© 2008 brett jameson dooies - university of...
TRANSCRIPT
1
ENHANCEMENT OF URANIUM DIOXIDE THERMAL AND MECHANICAL PROPERTIES BY OXIDE DOPANTS
By
BRETT JAMESON DOOIES
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2008
2
© 2008 Brett Jameson Dooies
3
To my mom
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ACKNOWLEDGMENTS
I would first like to acknowledge my graduate advisor, Dr. Anghaie for his continued
encouragement. I am lucky to be his student. Also, I thank the other members of my committee,
Dr. Dugan and Dr. Sigmund. Special thanks goes out to Dr. Dugan for being an excellent
teacher and undergraduate advisor.
This work was performed under the Department of Energy’s Advanced Fuel Cycle
Initiative Fellowship. The funding they provided enabled me to finish this research in a timely
manner. I would like to thank Dr. James Bresee and Dr. Tom Ward, for their technical feedback
on my research proposal. I would also like to thank Cathy Dixon and Donna Knight, for the
excellent work that they do for this program.
I would like to acknowledge the facilities that I used to perform my research. My samples
were made at the Innovative Nuclear Space Power and Propulsion Institute at the University of
Florida. Sample characterization was performed at the Major Analytical Instrumentation Center
at UF. I extend thanks to Dr. Travis Knight for his extremely helpful insights and to Dr. Jiwei
Wang for sharing his materials and equipment.
Finally, thanks go out to my family, my girlfriend, and my friends, whose support has
meant everything.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES ...........................................................................................................................7
LIST OF FIGURES .........................................................................................................................8
ABSTRACT ...................................................................................................................................10
CHAPTER
1 INTRODUCTION ..................................................................................................................12
1.1 Background Information ...............................................................................................12 1.1.1 Advanced Fuel Cycle Initiative .........................................................................12 1.1.2 Near-Term Goals of AFCI ................................................................................13 1.1.3 Long-Term Goals of AFCI ...............................................................................13
1.2 Waste Reduction in Current and Generation III+ Reactors ..........................................13 1.3 Objectives and Scope ....................................................................................................14
1.3.1 Objectives of Research ......................................................................................14 1.3.1.1 Grain size modification of UO2 ..........................................................14 1.3.1.2 Doping UO2 ........................................................................................14
1.3.2 Scope of Research .............................................................................................15
2 LITERATURE REVIEW .......................................................................................................16
2.1 Theoretical Foundation .................................................................................................16 2.2 Previous Experimental Work ........................................................................................16
2.2.1 Production of Doped Fuel Pellets .....................................................................17 2.2.2 Results of Previous Studies ...............................................................................18
3 MATERIALS AND METHODS ...........................................................................................24
3.1 Processing .....................................................................................................................24 3.1.1 Powders .............................................................................................................24 3.1.2 Cold Uniaxial Pressing ......................................................................................25 3.1.3 Pellet Sintering ..................................................................................................26
3.1.3.1 Induction furnace ................................................................................26 3.1.3.2 Sintering ..............................................................................................27
3.2 Characterization ............................................................................................................27 3.2.1 Preparation for Analysis ....................................................................................27
3.2.1.1 Scanning electron microscopy ............................................................27 3.2.1.2 X-ray diffraction .................................................................................29
3.2.2 Analysis of Samples ..........................................................................................29 3.2.2.1 Density measurements ........................................................................29
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3.2.2.2 Scanning electron microscopy and energy dispersive spectroscopy .......................................................................................29
3.2.2.3 Grain size determination .....................................................................30 3.2.2.4 X-ray diffraction .................................................................................30
4 RESULTS AND DISCUSSION .............................................................................................38
4.1 Density Measurements ..................................................................................................38 4.2 Pellet Characterization ..................................................................................................38
4.2.1 Optical Microscopy ...........................................................................................38 4.2.2 Scanning Electron Microscopy .........................................................................39 4.2.3 Electron Dispersive Spectroscopy.....................................................................43 4.2.4 X-Ray Diffraction .............................................................................................43
5 CONCLUSIONS AND FUTURE WORK .............................................................................60
5.1 Pellet Processing ...........................................................................................................60 5.2 Grain Size Analysis .......................................................................................................60 5.3 Future Work ..................................................................................................................61 5.4 Final Comments ............................................................................................................62
APPENDIX POWDER CHARACTERISTICS .......................................................................64
BIOGRAPHICAL SKETCH .........................................................................................................67
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LIST OF TABLES
Table page
2-1 Grain sizes obtained for doped UO2 in previous studies ........................................................22
3-1 Summary of dopant powders. .................................................................................................32
3-2 Masses of UO2 and dopant powders in sample mixtures. ......................................................32
3-3 Grinding and polishing sequence. ..........................................................................................32
4-1 TD’s of doped samples ...........................................................................................................45
4-2 Pellet masses and densities. ....................................................................................................45
4-3 Results of statistical analysis for pellet grain sizes.................................................................45
4-4 Lattice parameters of sintered pellets .....................................................................................46
5-1 Summary of grain size analysis. .............................................................................................63
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LIST OF FIGURES
Figure page
3-1 Sartorius R180D analytic balance used for mass measurements. ......................................33
3-2 Hydraulic press used for cold uniaxial pressing of pellets. ...............................................33
3-3 Stainless steel die set used for cold pressing powders. ......................................................34
3-4 Base used to hold pellet during sintering. ..........................................................................34
3-5 Induction furnace, controller, pyrometer, and vacuum chamber. ......................................35
3-6 LECO VC-50 precision low-speed diamond saw. .............................................................35
3-7 LECO Spectrum System 1000 grinder/polisher with semi-automatic head. .....................36
3-8 Setup for measuring apparent mass of sample while immersed in water. .........................36
3-9 JEOL JSM 6400 scanning electron microscope. ...............................................................37
3-10 Philips APD 3720 x-ray diffractometer. ............................................................................37
4-1 Optical images of pellet surfaces post-sinter .....................................................................47
4-2 Undoped UO2. ....................................................................................................................48
4-3 Nb2O5-doped UO2 ..............................................................................................................49
4-4 Al2O3-doped UO2 ...............................................................................................................50
4-5 Cr2O3-doped UO2. ..............................................................................................................51
4-6 Sc2O3-doped UO2 ...............................................................................................................52
4-7 Y2O3-doped UO2 ................................................................................................................53
4-8 V2O5-doped UO2 ................................................................................................................54
4-9 TiO2-doped UO2.................................................................................................................55
4-10 EDS spectrum of undoped UO2 sample. ............................................................................56
4-11 EDS spectrum of Nb2O5-doped sample. ............................................................................56
4-12 EDS spectrum of Al2O3-doped sample. .............................................................................57
4-13 EDS spectrum of Cr2O3-doped sample. .............................................................................57
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4-14 EDS spectrum of Sc2O3-doped sample. .............................................................................58
4-15 EDS spectrum of Y2O3-doped sample. ..............................................................................58
4-16 EDS spectrum of V2O5-doped sample. ..............................................................................59
4-17 EDS spectrum of TiO2-doped sample. ...............................................................................59
A-1 Particle size distribution of received uranium dioxide powder. ........................................64
10
Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
ENHANCEMENT OF URANIUM DIOXIDE THERMAL AND MECHANICAL
PROPERTIES BY OXIDE DOPANTS
By
Brett Jameson Dooies
August 2008 Chair: Samim Anghaie Major: Nuclear Engineering Sciences
The Advanced Fuel Cycle Initiative (AFCI) program is funding the development of high-
burnup fuels for use in current and future reactors. These fuels have the capacity to reduce the
rate at which spent fuel is created and thus reduce the amount of spent fuel requiring long-term
geologic disposal. One of the factors limiting the burnup of current reactor fuel is the buildup of
internal pin pressure due to the release of gaseous fission products from the fuel matrix.
Theoretical calculations have shown that increasing the grain size of uranium dioxide fuel could
help to reduce the release of fission products and thus slow the buildup of internal pin pressure.
This theory has been proven by numerous irradiation studies of large-grained UO2 which have
shown a decreased release rate of fission product gases. Obtaining large-grained fuel can be
accomplished by several methods including doping UO2 with a small amount (< 1 wt%) of
another metal oxide. In this research, UO2 was doped with oxides of niobium (Nb), aluminum
(Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and titanium (Ti) at a
concentration of 0.5 wt%. The pellets were sintered to greater than 93% of their theoretical
density and microstructural analysis was performed to ascertain the effects of the dopants on the
sintered pellets. The grain sizes were measured after chemically etching polished sections of the
11
samples. Titania showed the most substantial promotion of larger grain sizes with a 281%
increase over undoped UO2. Other dopants that showed potential for grain size increase were
niobia, alumina, chromia, and vanadia. Scandia had no significant effect on the grain size of the
fuel. The grain size of the yttria-doped fuel was unable to be determined due to inconsistencies
with the chemical etching process. Overall, the ability of a small amount of dopant to promote
larger grains in uranium dioxide fuel was shown, with titania having the most potential.
