must be avoided. As with other 

polymeric materials, chemical stability must be such that there is minimal outgassing induced by elevated 

temperature, radiation, or a combination thereof. In terrestrial Stirling systems, polytetrafluoroethylene 

(PTFE/Teflon, Dupont), polythermaleze, and polyimide wire coatings are common choices. 

Commercially available heat-shrink coatings are typically based on vinylidene fluoride. 

Modern Stirling convertors perform without contact at piston and displacer running surfaces during 

normal operation. Typically, solid lubricant coatings are applied to these running surfaces to minimize 

friction or stiction at start-up or under severe service conditions. The lubricant coating system must 

maintain adhesion, lubricity, impact resistance, and scratch resistance to function in the power convertor. 

The coatings must not generate debris that could interfere with the close clearance seals. There are a 

number of high temperature, ceramic-based solid lubricant systems, but polymer-based coatings such as 

Teflon fluoropolymer (DuPont), Emralon fluorocarbon (Acheson Colloids), and Xylan fluoropolymer 

(Whitford Corporation) are available for the temperatures currently required. 

Stirling convertor designs can use composites, such as glass-reinforced epoxies, as structures within 

the convertor. Dimensional stability, electrical insulation and minimal outgassing would be the primary 

performance requirements for this type of application. 

The definition of the radiation environment and the irradiation response in polymeric component 

materials is important for the FSPS because polymers can be damaged by radiation levels that are much 

lower than the levels affecting metallic and ceramic materials. Organic polymers are long, complex chains 

of covalently bonded atoms. The precise molecular structure including chain length, atom arrangement, 

and side-bonding (also known as cross-linking) determines the physical properties of the polymer. 

Ionizing radiation, which includes high energy protons, electrons, heavy particles, gamma-rays, and 

x-rays, can interact with polymers in several ways. The typical primary event is the ionization of an atom 

which locally breaks a covalent bond and creates a free radical that can then recombine somewhere else in 

the molecule (Ref. 6). In fact, the propensity for secondary and tertiary events from each primary 

radiation interaction is one of the reasons that radiation can be so damaging in all organic materials. The 

physical manifestation of the polymeric radiation damage is either scission or cross-linking of bonds. The 

scission or cross-linking can result in either softening or embrittling. The temperature of the polymer 

during irradiation is important since chemical mobility is a function of temperature. Furthermore, the 

gaseous environment surrounding the polymer is also key because the free radicals can combine with 

oxygen and accelerate material wastage. 

A perusal of the polymer irradiation literature suggested that the component radiation tolerances 

should be acceptable for this application. However there were certain concerns. The general radiation 

tolerance of polytetra-fluoroethylene (PTFE/Teflon) has been demonstrated to be quite low, especially in 

the presence of oxygen (Ref. 7). Though it has been shown that the specific chemistry of the PTFE can 

have a significant impact on the radiation resistance (Ref. 8). Also it is important to note that most of the 

irradiation literature is based on experiments performed in air and at room temperature. Drawing 

conclusions for the Stirling system radiation tolerance from the literature was problematic since both 

oxygen and temperature impact the radiation response. Furthermore, numerous studies have shown that 

additives or changes in formulation of the same basic polymer have a notable affect on the radiation 

resistance (see for example Refs. 9 and 10). It was deemed appropriate to confirm the radiation tolerance 

of mission-relevant polymeric materials under the correct environmental conditions of elevated 

temperature and inert cover gas, as recommended in the third-step of the approach for assessing 

spacecraft radiation protection (Ref. 2). Moreover, the changes brought about by irradiation were 

characterized both in terms of general chemical and physical response and in terms of component-specific 

characterization, such as shear-testing or compression-set, where applicable. 
NASA/TM—2011-216996 4 

Magnetic Components 

Magnetic materials are necessary in the alternator section of the Stirling engine where the relative 

motion of permanent and soft magnets generates an alternating current. A hard, or permanent, magnetic 

material is one that requires a large magnetic field to induce magnetization and then retains this 

magnetization in the absence of the applied field. However, magnetic permanence is a bit of a misnomer 

because certain combinations of elevated temperature and reverse magnetic field will erase the electronspin alignment that produces magnetism. The permanent magnetic materials required in the FSPS 

alternator must have high coercive force strength (coercivity) and high remanent magnetic flux density 

