Appendix 12: Oxidation Tests on U3Si2, UN and UO2 Powders in 20% O2/80% He and Steam Thermal Analysis of Accident Tolerant Fuel Materials
Introduction and Experimental Approach
The proposed U bearing Accident Tolerant Fuel (ATF) forms UN, U3Si2, and composites of the
two compounds, offer significant potential for improvement as LWR fuel over the current nuclear
fuel standard UO2, specifically in thermal conductivity and fissile content. However these
compounds have never before been applied as fuel for LWR use, specifically in high density
pellet form. Significant investigation and understanding of these compounds is required to
transition them to functional LWR fissile fuel.
To this effect, an experimental thermal analysis investigation of these two compounds was
undertaken to quantitatively determine the response of these compounds to elevated
temperatures in inert and oxidizing atmospheres. A Netzsch 449 F3 Jupiter simultaneous
thermal analyzer (STA) was used to measure reaction temperatures and energetics of these
reactions as a function of temperature and atmosphere using thermogravimetry (TG) and
differential scanning calorimetry (DSC). TG and DSC signals were collected individually (TG) or
simultaneously (TG + DSC) in gettered He, synthetic air (20% O2 - balance He), and ~100%
water vapor. Fuel material samples were heated at 10oC/min to 1250oC, held for ~10 minutes,
and then cooled to room temperature. This constant heating rate experiment allows for
quantitative determination of reaction temperatures and energies, which can then be used to
guide isothermal hold thermal analysis experiments at temperatures around the reaction
temperatures. For this investigation, samples of UO2 were analyzed in addition to U3Si2 and UN
for relative comparison. Considering the historic and continued future standard that UO2
represents as a LWR fuel in both fabrication and operation, it is important to understand the
differences between UO2 and the two proposed non-oxide fuel materials.
Experimental Results
Figure 1 presents typical TG and DSC signals as a function of time for a constant 10oC/min
ramp to 1250oC, 10 min. hold at 1250oC, and subsequent cooling for UO2 powder. Note that
there are 2 mass increase steps in the TG signal and 2 energy valleys in the DSC signal. These
results confirm previous results that the oxidation of UO2 to U3O8 is a 2-step reaction and that
both reactions are exothermic. (1) Figure 2 presents typical TG and DSC signals as a function
of time for a constant 10oC/min ramp to 1250oC for U3Si2 powder and Figure 3 presents similar
TG and DSC signals for UN powder. Note that in Figures 2 and 3, both TG and DSC signals
show single mass increase steps and 1 energy valley indicating that U3Si2 and UN oxidize to
U3O8 as single step, exothermic reactions.
Table 1 presents thermal analysis results for UO2 in powder and pellet form and Table 2
presents thermal analysis results for U3Si2 and UN in powder form only. These results include
the following measured values using the indicated techniques.
value measured
technique
 mass, in % of initial
Tox, i, oxidation reaction initiation temperature in oC
RXN enthalpy, reaction enthalpy in Vs/mg
TG
TG and DSC
DSC
Pellet material analysis was performed using TG only due to the size of typical sectioned pellet
samples. Additionally, TG only was used for analysis in water vapor due to equipment
restrictions when using the water vapor furnace.
Figure 1. Typical TG + DSC signal for 10oC/min
ramp to 1250oC for UO2 powder. Red plot is
sample temperature in oC, green plot is TG data
mass gain in mg, and blue plot is DSC data
in V/mg.
Figure 2. Typical TG + DSC signal for 10oC/min
ramp to 1250oC for U3Si2 powder. Red plot is
sample temperature in oC, green plot is TG data
mass gain in mg, and blue plot is DSC data
in V/mg.
Figure 3. Typical TG + DSC signal for 10oC/min
ramp to 1250oC for UN powder. Red plot is
sample temperature in oC, green plot is TG data
mass gain in mg, and blue plot is DSC data in V/mg.
Table 1. Thermal analysis results for UO2 powder and sintered pellet form.
Table 2. Thermal analysis results for U3Si2 and UN in powder form.
Results Discussion
For UO2 tested in both powder and pellet forms, no oxidation reaction was observed when
heating to 1250oC in gettered He. The He cover gas used is actively gettered to an O2 level on
the order of 10-12 ppm of O2. These results prove that it is possible to suppress the oxidation
reaction of U at elevated temperatures in a sufficiently O2 free environment.
Both U3Si2 and UN are shown to exhibit single step oxidation reactions in synthetic air to the
stable U oxide U3O8. The reaction sequence for these 2 compounds can be expressed as
follows.
U3Si2, UN --> U3O8
(i)
In comparison, UO2 exhibits a two-step oxidation reaction to U3O8 and this reaction sequence
can be expressed as follows. (1)
UO2 --> U3O7 -->
U3O8
(ii)
For the work performed here, only UO2 was tested in sintered pellet form and this material
exhibited a single step oxidation reaction as shown in table 1. A speculative reason for this
result is that a certain amount of U3O8 powder is added to UO2 powder as a sintering aide
during fuel pellet fabrication and the U3O8 content could suppress the UO2 --> U3O7 reaction
step. U3Si2 and UN were not analyzed in sintered pellet form because it was determined that
oxidation of these materials in solid form was too exothermic and literally threw sample material
out/off of the sample holder in the STA during the oxidation reaction.