12
CHAPTER 1 INTRODUCTION
1.1 Background Information
The United States currently has 104 operating nuclear reactors. In 2007 these reactors
safely produced over 806 billion kWh of electricity. Unlike many other major energy sources,
nuclear energy does not emit any greenhouse gases into the atmosphere. Currently, about one-
third of energy produced in the US is from carbon-free sources, and nuclear power makes up
about 70% of that energy (NEI, 2008).
The majority of the reactors currently operating utilize an 18-month refueling cycle, with
average fuel assembly discharge burnups in the range of 35 – 45 GWd/kgU. With new
Generation III+ reactors on the horizon, many utilities are hoping to switch to a 24-month
refueling cycle. This would be beneficial economically and from a fuel efficiency standpoint,
reducing the overall amount of spent fuel and high level waste being produced. As such, reactor
fuel will soon be pushed to new, higher burnups and will be subjected to increasingly rigorous
operating conditions. The reliability of high-burnup nuclear fuel must be assured in order for
these goals to be realized.
1.1.1 Advanced Fuel Cycle Initiative
The Advanced Fuel Cycle Initiative (AFCI) is a program of the United States Department
of Energy. The mission of AFCI is “to develop fuel cycle technologies that will meet the need
for economic and sustained nuclear energy production while satisfying requirements for
controlled, proliferation-resistant nuclear materials management system.” (DOE, 2008) AFCI is
meant to develop new technologies to aid the current reactor fleet, as well as future Generation
III+ and Generation IV reactors. Mission success would result in a reduction of the amount of
13
high level radioactive waste requiring geologic disposal, a reduction of the plutonium content of
civilian spent fuel, and increased energy extraction from the fuel.
1.1.2 Near-Term Goals of AFCI
The near-term goals of AFCI are to develop and demonstrate a proliferation-resistant spent
fuel recycling program. This pursuit would reduce the volume and heat content of high level
waste that requires long-term storage, and exploit the large amount of fissile material still present
in spent fuel. The Secretary of Energy is required to advise Congress on the need for an
additional geologic repository (to follow Yucca Mountain) by 2010. This recommendation will
be driven by the ability to establish a viable spent fuel recycling program for commercial spent
nuclear fuel.
1.1.3 Long-Term Goals of AFCI
The long-term goals of AFCI are to develop a system for recycling spent fuel that would
separate the fuel and allow for the destruction of actinides and other long-lived fission products
in fast reactors through transmutation. The removal of these products would increase the
capacity of the planned Yucca Mountain repository up to fifty-fold due to the decrease in the
heat load of the remaining spent fuel. By successfully increasing the technical capacity by such
a large factor, Yucca Mountain would be sufficient for the storage of all current spent fuel as
well as all the fuel produced in the next century (DOE, 2008).
1.2 Waste Reduction in Current and Generation III+ Reactors
The long-term goals of the AFCI could take upwards of 30 years to be completely
developed and implemented. Until the technology for reprocessing is available, current reactors
(and those currently being planned for construction) must be concerned with the waste that they
are producing now. Many reactor sites have recently run out of space for the storage of spent
fuel on site, and with the opening of Yucca Mountain still years away, dry cask storage is being
14
used all over the country to alleviate the overcrowded spent fuel pools. Increased fuel cycle
length for Generation III+ reactors will reduce the rate at which spent fuel is produced. This is
directly in line with meeting the goals of the AFCI program. Current research is supporting the
development of reliable, high-burnup fuels for immediate deployment.
1.3 Objectives and Scope
1.3.1 Objectives of Research
One of the factors limiting fuel burnup is the accumulation of internal pin pressure due to
the release of fission gases from the fuel and into the gap region during irradiation. It was first
shown by Turnbull (Turnbull, 1974) that an increase in the fuel’s grain size can slow the fission
gas release (FGR) and fuel swelling rates of irradiated fuel. By limiting the release of the fission
gases, the buildup of internal pin pressure is slowed and burnup can be increased. Thus, the goal
of this research is to increase the burnup capabilities of uranium dioxide fuel by increasing the
grain size of the fuel.
1.3.1.1 Grain size modification of UO2
There are several methods available for increasing the grain size of sintered UO2 fuel. One
method is to subject the sintered fuel to a long heat treatment to promote continued grain growth.
This has the disadvantages of being time consuming and expensive. Another method involves
“doping” the UO2 powder with a small amount (typically < 1 wt%) of another oxide powder.
The dopants can promote grain growth through several different mechanisms and can facilitate
an increase in the burnup capabilities of the fuel.
1.3.1.2 Doping UO2
This research project aims to investigate the effects of several oxide dopants on the grain
sizes of sintered uranium dioxide fuel. This research will require the production of eight
different uranium dioxide fuel samples. Seven different oxide dopants will be considered and
15
one undoped pellet will be produced for comparison. The dopants that will be used are oxides of
niobium (Nb), aluminum (Al), chromium (Cr), scandium (Sc), yttrium (Y), vanadium (V), and
titanium (Ti). The dopant concentration for each dopant will be set at 0.5 wt%. The average
grain sizes will be determined and the dopants that are most effective at increasing grain size will
be recommended for further consideration. This study represents the first investigation of
scandia and yttria as dopants for UO2. For the remaining dopants, a comparison will be made
with results found in other publications.
1.3.2 Scope of Research
This research aims to characterize the grain sizes of doped UO2 fuel using equivalent
conditions. The dopant concentrations and the sintering process will be equivalent for each
sample. The goal is exclusively to identify each dopant’s ability to promote grain size growth
during sintering. Future research should aim to optimize the parameters for a specific dopant,
including the concentration of the dopant, the oxygen potential of the sintering atmosphere, and
the sintering temperature. Ideally, this study will allow future research to focus on the dopants
that show the most promise of increasing the grain size of UO2.
16
CHAPTER 2 LITERATURE REVIEW
2.1 Theoretical Foundation
The diffusion of fission gases in fuel under irradiation was first described by A. H. Booth
(1957). The expression for F, the fractional release of stable fission gas from the fuel, is shown
in (2-1).
2 2 2
4 4 21
6 11 1 exp( )n
a n DtFDt n a
ππ
∞
=
⎧ ⎫−= − −⎨ ⎬
⎩ ⎭∑ (2-1)
In this equation, D is the diffusion coefficient, t is the time, and a is the radius of a hypothetical
spherical volume. For a small gas release, F ∂ 1/a. If the hypothetical volume is assumed to be a
grain in the fuel, increasing the grain size can reduce the fraction of fission gas released from the
fuel matrix into the pin volume. This model is the basis for the proceeding experimental
research.
The Booth model was later modified to include the effects of gas re-solution from
intragranular bubbles and interlinked porosity. A simple relation for low quantity gas release, as
proposed by Killeen (1975), is given in (2-2).
43
S DtFV π
= ⋅ (2-2)
In this case, the release fraction is related to the surface-to-volume ratio of the sample.
Again, this relationship demonstrates that by increasing the grain size (and thus decreasing the
surface-to-volume ratio), the fission gas release rate will decrease.
2.2 Previous Experimental Work
Several of the dopants investigated in this study have been researched previously,
specifically oxides of niobium, aluminum, chromium, vanadium, and titanium. Scandia and
17
yttria have not previously been used as dopants for UO2. It is desirable to review the results of
previous research in order to identify the parameters that must be considered when doping UO2.
Prior experiments can be split into pre-irradiation and post-irradiation results. In this
research project, no irradiation testing was conducted. Therefore, this review will focus on
sample production methods and pre-irradiation analyses.
2.2.1 Production of Doped Fuel Pellets
The process for producing UO2 fuel pellets is well established and is used in wide scale
production throughout the world. The only steps typically required are cold pressing the powder
into a green pellet and sintering the pellet to achieve densification. Binders and lubricants are
sometimes used, though not usually necessary. Sintering is typically performed in a reducing
environment at around 1700ºC for approximately four hours. When working with doped UO2,
an initial step of mixing the powders is also necessary. The effects of manipulating these
variables, as well as dopant concentrations, have been investigated in several studies of doped
UO2 fuel (Ainscough et al., 1974; Bourgeois et al., 2001).