(remanence) since the alternator performance is maximized for high values of both coercivity and 

remanence. A soft magnet is a material that will be magnetized in the presence of a magnetic flux and 

thereby affect the shape and strength of the overall magnetic field. Yet the remanent magnetic field in the 

soft magnet will be low once the applied field is removed. Soft magnetic materials thus are characterized 

by high permeability and low coercivity such that the material can be magnetized and demagnetized with 

minimal energy loss or hysteresis energy (Ref. 11). 

The primary irradiation damage mechanism in all metals is atomic displacement that can occur when 

a neutron or heavy ion strikes a metal atom with sufficient energy to cause an elastic or non-elastic 

reaction within the crystal structure. The recoiling atom is referred to as the primary knock-on atom and 

its energy is distributed to other nearby atoms by a series of secondary collisions in what is called a 

displacement cascade. This produces empty lattice sites (vacancies) and atoms injected into the interstices 

between lattice sites (interstitials). Fortunately, most vacancies and interstitials recombine along the 

original lattice structure. Those that do not recombine are responsible for radiation-induced 

microstructural and property changes. Materials with metallic bonding are insensitive to ionizing 

radiation because the electron cloud easily redistributes in response to electron orbit changes. As 

mentioned previously, mechanical property changes in metallic alloys do not become significant until 

neutron fluences exceed 1017 neutron/cm2

. However, permanent magnets can be affected by irradiation at 

levels as low as 1012 neutron/cm2

 with both temperature and demagnetization field being important 

correlating factors (Ref 12). Numerous studies have shown that SmCo-based magnets also have greater 

resistance to neutron irradiation than NdFeB-based magnets possibly due to a combination of thermal and 

radiation-induced degradation (Refs. 12 to 14). Gordon and Sery reported the onset of degradation in Nibased soft magnetic material around 1017 neutron/cm2

 and the onset of degradation around 1018

neutron/cm2

 in a Fe-49Co alloy (Ref. 15). The review of literature irradiation data was deemed sufficient 

for ensuring that Fe-based soft magnetic materials and SmCo-based permanent magnets would perform as 

required in FSPS radiation environments. 

Radiation Hardness Testing 

The linear alternator candidate materials must be capable of tolerating a mixed neutron and gammaradiation environment to successfully complete the envisioned mission scenarios. The objective was to 

evaluate the performance of candidate polymeric materials used in the alternator by subjecting material 

coupons to similar neutron fluences and ionizing doses at the appropriate operating temperature and gas 

environment. A mixed, neutron and gamma, irradiation environment best simulates the anticipated FSPS 

mission environment. However, the neutron activation of certain materials greatly increases the safety 

risk, handling protocols, and hence the cost of those experiments. Therefore, experiments using a 

combination of test facilities generated a broad range of information in a cost-effective approach. 

Individual material coupons were exposed to ionizing radiation at a Oak Ridge National Laboratory 

(ORNL) gamma irradiation facility (GIF). The ORNL coupon-test matrix was augmented with a smaller 

matrix of samples exposed to a mixed-fluence environment at the Texas A&M University (TAMU) 
NASA/TM—2011-216996 5 

Figure 1.—Gamma flux variation along the axial length of the test canister. 

Highlighted regions show the target specimen dose examples. 

research reactor. Also a prototypic alternator, the Stirling Alternator Radiation Test Article (SARTA), 

was tested at a Sandia National Laboratories (SNL) GIF. Accelerated dose rates could be used in these 

experiments since oxidative processes are the major contributor to time-dependent radiation degradation 

in polymers and an inert-gas environment is relevant for the FSPS power convertor system. 

