The plasma spheroidization system (PSS) is the first process in the development of tungsten-uranium dioxide (W-UO2) fuel cermets. The PSS process improves particle spherocity and surface morphology for coating by chemical vapor deposition (CVD) process. Angular fully dense particles melt in an argon-hydrogen plasma jet at between 32-36 kW, and become spherical due to surface tension. Surrogate CeO2 powder was used in place of UO2 for system and process parameter development. Particles range in size from 100 - 50 microns in diameter. Student s t-test and hypothesis testing of two proportions statistical methods were applied to characterize and compare the spherocity of pre and post process powders. Particle spherocity was determined by irregularity parameter. Processed powders show great than 800% increase in the number of spherical particles over the stock powder with the mean spherocity only mildly improved. It is recommended that powders be processed two-three times in order to reach the desired spherocity, and that process parameters be optimized for a more narrow particles size range. Keywords: spherocity, spheroidization, plasma, uranium-dioxide, cermet, nuclear, propulsio

Cavender, Daniel P.

Frendi, Abdelkader

Mireles, Omar R.

NASA Technical Reports Server

Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
Design of a Uranium Dioxide Spheroidization System 
Daniel P. Cavender
1
, Omar R. Mireles
1
, Abdelkader Frendi
2
 
1
NASA Marshall Space Flight Center, Huntsville AL 35812 
 
2
University of Alabama-Huntsville, Huntsville AL 35899 
 
Abstract. The plasma spheroidization system (PSS) is the first process in the development of tungsten-uranium 
dioxide (W-UO2) fuel cermets. The PSS process improves particle spherocity and surface morphology for coating 
by chemical vapor deposition (CVD) process. Angular fully dense particles melt in an argon-hydrogen plasma jet at 
between 32-36 kW, and become spherical due to surface tension. Surrogate CeO2 powder was used in place of UO2 
for system and process parameter development. Particles range in size from 100 – 50 microns in diameter. Student’s 
t-test and hypothesis testing of two proportions statistical methods were applied to characterize and compare the 
spherocity of pre and post process powders. Particle spherocity was determined by irregularity parameter. Processed 
powders show great than 800% increase in the number of spherical particles over the stock powder with the mean 
spherocity only mildly improved. It is recommended that powders be processed two-three times in order to reach the 
desired spherocity, and that process parameters be optimized for a more narrow particles size range.  
Keywords: spherocity, spheroidization, plasma, uranium-dioxide, cermet, nuclear, propulsion  
INTRODUCTION 
Conventional chemical propulsion systems alone cannot provide the energy density and efficiency needed for 
successful long duration manned and robotic space flight missions; therefore new propulsion technologies must be 
leveraged if mankind is to continue pioneering the frontier of space. Nuclear fission type power generation and 
propulsion systems are a necessary evolution in realizing near term space exploration goals. NASA’s MSFC is 
developing tungsten-uranium dioxide fissile fuel rod samples for solid-core nuclear thermal rocket (NTR) 
technology applying new manufacturing and material processing technologies to build on the success of Rover-
NERVA research.  
An NTR’s solid core may be made of an arrangement of bundled hexagonal fuel rods2. These fuel rods are made of 
highly enriched UO2 fuel powders encapsulated in Tungsten (W) to form a cermet (ceramic-metallic) matrix 
composite. During the 1960’s and 1970’s, the joint NASA and Atomic Energy Commission’s (AEC) Rover-
NERVA program spent extensive time on the development of graphite based composite fuels for NTR engines
10
. 
Subsequent efforts such as the ANL 200 and GE 710 programs went on to investigate W-UO2 cermet fuel forms.  
Fissile fuel loss was an issue heavily researched with no simple solution. The prevalent theory, according to 
Haertling
5
, was that UO2 fissile fuel loss occurs due to mechanical failures of the fuel rod’s tungsten cladding caused 
by formation of UH3 resulting in large volumetric expansion. Hydrogen penetrates into the cermet matrix by bulk or 
grain boundary diffusion at high temperatures and reacts with the UO2 producing oxygen deficient UO2 as seen in 
Eq. (i). Upon cooling, substoichiometric UO2-x dissociates releasing free U as seen in Eq. (ii).  
                    (i) 
        (   )       (ii) 
             (iii) 
https://ntrs.nasa.gov/search.jsp?R=20130011185 2019-08-31T00:17:20+00:00Z
Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
The free uranium, which has a melting point well below 
the NTR core temperature, migrates along grain 
