Thermogravimetric study of the reaction of uranium oxides with fluorine

Article abstract of DOI:10.1016/j.jallcom.2007.04.099

Thermogravimetric study of the reaction of uranium oxides with fluorine was conducted by TG-DTA method using anti-corrosion type differential thermobalance. When UO₂ was heated from R.T. to 500 °C in 5% F₂/He atmosphere, the weight increase appeared at ca. 250 °C with an exothermic peak. Then the weight decreased slightly with a small exothermic peak followed by the complete volatilization with a large exothermic peak at ca. 350 °C. At a flow rate of 15, 30, 60 ml min^-1, there seemed to be no significant change for the fluorination of UO₂. With the different heating rates of 1, 2, 5 and 10 °C min^-1, the fluorination peak shifted to higher temperature with increasing heating rates. For the comparison with thermogravimetric results, phase analysis by XRD method was conducted for the products obtained at different temperatures. At 260 °C, the product was UO₂ with a small amount of the intermediate compound, UO₂F. The amount of this compound increased with increasing temperature up to 320 °C. Then another phase of UO₂F₂ appeared at 340 °C but it was immediately fluorinated to the volatile fluoride. When U₃O₈ was used as a starting material, it was found that the steep weight decrease peak appeared at ca. 350 °C and the uranium volatilized completely. This result suggests that fluorination of U₃O₈ occurs at this temperature forming UF₆. Uranium trioxide showed the similar fluorination behavior to that of U₃O₈.

Full text of DOI:10.1016/j.jallcom.2007.04.099

Journal of Alloys and Compounds 451 (2008) 673–675
Thermogravimetric study of the reaction of uranium oxides with fluorine
∗
Motohiro Komura, Nobuaki Sato , Akira Kirishima, Osamu Tochiyama
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai 980-8577, Japan
Available online 19 April 2007
Abstract
Thermogravimetric study of the reaction of uranium oxides with fluorine was conducted by TG-DTA method using anti-corrosion type differential
◦ ◦
2 2
was heated from R.T. to 500 C in 5% F /He atmosphere, the weight increase appeared at ca. 250 C with an exothermic
thermobalance. When UO
peak. Then the weight decreased slightly with a small exothermic peak followed by the complete volatilization with a large exothermic peak
◦
−1
2
at ca. 350 C. At a flow rate of 15, 30, 60 ml min , there seemed to be no significant change for the fluorination of UO . With the different
◦
−1
heating rates of 1, 2, 5 and 10 C min , the fluorination peak shifted to higher temperature with increasing heating rates. For the comparison
with thermogravimetric results, phase analysis by XRD method was conducted for the products obtained at different temperatures. At 260 C, the
◦
product was UO
up to 320 C. Then another phase of UO
as a starting material, it was found that the steep weight decrease peak appeared at ca. 350 C and the uranium volatilized completely. This result
2
with a small amount of the intermediate compound, UO
2
F. The amount of this compound increased with increasing temperature
◦
◦
2
F
2
appeared at 340 C but it was immediately fluorinated to the volatile fluoride. When U
3
O
8
was used
◦
suggests that fluorination of U
3
O
8
occurs at this temperature forming UF
6
. Uranium trioxide showed the similar fluorination behavior to that of
U
3 8
O .
©
2007 Elsevier B.V. All rights reserved.
Keywords: Uranium; Oxide; Fluoride; Fluorine; Thermogravimetry
◦
1
. Introduction
of UO3 with HF gas at 350 C. And this UO2F2 is fluorinated
◦
to UF by fluorine at 270 C. UF is obtained by the reaction of
6
6
◦
For overcoming the problems associated with the current
UO2 with fluorine at 500 C. However, the reaction of UO2 with
fluorine at lower temperature has not been studied yet. In this
paper, fluorination behavior of uranium oxides, UO2, U3O8 and
UO3 with fluorine was studied by thermogravimetric method
using anti-corrosion type differential thermobalance.
hydrochemical reprocessing process for spent nuclear fuels,
severalpyrochemicalprocessesareproposedandexamined. Flu-
oride volatility process is one of such processes, which are in
the plant scale development stage. In this process, fluorination
of spent UO2 fuel by fluorine is considered followed by the sep-
aration of UF and PuF from fission products by volatilization.
2. Experimental
6
6
Many studies on the formation of uranium fluoride such as UF4
2
.1. Sample preparation
and UF from oxides have been reported [1,2]. However, for-
6
mation and fluorination of the U–O–F compounds which may
appear in the fluorination of UO2 are not well studied. Uranyl
◦
U3O8 was obtained by the oxidation of U metal turnings in air at 800 C.
◦
Stoichiometric UO2 was prepared by the H2 reduction of U3O8 at 1000 C. UO3
was obtained by the dissolution of U O in 1-M HNO followed by heating in air
at 350 C. Analytical grade 5% F –He gas obtained from CENTRAL GLASS
fluoride, UO2F2 is usually formed by the hydrolysis of UF with
6
3
8
3
◦
H2O either in the gaseous or liquid states. During the hydrol-
2
ysis, several compounds such as UOF4 [3,4], U2O3F [5] and
Co., Ltd. and argon gas of 99.9999% purity from Nippon Sanso Co., Ltd. were
6
used as received.
U3O F8 [5] are formed by controlling the steam pressure pre-
5
cisely. But uranium only takes on a hexavalent state in the above
2
.2. Thermogravimetric analysis
compounds since they are formed from UF in oxidation condi-
6
tions. On the other hand, UO2F2 is also formed by the reaction
For the thermogravimetry of the reaction of UOx with F2, the RIGAKU TG-
DTA system, Thermoplus 2, which was installed in a high purity argon glove
box, was used. About 10 mg of uranium oxide sample was put in a Ni sample pan
and set in the sample holder of TG-DTA apparatus. Before heating, the TG-DTA
system was evacuated to approximately 100 Pa for 30 min and then refilled with
∗
Corresponding author. Tel.: +81 22 217 5142; fax: +81 22 217 5143.
E-mail address: dragon@tagen.tohoku.ac.jp (N. Sato).
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.04.099