The first coupon irradiation testing was conducted at ORNL’s High Flux Isotope Reactor (HFIR) 

GIF. The HFIR spent-fuel assemblies decay in a water holding facility. Adjacent locations in the pool are 

available for gamma exposure testing. The advantage of this test facility is that the flux distribution along 

the length of the test chambers allows for the accumulation of more than one dose in a single 

experimental run. Figure 1 illustrates the relative gamma flux profile along with approximate sample 

locations used to achieve three target doses at once. Dose rates in the irradiation canister vary with the age 

of the fuel assembly, but can be as high as 105 rad/hr. Radiachromic film dosimeters were used to 

measure the actual gamma dose at the test specimen locations in the sample holders. The inner 

dimensions of the canister available for irradiation testing are approximately 7.5 cm diameter and 63 cm 

length. The cap of the irradiation canister was fitted with gas handling and electric power feed-through 

junctions to incorporate the environmental requirements for the irradiation tests. The canister insert 

consisted of a multi-zone heater cartridge to which the sample holder assemblies were attached. The 

canisters were filled with argon to provide an inert cover gas. The sample holder assemblies were made of 

aluminum to maximize thermal conduction and minimize radiation absorption. Different sample holders 

were fabricated to accommodate various material specimens. 

Table 1 lists the coupon descriptions, the exposure temperatures, and the dose calculated from the 

dosimetry calibrations and Monte Carlo N-Particle Transport Code (MCNP) calculation with respect to 

position, exposure time, and maximum dose. All exposures were performed under inert, argon cover gas. 

Two types of epoxy, Hysol (Loctite) and Duralco (Cotronics), were tested. Two types of elastomer 

o-rings, silicon and Kalrez (DuPont), were tested. Commerically available coated wires were tested. Viton 

(DuPont) and Kynar (Arkema Group) heat shrink materials were teste. A Xylan (Whitford Corporation) 

solid lubricant was tested. 

A second set of coupon irradiation tests was conducted at the TAMU TRIGA (General Atomics) 

Mark I reactor. The samples were exposed in the confinement building, which is a dry, irradiation cell 

adjacent to the reactor pool. The reactor core is coupled to the irradiation cell through an irradiation cell 

window. The average thermal and fast neutron fluxes in the vicinity of the irradiation cell window were 

experimentally characterized through the use of threshold activation foils. The use of several types of Normalized Flux

Height from Bottom (inches)
NASA/TM—2011-216996 6 

TABLE 1.—TEST MATRIX FOR ORNL IRRADIATED 

COUPONS AND CALCULATED DOSE 

Coupon Type Temperature,

°C 

Dose, 

Mrad 

Epoxy and epoxy shear assembly 125, 150 3, 12, 14 

 125 15, 71 

Elastomeric o-rings 125, 150 3, 12, 14 

PTFE-insulated wire 125, 150 3, 12, 14 

 125 15, 71 

Polyimide-insulated wire 125 3, 12, 14 

Heat shrink tubes over Al rods 125, 150 3, 12, 14 

Solid Lubricant Coating 125, 150 3, 12, 14 

TABLE 2.—TEST MATRIX FOR TAMU IRRADIATED 

COUPONS AND CALCULATED DOSE 

Coupon type Temperature,

°C 

Dose,

Mrad 

Fluence, 

n/cm2

Epoxy shear assembly 125, 150 1.4, 5.4 1×1014, 5×1014

Elastomeric o-rings 125, 150 1.4, 5.4 1×1014, 5×1014

PTFE-insulated wire 125, 150 1.4, 5.4 1×1014, 5×1014

Polyimide-insulated wire 125, 150 1.4, 5.4 1×1014, 5×1014

Heat shrink tubes 125, 150 1.4, 5.4 1×1014, 5×1014

Solid lubricant coating 125, 150 1.4, 5.4 1×1014, 5×1014

activation foils allowed the neutron spectrum to be estimated at discrete energies. The spectra were then 

used to benchmark the existing MCNP models in the vicinity of the irradiation cell window. Two types 

of gamma dosimeters were used to characterize the spatial dose rates as a function of distance from the 

window. A location 11 cm from the window face was selected as best to achieve the required operational neutron fluence while staying within a mission-relevant dose limit of 10 Mrad. The combined 

measurements and modeling provided an average total neutron flux estimate of approximately 

2.9×1011 neutrons/cm2

/sec and a corresponding gamma-ray dose rate estimate of 0.12 to 0.14 Mrad/min 

at the test location. Aluminum pressure vessels were used to house the specimens during irradiation. 