boundaries where it reacts with an excess of hydrogen and 
can form UH3 within the cermet; Eq.(iii). Rover-NERVA 
era investigations found that fissile fuel loss could be 
mitigated through process control techniques to improve 
particle density and purity, particle packing density and 
particle surface morphology. Spherocity, density, purity, 
and surface morphology of UO2 powder can be improved 
using thermal plasma processing.  
EXPERIMENT DESIGN 
The PSS is a 14 foot tall vertically oriented water cooled 
stainless steel vacuum chamber mounted inside of a 
carbon steel frame with a commercial spray plasma torch 
mounted inside the top of the chamber. A nearby control 
console manages the plasma forming gases, DC power 
supply for the plasma gun, and chamber vacuum level. 
Powder is metered and transported to the plasma gun by a 
commercial powder feeder. Operations require two 
operators.  
The three 12 inch diameter 304 stainless steel (SS) pipe 
sections that compose the 100 inch long chamber have an 
internal volume of 7.5 cubic feet. The chamber segments 
are mated together with Flexitallic 347 SS spiral-wound 
gaskets between ANSI B16.5 class 150 series weld neck 
flanges. Each chamber segment has 304 SS sheets rolled 
and welded together on each chamber segment seated on 
the weld neck of the flanges leaving a sealed volume in 
which cooling water flows.  
The Praxair model SG-100 multi-mode plasma spray gun 
is mounted on two brass feedthroughs that served as rigid 
mounts and supplied deionized cooling water to the 
anode, cathode, and gun face plate. The gun must be 
electrically isolated; therefore, the brass feedthroughs are 
mounted on a non-conductive garolite plate machined to 
mount to the PSS chambers top plate CF675-HN. Argon 
is the primary arc forming gas and a secondary gas of 
either hydrogen or helium was used to increase the heat 
content and velocity of the plasma. The SG-100 operates 
at maximum power level of 100 kW and must be water 
cooled
9
. The SG-100 is powered by a Halmer Robicon 
120 kW power supply and a deionized water supply pump 
with reservoir supplies cooling water. The plasma gun 
starter current is provided by a Praxair HF-2000 high 
frequency starter box.  
Stock powder and plasma-forming gases are routed to the SG-100 by 0.25 inch SS tubes inside of the chamber and 
0.25 inch teflon tubes outside of the chamber. Powder can either be internally or externally injected into the plasma 
jet though external injectors mitigate the risk of powder clogging inside of the gun.  
Figure 1. NASA MSFC UO2 PSS 
Figure 2. Praxair SG-100 setup 
Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
Figure 2. PSS EGMS Diagram 
The top chamber segment (TCS) has two 600CF-HN 
(con-flat half nipples) clocked 90 degrees apart where 
Quartz viewports are fastened allowing the plasma torch 
to be viewed from two perspectives, which offer relevant 
operator based information about how the powder 
interacts with the plasma. The bottom chamber segment 
(BCS) reduces the chamber’s internal diameter from 12 
inches to 2 inches. The conical transition was machined 
from a single billet of 304 SS to a 30 degree half angle to 
allow the powders to efficiently flow towards the center 
and bottom of the chamber to be collected in a removable 
hopper. The transition matched the wall thickness of the 
12 inch pipe segments. The reduced end of the transition 
is machined to accept a counter bored 450CF flange. An 
auxiliary threaded 304 SS coupling is welded onto the 
bottom of the BCS to allow the PSS chamber water 
jackets to be drained. 
The Effluent Gas Management System (EGMS) has an 
Edwards RV8 rotary vane pump, two custom designed 
ASME AG-1 compliant HEPA filter assemblies with pre-
screens, a 1 inch Swagelok 0.334 psig relief valve, and a 
vacuum control valve. The EGMS mounts to the 600CF-
NH on the middle chamber segment of the PSS. Chamber 
vacuum level is measures by analogue vacuum dial 
gauge. The vacuum level is controlled by a manual 
vacuum pump bypass valve. The PSS typically operates at 
350 - 600 torr. Typical total gas flow into the chamber 
from the gun and powder feeder is 2 - 4 scfm. The flow 
rates of the plasma-forming gases are regulated by flow 
control orifices and upstream gas pressure
4
.  
Surrogate powder spheroidization tests used the EPPI 
171D powder feeder. The UO2 spheroidization tests will 
use a rebuilt EPI 60C powder feeder. The powder feeder 
meters the powder into a carrier gas at 0.34 – 1.0 oz./min. 
The 60C powder hopper was modified to support a 
butterfly valve and quick flange on the top for isolated 
transfer of the UO2 powders into the hopper. Three 
custom 2 lb. capacity powder transfer and loading 
hoppers were built to attach to the 60C as well as the 
bottom of the PSS for handling UO2 safely. 
 
