Study of a Tricarbide Grooved Ring Fuel Element for Nuclear Thermal Propulsion

Brian Taylor1, Dr. Bill Emrich1, Dr. Dennis Tucker1, Marvin Barnes1, Nicolas Donders2, and Kelsa Benensky3

1NASA Marshall Space Flight Center, Huntsville, AL, 35812 2Kettering University, Flint, MI, 48504 3University of Tennessee, Knoxville, TN, 37996

Abstract the U.S. Atomic Energy Commission and NASA. Sev- eral engines were built and tested under this program Deep space exploration, especially that of Mars, is on the in order to develop this technology for a manned mis- horizon as the next big challenge for space exploration. sion to Mars. This work ran until the early 1970’s.[1, 2] Nuclear propulsion, through which high thrust and effi- The reactors of the NERVA engines consisted of fuel el- ciency can be achieved, is a promising option for decreas- ements formed into hexagonal rods with cylindrical flow ing the cost and logistics of such a mission. Work on nu- passages through which propellant was flowed. The nu- clear thermal engines goes back to the days of the NERVA clear fission reactions heated the material in the rod and program. Currently, nuclear thermal propulsion is under the heat was transferred to the propellant. This type of development again in various forms to provide a supe- nuclear propulsion is called nuclear thermal propulsion rior propulsion system for deep space exploration. The (NTP). Later, particle bed reactors were considered in authors have been working to develop a concept nuclear a defense project called Timberwind. This reactor ge- thermal engine that uses a grooved ring fuel element as an ometry flowed propellant through a bed of spherical fuel alternative to the traditional hexagonal rod design. The particles to greatly increase surface area and thus heat authors are also studying the use of carbide fuels. The transfer. This design showed promise for significantly im- concept was developed in order to increase surface area proving thrust to weight ratios and specific impulse com- and heat transfer to the propellant. The use of carbides pared to the NERVA engines. It suffered; however, from would also raise the operating temperature of the reactor. nuclear thermal instabilities.[2] It is hoped that this could lead to a higher thrust to weight nuclear thermal engine. This paper describes the model- ing of neutronics, heat transfer, and fluid dynamics of this alternative nuclear fuel element geometry. Fabrica- tion experiments of grooved rings from carbide refractory metals are also presented along with material characteri- zation and interactions with a hot hydrogen environment. Results of experiments and associated analysis are dis- cussed. The authors demonstrated success in reaching de- sired densities with some success in material distribution and reaching a solid solution. Future work is needed to improve distribution of material, minimize oxidation dur- ing the milling process, and define a fabrication process that will serve for constructing grooved ring fuel rods for Figure 1: Illustration of a Grooved Ring Fuel Element large system tests. Recently, there has been a renewed effort to develop I Foreword NTP as NASA has been working seriously toward manned Mars mission planned for the 2030s. Much of the fuel fab- Propulsion systems that derive their power from nuclear rication and engine modelling efforts have been focused on reactions have been conceptualized for many decades. Se- the tungsten and graphite based fuel elements in hexago- rious efforts to develop this technology were made in both nal rod geometries. Cermet and Graphite based fuels are the United States and the Soviet Union. The most well being studied for fuel element fabrication. This work has known program was that of the Nuclear Engine for Rocket been moving toward the hexagonal rod geometry. The au- Vehicle Application (NERVA) that was a joint effort of thors of this paper have been working on a center innova-