The pressure vessels were evacuated then charged with high purity helium and heated to the desire 

exposure temperature. Additional details on the test configurations are given in References 16 and 17. 

Table 2 lists the coupon material types exposed in the TAMU testing. 

The final series of experiments involved irradiation of a motoring test article which captured the 

functionality of a Stirling linear alternator. The Stirling Alternator Radiation Test Article (SARTA) was 

fabricated by Sunpower, Incorporated. The potentially radiation-sensitive polymeric materials and magnetic 

materials were represented in their expected component applications. The SARTA utilized typical hardmagnetic material, soft-magnetic-material lamination cartridge, cylinder, and cylinder o-rings. Two, metalsheathed, type-K thermocouples were bonded within the alternator. The motor was encased in a stainless 

steel pressure vessel mounted on a transition plate with power, gas, and thermocouple feed-through 

junctions. Motoring control for the SARTA was provided by a manually-operated, variable-frequency 

power supply. Temperature control was provided by a combination of external cooling and closed-loop 

heating. The operating conditions of alternator power, alternator current, alternator voltage, stroke 

magnitude, pressure, internal temperature, and external temperature were continuously recorded during 

motoring. Engine noise was monitored through a microphone for audible feedback. 
NASA/TM—2011-216996 7 

 

Figure 2.—SARTA mounted in low-dose-rate configuration, source down (left). SARTA mounted in 

high-dose-rate configuration, source up (right). 

TABLE 3.—TEST MATRIX SUMMARY FOR SARTA EXPOSURES INCLUDING 

CALCULATED DOSE RATE AND CUMULATIVE DOSE 

Irradiation runs Temperature,

°C 

Dose rate, 

rad/s 

Approximate dose/step, 

Mrad 

Cumulative dose, 

Mrad 

1 to 4 90 80 0.14 to 0.35 0.98 

5 to 7 90 80 1 to 1.3 4.0 

8 to 9 90 850 0.25 4.5 

10 to 13 90 850 2 to 10 22.4 

14 125 80 0.3 22.7 

15 to 18 125 850 1 to 10 40.1 

The SARTA hardware was irradiated at SNL. This facility uses a 155.6 kCi 60Co source (source 

strength at the time of exposure) that consisted of a 20 pin, 32 cm diameter circular array. Two different 

locations were used in the test cell to provide two different dose rates. The SARTA test article was placed 

in the test cell approximately a meter outside the Co60 array for the low-dose-rate configuration. The test 

article was positioned inside the array for the high-dose-rate configuration. Figure 2 shows the test article 

in each configuration; the high-dose-rate configuration is shown as viewed through a leaded-glass 

window. CaF2 thermo-luminescent dosimeters were used to measure the actual dose for six, shortduration test runs. The calculated average dose rate was 78.9 rad/sec for the low-dose-rate configuration 

and 854 rad/sec for the high-dose-rate configuration. 

The operating performance of the alternator was measured continuously during irradiation. Between 

each irradiation test the inductance, resistance, and resistance-to-ground were measured for the alternator 

coil and the fast linear displacement transducer coil. Table 3 summarizes the operating temperature and 

dose accumulated for the SARTA irradiation testing runs (Ref. 18). Itterative step changes were made to 

increase the test condition severity based on the component temperature, the dose rate, and the 

accumulated dose per step. Although the variation in operating parameters between the baseline testing at 