6
74
M. Komura et al. / Journal of Alloys and Compounds 451 (2008) 673–675
Ar up to ambient pressure. After that, 5% F2–He gas mixture was introduced.
◦
Then the sample was heated from room temperature to 500 C at constant rates.
◦
−1
The heating rates were 1, 2, 5 and 10 C min , and the flow rates 15, 30 and
−
1
6
0 ml min
.
2
.3. Phase analysis
For the phase analysis of the products appeared in the TG-DTA experiments,
fluorination experiment with ca. 500 mg sample was carried out with the same
reaction condition as those of TG-DTA. 2.4 X-ray diffraction analysis. Then
the phase relation of the products was carried out by the X-ray powder diffrac-
tion with a Rigaku Type RAD-IC diffractometer using Cu K␣ radiation(40 kV,
2
0 mA) monochromatized by curved pyrolytic graphite.
3
. Results and discussions
Fig. 2. XRD results of the products obtained by the reaction of UO2 with F2 at
different temperatures. (᭹) UO2(JCPDS41-1442), (ꢀ) UO2F2, (ꢁ) cover film.
3
.1. Thermogravimetry of UO2
Fig. 1 shows the TG-DTA curves for the reaction of UO2 with
◦
−1
−1
phase (᭹) and the cover film (ꢁ). The peaks for new phase
F2 ataheatingrateof5 C min withaflowrateof30 ml min
.
◦
seem to be anhydrous UO2F2 [5]. In the paper of U3O F8 [6],
5
It is seen that the weight increases slightly (ca. 7%) at ca. 200 C
◦
the d values for this compound were reported. The XRD date
of the prepared UO2F2 is given in the Fig. 3 compared with
with a middle exothermic peak at 250 C. Then a slow weight
◦
decrease with small exothermic peak between 300 and 350 C
◦
the reported values. The strongest peak of UO2F2 at around 16
was observed followed by the rapid weight decrease between
◦
◦ ◦
was not observed in the 350 C product. This suggests that once
3
50 and 400 C. At 400 C, the weight change value is closed
the UO2F2 is formed, fluorination and volatilization seems to be
fast.
to −100%, i.e. the sample volatilized completely. The weight
◦
increase at 200 C is very close to that (7.04%) of formation
Since such oxyfluorides were prepared by the reaction of
of UO2F from UO2. As mentioned in Section 1, if UO2F2 is
formed in F2 atmosphere, it would be immediately fluorinated
UF with H2O, the number of fluorine decreases from 6 to 2.
6
In the above UOxFy compounds, uranium takes hexavalent and
the number of fluorine, y, is larger than 2. But in the case of
fluorination of UO2, the obtained UOxFy should be x > 2 and
to UF and be removed by sublimation. In the temperature region
6
◦
between 200 and 350 C, formation of intermediate UOxFy and
fluorination of such UOxFy to UO2F2 seems to be competitive.
So, in this region, the phase relation of the product obtained at
different temperatures was analyzed by powder X-ray diffrac-
◦
y < 2, since UO2F2 is formed at ca. 350 C and it immediately
transformed to UF . From the above results, the fluorination of
6
◦
UO2 to UF by F2 may proceeds via UO2F, UO2F2.
6
tion method. The results are given in Fig. 2. From 280 to 320 C,
the main phase was UO2, while small peaks at the high angle
side of the UO2 peaks seemed to appear. From the TG results,
the amount of fluorine increased in this region. At 340 C, the
3.2. Effects of conditions on thermogravimetry
◦
obtained sample was too hygroscopic to conduct the XRD anal-
ysis, the sample was handled in the argon glove box, i.e. sample
on the XRD plate was covered by the film for avoiding the con-
tact with moisture during the measurements. In the XRD result
Fig. 4 shows the effect of flow rates on the TG curves of the
◦
−1
reaction of UO2 with F2 with a heating rate of 5 C min . The
different flow rates of F2 were 15, 30 and 60 ml min 1. Since no
significant difference was observed in these three TG curves, the
amount of F2 was found to be enough for the TG of the reaction
of UO2 with F2 even at a low flow rate.
−
◦
◦
of 340 C sample, new peaks at around 2θ = 43, 46, 50 and 55
ꢀ) were observed as well as the peaks correspond to the UO2
(
Fig. 5 shows the effect of heating rates on the TG curves of
the same reaction as mentioned above. As seen in Fig. 5, these
three TG curves are similar, while the curve itself shifted to
Fig. 1. TG curves obtained by the reaction of UO2 with F2 at a heating rate of
◦
−1
2 2
Fig. 3. XRD profile of the prepared UO F compared with reported value.
5
C min
.