1 ANS NETS 2018 Nuclear and Emerging Technologies for Space Las Vegas, NV, February 26 March 1, 2018, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2018) tion fund project to investigate an alternative fuel element geometry proposed by Dr. Bill Emrich at Marshall Space Flight Center, a co-author of this paper. This concept is centered on the idea of increased surface area and heat transfer to the propellant, much as particle bed reactors do, while eliminating the thermal instabilities by creating a defined flow path. This concept is known as the grooved fuel ring element. The idea is to build a fuel element from a stack of washer like rings. Each ring has grooves cut into the surface to allow propellant, constrained by an outer structure, to flow through the grooves to the cen- ter where it can flow down and out of the reactor. This provides a large increase in surface area which is directly proportional to heat transfer. These elements would be Figure 2: Cross Section of Several Refractory Metals put together to make up a reactor much in the same was as traditional elements do. Figure 1 illustrates the con- cept. out was developed for different material combinations for In addition to the alternative geometry, the authors are a given power and thrust level. This drove the selec- investigating the use of mixed carbide fuels. These fuels tion of materials for fabrication experiments and served are composed of fissile uranium carbide (UC) the fuel as a guide for how the manufactured carbides would affect and additional refractory metal carbides (e.g. niobium criticality and operating temperature. carbide (NbC) and zirconium carbide (ZrC)) in solid so- lution to increase the melting temperature of UC. The Soviet Union conducted tests of carbide fuels and reached II.A Material Selection reactor temperature greater than 3000 K.[1] A combina- The selection of carbides is a trade between high melting tion of carbide materials has the potential to allow max- temperature properties and low neutron absorption cross imum fuel operating temperatures in the vicinity of 3500 sections. While the highest melting temperature are de- K. This is in contrast to the lower temperature limita- sired, the neutron cross section determines the quantity tions of other fuel forms. The use of carbide fuels could of uranium (UC) required for the reactor to be critical. allow the reactor to run at higher temperature and pro- The melting point of UC is lower than the refractory car- vide more thrust and specific impulse to the propulsion bide compounds of interest; therefore, increasing the UC system. content lowers reactor temperature limits. Several refrac- In order to develop this alternate geometry carbide fuel tory metals were chosen as the most promising material element, data is needed to develop the fabrication pro- candidates based on past research. cess, understand the fuels performance in a hot hydrogen Figure 2 compares the cross section of Hafnium, Tan- environment and characterize material properties. This talum, Niobium and Zirconium. One can see a significant has been the primary goal of the work discussed in this difference between the materials. While hafnium carbide paper. Past work has been studied to provide the founda- (HfC) and tantalum carbide (TaC) have the higher melt- tion for this work.[3, 4, 5, 6] Additionally, modeling has ing temperatures, this would be countered by the need been done to understand the neutronics, fluid, and heat for more UC. NbC and ZrC were chosen as the primary transfer processes in such a reactor. Model results have candidates of interest due to their lower absorption cross been used to guide material selection and fabrication ex- section. Since solid solutions containing ZrC, NbC will periments. Processes and results to date are discussed in capture fewer neutrons, less UC is needed within the re- this document. actor. Reducing UC content will ultimately maximize the operating temperature of the fuel by allowing for the II Modeling of the Neutronics for a Grooved highest melting points. Fuel Ring Reactor II.B Reactor Model In order to guide the selection of materials for fabrication tests, the reactor concept must be developed. The crit- The reactor model was built using MCNP, a common icality and operation of the reactor is highly dependent nuclear physics code. The reactor model employs the upon material density, material selection, moderating ma- grooved ring element design in a cylindrical reactor. Pro- terials, structure, etc. The model was developed with the pellant is assumed to be hydrogen. Beryllium is used a Monte Carlo N-Particle coede (MCNP). A concept lay- moderator in the core and a reflector at the boundary.

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Figure 3: (U-Zr-Nb)C Fuel Reactor with fuel to modera- tor ratio of 0.261 Figure 5: Uranium Carbide Requirements for Criticality

ing the amount of UC needed is important to raising the operational temperature of the reactor. While the plot in Figure 5 is done for an enrichment of 95%, a better estimate of enrichment may be determined with further analysis. The less than theoretical density fuel and high neutron cross sections of the other carbides lead toward higher enrichment to counter these effects. Also, less uranium is desired to take advantage of the high melting temperatures of the other carbides. The lower uranium loading as compared with other fuel forms Figure 4: (U-Zr-Ta)C Fuel Reactor with fuel to modera- leads to higher enrichment. It may be, however, that if tor ratio of 2.95 the neutron spectrum is adjusted correctly by varying the amount of moderator in the fuel element, it might be that you can still design to 20% enrichment. The concept reactor was sized for 25,000 N of thrust with a power output of 8 kW/cm3. Two combinations of carbides were modeled. The first III Thermal/Fluid Model configuration used a fuel mixture of UC, ZrC and NbC. The thermal and fluid physics in a grooved ring fuel el- The second configuration changed the mixture to UC, ement were modeled in order to better understand the ZrC and TaC. The reactivity of the reactor models were conditions needed for the propulsion system. Comsol was analyzed to determine the amount of fuel required to op- used to create this model. Thermal and fluid physics were erate effectively. The first carbide mixture requires a fuel coupled together in this model of a representative fuel el- to moderator ratio of 0.261 while the second carbide mix- ement. Overall, the authors were looking for the pressure ture requires a much higher fuel to moderator ratio of differential driving the flow that would be ideal for heat- 2.95. Side and top views of these reactor configurations ing the propellant as well as determining the number of are shown in fig. 3 and fig. 4 grooves needed to reach a reasonable mass flow rate. Con- Additionally, calculations were carried out to determine ditions were adjusted based on the concept engine size of the impact of density on the required quantity of Uranium 25,000 N and 8 kW/cm3. fuel. The flow paths through the grooves and center of the rings reduce the available volume of fissionable mate- III.A Model Description rial. Also, the manufacturing process will result in small amounts of porosity. A small amount of porosity is ac- The model attempts to represent a fuel element consist- tually required to allow room for the fission products. ing of a stack of grooved fuel rings enclosed by a struc- This is represented in terms of the percent of theoretical ture. The stack is limited to two rings in order to reduce density. Theoretical density is 100% fuel in the ring vol- computational time. This is expected to be adequate to ume. This is plotted in fig. 5. The fraction of the carbide characterize the physics of the model. The outer struc- mixture that is UC is plotted against the fraction of the- ture consists of a beryllium hexagon. The central stack oretical density. One can see that the mixture containing of rings are assigned the material ZrC. As of the writ- TaC requires much larger amounts of uranium than the ing of this report the required properties of the carbide NbC mixture. As the density drops below 1, the amount mixture have not been measured. This will be done in of uranium needed to achieve criticality climbs. Reduc- order to update the model at a future date. The fluid is