NASA Glenn Research Center (GRC) and the testing at SNL were greater than anticipated, the operating 

conditions measured during all 18 irradiation experimental runs appeared to be quite stable. Differences 

in the support systems at GRC and SNL resulted in larger pressure leak rates during the testing at SNL, 

presumably due in part to leakage around thermocouple fittings and at the pressure regulator. In fact, the 

biggest influence in electrical performance found during the irradiation runs appeared to result from the 

fluctuation in system pressure. 
NASA/TM—2011-216996 8 

Post-Irradiation Evaluation Techniques 

A range of techniques were used to evaluate possible physical and chemical changes due to the 

combined thermal and radiation exposures. Specimen weights were measured before and after irradiation 

using standard, calibrated scales to assess possible weight loss or gain due to mechanisms such as 

oxidative processes. Specimen dimensions were measured before and after irradiation using standard, 

calibrated calipers to assess possible shrinking or swelling. A standard drying process was used prior to 

weight or dimension measurement. Visual changes were assessed using manual inspection as well as 

optical microscopy up to 1000× magnification. Scanning electron microscopy was used for higher 

magnification visualization of select samples as well as qualitative surface chemistry using energy 

dispersive spectroscopy. The bonding strength of the candidate epoxies was assessed using sub-scale, 

lap-shear tensile specimens. Lap-shear tests were performed at 120 °C in air with a standard, screwdriven, universal load frame. The o-ring samples were evaluated through a combination of leak testing, 

compression set, and residual tensile strength. Various micro and nano-scale tests were employed to 

determine potential radiation-induced mechanical property changes in the polymers used as insulation 

sleeves, insulation coating, shrink tubing, and rub-surface coating. The nano-scale coating tests were 

performed by Nanovea, Irvine, California. A Thermo Electron Nicolet 380 FTIR was used to obtain 

surface chemical spectra for material samples. This technique provides a good indication of the changes 

in surface chemistry, as indicated by the presence of different peaks, peak shifting, or large changes in 

peak intensity. A TA Instruments Q-1000 Differential Scanning Calorimeter (DSC) was used to produce 

heat-flow-difference curves from specimens weighing 5 to 10 mg. This technique identifies exothermic or 

endothermic reactions, phase transitions such as melting, glassy-rubbery transitions, and the degree of 

curing. A TA Instruments Q-500 Thermo Gravimetric Analysis (TGA) system was used to measure 

outgassing through weight change as a function of temperature and the thermal degradation temperature 

(Td). For all physical and chemical measurements, control specimens were tested which had experienced 

thermal aging comparable to the time-at-temperature experienced in the elevated-temperature irradiation 

testing. 

Post-Irradiation Evaluation Results 

Epoxy coupons experienced no systematic or statistically significant changes in weight, dimensions, 

appearance, or physical/chemical properties as a function of irradiation under the conditions studied. The 

epoxy-bonded lap-shear test results exhibited a high degree of variability, which made trends in strength 

or strain-to-failure difficult to isolate. There did appear to be a slight decrease in both strength and strainto-failure as the result of thermal aging and, possibly, an additional decrement due to irradiation at 

elevated temperature. However, it must be emphasized that the changes between average values were less 

than the scatter as defined by standard deviation. 

Elastomeric o-ring samples experienced no systematic or statistically significant changes in weight, 

appearance, or physical/chemical properties as a function of the irradiation under the conditions studied. 

There was a slight compression-set for the o-rings irradiated under load (ORNL gamma testing) and 

slight swelling for o-rings irradiated not under load (TAMU mixed-fluence testing). In post-irradiation 

inspection of the SARTA hardware, there was considerable compression-set observed for a silicon o-ring, 

but no loss of sealing was apparent. In the ORNL gamma-irradiated coupons there was no loss in sealing 

as a result of irradiation but there were slight changes in residual tensile strength. In post-irradiation 

tensile testing of the o-rings, gamma radiation appeared to decrease the strain-to-failure and increase the 

tensile strength for silicon o-rings. However, gamma radiation exposure decreased both the strain-tofailure and the tensile strength for Kalrez o-rings. Tensile testing was not performed after mixed fluence 

exposures. 

Compression-set of the elastomers was a particular area of concern based on the previous irradiation 

literature. The compression-set property, CB, was defined by CB = (to – tf)/(to – tshim), where: to = original 

specimen thickness, tf = final specimen thickness, and tshim = shim thickness (i.e., tshim = 0.75 to). The 
NASA/TM—2011-216996 9 

residual compression set was measured at 30 min after unloading and also measured at other times up to 

48 hr after unloading to monitor recovery. Figure 3 shows the compression-set results for the two types of 

elastomeric o-rings tested. Here open symbols represent post-gamma-only irradiation and filled symbols 

represent post-mixed-fluence irradiation. Cross symbols represent as-received or after thermal aging. 