RESULTS AND DISCUSSION 
The purpose of the PSS is to improve the spherocity of UO2 powder by plasma processing. Simultaneously, surface 
morphology should be improved making the particles a better substrate for CVD coating. In theory, angular powder 
is transported through a plasma jet by a carrier gas, where the powder is melted in the plasma jet. The particle 
consolidates and becomes spherical due to surface tension. Undesired trace elements are vaporized in the plasma 
flame yielding a higher purity product. The particle solidifies as it exits the plasma jet. The result is a highly dense, 
pure, and spherical particle with ideal surface morphology for coating
1
.  
Figure 3. PSS Top Chamber Segment 
Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
These experiments used F. J. Brodmann & Co. L.L.C. 
manufactured Cerium Oxide (CeO2) as a surrogate for 
UO2. CeO2 melts at 2673 K, which is lower than the 
melting point of UO2 at 3138 K, but has a phase 
composition that closely matches that of UO2, hence the 
motivation for developing CeO2 spheroidization 
parameters and process controls. The Brodmann CeO2 
powder is angular, fully dense, sintered, and crushed 
powder that is more like the stock UO2 powders that will 
be received from Idaho National Laboratories (INL) for 
W-UO2 cermet fuel development. The powder was 
classified into ranges (63-75microns, 75-90 microns, and 
90-106 microns) and photographed by a LEICA 
stereomicroscope like that illustrated in Figure 5.  
Statistical inference between the population and the samples is established by first assuming that the data is normally 
distributed, that there exists an equality of variance between the population and the powder sample, and that sample 
particle’s data is independent of others. It is therefore possible to apply the Central Limit Theorem6. 50 particles 
were randomly selected from each range and evaluated for minimum circumscribed diameter (d1), maximum 
inscribed diameter (d2), evidence of shelling and/or agglomeration, color, and surface morphology. The particles 
mean diameter was calculated using Eq. (1), and the particle’s spherocity was calculated using the irregularity 
parameter (IPi) in Eq. (2)
4
.
 
The sample’s mean particle size (davg) and irregularity parameter (  ̅̅ ̅) may be calculated 
using Eq. (3), and the standard deviation for mean diameter (sd)and irregularity parameter (sIP) may be calculated 
using Eq. (4). Utilizing Student’s t-distribution testing6, and accepting an   ̅̅ ̅ ≤ 1.2 as the criteria for spherocity8, it 
was shown that the stock Brodmann CeO2 powder is not spherical and that it is a very low probability that it is by 
chance; Table 1. Now, the goal of the PSS runs is to improve the spherocity of the CeO2 powder and to do so with a 
low probability that it is by chance.   
  