Dopant mixing. The large majority of doping studies have integrated the dopant powder
into the UO2 powder by a mechanical mixing method. This is typically done without organic
additions by rotary ball milling with steel balls for two to 72 hours. In some cases this was
preceded or followed by sieving of the powder mixture.
While mechanical mixing is the most common method of mixing the dopant and UO2
powders, one other noteworthy method was used by Bourgeois et al. (2001) for doping UO2 with
Cr2O3. In order to more homogeneously mix the dopant into the powder, spray-drying of a
suspension containing UO2 powder in solution with distilled water and (NH4)2CrO4 was
performed. The spray-drying was followed by calcination of the powders in argon to transform
the chromate into Cr2O3. This process did not disturb the specific surface area of the powders,
18
nor their O/U ratios. It was found that this mixing method resulted in larger final grain sizes for
Cr2O3-doped UO2 than previous studies, presumably due to the improved homogeneity of the
mixing.
Cold pressing. The method used almost exclusively for cold pressing doped UO2 powders
is uniaxial pressing, often bilaterally. Typical pressures for this process are from 100–350 MPa.
Green densities are normally on the order of 50–60 %TD. Due to the high compressibility of the
powders, large differences in cold pressures do not yield large differences in green densities. A
study by Bourgeois et al. (2001) found that a 5–6% difference in green density resulted in less
than 1% difference in sintered density. Due to the relative unimportance of the cold pressure and
green pellet density on the final sintered density of samples, the effects of varying the cold
pressure were not studied in this project.
Other parameters. There are several other parameters that play an important role in the
final sintered pellet characteristics. Specific examples include the sintering temperature, the
sintering time, the oxygen potential of the sintering environment, and any heat treatments that
may be applied after sintering. Table 2-1 shows several variables from previous research and
illustrates the large variety of options available. Some of the effects of changing these
parameters will be discussed with regards to the results that follow.
2.2.2 Results of Previous Studies
There have been a multitude of studies concerning the effects of dopants on the
characteristics of sintered UO2 pellets. The majority of these studies have focused on the
abilities of different dopants to promote an increase in grain size. Table 2-1 lists grain sizes
found for doped-UO2 pellets in previous studies that have used Cr2O3, TiO2, Nb2O5, Al2O3 and
V2O5, all of which were investigated in this research. The results of these studies vary widely
depending on the dopant concentrations and the sintering conditions used. Additionally, most
19
studies do not state whether they have reported 2-D grain sizes or have corrected for and reported
3-D grain sizes. The far right column shows the relative percent increase of the doped samples
over the control (undoped) samples in each study. Dashes indicate that the control grain size was
not reported. Since different studies have different powders and parameters, the percent increase
in grain size provides a better measure for comparison of different studies than the absolute grain
sizes.
There is no published literature on the use of Sc2O3 or Y2O3 as dopants for UO2. Thus,
there are no previous results with which to compare.
Chromia. For Cr2O3-doped UO2, the average grain sizes quoted vary between 12 and 126
μm. Using a concentration of 0.5 wt% and similar sintering conditions as the current research,
the reported grain sizes vary between 50 and 126 μm, or an average relative increase of
approximately 550%. The grain size increase due to doping with Cr2O3 seems to be limited to a
dopant concentration of 0.3–0.5 wt%. At concentrations above 0.5 wt%, no additional grain size
increase is seen.
Solubility limits have been quoted by several sources. Leenaers et al. (2003) found the
solubility at 1600, 1660, and 1760°C to be 0.065, 0.086, and 0.102 wt%, respectively. Kashibe
et al. (1998) found that the solubility at 1750°C was only 0.012 wt%, although in this case a very
small amount (0.065 wt%) was initially added to the powder. Bourgeois et al. (2001) found the
solubility to be 0.07 wt% in the range of 1500–1700°C. Solubility varies with both temperature
and oxygen potential, which could explain some of the differences above.
The actual amount of Cr2O3 added is well above these limits in most research published.
Above the solubility limit, Cr and Cr2O3 form a eutectic at around 1550°C and the liquid phase
enhances grain growth and densification. This provides some explanation for the mechanism of
20
increased grain size in Cr2O3-doped UO2. It is also seen in Table 2-1 that for pellets sintered
below 1550°C, the grain sizes achieved are generally much smaller than for those sintered at
temperatures above 1600°C.
Titania. Titania-doped UO2 grain sizes range from 5 to 133 μm. For a concentration of
0.5 wt% and similar sintering conditions to the current research, the grain sizes vary from 70 to
133 μm, or an average relative increase of approximately 350%.
Ainscough et al. (1974) studied the effect of sintering temperature and additive content on
grain size and found that for concentrations of 0.03–0.33 wt% TiO2, grain sizes varied from 5–45
μm at 1550°C sintering temperature. At 0.13 wt% dopant, the maximum grain sizes were
obtained. At 1650 and 1750°C, the samples containing ≥0.13 wt% TiO2 experienced enhanced
grain growth due to the formation of a UO2-TiO2 eutectic that separated to the grain boundaries
during sintering. The eutectic was never formed for concentrations of 0.07 wt% or less and was
occasionally formed for 0.13 wt%. This led to the conclusion that TiO2 solubility in UO2 is
between 0.07 and 0.13 wt% in the range of 1650–1750°C.
Niobia. Niobia-doped UO2 grain sizes range from 7 to 56 μm. For a concentration of 0.5
wt% and similar sintering conditions to the current research, the grain sizes vary from 40 to 56
μm, or an average relative increase of approximately 230%.
The solubility of Nb2O5 in UO2 has been quoted as being approximately 0.5 wt% in several
studies. Harada (1996) showed the variation of the lattice parameter as the niobia content is
increased and found that the variation halted very near 0.5 wt%. Harada (1996) also showed the
variation of the lattice parameter as a function of temperature and reported that a solid solution
occurs between 1400 and 1700°C.
21
Alumina. There is significantly less published research using alumina as a dopant for UO2
than there is for chromia, titania, and niobia. The range of grain sizes found in the two studies
shown in Table 2-1 is from 10 to 30 μm. Both of the studies have dopant concentrations lower
than the 0.5 wt% used in this research.
Vanadia. Vanadia was shown in the study by Amato et al. (1967) to reduce the grain size
of sintered UO2 pellets when used as a dopant. On the other hand, Radford and Pope (1983) and
Aybers et al. (2004) found that grain size increased with increasing V2O5 concentration up to 1.0
mol%. Without additional studies, it is hard to say whether or not vanadia is effective at
promoting increased grain size. This research will try to provide further insight on this dopant.
22
Tabl
e 2-
1. G
rain
size
s obt
aine
d fo
r dop
ed U
O2 i
n pr
evio
us st
udie
s D
opan
t R
efer
ence
D
opan
t Con
cent
ratio
n Si
nter
ing
Atm
osph
ere
Tem
pera
ture
(°C
) Ti
me
(h)
Gra
in S
ize
(μm
) %
Incr
ease
K
illee
n (1
980)
0.
5 w
t%
Not
quo
ted
Not
quo
ted
Not
quo
ted
5
0–55
76
7%
O
hai (
2003
) 0.
1 w
t%
Hyd
roge
n 17
00
4
45–6
0
42
0%
0.3
wt%
H
ydro
gen
1700
4
65–
110
770%
0.5
wt%
H
ydro
gen
1700
4
80–
126
930%
1.0
wt%
H
ydro
gen
1700
4
80–
126
930%
Cr 2
O3
Ayb
ers e
t al.
(200
4)
100
ppm
90
% A
r + 1
0% H
2 17
00
4
12–2
0
6
0%
1000
ppm
90
% A
r + 1
0% H
2 17
00
4
13–2
3
8
0%
2000
ppm
90
% A
r + 1
0% H
2 17
00
4
13–2
5
9
0%
3000
ppm
90
% A
r + 1
0% H
2 17
00
4
15–2
7
11
0%
K
ashi
be e
t al.
(199
8)
0.06
5 w
t%
Hyd
roge
n 17
50
2
1
5
0
%
A
insc
ough
et a
l. (1
978)
0.
3 w
t%
Not
quo
ted
Not
quo
ted
Not
quo
ted
80
-
B
ourg
eois
et a
l. (2
001)
0.
05–0
.7 w
t%
H2
+ 1
vol%
H2O
15
25
4
15–2
8
250
%
0.05
–0.7
wt%
H
2 +
1 vo
l% H
2O
1625
4
29
–50
3
88%
0.05
–0.7
wt%
H
2 +
1 vo
l% H
2O
1700
4
15
–87
410%
D
elaf
oy e
t al.