Scatter bands are shown for the as-received material data set. The silicon material experienced less 

compression-set than the Kalrez. For the silicon, there might be some increase in compression-set for 

exposures at the highest temperature, but more data would be required to confirm that the trend is real and 

not an artifact of scatter within the data. The irradiated Kalrez samples experienced relatively the same 

compression-set as the controls specimens. 

The PTFE-coated wires were the component most affected by irradiation. Although there were no 

systematic or statistically significant changes in weight or dimensions, visible cracks were seen after 

15 Mrad at 125 and 150 °C in gamma-only ORNL testing. Cracks were also found in the PFTE-insulated 

wires during SARTA disassembly and inspection (Ref. 19). This cracking can be seen most readily when 

the wires are cut. Examples are shown in Figure 4. DSC results appeared to indicate an increase in heat-ofmelting which would suggest an increase in crystallinity after 3 Mrad exposure, but the data variation was 

too large to be conclusive. Limited nano-indentation data suggests an increase in hardness due to thermal 

history alone, followed by an additional increase in hardness due to irradiation. The limited data 

generated between 3 and 15 Mrad makes it difficult to assign a safe radiation limit based on these results. 

PTFE insulation is not recommend for FSPS applications above 5 Mrad without additional experimental 

verification. 

 

Figure 3.—Percent compression-set versus relaxation time for (a) silicon and (b) Kalrez o-rings. 

 

Figure 4.—Cracks developed in PFTE wire insulation are readily observed when irradiated 

wires were cut after (a) 15 Mrad gamma exposure at 125 oC and (b) the cumulative 

SARTA exposure. 

Time (hr)

0 10 20 30 40 50

Silicon C

B (%)

0

5

10

15

20

25

30

AR 

125C 

125C

125C

125C

125C

125C

150C 

150C

150C

150C

150C

150C 

3 Mrad: (a)

 5x1014 n/cm2

:

12 Mrad:

14 Mrad:

1014 n/cm2

:

Time (hr)

0 10 20 30 40 50

Kalrez C

B (%)

0

5

10

15

20

25

30

AR 

125C 

125C 

125C

125C

125C

125C

150C 

150C 

150C 

150C 

150C 

150C 

3 Mrad: (b)

 5x1014 n/cm2

:

12 Mrad:

14 Mrad:

1014 n/cm2

:
NASA/TM—2011-216996 10 

Polyimide-coated wires had very thin coatings which were difficult to interrogate by visible 

inspection. Limited nano-indentation data on the polyimide-coated wires suggested an increase in 

polyimide hardness due to thermal history alone, followed by a slight decrease in hardness after 3 Mrad, a 

larger decrease after 12 Mrad and then an additional increase after 14 Mrad. More analysis is underway to 

attempt to understand this apparent change in mechanism. Load-to-initiate delamination appeared to 

increase as a function irradiation for the polyimide-coated wires. The SARTA performed within expected 

variation through motoring up to 40 Mrad. However, a short-circuit was detected in post-radiation 

inspection after the unit was removed from the test cell. Post-irradiation disassembly suggested that the 

short-circuit occurred in the SARTA alternator windings, which are conductive wires coated with 

polyimide insulation. The alternator wire could not be unwound for detailed inspection because the wire 

was encased in potting material. It was determined that removing the potting material might damage the 

coating and it would be impossible to discriminate between disassembly damage and irradiation damage. 

Therefore the source of short-circuit could not be identified. It should be noted that the SARTA was a 

one-offering, proto-type designed specifically for this test and the steady-state operational life of the unit 

in absence of irradiation is likewise unknown. Even though literature data suggests that polyimide 

materials in general have high radiation resistance, the current work suggests that alternator windings 

with thin polyimide coatings should not be used for FSPS applications above 10 Mrad without additional 

experimental verification. 