 
  
IPavg [StDev] tcalc tcrit Prop
1.49 [0.22] 9.195 1.676 0.01%
1.53 [0.21] 11.302 1.676 0.01%
1.51 [0.22] 10.195 1.676 0.01%
C.I. (95%)
1.421 - 1.544
1.464 - 1.578
1.447 - 1.567
PSS Run #, Powder Type, Size Range
Stock; Brodmann CeO2; 90µm-106µm
Stock; Brodmann CeO2; 75µm-90µm
Stock; Brodmann CeO2; 63µm-75µm
Table 1. Summary of Stock Brodmann CeO2 Statistical Analysis 
Figure 3. Stock Brodmann CeO2 (75-90 micron) 
Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
PSS runs #8 and #9 were conducted on 10 Oct 2012, and were 
the first to process fully dense and angular powders. Figure 6 
shows the PSS in operation. The powder can be seen entrained 
in the plasma clearly near the bottom of the jet. The process 
parameters used for Runs #8 and #9 where referenced from 
recommended operational parameters for processing similar 
powders
9
 and are listed in Table 2. 
 
Table 2. PSS processing parameters 
 
The Run #8 and #9 powders were separately classified into 
three ranges (63-75 microns, 75-90 microns, and 90-106 
microns) and a sample was prepared for each range for 
imaging. Run #8 and #9 powders were progressively darker as 
particle size decreases as shown in Figure 7.  This observation 
may be evidence of dissociation, which may indicate that the 
powders were processed a too high a temperature. It is 
believed that the darkening is only on the surface and due to 
oxidation of the free cerium, but further analysis is needed to 
determine the nature of the darkening of processed particles. 
As with the stock Brodmann powders, 50 randomly selected 
particles from each range were evaluated for minimum 
circumscribed diameter (d1), maximum inscribed diameter 
(d2), evidence of shelling or agglomeration, color, and surface 
morphology.  
Hypothesis testing of two-proportion
6
 showed that the Run #8 and #9 powders exhibited a statistically significant 
improvement in spherocity. It was also observed that while the powders are more spherical after processing, the 
mean irregularity parameters for the run #8 and #9 powders exceeded the desired maximum of 1.2. This observation 
is supported by applying the same statistical method utilized to characterize the stock Brodmann powders. The Run 
#8 and #9 powders are also not spherical according to the established criteria; but are significantly closer; Table 3. 
 
 
 