M. Komura et al. / Journal of Alloys and Compounds 451 (2008) 673–675
675
◦
−1
of 5 C min . TG curve for UO2 obtained with the same con-
ditions is also shown for comparison. In the case of UO3, it is
◦
seen that sharp weight decrease appears at ca. 350 C, and the
sample volatilizes completely, i.e. formation and sublimation of
UF . If oxygen in UO3 is replaced by fluorine without vapor-
6
ization, the weight should be increased. However, no significant
weight increase was observed before the weight decrease at ca.
◦
3
50 C. When the sample was UO2F2, similar TG curve was
obtained. From these results, it is suggested that the UO3 reacts
with F2 forming UO2F2 first, but the fluorination of UO2F2 to
UF is too fast to keep it as a stable intermediate product. In
the case of U3O8, TG curve was found to be similar to that for
6
Fig. 4. Effect of flow rates on the TG curves of the reaction of UO2 with F2.
UO3. However, the slight weight increase of the TG curve for
◦
U O8 at temperatures between 300 and 350 C suggests the dif-
3
ference in fluorination behavior between U3O8 and UO3. This
is caused by the oxidation of tetravalent uranium in U3O8 to
hexavalent.
4. Conclusion
To investigate the fluorination of UO2 in spent fuel by fluo-
ride volatility process, fluorination behavior of uranium oxides,
such as UO2, U3O8 and UO3 with fluorine was studied by
thermogravimetric method using anti-corrosion type differen-
Fig. 5. Effect of heating rates on the TG curves of the reaction of UO2 with F2.
◦
tial thermobalance at temperatures from R.T. to 500 C in 5%
F2/He atmosphere. The obtained results are as follows;
(
1) Formation of an intermediate oxyfluoride UOxFy (x > 2,
◦
y < 2) seemed to occur at ca. 300 C with an exothermic
reaction.
(
2) Complete volatilization with a large exothermic reaction
◦
was observed at ca. 350 C, which corresponds to the for-
mation and sublimation of UF6.
(
(
3) The effect of heating rate on thermogravimetry was signifi-
cant but that of flow rate was not.
4) Both U3O8 and UO3 show similar fluorination behaviours
which have the formation and sublimation of UF at ca.
350 C.
Fig. 6. TG curves obtained by the reaction of U3O8 and UO3 with F2 at a heating
6
◦
−1
◦
rate of 5 C min
.
the higher temperate region with increasing heating rate. This
suggests that the reaction rate of fluorination is slow compared
with the heating rate.
References
[1] J. Katz, E. Rabiowitch, The Chemistry of Uranium, 1951, 387–400.
[
2] L.R. Morss, N.M. Edelstein, J. Fuger (Eds.), The Chemistry of the Actinide
and Transactinide Elements, 1, third ed., Springer, 2006, p. 558.
3] P.A.G. O’Hare, J.G. Malm, J. Chem. Thermodyn. 14 (1982) 331–336.
4] P.W. Wilson, J. Inorg. Nucl. Chem. 36 (1974) 303–305.
3
.3. Thermogravimetry of UO3 and U3O8
[
[
Fig. 6 shows the TG curves for the reaction of UO3 and
U3O8 by F2 with a flow rate of 30 ml min at a heating rate
[5] A. Atoji, O. Dermatt, Acta Crystallogr. B 26 (1970) 1540–1543.
[6] M.G. Otey, R.A. LeDoux, J. Inorg. Nucl. Chem. 29 (1967) 2249–2256.
−
1