The heat-shrink materials irradiated in the ORNL gamma-exposure testing had been pre-shrunk on 

aluminum rods under controlled furnace conditions. These heat-shrink samples experienced no systematic 

or statistically significant changes in weight, appearance, or physical/chemical properties as a function of 

irradiation after ORNL gamma exposures. The heat-shrink samples irradiated in the TAMU mixed 

fluence testing were not thermally aged before irradiation. The thermal changes experienced during 

irradiation at elevated temperature overwhelmed any additional changes due to irradiation. Heat shrinking 

in SARTA assembly was performed using a heat gun. This standard assembly practice introduces 

variation in the amount of heat applied to the heat shrink material. Therefore, it was problematic to make 

physical-state comparisons between the SARTA, post-irradiated heat shrink materials and various 

controls. 

The solid lubricant-coated samples experienced no systematic or statistically significant changes in 

weight, appearance, or physical/chemical properties as a function of irradiation under the conditions 

studied. 

Conclusion 

Acceptable levels of radiation tolerance have been established for a candidate set of FSPS power 

convertor materials through a combination of coupon testing and experiments with an alternator-like test 

article. Experiments were performed in inert gas and at elevated temperatures up to 150 °C. Extensive 

data was collected up to 15 Mrad of ionizing radiation from different sources and limited data was 

generated at higher doses. Most of the samples were functionally unaffected by the irradiation conditions. 

Ionizing radiation up to 10 Mrad and neutron exposures up to 5×1014 neutron/cm2

 produced insignificant 

degradation for all of the materials tested except the PTFE-based wire insulation. Therefore, it is 

recommended that future space fission power concept design iterations can proceed with higher neutron 

and gamma fluence limits for the power conversion units than those previously proposed (Ref. 1). Power 

conversion system designers must be cognizant of the fact that the radiation response of polymers can be 

greatly affected by specific additives (Ref. 9) and deviations from the specific materials tested must be 

made judiciously. 
NASA/TM—2011-216996 11 

References 

1. “Fission Surface Power System Initial Concept Definition,” NASA/TM—2010-216772, August 

2010. 

2. “Nuclear and Space Radiation Effects on Materials,” NASA SP–8053, June 1970. 

3. Cheryl L. Bowman, Steven M. Geng, Janis M. Niedra, Ali Sayir, Eugene E. Shin, James K. Sutter, 

and Lanny G. Thieme, “Materials-of-Construction Radiation Sensitivity for a Fission Surface Power 

Convertor,” paper 2035 in Proceedings of Space Nuclear Conference 2007 (ANS), Boston, MA, 

June 24–28 2007. 

4. Omar Mireles and Samim Anghaie, “Review of Potential Degradation Mechanisms for the 

Upcoming Experimental Demonstration of Radiation Effects On the Performance of a StirlingAlternator Converter,” Proceedings of Nuclear and Emerging Technologies for Space 2009, 

CP208282, Vol. 1, ANS, Atlanta, GA, 2009. 

5. Bruce Chalmer, Physical Metallurgy, John Wiley & Sons, Inc., New York, NY, 1962, p. 314. 

6. Miyuki Hagiwara and Tsutomu Kagiya, “High Energy Degradation and Stabilization of Polymers” 

in Degradation and Stabilization of Polymers, H.H.G. Jellinek, ed., Elsevier Science Publishing Co., 

New York, NY, 1983. 

7. M. Dole, in The Radiation Chemistry of Macromolecules II, edited by M. Dole, Chap. 9, Academic 

Press, 1973, pp. 167–178. 

8. Akihiro Oshima, Shigetoshi Ikeda, Tadao Seguchi, and Yoneho Tabata, “Improvement of Radiation 

Resistance for Polytetrafluoroethylene (PTFE) by Radiation Crosslinking,” Radiat. Phys. Chem., 

49(2), 1997, pp. 279–284. 

9. Shigehiko Tabuse, et al., “Radiation protection effects by addition of aromatic compounds to 

n-dodecane,” Radiat. Phys. Chem., 62(1), 2001 pp. 179–187. 