  
Run
Power 
(kW)
Current 
(Amps)
Vacuum 
(Torr)
Arc Gas
Flow Rate 
(SCFM)
Powder
8 32.8 820 400 Ar-H2 105 Brodmann CeO2
9 36.9 820 760 Ar-H2 105 Brodmann CeO2
IPavg [StDev] Spherical Zcalc Zcrit Prop
1.2 [0.21] 66% -5.769 1.645 < 0.01%
1.39 [0.29] 32% -3.993 1.645 < 0.01%
1.37 [0.24] 28% -3.641 1.645 < 0.01%
1.3 [0.18] 30% -2.73 1.645 < 0.33 %
1.29 [0.2] 34% -4.165 1.645 < 0.01 %
1.34 [0.3] 38% -4.500 1.645 < 0.01 %
Run #8; Brodmann CeO2; 63µm-75µm 1.297 - 1.429
PSS Run #, Powder Type, Size Range C.I. (95%)
Run #9; Brodmann CeO2; 90µm-106µm 1.243 - 1.345
Run #9; Brodmann CeO2; 75µm-90µm 1.233 - 1.343
Run #9; Brodmann CeO2; 63µm-75µm 1.249 - 1.415
Run #8; Brodmann CeO2; 90µm-106µm 1.136 - 1.253
Run #8; Brodmann CeO2; 75µm-90µm 1.305 - 1.465
Table 3. PSS Run #8 and #9 Statistical Data 
Figure 5. Sieve Run #8 powders before imaging 
Figure 4. PSS in operation 
Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
The Run #8 Brodmann CeO2 powder processed at 32.8 
kW in argon-hydrogen plasma showed significant 
improvement with 42% of the particles passing as 
spherical (compared to <5% of stock) and the mean IP 
was improved from 1.51 to 1.32, which is a 60% 
improvement, but it is still above the desired mean IP. 
Also, 40% of the powder was noted as having a good 
surface morphology for coating. The Run #9 Brodmann 
CeO2 powder was processed at 36.9 kW in argon-
hydrogen plasma and did not show as significant an 
improvement as Run #8 with 34% of the particles passing 
as spherical, but the mean IP was slightly more improved 
from 1.51 to 1.31, which is a 65% improvement, but it is 
still above the desired mean IP. Nearly 60% of the 
powder was noted as having a good surface morphology.  
CONCLUSIONS 
The PSS successfully improved the spherocity of angular stock CeO2 surrogate powder The Run #8 Brodmann CeO2 
powder was processed at 32.8 kW in argon-hydrogen plasma and showed significant improvement with 42% of the 
particles passing as spherical (compared to <5% of stock) and the mean IP was improved from 1.51 to 1.32. The 
Run #9 Brodmann CeO2 powder was processed at 36.9 kW in argon-hydrogen plasma and did not show as 
significant an improvement as Run #8 with 34% of the particles passing as spherical. It is evident that the 
operational parameter for run #8 and #9 will not yield the desired spherocity of the surrogate powders in one pass. 
Multiple passes through the PSS would be required to lower the mean IP to below 1.2 and further improve the 
surface morphology. Further development is needed to optimize spheroidization parameters.  
ACKNOWLEDGMENTS 
I would like to thank the NASA Marshall Space Flight Center personnel who were involved in the fabrication of the 
PSS hardware: Keith Chavers, Ronnie Renfroe, Bart Suggs, and Roger Harper. Thank you to Robert Hickman, 
Jeramie Broadway, and Omar Mireles for assigning me to this project. 
REFERENCES 
[1]  Ananthapadmanabhan, P. V., “Chapter 6: Thermal plasma processing.” in Pergamon Materials Series, 1999, Vol. 2, pp. 
121-150. 
[2] Angelo, J. A., Space Nuclear Power, Malabar, FL: Orbit Book Company, Inc., 1985. 
[3] Cochran, Final report for FY 1965 on Tungsten-Uranium dioxide Materials for NASA: Oak Ridge National Laboratory, 
1965. 
[4] Crane Co., Flow of fluids through valves, fittings, and pipe. Chicago: Crane Co., 1957. 
[5] Haertling, C., Literature review of thermal and radiation performance parameters for high-temperature, uranium dioxide 
fueled cermet materials. Journal of Nuclear Materials, 2007, 366(3), 317-335.  
[6] Holman, J. P. (2001). Experimental methods for engineers. Boston: McGraw-Hill. 
[7] Károly, Z., Plasma spheroidization of ceramic particles. Chemical Engineering and Processing: Process Intensification, 
2005, 44(2), 221-224. 
[8] Kumar, S., “Spheroidization of metal and ceramic powders in thermal plasma jet” in Computational Materials Science, 
2006, 36(4), 451-456. 
[9]  Praxair Surface Technologies, Inc., Model SG-100 Plasma Spray Gun Operator’s Manual, 2010. 
[10] Saunders, N. T., Feasibility study of a Tungsten Water-Moderated Nuclear Rocket II. Fueled Materials: NASA Lewis 
Research Center, 1968. 
 
 
  
Figure 6. PSS Run #8 CeO2 (90-106 microns) 
Proceedings of Nuclear and Emerging Technologies for Space 2013 
Albuquerque, NM, February 25-28, 2013 
Paper xxxx 
 
 
EQUATIONS 
 
  ̅  
         
 
⁄  (1) 
     
    
    
⁄   (2) 
  ̅  
 
 
∑   (3) 
    √ (    ̅) (   )⁄  (4) 
        (5) 
        (6) 
       
(  ̅̅ ̅    )
( √ ⁄ )
⁄  (7) 
       ̅   (        )  
 
√ 
⁄  (8) 
      ̂       ̂       or    ̂       ̂        (9) 
      ̂       ̂       or    ̂       ̂        (10) 
  ̅  
     
     
 (11) 
  ̅     ̅ (12) 
   
( ̂   ̂ )
√
 ̅(   ̅)
  
 
 ̅(   ̅)
  
 (13) 
 
 
 
 
  
 


Design of a Uranium Dioxide Spheroidization System

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