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assigned as H2 and given an inlet temperature of 500 K. Initial temperatures are set near the inlet temperature, except for the fuel which is set to 3000 K. The pressure differential between inlet and outlet is set to 4 psi. The representative picture of the model and the corresponding mesh can be seen in figs. 6 and 7.

Figure 8: Top Down View of Temperature Distribution Slice

Figure 6: Wire Frame Model

III.B Analysis of Results Several runs of the model were performed to determine the conditions required to drive the flow through the grooves such that the propellant is heated to approxi- Figure 9: Side View of Temperature Distribution Slice mately 3000 K. The pressure differential for this geometry is 4 psi. Ideally the propellant should flow fast enough to remove enough heat that the fuel does not over heat while order of several hundred m/s and the flow remains lam- not moving so fast that the temperature at the outlet is inar. There is higher velocity around the corners of the too low. Figure 8 and fig. 9 display the temperature dis- inlet prior to filling the outer passage and passing through tribution over slices of the element cut horizontally and the grooves. The flow is seen to accelerate through the vertically. One can see in Figure 8 the flow is heated as grooves and mix in the central passage. As mentioned it passes through the grooves and exits near the max fuel before a longer more realistic stack will allow for more temperature of 3000 K. One can examine fig. 9 to see how mixing and uniform conditions at the outlet. the central passage heats up from the flow through the grooved rings. Since this model is limited to two rings, IV Fabrication of a Grooved Ring Fuel Element the exit temperature is low near the axis due to the heat sink through the top of the stack to the cold propellant. The optimal methods for constructing carbide fuel ele- A more realistic and taller stack would reach a more uni- ments is not well understood. Refractory carbides are form exit temperature as more flow is heated and a longer some of the highest melting temperature compounds path exists for mixing. making their production very difficult and energy inten- The velocity distribution is shown in fig. 10 and fig. 11 sive. In previous carbide fuel development programs, pro- as horizontal and vertical slices. Max velocities are on the duced fuels were very brittle and susceptible to cracking.

Figure 7: Mesh Figure 10: Top Down View of Velocity Distribution Slice

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Figure 11: Side View of Velocity Distribution Slice