(200
7)
0.16
wt%
N
ot q
uote
d N
ot q
uote
d N
ot q
uote
d
50–6
0
58
8%
A
rbor
eliu
s et a
l. (2
006)
≤
1,00
0 pp
ma
H2/
CO
2 ≤1
800
14
40
–55
336%
A
mat
o et
al.
(196
6)
0.5
wt%
H
ydro
gen
1400
+ h
eat t
reat
b 1
4
.4–5
1.5
2
50%
A
isco
ugh
et a
l. (1
974)
0.
03–0
.33
wt%
H
ydro
gen
1550
12
5
20–4
5
-
0.03
–0.3
3 w
t%
Hyd
roge
n 16
50
16
10–
105
10
40%
0.03
–0.3
3 w
t%
Hyd
roge
n 17
50
4
1
0–90
-
TiO
2 R
adfo
rd a
nd P
ope
(198
3)
0.05
–1.0
mol
% m
etal
H
ydro
gen
Six
step
s fro
m 9
25 -
1780
1
hour
at e
ach
step
1
0.9–
69.7
410
%
Y
uda
et a
l. (1
997)
0.
2 w
t%
Not
quo
ted
Not
quo
ted
Not
quo
ted
68
656%
O
hai (
2003
) 0.
05–1
.0 w
t% m
etal
H
ydro
gen
1700
4
65–1
33
880%
A
yber
s et a
l. (2
004)
10
0 pp
m
90%
Ar +
10%
H2
1700
4
11–
19
50%
1000
ppm
90
% A
r + 1
0% H
2 17
00
4
1
5–33
14
0%
2000
ppm
90
% A
r + 1
0% H
2 17
00
4
2
3–43
23
0%
3000
ppm
90
% A
r + 1
0% H
2 17
00
4
3
1–59
35
0%
23
Tabl
e 2-
1. C
ontin
ued
Dop
ant
Ref
eren
ce
Dop
ant C
once
ntra
tion
Sint
erin
g A
tmos
pher
e Te
mpe
ratu
re (°
C)
Tim
e (h
) G
rain
Siz
e (μ
m)
% In
crea
se
K
illee
n 0.
1 at
%
Not
quo
ted
Not
quo
ted
Not
quo
ted
7
0%
1.0
at%
N
ot q
uote
d N
ot q
uote
d N
ot q
uote
d
28
300
%
Sa
wbr
idge
0.
25 m
ol%
N
ot q
uote
d N
ot q
uote
d N
ot q
uote
d
24–
30
-
0.4
mol
%
Not
quo
ted
Not
quo
ted
Not
quo
ted
2
8–32
-
0.5
mol
%
Not
quo
ted
Not
quo
ted
Not
quo
ted
4
3
-
0.6
mol
%
Not
quo
ted
Not
quo
ted
Not
quo
ted
2
8–32
-
0.8
mol
%
Not
quo
ted
Not
quo
ted
Not
quo
ted
2
8–32
-
Nb 2
O5
1.
0 m
ol%
N
ot q
uote
d N
ot q
uote
d N
ot q
uote
d
25
-
0.3
wt%
R
educ
ing
1750
2
3
0–35
2
20%
A
ssm
ann
0.5
wt%
R
educ
ing
1750
2
4
0–50
3
50%
0.5
wt%
O
xida
tive
1100
1
2–15
-
20%
0.05
–0.5
mol
% m
etal
H
ydro
gen
Six
step
s fro
m 9
25 -
1780
1
hour
at e
ach
step
20.
1–50
.5
343
%
R
adfo
rd
0.3
wt%
W
et h
ydro
gen
1700
3
3
0
5
00%
H
arad
a 0.
05–1
.0 w
t% m
etal
H
ydro
gen
1700
4
1
4–56
2
50%
O
hai
0.05
–1.0
wt%
met
al
Hyd
roge
n 16
00
4
9–
31
100
%
K
ashi
be
0.07
6 w
t%
Hyd
roge
n 17
50
2
30
100
%
Al 2O
3 A
yber
s 10
0 pp
m
90%
Ar +
10%
H2
1700
4
1
0–24
60%
1000
ppm
90
% A
r + 1
0% H
2 17
00
4
12–2
6
90%
A
mat
o 0.
68 w
t%
Hyd
roge
n 14
00–1
700
0.5–
10
2
.5–2
4.0
-3
2%
1.38
wt%
H
ydro
gen
1400
–170
0 0.
5–10
2.5
–17.
0
-40%
2.10
wt%
H
ydro
gen
1400
–170
0 0.
5–10
2.5
–11.
2
-50%
V2O
5 R
adfo
rd
0.05
– 1
.0 m
ol%
met
al
Hyd
roge
n Si
x st
eps f
rom
925
- 17
80
1 ho
ur a
t eac
h st
ep
12
.4–5
7.1
343
%
A
yber
s 10
0 pp
m
90%
Ar +
10%
H2
1700
4
14
–22
80%
1000
ppm
90
% A
r + 1
0% H
2 17
00
4
16–3
0
1
30%
2000
ppm
90
% A
r + 1
0% H
2 17
00
4
14–3
2
1
30%
a A
lso
incl
uded
≤ 1
000
ppm
alu
min
a ad
ditiv
e b H
eat t
reat
men
ts w
ere
in h
ydro
gen
atm
osph
ere
from
145
0 - 1
630
ºC fo
r 0.5
- 10
hou
rs
24
CHAPTER 3 MATERIALS AND METHODS
The methodologies employed in this research focused on the processing of the doped UO2
pellets and the characterization of the sintered pellets. The materials required for this research
included the uranium dioxide powder, the dopant powders, the cold uniaxial press, the induction
furnace and associated parts, the equipment used to prepare the samples for analysis, and the
analysis equipment.
3.1 Processing
Pellet processing was performed at the Applied Ultra High Temperature Research
Laboratory at the Innovative Nuclear Space Power and Propulsion Institute at the University of
Florida. The production of the doped fuel pellets entailed several steps. The first step was to
measure and mix each of the dopants with the uranium dioxide powder. Next, the powder
mixtures were cold pressed into semi-dense compacts (green pellets). Then the green pellets
were sintered to achieve final densification.
3.1.1 Powders
The powders used in this research were the uranium dioxide powder and the seven dopant
powders. The UO2 powder was comprised of depleted uranium (DU) obtained from
Framatome/AREVA. Appendix A shows some properties of the initial UO2 powder. Table 3-1
shows a summary of the dopant powders used in this research.
Powder mixing. The dopants were added to the UO2 powder at an optimum concentration
of 0.5 wt%. Table 3-2 shows the actual measured values for the UO2 and dopant powder masses
prior to mixing. The powder masses were measured using a Sartorius R180D analytic balance
seen in Figure 3-1.
25
The powder mixtures were combined in HDPE bottles and six stainless steel shots were
added. The powder mixing was performed in air at atmospheric pressure for all of the dopants
except for Cr2O3 and V2O5, which were mixed under argon at atmospheric pressure due to their
volatility. The mixtures were shaken for thirty minutes in a Spex Certiprep 8000M Mixer/Mill.
The stainless steel shots were added to help break up the powder particles during shaking and to
induce good mixing.
3.1.2 Cold Uniaxial Pressing
The cold press used in this research was a Carver model #3912 hydraulic press, shown in
Figure 3-2. The cold uniaxial pressing of powders requires the use of a punch and die. Initially,
a simple stainless steel rod and tube were used for cold pressing the powders. One rod was
inserted into the tube, powder was poured in, and a second rod was inserted above. The
collection was then punched in the hydraulic press. This initial process did not produce green
pellets of sufficient quality to be sintered. It was evident that the ends of the rods were not
polished well enough, as the green pellets were sticking and breaking. Additionally, the tubes
were not strong enough to withstand the outward pressure exerted by the powder upon being
pressed. This resulted in bowing of the sides and poor compaction of the powder.
Due to the failure of the stainless steel punch and die, a polished die set made specifically
for cold pressing pellets was obtained. The set was an International Crystal Laboratories 13mm
KBr Die Set consisting of a base, a stainless steel die, and two stainless steel anvils for punches,
shown in Figure 3-3. The process for cold pressing a pellet using this die set was as follows:
• Measure 3.5–4.5 g of doped UO2 powder
• Clean each piece of the die set and attach the die to the base
• Coat the inside of the die with stearic acid solution, which acts as a lubricant, using the long stainless steel punch
26
• Coat the first anvil in stearic acid solution and insert it into the die, polished side up
• Pour the powder into the die
• Coat the second anvil in stearic acid solution and insert it into the die, polished side down
• Place the punch on top of the second anvil and press in the hydraulic press for ten minutes
• Remove the punch and separate the die from the base, removing the first (bottom) anvil
• Flip the die over and reinsert the punch
• Place a metal ring on top of the die and use the hydraulic press to punch out the pellet through the top
Typical pressures for cold pressing range between 200 and 350 MPa. Each pellet in this
study was pressed at approximately 300 MPa and green pellet densities were all measured to be
between 50% and 60% TD. The green pellet densities were calculated using their measured
dimensions to estimate their volume.