10. EI-Sayed A. Hegazy, et al., “Irradiation effects on aromatic polymers: 1. Gas evolution by gamma 

irradiation,” Polymer, 33(14), 1992, p. 2897–2903. 

11. Robert C. O’Handley, Modern Magnetic Materials: Principles and Applications, (John Wiley & 

Sons, Inc., 2000), chapter 10. 

12. R.D. Brown and J.R. Cost, “Radiation-Induced Changes in Magnetic Properties of Nd-Fe-B 

Permanent Magnets,” IEEE Trans. on Magnetics, 25(4), 1989, pp. 3117–3120. 

13. J.R. Cost and R.D. Brown, “Sm-Co Permanenet Magnets: Effects of Fast Neutron Irradiation,” 

Metall. Trans. A, 21A, 1990, pp. 1817–1821. 

14. C.H. Chen, et al., “The Effect of neutron Irradiation on Nd-Fe-B and Sm2Co17 Based high 

Temperature Magnets,” IEEE Trans. on Magnetics, 41(10), 2005, pp. 3832–3834. 

15. D.I. Gordon and R.S. Sery, “Nuclear Irradiation Effects on Ferromagnetic Core Materials,” in Solid 

State Physics in Electronics and Telecommunications, Vol. 4, M. Désirant and J.L. Michiels, eds., 

Academic Press, New York, NY, 1960. 

16. Omar R. Mireles, “Experimental Demonstration Of Radiation Effects On The Performance Of A 

Stirling-Alternator Convertor And Candidate Materials Evaluation,” Ph.D. Dissertation, Univ. of 

Florida, May 2010. 

17. Omar R. Mireles, E. Eugene Shin, Cheryl L. Bowman, and Latha Vasudevan, “Mixed Neutron and 

Gamma-Ray Testing of Stirling-Alternator Candidate Organic Materials,” paper 769917 in 

Proceedings of 8th International Energy Conversion Engineering Conference (AIAA), Nashville, 

TN, July 25–28, 2010. 

18. Omar Mireles, Cheryl Bowman, and Ross Radel, “Radiation Testing of a Stirling-Alternator 

Convertor,” Proceedings of the American Nuclear Society Conference, CP211099, Vol. 1, ANS, 

Washington D.C., 2009. 

19. Omar R. Mireles, E. Eugene Shin, and Cheryl L. Bowman, “Post-Irradiation Evaluation of the 

Stirling Alternator Radiation Test Article,” Paper 10008 in Proceedings of International Congress 

on Advances in Nuclear Power Plants (ICAPP’10, ANS), San Diego, CA, June 13–17, 2010. 
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Radiation Specifications for Fission Power Conversion Component Materials 

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Bowman, Cheryl, L.; Shin, E., Eugene; Mireles, Omar, R.; Radel, Ross, F.; Qualls, A., Louis 

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13. SUPPLEMENTARY NOTES

Ross F. Radel, Phoenix Nuclear Labs, Middleton, Wisconsin 53562 (formerly of Sandia National Laboratories). 

14. ABSTRACT

NASA has been supporting design studies and technology development that could provide power to an outpost on the moon, Mars, or an 

asteroid. One power-generation system that is independent of sunlight or power-storage limitations is a fission-based power plant. There is a 

wealth of terrestrial system heritage that can be transferred to the design and fabrication of a fission power system for space missions, but 

there are certain design aspects that require qualification. The radiation tolerance of the power conversion system requires scrutiny because 

the compact nature of a space power plant restricts the dose reduction methodologies compared to those used in terrestrial systems. An 

integrated research program has been conducted to establish the radiation tolerance of power conversion system-component materials. The 

radiation limit specifications proposed for a Fission Power System power convertor is 10 Mrad ionizing dose and 5×1014 neutron/cm2

fluence for a convertor operating at 150 °C. Specific component materials and their radiation tolerances are discussed. This assessment is for 

the power convertor hardware; electronic components are not covered here. 

15. SUBJECT TERMS

Polymers; Radiation effects; Gamma rays; Neutron irradiation 

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