It is important to establish the baseline process for the Figure 12: Direct Current Sintering Machine at Marshall manufacture of solid solution carbides prior to proceed- Space Flight Center ings with fuel element construction. To address this, the authors conducted a series of tests to determine the best bution is unknown outside of vendor specifications. This processing conditions for manufacture. The authors chose occurred due to milling equipment being out of service to first employ direct current sintering (DCS) to create while being repaired. Some screening of particulate was sample carbides. Hot isostatic press was determined to done in later experiments to reduce the maximum par- be a possible additional step to increase density. After ticle size and limit the variability of particle diameter. sintering the sample pieces were analyzed to determine The micro mill became operational during the summer of density and material distribution. Control variables were 2017. At this point a few samples of powder were milled adjusted from test to test in order to work toward an to reduce particle size prior to the sintering process. The optimal density and material distribution. milling process is intended to improve material distribu- tion through the material. IV.A Uranium Surrogate and Material Selection The first step in the processes of learning how to make the IV.C Direct Current Sintering Experiments carbide fuels was selecting the appropriate materials. As The fabrication of carbide material samples were con- discussed earlier, the neutron cross section has a signifi- ducted using a DCS machine. This can be seen in fig. 12. cant impact upon the engine operation. For this reason, Powder is loaded into a die prior to being placed in the NbC and ZrC were chosen for fabrication experiments. A machine. Once installed, the DCS places the sample un- key component to the carbide fuel is of course the UC. der a pressurized inert atmosphere. It then runs a large The use of uranium, however, comes with regulatory bur- current through the powder to raise its temperature. The dens that translate into additional cost. It was practical machine can be controlled in several ways. Pressure, rise to use a surrogate in place of uranium to reduce cost. A time, dwell time (at max temperature), and cooling rate surrogate material with a similar crystal structure would can all be controlled. These variables were adjusted in behave in a similar manner as uranium carbide in the for- an effort to work toward a sample with high density and mation of the material, but offered significant savings and good material distribution. ease of use. Vanadium carbide (VC) was chosen as a sur- rogate since it is relatively inexpensive and forms a similar IV.C.1 Density of Samples crystal structure. The materials tested in the fabrication experiments were NbC, ZrC and VC. The quantities used High densities were achieved earlier than anticipated. were approximately 61% ZrC, 31% NbC and 8% VC by The authors were successfully able to achieve densities weight. up to 98%. This is above the required density which is expected to be approximately 95% to accommodate fis- IV.B Powder Preparation sion product production. Dwell time, cooling rate, and sintering temperature were critical variables to achieving The carbide powders as provided by the vendors were of a density goals. Sintering temperatures of approximately nano particle size that varied within some diameter range 1600 C were adequate to reach high densities (>95%TD). per quality specifications. Early fabrication experiments The carbides also achieved high densities at fast cooling in the DCS machine were done with raw powder as pro- rates. In fact the machine was turned off for a maximum vided by the manufacturer. This created some degree of cooling rate of approximately 200 degC/min. The highest uncertainty in the results since exact particle size distri- densities were achieved at this cooling rate. Additionally

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Table 1: X-Ray Spectroscopy Analysis of fig. 16 Figure 13: Percent Theoretical Density vs Sintering Tem- perature

Figure 14: Percent Theoretical Density vs Cooling Rate Figure 16: X-Ray Spectroscopy of First DCS Sample dwell times on the order of 20 minutes were adequate to give the best results. Plots of these variables can be X-ray spectroscopy is employed to identify the materi- seen in fig. 13, fig. 14, and fig. 15. These figures plot the als in the samples and their distribution throughout the percent of theoretical density as a function of sintering crystal structure. temperature, cooling rate, and dwell time respectively. Early samples showed large grain boundaries. In these Note that there is a good degree of variability believed to early samples the particle distribution is less than ideal. be in large part due to the variation in particle size. Material can be seen to be clustering together. This is be- lieved to be in large part due to the variability in particle IV.C.2 Material Distribution size with a non-optimal direct current sintering process contributing. The x-ray spectroscopy at one location of In order to achieve carbide fuel elements with uniform the first sample fun at 1600 degrees Celsius for 10 minutes properties and best performance the distribution of the with a 100 C/min cooling rate can be seen in fig. 16 with carbide elements must be as close as possible to uniform. table 1 showing the amount of material at the various Particle distribution of test samples were studied in ad- locations. dition to sample densities. Measurements were made After analyzing several samples the authors decided to using a scanning electron microscope and x-ray spec- sift the materials through a screen in order to limit the troscopy. The scanning electron microscope allows one maximum particle size. This limited the particle size to to see the grain structures formed in the carbide sample. 45 micron. Material screening showed improvement to the particle distribution. This can be seen in fig. 17 and table 2.

IV.D Cutting Grooves Important to the manufacturing process is the method for creating the grooves in the carbide rings as well as the center hole. Two tools are being investigated for use in this effort; a diamond wire saw and a water jet. The saw was tested on a carbide sample and successfully cut several grooves through the material. The saw has sev- Figure 15: Percent Theoretical Density vs Dwell Time eral limitations however. The saw is limits the groove

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diameter to the width of the blade. It is difficult to work with due to the small size of the carbide rings. Also, it is not effective at cutting the center hole. The water jet is thought to be a promising candidate. Current water jet capabilities at Marshall Space Flight Center allow the groove width to be cut as small as 0.03 inches. The water jet is capable of cutting curves and can cut out the center hole. Adjusting the pressure should allow the operator to cut the grooves without cutting all the way through the ring. Water jet tests are planned future work. Test cuts will be performed on a carbide sample. This will be fol- lowed by an attempt to produce several full size grooved fuel rings with the DCS and water jet.