Results presented in table 1 show that UO2 gains 3 to 4% of its original mass during oxidation to
U3O8 as either powder or pellet form in synthetic air. Compared to UO2, U3Si2 and UN powders
gain significantly more mass during oxidation to U3O8, approximately 21 and 11% of initial mass
respectively in synthetic air as shown in table 2. Additionally, U3Si2 and UN powders gain
approximately 17 and 7 % of initial mass during oxidation to U3O8 in steam also shown in table
2. Qualitatively, the larger mass gain of U3Si2 and UN during oxidation compared to that of UO2
can be explained in that U3Si2 and UN are non-oxide compounds of U. UO2 is already an oxide
form of U, however not the equilibrium oxide U3O8. Thus, more O2 is consumed during the
oxidation of U3Si2 and UN than during the oxidation of UO2 to U3O8 because UO2 is in a nonequilibrium oxide state. Materials balance calculations would confirm these experimental results.
Tables 1 and 2 present the temperatures at which the oxidation reaction initiates (Tox, i) for UO2,
U3Si2, and UN in synthetic air, and in steam for U3Si2 and UN. For UO2, Tox, i is presented for
both the first and second steps of the reaction. Also shown in Table 1, the enthalpy of these 2
oxidation reaction steps are relatively the same. So it can be stated that oxidation of UO2
powder begins at approximately 165oC with a second step in the oxidation reaction at 348oC.
Note that the single step oxidation reaction in synthetic air for sintered UO2 in pellet form was
determined to be 453oC. Presently, it is not clear why Tox, i for solid UO2 is so much higher than
that of powdered UO2. The Tox, i of U3Si2 is 351oC and that of UN is 253oC both in synthetic air
with values shown in table 2. Additionally, the Tox, i of U3Si2 and UN in steam are 429 and 351oC
respectively. Thus the temperatures at which UO2, U3Si2, and UN in powder form oxidize can be
ranked as follows.
Tox, i (syn. air) = UO2 < UN < U3Si2
Tox, i (steam) = UN < U3Si2
These results indicate that of the fuel compounds analyzed in this work, U3Si2 has the highest
resistance to oxidation as a function of temperature in both synthetic air and steam. As stated
previously, solid sintered pellet form of U3Si2 and UN were not analyzed in this work because
their oxidation reactions are too exothermic. Future analysis of these materials in solid, sintered
form would be most beneficial since this is the form of an LWR fuel pellet,
The enthalpy of the oxidation reactions for all three fuel compounds in synthetic air are
presented in tables 1 and 2. All oxidation reactions are exothermic; i.e.; negative reaction
enthalpy values. These values for UO2, U3Si2, and UN in powder form oxidized in synthetic air
can be ranked as follows.
RXN enthalpy (syn. air) = UO2 << UN < U3Si2
Here the reaction enthalpy of UO2 is presented as the sum of the values of the first and second
steps of the oxidation reaction to U3O8 and this value is one order of magnitude smaller than the
reaction enthalpy’s of U3Si2 and UN. Note that reaction enthalpy’s in steam were not determined
because these values are measured using DSC and the DSC sample support cannot be used in
the STA water vapor furnace. Clearly, all three U bearing fuel compounds analyzed here oxidize
exothermically with the non-oxide fuel compounds (U3Si2 and UN) exhibiting very exothermic
oxidation reactions.
Summary
The experimental analysis results presented here clearly show the reactive nature of U bearing
fuel compounds. While the current LWR fuel standard UO2 does oxidize exothermically at
relatively low temperature, the proposed ATF fuel compounds U3Si2 and UN oxidize at higher
temperatures but are one order of magnitude more reactive as measured by the oxidation
reaction enthalpy. While these results were generated on powder form of these compounds, it is
offered that the relative Tox, i and oxidation reaction enthalpy’s should extrapolate to solid pellet
form. Obviously this requires experimental proof.
All experiments in this study were performed on monolithic fuel compound materials. It has been
proposed that composites of U3Si2 and UN could improve the resistance to reaction with water
of these fuel materials. Based on the results of this work, compositing U3Si2 and UN will have
very little or no effect on the oxidation resistance of these two compounds. While U3Si2 does
oxidize at a higher temperature than UN, the reaction of U3Si2 to U3O8 is more exothermic than
UN. Thermal analysis work to determine the Tox, i and oxidation reaction enthalpy of composites
of U3Si2 and UN, either in powder or sintered pellet form, should be performed.
An important conclusion from this work is that, while UO2 is a reactive fuel material and requires
proper handling during processing, the proposed ATF fuel compounds U3Si2 and UN are
significantly more reactive. Thus proper precautions should be taken during the processing and
handling of these compounds and experimental efforts such as presented here can be helpful.
References
1. McEachern R.J. and Taylor, P., “A Review of the Oxidation of Uranium Dioxide at
Temperatures Below 400oC”, AECL-11335, January 1997, Atomic Energy of Canada Limited.