3.1.3 Pellet Sintering
3.1.3.1 Induction furnace
The furnace used for heating and sintering the green pellets was a 20 kW Taylor Winfield
Thermonic generator, model CE2000. The heating was accomplished with a water-cooled
copper coil inside of a vacuum chamber inductively heating a tungsten tube that surrounded the
sample. The pellet sat on a piece of tantalum carbide foam which sat atop a boron nitride base.
The setup is shown in Figure 3-4. Both the BN base and the TaC foam were used for their high
temperature capabilities and chemical stability. At the bottom of the BN stand was a hole that
allowed gas to enter and flow up through the TaC foam, across the pellet, and out the top of the
apparatus. A mixture of Ar-5%H2 was used to reduce the samples during sintering.
Temperature was controlled using a Maxline IRCON active controller and measured using a
two-color infrared pyrometer. The furnace system is shown in Figure 3-5.
27
3.1.3.2 Sintering
The pellets were sintered at 1700°C for approximately four hours while exposed to a
mixture of Ar - 5%H2. The heat up and cool down rates were controlled manually and were thus
subject to some amount of variation. Due to this variation, several of the pellets experienced
cracking during sintering. It is surmised that excessively fast heating up or cooling down was
the cause of the cracking. This did not affect the results of this study since microstructural
characterization was still possible despite the cracking. It is not presumed that the cracking
indicates any real difficulties that would be associated with producing doped pellets.
3.2 Characterization
Characterization of the sintered pellets was performed in order to compare the
microstructures and grain sizes of the doped pellets. Density measurements, optical microscopy,
and preparations for other analyses were performed at the Applied Ultra High Temperature
Research Laboratory at the Innovative Nuclear Space Power and Propulsion Institute at the
University of Florida. SEM, EDS, and XRD were performed at the Major Analytical
Instrumentation Center at the University of Florida.
3.2.1 Preparation for Analysis
3.2.1.1 Scanning electron microscopy
Cutting. The sintered fuel pellets first needed to be cut into cross-sectional pieces to allow
for internal microstructural examination. Cutting was performed with a LECO VC-50 precision
low-speed diamond saw shown in Figure 3-6. Samples were cut into eighths and the slices were
approximately 1–3 mm in thickness.
Setting in epoxy. After cutting the samples, the slices needed to be set in epoxy to allow
for grinding and polishing. The epoxy is a 10:2 mixture of LECO epoxy resin and LECO
hardener. This was poured over the slices and allowed to set overnight.
28
Grinding and polishing. The samples were ground and polished using a LECO Spectrum
System 1000 grinder/polisher with semi-automatic head shown in Figure 3-7. The
grinder/polisher head attachment enabled semi-automatic operation for six samples at a time with
constant applied pneumatic pressure. Table 3-3 shows the grinding and polishing parameters.
Chemical etching. After grinding and polishing, a chemical etch was used to reveal the
grain structure of the sample. There are a wide variety of etchants that have been reported in the
literature (Petzow, 1978). The most common etchant for UO2 seems to be a 1:1:1 mixture of
H2SO4, H2O2, and H2O. This was used for several of the samples in this study. The etchant was
applied with a cotton tipped applicator, allowed to sit for anywhere from one minute to two
hours, and then brushed off with the cotton swab. The samples were ultrasonically cleaned
following the etching process. This etchant was mildly effective but did not work for all of the
samples.
Another etchant that was used was a 1:1 mixture of distilled water and 90% nitric acid.
This etchant was applied to the surface and allowed to sit for five seconds to 2 minutes before
being brushed off with a cotton swab. The reasons for different etchants yielding different
results are unknown. Many other methods for etching could have been tried, possibly to greater
effect, including thermal etching and electrolytic etching techniques.
Conductive paint. Investigation by SEM requires that the sample be electrically
conductive, lest a charge build up in the sample and cause burning and image distortion. Thus, a
coating of conductive graphite paint was applied to the epoxy region surrounding the samples to
establish an electrical contact with the specimen holder.
29
3.2.1.2 X-ray diffraction
A small amount of each sample was ground into powder and prepared for investigation by
XRD. The powder was placed on slides and adhered using a solution of one part collodian and
seven parts amyl acetate.
3.2.2 Analysis of Samples
3.2.2.1 Density measurements
The density of each sample was measured after sintering. Density measurements were
performed using an immersion technique. First, the mass of the sample in air was measured.
Then the apparent mass of the sample while submersed in water was measured. This
measurement was made by filling a beaker with water so that the sample holder was immersed,
tearing the scale, and then placing the pellet on the holder. It is important to tear the scale after
immersing the sample holder in order to account for its change in apparent mass. Though small,
this could introduce a significant discrepancy in the measurement if done improperly. The red
platform kept the beaker isolated from the scale. Figure 3-8 shows the apparatus used for
measuring the apparent mass while submersed in water.
Since the density of water at room temperature is 1 g/cc, the difference in the mass in air
and the mass in water is equal to the sample volume. In this way, the densities of the samples
were calculated by dividing the mass in air by the volume.
3.2.2.2 Scanning electron microscopy and energy dispersive spectroscopy
Secondary and backscatter electron images were taken using a JEOL JSM 6400 SEM with
Link ISIS digital image capturing system shown in Figure 3-9. Secondary electron images
reveal topographical information while backscatter electron images are used to show
compositional contrast. Images were taken of both polished and etched samples to reveal
microstructural characteristics and grain structures.
30
Energy dispersive spectroscopy was also performed with the JEOL JSM 6400 SEM. This
gives a qualitative assessment of the elements present in the sample. Under ideal conditions,
EDS can reveal elements at concentrations down to 0.1 wt%. Thus, it is theoretically possible to
see the dopant elements (present as oxides at 0.5 wt% nominally) as long as their spectra do not
overlap with the uranium spectrum.
3.2.2.3 Grain size determination
The most desirable method for analyzing the grain size of a material is to use a computer
program to automate the process. This is only possible for very high quality images of grain
structure as the programs identify the grain boundaries based on contrast within the image. None
of the images of etched samples were of sufficient quality to be analyzed by software. Instead,
grain sizes were calculated using a linear intercept method (Abrams, 1971). This involved
placing multiple lines at different orientations over images of the etched samples and counting
the number of intercepts along the line. Each grain that is crossed is counted as 1 and if the test
length ends inside of a grain it is counted as ½. For each sample, at least two images were used,
with approximately 15 test lengths used on each image to determine the grain size of the
specimen. For each image, the total number of intercepts counted was greater than 50. The
mean grain size result found in each image was averaged and the standard deviation was
computed.
3.2.2.4 X-ray diffraction
Samples were investigated using a Philips APD 3720 x-ray diffractometer, shown in
Figure 3-10. The XRD was operated at a voltage of 40 kV and current of 20 mA. The resultant
peaks were compared with known data for information about the constituents and phases present.
XRD spectra were taken of both the mixed powders prior to sintering and of the sintered pellets
31
(after pulverizing a small portion of them into powder). Lattice parameters were found for each
of the sintered pellets and compared to undoped UO2.
32
Table 3-1. Summary of dopant powders. Dopant # Material Supplier Purity
1 Nb2O5 Sigma Aldrich 99.99%2 Al2O3 Alfa 99.60%3 Cr2O3 Sigma Aldrich 99.90%4 Sc2O3 Alfa 99.90%5 Y2O3 Fisher Scientific 99.99%6 V2O5 Sigma Aldrich 99.99%7 TiO2 Sigma Aldrich 99.99%
Table 3-2. Masses of UO2 and dopant powders in sample mixtures. Pellet # Dopant Measured Mass UO2 (g) Measured Mass Dopant (g) Dopant wt%
1 - 9.16463 0 0.000%2 Nb2O5 4.13159 0.02076 0.500%3 Al2O3 9.10194 0.04579 0.501%4 Cr2O3 9.39934 0.04716 0.499%5 Sc2O3 9.11687 0.04580 0.500%6 Y2O3 9.11591 0.04576 0.499%7 V2O5 9.11214 0.04576 0.500%8 TiO2 9.10587 0.04540 0.496%
Table 3-3. Grinding and polishing sequence. Grit/Micron size Time (minutes) Pressure (psi) 120 grit 2–4 40400 grit 2 35600 grit 2 351200 grit 1.5 256 micron 1.5 Variable*1 micron 1.5 Variable*0.5 micron 1 Variable**Manual pressure applied
33
Figure 3-1. Sartorius R180D analytic balance used for mass measurements.