V Carbide Material Characterization

Characerizing the material properties of the carbides used in this study is necessary to understand their performance in a reactor environment. We have reviewed previous work after reviewing available literature and have built upon past work. The authors are working to measure the thermal diffusivity of the carbide mixtures. Also, the Table 2: X-Ray Spectroscopy Analysis of fig. 17 material samples created using DCS are being tested in a hot hydrogen environment (T > 2000K). Testing in a hot hydrogen environment is used to verify the structural integrity and chemical stability of the materials at rele- vant temperatures while exposed to the proposed hydro- gen propellant.

V.A Thermal Diffusivity Thermal diffusivity measurements were attempted. Two samples were measured by heating the material with a light source to a set temperature. The material is then allowed to cool back to room temperature. The cooling rate is measured and used to determine thermal diffu- sivity. Multiple measurements were taken and average was found. Thermal diffusivity numbers were obtained and plotted for the two samples. For reasons unknown the samples disintegrated at relatively low temperatures. Testing in a hot hydrogen environment showed survivabil- ity at much higher temperatures, so this result is as of yet unexplained. It could possibly be the result of a preex- isting fracture or other outside factor. The data obtained is presented in fig. 18. Future work is planned to take new measurements of thermal diffusivity at much higher temperatures. Figure 17: X-Ray Spectroscopy of Carbide Sample after Screening to 45 microns V.B Hot Hydrogen Environment Testing Hot hydrogen environmental testing was conducted in the Compact Fuel Element Environmental Test (CFEET) system at NASA Marshall Space Flight Center (fig. 19). CFEET has a 50 kW induction power supply and two- color pyrometers for temperature measurements up to

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the presence of unidentified peaks post-CFEET testing. Further analysis is needed to verify if unidentified peaks are due to the formation of free carbon, Nb2C, or other lower melting temperature compounds.

Figure 18: Thermal Diffusivity Measurements of Two Carbide Samples

3000 ◦C. It is designed to flow hydrogen across subscale fuel materials for testing at high temperatures for up to ten hours.

Figure 20: X-Ray Spectroscopy Analysis Before and After Testing in CFEET

Additional CFEET tests were conducted once the mi- cro mill was operational. This allowed for carbide samples to be made from milled powders in an effort to produce improved distribution in the sintered carbide pieces. It was observed that cracking was evident in the sintered carbides for powders that had been milled. Two tests were run on milled carbide units. Each test was run for 45 minutes. The peak operational temper- atures were 2500 K for the first test and 2750 K for the second. The formation of blisters as seen in fig. 21 were observed in the samples following exposure to the hot hydrogen environment. The cracks were also seen to worsen(fig. 22). EDX analysis revealed large quantities of carbon in the blisters. The samples were also shown to contain large amounts of Zirconium Oxide (ZrO2). The formation of oxides in the carbides is likely the cause of the formation Figure 19: Compact Fuel Element Environmental Test of these two features. One can see the XRD analysis, system shown in fig. 23 of the tested units, that spikes corre-

An initial sample was run at a relatively low tempera- ture to verify if the sample would fall apart similar to observed in the thermal diffusivity testing. The sam- ple maintained structural integrity when exposed to a maximum temperature of 2000 K for 30 minutes. Sub- sequently, three samples were run for 30 minutes (rough timescale of a single engine burn on a Mars mission) at 2250 K. X-ray diffraction (XRD) analysis appears to show the tricarbides moving toward a solid solution. Solid so- lutions refer to an ideal mixture of the tricarbides as op- posed to discreet clusters of compacted bianary carbide powders. This is indicated by the shifting toward a sin- gle defined peak at an intermediate 2θ values. This XRD data can be seen in fig. 20. There does however, exist Figure 21: Carbon rich blister formation

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Figure 22: Crack formation in carbide samples following Figure 24: XRD Analysis Prior to Milling Carbide Pow- CFEET Test der sponding to ZrO2 are present.