Figure 3-2. Hydraulic press used for cold uniaxial pressing of pellets.
34
Figure 3-3. Stainless steel die set used for cold pressing powders.
Figure 3-4. Base used to hold pellet during sintering.
35
Figure 3-5. Induction furnace, controller, pyrometer, and vacuum chamber.
Figure 3-6. LECO VC-50 precision low-speed diamond saw.
36
Figure 3-7. LECO Spectrum System 1000 grinder/polisher with semi-automatic head.
Figure 3-8. Setup for measuring apparent mass of sample while immersed in water.
37
Figure 3-9. JEOL JSM 6400 scanning electron microscope.
Figure 3-10. Philips APD 3720 x-ray diffractometer.
38
CHAPTER 4 RESULTS AND DISCUSSION
4.1 Density Measurements
The theoretical density of the fuel is slightly affected by the presence of the dopant species.
Thus, calculations of the theoretical densities of the samples were made by mass averaging the
densities of the UO2 and the dopant. The theoretical densities for each sample are presented in
Table 4-1.
The measured masses and calculated densities of the final sintered pellets are shown in
Table 4-2. The average density of the pellets is 10.39 g/cc. None of the samples had a density
lower than 93.35 %TD. Thus, the processing method used was successful in producing dense
pellets suitable for further analysis.
4.2 Pellet Characterization
4.2.1 Optical Microscopy
Optical microscopy was used to image the surface of the pellets after sintering. Figure 4-1
shows all eight pellets.
It is seen that the chromia-doped, scandia-doped, and titania-doped pellets each had a large
crack that propagated from the center to the edge of the pellet. The vanadia-doped pellet also
had a crack but it was less severe. These cracks are most likely due to excessively quick heat up
and cool down rates during the sintering process. Future processing methods must consider this
problem to avoid cracking the pellets, as cracked pellets would not be mechanically sound for
use in reactors. However, since this research was only concerned with microstructural
characterization, the cracked pellets are not a major issue for this project.
39
The surfaces of many of the pellets appear to contain multiple scratches and other flaws.
These arose from handling the pellets before and after sintering. All microstructural
investigation was performed on ground and polished internal cross-sections of the pellets.
4.2.2 Scanning Electron Microscopy
Images were taken of the samples before and after chemical etching. Prior to chemically
etching the samples, topographical and compositional features could be investigated to look for
any unexpected features or phases. After etching, the grain structure was revealed and grain
sizes could be analyzed. Figures 4-2 through 4-9 present the results.
Backscatter electron images showed no signs of compositional heterogeneity in any of the
doped samples. This does not mean that the mixture was perfectly homogeneous throughout the
sample as only small sections were imaged. In any case, it does not seem that any of the dopants
segregated severely within the samples.
Undoped UO2. For the undoped UO2 pellet, the microstructure reveals a low porosity
sample as expected from the high density measured. The lines seen across the sample in Figure
4-2 A are remnants of the grinding and polishing process and not a microstructural feature.
Figure 4-2 B shows two regions of the etched surface with grains revealed. A sulfuric acid
solution was used to etch the sample. The etchant did not work exceptionally well on the
undoped sample as seen by the roughness of the grain structure. Despite the poor etching, it was
still possible to measure the grain sizes with this sample. The average grain size of the undoped
UO2 sample was 2.73 μm with a standard deviation of 0.35 μm. This is significantly smaller
than typical grain sizes for undoped UO2 quoted in the literature, which average nearly 10 μm in
the studies shown in Table 2-1. The reason for this discrepancy is not known but could be a
result of differences in initial powder size distribution, processing parameters, or sintering
procedures. Due to this difference, the relative increase in grain size with each dopant will be
40
used as a more meaningful measurement, rather than a direct comparison of grain sizes with
other studies.
Nb2O5-doped UO2. The niobia-doped pellet seen in Figure 4-3 shows a low porosity
microstructure as well. This sample was etched with sulfuric acid and shows a more defined
grain structure than the undoped UO2. It is seen that some of the grains appear to have bumps on
the surface. This occurred with several samples and is thought to be a consequence of the
chemical etching process. The average grain size of the niobia-doped pellet was 5.94 μm with a
standard deviation of 0.58 μm. This is a relative increase of 117% over undoped UO2, a very
substantial increase. However, it is less than the average grain size increase found for other
studies using 0.5 wt% niobia, which resulted in increases of nearly 230% under similar
processing conditions. This could again be due to differences in powders or methodologies.
However, despite being less than the changes quoted by other studies, 117% increase in grain
size is significant for a 0.5 wt% addition of Nb2O5.
Al2O3-doped UO2. Figure 4-4 shows the alumina-doped pellet. The topography is again
seen to be low porosity with no unexpected features present. The etchant used was sulfuric acid.
Figure 4-4 B shows the etched regions with a relatively well defined grain structure. The
bumpiness noted for the niobia-doped pellet is also seen here. The average grain size of this
sample was 5.14 μm with a standard deviation of 0.50 μm. This is a relative increase of 88%
over the undoped UO2 pellet. Previous studies found an average increase of approximately 80%
with alumina, although these were done with lower concentrations than 0.5 wt%.
Cr2O3-doped UO2. Figure 4-5 A shows the chromia-doped UO2 prior to etching at two
magnifications. The topography of this sample is slightly more porous than other samples. This
pellet had the second lowest measured density at 93.61 %TD. Figure 4-5 B shows the etched
41
regions of the chromia-doped pellet. Sulfuric acid was used to etch this sample. The average
grain size of the pellet was 4.77 μm with a standard deviation of 0.58 μm. This is a 74% increase
over the undoped sample, significantly smaller than the increase seen by other studies using
chromia as a dopant. Studies using 0.5 wt% chromia and similar processing tactics found an
average relative increase in grain size of 550%. Previous studies have found evidence of a Cr-
Cr2O3 eutectic forming above 1550°C, which creates a liquid phase that enhances grain growth
during sintering. There has been evidence of Cr-rich spots appearing in the sintered pellet as a
result (Bourgeois et al., 2001). These Cr-rich spots were not found in this study using BSE
compositional contrast imaging, however EMPA may better elucidate this sort of phenomenon.
Sc2O3-doped UO2. No previous results for scandia-doped UO2 have been reported in the
literature. Figure 4-6 shows the microstructure of this pellet. In Figure 4-6 A, no unusual
microstructural features are observed. Figure 4-6 B shows the etched microstructure for this
sample. The etching was attempted with sulfuric acid four times, with the acid allowed to sit
different lengths of time varying from 10 seconds to 2 hours. None of these attempts provided
sufficient quality images to reveal grain structures. The images seen in Figure 4-6 B were
acquired after etching with a 1:1 mixture of 90% nitric acid and distilled water for approximately
20 seconds. While the quality of these images is still poor, grain size analysis could be executed.
The average grain size measured for the scandia-doped sample was 2.63 μm with a
standard deviation of 0.35 μm. This is a 4% decrease from the undoped UO2 sample. This
difference, however, is not statistically significant. As such, it is concluded that scandia-doping
had no significant effect on the grain size of UO2.
Y2O3-doped UO2. Figure 4-7 shows the yttria-doped pellet. It is seen in Figure 4-7 B that
the chemical etching failed to reveal the grain structure for this pellet. Both sulfuric and nitric
42
acid etchings were tried, but to no avail. As such, no grain size analysis can be performed on this
sample.
V2O5-doped UO2. Figure 4-8 shows the vanadia-doped UO2 sample. Again, no
unexpected microstructural features are seen in Figure 4-8 A. The grain structure in Figure 4-8
B was revealed by sulfuric acid etching. The average grain size of this sample was 3.41 μm with
a standard deviation of 0.13 μm. This is an increase over undoped UO2 of 25%. Previous
studies have reported results ranging from 50% decreases up to nearly 350% increases in grain
size due to vanadia additions. This study finds that the increase is modest at 25%, but it is a
statistically significant increase. This lends more evidence to the argument that vanadia could be
used as a dopant to increase grain sizes in UO2.
TiO2-doped UO2. Figure 4-9 shows the titania-doped pellet. It is seen in Figure 4-9 A
that the polish of this sample was not as successful as some of the others. However, it is still
possible to conclude that no unusual features are present. Figure 4-9 B shows the etched regions
of the sample. The nitric acid etchant was used on the pellet and the results are the best of any of
the etched samples. The grain boundaries are well defined and the grain structure is easily
visible throughout the images. It is not clear why the nitric acid was so successful in this
instance. The average grain size of this sample was 10.37 μm with a standard deviation of 1.19
μm. This is a 279% relative increase in grain size over the undoped UO2 pellet, by far the largest
increase seen for any of the dopants in this study. This is still slightly less than the average
increase seen by similar studies of 330%, but it is reasonably close.
Grain size statistical analysis. The doped samples’ grain sizes found in this study were
subjected to statistical analysis to confirm that they were significantly different from the undoped
sample’s grain size. This was done by using a two-sample t-test assuming equal population
43
variances. The assumption of equal variances is justified because while the grain sizes differ, the
distribution of sizes is probably relatively constant for each sample. Each doped sample was
tested against the undoped sample. The null hypothesis was that the mean grain size of the
doped sample was equal to the mean grain size of the undoped sample. The alternative
hypothesis was that the doped sample’s mean grain size was greater than that of the undoped
sample. In each case, the pooled standard deviation was used. This was calculated using (4-1).
s2 = ((n1-1)*s12 + (n2-1)*s2
2) / (n1 + n2 - 2) (4-1)
The results of the statistical analysis showed that the differences in mean grain sizes were
significant for the niobia-doped, alumina-doped, chromia-doped, vanadia-doped, and titania-
doped samples with respect to the undoped sample at a confidence level of 95%. The scandia-
doped sample was not significantly different. The results of this analysis are summarized in
Table 4-3.
4.2.3 Electron Dispersive Spectroscopy
EDS was used to identify composition of the samples after sintering. The spectra are
shown in Figures 4-10 through 4-17. EDS spectra were taken at an operating voltage of 25 kV
and operated for a live time of 300 seconds. All of the dopants are at least marginally visible in
the spectra except for vanadium. As mentioned earlier, the minimum elemental concentration
that can be detected by EDS is 0.1 wt%. The dopants are present at 0.5 wt% as oxides, with
metal contents therefore being even lower. It is thus very impressive that the dopants are seen in
these spectra. As EDS is only a qualitative tool, no quantitative information can be found from
these spectra.
4.2.4 X-Ray Diffraction
Powder XRD measurements were made before and after sintering. Several of the pre-
sinter measurements showed small traces of the dopant powders. Additionally, the UO2 powder
44
was slightly hyperstoichiometric prior to sintering, as expected from the powder analysis shown
in Appendix A which measured the received powder to be UO2.10. The post-sinter results show
only stoichiometric UO2 peaks for all of the samples, signifying that the powder was reduced by
the hydrogen in the sintering environment and that the dopant powders were integrated into the
matrix. Any amount of dopant that remained segregated, if any, was too small an amount to be
detected using XRD. The lattice parameters were calculated from the spectra using measured d-
values from the (331) plane of UO2. The lattice parameter a0 is related to the d-value and (hkl)
plane in cubic lattices by (4-2) (Brundle et al., 1992).
a0 dhkl h2 + k2 + l2)1/2 (4-2)
The lattice parameters are shown in Table 4-4. In all of the doped samples, the lattice
parameter decreased compared to undoped UO2. This change in lattice parameter further
confirmed that species other than pure UO2 are present in the samples.
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Table 4-1. TD’s of doped samples Sample Dopant Dopant Density (g/cc) TD of doped sample (g/cc)
1 Undoped - 10.9602 Nb2O5 4.60 10.9283 Al2O3 3.96 10.9254 Cr2O3 5.22 10.9315 Sc2O3 3.86 10.9256 Y2O3 5.03 10.9307 V2O5 3.35 10.9228 TiO2 4.23 10.926
Table 4-2. Pellet masses and densities. Sample # Dopant Mass in air (g) Mass in water (g) Volume [=Δm] (cc) Density (g/cc) %TD
1 Undoped 4.47979 4.05572 0.42407 10.56 96.39%2 Nb2O5 3.59458 3.24769 0.34689 10.36 94.82%3 Al2O3 4.33465 3.91698 0.41767 10.38 94.99%4 Cr2O3 3.98003 3.59108 0.38895 10.23 93.61%5 Sc2O3 3.61022 3.2562 0.35402 10.20 93.35%6 Y2O3 4.55315 4.12305 0.43010 10.59 96.85%7 V2O5 4.09821 3.70537 0.39284 10.43 95.52%8 TiO2 3.89407 3.51828 0.37579 10.36 94.84%
Table 4-3. Results of statistical analysis for pellet grain sizes Sample Dopant Pooled St Dev p-value (95% confidence) Confidence Interval
2 Nb2O5 0.474 0.0000 (2.39, 4.03)3 Al2O3 0.431 0.0001 (1.66, 3.15)4 Cr2O3 0.416 0.0024 (1.04, 3.04)5 Sc2O3 0.347 0.6200 (-0.93, 0.73)7 V2O5 0.259 0.0051 (0.28, 1.13)8 TiO2 0.796 0.0000 (6.08, 9.21)
46
Table 4-4. Lattice parameters of sintered pellets Sample Dopant a0 (Å)
1 Undoped 5.467062 Nb2O5 5.466893 Al2O3 5.462754 Cr2O3 5.462145 Sc2O3 5.462146 Y2O3 5.459397 V2O5 5.457218 TiO2 5.45756
47
Figure 4-1. Optical images of pellet surfaces post-sinter. Pellet labels in top left corners.
48
A
B
Figure 4-2. Undoped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
49
A
B
Figure 4-3. Nb2O5-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
50
A
B
Figure 4-4. Al2O3-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
51
A
B
Figure 4-5. Cr2O3-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
52
A
B
Figure 4-6. Sc2O3-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
53
A
B
Figure 4-7. Y2O3-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
54
A
B
Figure 4-8. V2O5-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
55
A
B
Figure 4-9. TiO2-doped UO2. A) Before etching. B) After etching. Images on the right are at higher magnifications than the images to their left.
56
Figure 4-10. EDS spectrum of undoped UO2 sample.
Figure 4-11. EDS spectrum of Nb2O5-doped sample.
57
Figure 4-12. EDS spectrum of Al2O3-doped sample.
Figure 4-13. EDS spectrum of Cr2O3-doped sample.
58
Figure 4-14. EDS spectrum of Sc2O3-doped sample.
Figure 4-15. EDS spectrum of Y2O3-doped sample.
59
Figure 4-16. EDS spectrum of V2O5-doped sample.
Figure 4-17. EDS spectrum of TiO2-doped sample.
60
CHAPTER 5 CONCLUSIONS AND FUTURE WORK
5.1 Pellet Processing
The processing method used in this research was successful in making dense pellets
suitable for microstructural examination. The process was streamlined and repeatable and the
resulting bulk pellet properties did not vary significantly. The initial cold pressing was
unsuccessful as the stainless steel pieces were neither polished enough nor strong enough to
create green pellets of acceptable quality. The ICL KBr die set remedied this issue, producing
high quality green pellets from the powders. The sintering process caused cracking in several
pellets, most likely due to excessively quick heat up and cool down rates. This was not a
hindrance for the remaining characterization but would be unacceptable for reactor fuel.
5.2 Grain Size Analysis
The etching process for revealing grain boundaries in UO2 was wildly inconsistent
between samples, even for different sections of the same sample. It was found that an excellent
polish is required to have any chance of revealing grains, but even this does not guarantee a
successful etch. Both the sulfuric acid and nitric acid solutions were used successfully, but both
of them failed more often than they succeeded. The discovery of an etchant that could provide
repeated, high quality results would be extremely useful for this kind of analysis.
Table 5-1 summarizes the results of the grain size analysis. As mentioned above, all of the
average grain sizes measured in this study are smaller than typical values found in other studies.
The relative increases in grain sizes for each dopant are typically smaller as well. However, the
increases do show that a very small addition of a dopant to UO2 powder can significantly
increase the grain size of the sintered pellet, which in turn can improve fission gas retention and
swelling properties. This result should be kept in mind when designing high-burnup fuel for
61
current and next generation reactors that utilize uranium dioxide as their fuel. The dopants that
seemed to be most effective in this study were niobia and titania, with titania showing the largest
grain size increase of 279% over the undoped sample. The yttria-doped pellet did not yield grain
size measurements due to ineffective etching. However, this dopant cannot be discounted as it
has never been investigated. Scandia proved to have no significant effect on the grain size,
therefore it should not be considered further.