Figure 25: XRD Analysis following Carbide Powder Milling

element of a nuclear thermal rocket. The team ran the initial calculations and models to drive the carbide fuels Figure 23: X-Ray Spectroscopy Analysis Before and After experiments. The models built will serve as a foundation Testing in CFEET upon which more thorough modeling can be completed following additional data collection. Zirconium oxide formation hinders the construction of Direct current sintering experiments were shown to be a structurally sound high melting temperature carbide quite effective at achieve high density carbide samples fuel element. For this reason the authors are interested and reasonable material distribution. Testing of sintered in determining the root cause of the oxide formation and carbides in a hot hydrogen environment also showed posi- modifying the fabrication process to prevent its forma- tive results. The tests showed the carbide samples moving tion. toward a solid solution which is optimal for fuel design. The powders were studied before and after the milling They also showed the carbide could withstand the hot process. It was thought this was a likely cause of the hydrogen environment when there was low oxide forma- oxide formation since little oxide was seen in the pieces tion. Although unmilled sintered tricarbide samples per- that were not milled. XRD analysis was performed on formefed well when exposed to hydrogen at high tempera- powders before and after milling. The results of the before tures, oxidation of the carbide powders during the milling and after milling XRD analysis are plotted in fig. 24 and process resulted in severe cracking and blister formation fig. 25. The key material response points are plotted to in the hot hydrogen environment. illustrate materials present. The next step in developing the fabrication process is to One can see in fig. 24 that there is no oxide formation. address the oxidation of the carbide powders during the The points indicating ZrO2 are not present. In contrast to milling process. The milling process is employed to im- this fig. 25 indicates the formation of ZrO2 in the milled prove material distribution throughout the sintered car- carbide powder. This indicates the milling process results bide mixture by reducing particle size. Oxidation must in freeing carbon and the formation of ZrO2. be prevented in order to develop carbide fuels that will maintain structural and chemical integrity in a hot hy- VI Conclusions drogen environment as will be seen in a nuclear thermal rocket. This project set out to begin development of the fab- Additionally, experiments need to be run to determine rication process of carbide fuels for a grooved ring fuel a method for moving the carbide mixtures toward a solid

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solution. Heating in a graphite furnace under vacuum References may achieve this and is planned for testing. Machining of the grooves and central passage to form the carbides into [1] I. I. A G LANIN, “Selecting and using materials for grooved rings for the construction of a full size element a nuclear rocket engine reactor,” pp. 305–318. must also be addressed. Tests are planned for cutting the [2] W. EMRICH, Principles of Nuclear Rocket Propul- sintered carbides in a water jet. Finally, a full size pro- sion, Elsevier, 1st ed. totype grooved ring fuel element needs to be built and tested in the Nuclear Thermal Rocket Element Environ- [3] T. KNIGHT, “PROCESSING OF SOLID SO- ment Simulator (NTREES) for performance assessment. LUTION, MIXED URANIUM/REFRACTORY The team believes the results of the work conducted METAL CARBIDES FOR ADVANCED SPACE in FY17 to be encouraging. The carbide fuels and the NUCLEAR POWER AND PROPULSION SYS- grooved ring fuel element are a promising fuel form and TEMS,” . design geometry for future nuclear thermal rockets. [4] L. L. LYON, “Performance of (u, Zr)C-Graphite (Composite) and of (U, Zr)C (Carbide) Fuel Elements A Nomenclature in the Nuclear Furnace 1 Test Reactor,” .

K = Kelvin [5] M. A.-S.-F. S. A. GHAFFARI, “Diffusion and solid so- lution formation between the binary carbides of TaC, NbC = Niobium Carbide HfC and ZrC,” pp. 180–184.

ZrC = Zirconium Carbide [6] S. A. TRAVIS KNIGHT, “Processing and fabrication of mixed uranium/refractory metal carbide fuels with ZrO2 = Zirconium Oxide liquid-phase sintering,” pp. 54–60.

TaC = Tantalum Carbide

HfC = Hafnium Carbide

UC = Uranium Carbide

VC = Vanadium Carbide

DCS = Direct Current Sintering

H2 = Diatomic Hydrogen

XRD = X-Ray Diffraction

EDX = Energy Dispersive X-Ray Spectroscopy

NTREES = Nuclear Thermal Rocket Element Envi- ronment Simulator

B Acknowledgments

The authors would like to thank the leadership at Mar- shall Space Flight Center, the Space Technology and Mis- sion Directorate, and the Center Innovation Fund pro- gram for their support and funding of this work. The authors would also like to thank the contribution and support of the many people within the propulsion and materials engineering departments at MSFC. Kelsa Be- nensky is supported by a NASA Space Technology Re- search Fellowship

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