5.3 Future Work
There is no shortage of further work to be done in the study of doped fuel. Factors that
could affect the results include powder mixing methods, dopant concentrations and solid
solubility limits, cold pressing methods and pressures, and sintering conditions. For mixing
methods, it has been suggested by the work of Bourgeois et al. (2001) that more homogeneous
mixing can lead to further increases in grain size due to the better distribution of the dopant
within the matrix of the material. The ultimate attempt at homogenous mixing may be to coat
the UO2 powder particles with the dopant, either through chemical vapor deposition or atomic
layer deposition. This would provide further insight into the mechanisms of grain size increase
due to the dopants.
The obvious next step in this research is irradiation testing of doped fuel samples.
Previous studies that have looked at irradiating doped fuel have in many cases confirmed a
decrease in fission gas release and fuel swelling during irradiation, making possible higher
burnups (Arborelius et al., 2006; Delafoy et al., 2007; Harada, 1996; Kashibe et al., 1998;
Killeen, 1975; Turnbull, 1974; Yuda et al., 1997). Other improvements due to the presence of
dopants have been noted as well. Delafoy et al. (2007) found that doping UO2 with chromia
improved the pellet-clad interaction properties of the fuel.
62
Several possible drawbacks should also be looked for when investigating doped fuel under
irradiation. Niobia, while being a good promoter of grain size increase, has also been found to
increase the rare gas diffusion coefficient in UO2 (Killeen, 1975). This causes the fission gases
to diffuse more quickly within the fuel matrix, offsetting the benefits of the increased grain size.
5.4 Final Comments
The possibility of low quantity dopant additions providing an avenue to a high burnup UO2
based fuel is extremely attractive. Uranium dioxide is the most qualified fuel on the planet and
the ability to improve upon it by such a small modification must be appreciated. A fuel
qualification program for doped-UO2 would not be nearly as intensive as one for a new fuel
form. Thus, doped UO2 could improve fuel cycle economy for existing and future reactors in
less than a decade. This is an option that should be pursued with further research to fulfill its
enormous potential.
63
Table 5-1. Summary of grain size analysis. Sample Dopant Mean Grain Size (μm) St Dev (μm) % Error % Change
1 Undoped 2.73 0.35 12.6% -2 Nb2O5 5.94 0.58 9.7% 117%3 Al2O3 5.14 0.50 9.8% 88%4 Cr2O3 4.77 0.58 12.1% 74%5 Sc2O3 2.63 0.35 13.4% -4%6 Y2O3 - - - -7 V2O5 3.41 0.13 3.7% 25%8 TiO2 10.37 1.19 11.4% 279%
64
APPENDIX POWDER CHARACTERISTICS
The initial powder information was obtained from Dr. Jiwei Wang who analyzed the UO2
powder received from Framatome/AREVA. The initial oxygen-to-uranium ratio was found by
oxidizing the powder to U3O8 and measuring the weight change. The change indicated an initial
O/U ratio of 2.10.
The particle size distribution of the received UO2.10 powder was characterized by sieve
analysis. Ten grams of the powder were sieved through a series of screens, which were then
weighed. The analysis was done three times and the average values are plotted in Figure A-1.
Particle Size (micron)
Rel
ativ
e N
umbe
r
0 25 50 75 100 125 150 175 200 225 2500
0.2
0.4
0.6
0.8
1
1.2
Figure A-1. Particle size distribution of received uranium dioxide powder.
65
LIST OF REFERENCES Abrams, H., 1971. Grain size measurement by the intercept method. Metallography 4, 59-78. Ainscough, J., Raven, L., Sawbridge, P., 1978. Int. Symp. on water reactor fuel fabrication with special emphasis on its effect on fuel performance. Prague: IAEA-SM233. Ainscough, J., Rigby, F., Osborn, S., 1974. The effect of titania on grain growth and densification of sintered UO2. J. Nucl. Mater. 52, 191-203. Amato, I., Colombo, R., Petruccioli Balzari, 1966. Grain growth in pure and titania-doped uranium dioxide. J. Nucl. Mater. 18, 252-260. Amato, I., Ravizza, M., 1967. The effect of vanadium oxide additions on sintering and grain growth of uranium dioxide. J. Nucl. Mater. 23, 103-106. Arborelius, J., Backman, K., Hallstadius, L., Limback, M., Nilsson, J., Rebensdorff, B., et al., 2006. Advanced doped UO2 pellets in LWR applications. J. Nucl. Sci. Tech. 43 (9), 967-976. Assmann, H., Dorr, W., Gradel, G., Maier, G., Peehs, M., 1981. Doping UO2 with niobia - beneficial or not? J. Nucl. Mater. 98, 216-220. Aybers, M., Aksit, A., Akbal, S., Ekinci, S., Yayli, A., Colak, L., et al., 2004. Grain growth in corundum-oxides doped uranium dioxide and effects of grain growth to the mechanical properties of uranium dioxide such as elasticity determined by ultrasonic methods. Key Eng. Mater. 264-268, 985-988. Booth, A., 1957. A method of calculating fission gas diffusion from UO2 fuel and its application to the X-2-f test loop. AECL-496. Bourgeois, L., Dehaudt, Ph., Lemaignan, C., Hammou, A., 2001. Factors governing microstructure development of Cr2O3-doped UO2 during sintering. Journal of Nuclear Materials 297, 313-326. Brundle, C., Evans, C., Wilson, S., 1992. Encyclopedia of Materials Characterization. Greenwich: Manning. Delafoy, C., Dewes, P., Miles, T., 2007. AREVA NP Cr2O3-doped fuel development for BWRs. Proceedings of the 2007 International LWR Fuel performance Meeting. San Francisco, CA. DOE, 2008. Advanced Fuel Cycle Initiative. Retrieved from U. S. Department of Energy: http://www.ne.doe.gov/afci/neAFCI.html Harada, Y., 1996. Sintering behaviour of niobia-doped large grain UO2 pellet. J. Nucl. Mater. 238, 237-243.
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Kashibe, S., Une, K., 1998. Effect of additives (Cr2O3, Al2O3, SiO2, MgO) on diffusional release of Xe-133 from UO2 fuels. J. Nucl. Mater. 254, 234-242. Killeen, J., 1980. Fission gas release and swelling in UO2 doped with Cr2O3. J. Nucl. Mater. 88, 177-184. Killeen, J. 1975. The effect of additives on the irradiation behaviour of UO2. J. Nucl. Mater. 58, 39-46. Leenaers, A., de Tollenaere, L., Delafoy, Ch., Van den Berghe, S., 2003. On the solubility of chromium sesquioxide in uranium dioxide fuel. J. Nucl. Mater. 317, 62-68. NEI, 2008. Resources and Stats. Retrieved from U.S. Nuclear Power Plants: http://www.nei.org/resourcesandstats/nuclear_statistics/usnuclearpowerplants/ Ohai, D., 2003. Large grain size UO2 sintered pellets obtaining used for burnup extension. Transactions of the 17th International Conference on Structural Mechanics in Reactor Technology. Prague, Czech Republic. Petzow, 1978. Metallographic Etching. Metals Park, Ohio: American Society of Metals. Radford, K., Pope, J., 1983. UO2 fuel pellet microstrcuture modification through impurity additions. J. Nucl. Mater. 116, 305-313. Sawbridge, P., Reynolds, G., Burton, B., 1981. The creep of UO2 fuel doped with Nb2O5. J. Nucl. Mater. 97, 300-308. Turnbull, J., 1974. The effect of grain size on the swelling and gas release properties of UO2 during irradiation. J. Nucl. Mater. 50, 62-68. Yuda, R., Harada, H., Hirai, M., Hosokawa, T., Une, K., Kashibe, S., et. al., 1997. Effects of pellet microstructure on irradiation behavior of UO2 fuel. J. Nucl. Mater. 248, 262-267.
67
BIOGRAPHICAL SKETCH
Brett Jameson Dooies was born in West Palm Beach, FL, on March 3, 1984. In 2002, he
graduated from the Alexander W. Dreyfoos, Jr. School of the Arts ninth in his class. He attended
the University of Florida, as an honors student, from 2002 to 2006, where he participated in both
academic and leadership activities. Brett was awarded a Bachelor of Science degree in nuclear
and radiological engineering in December 2006, graduating cum laude. He spent the next year on
a graduate assistantship with the University of Florida, beginning the initial stages of this
research project. He was then awarded funding from the Advanced Fuel Cycle Initiative of the
Department of Energy, a generous fellowship that allowed him to complete this research and
present his findings at national conferences. From 2007–2008, Brett served as the student
conference proposal chair for the UF chapter of the American Nuclear Society, an endeavor that
secured the annual ANS student conference for UF in 2009. In August 2008, he received his
Master of Science degree in nuclear engineering sciences from the University of Florida. In July
2008, Brett started as a member of General Electric’s Edison Engineering Development Program
in Wilmington, NC.