Fluorination of niobium compounds with fluorine for fluoride volatility method

Article abstract of DOI:10.1016/j.jfluchem.2018.09.010

Niobium is one of the fission products contained in spent nuclear fuel. Since niobium pentafluoride (NbF₅) has high volatility, it is considered that niobium volatilizes with uranium hexafluoride when applying the fluoride volatility method, which is a promising pyro-reprocessing method. In this study, fluorination behavior characteristics of niobium compounds, such as reaction temperature, volatility, and reaction path, were investigated by thermogravimetric and differential thermal analyses and X-ray diffraction analysis. The target compounds were niobium metal, the niobium oxides NbO, Nb₂O₃, NbO₂ and Nb₂O₅, and niobium oxyfluoride (NbO₂F). All the niobium compounds reacted exothermically and were volatilized completely by the reaction with F₂. It was considered that niobium volatilized as NbF₅. The fluorination reactions started respectively at 180, 200, 300, and 300 °C for niobium metal, NbO, NbO₂ and Nb₂O₅. In the fluorination of niobium oxides, the intermediate product NbO₂F was also fluorinated above 300 °C and volatilized completely. Nb₂O₃, which seemed to be a mixture of NbO and NbO₂, reacted with F₂ as described by the summation of the fluorination reactions of NbO and NbO₂. The reaction mechanism for the fluorination of niobium compounds obtained in this study is applicable to evaluation of the niobium transfer phenomena in the reprocessing process of the fluoride volatility method.

Full text of DOI:10.1016/j.jfluchem.2018.09.010

Journal of Fluorine Chemistry 216 (2018) 1–6
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Fluorination of niobium compounds with fluorine for fluoride volatility
method
⁎
Daisuke Watanabe , Daisuke Akiyama, Nobuaki Sato
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Niobium is one of the fission products contained in spent nuclear fuel. Since niobium pentafluoride (NbF ) has
5
Reprocessing
Fluoride volatility method
Fluorination
high volatility, it is considered that niobium volatilizes with uranium hexafluoride when applying the fluoride
volatility method, which is a promising pyro-reprocessing method. In this study, fluorination behavior char-
acteristics of niobium compounds, such as reaction temperature, volatility, and reaction path, were investigated
by thermogravimetric and differential thermal analyses and X-ray diffraction analysis. The target compounds
Fission product
Niobium
were niobium metal, the niobium oxides NbO, Nb
2
O
3
, NbO
2
and Nb
2
O
5
, and niobium oxyfluoride (NbO
. It was
. The fluorination reactions started respectively at 180, 200, 300,
and Nb . In the fluorination of niobium oxides, the intermediate
F was also fluorinated above 300 °C and volatilized completely. Nb , which seemed to be a
mixture of NbO and NbO , reacted with F as described by the summation of the fluorination reactions of NbO
and NbO
2
F). All
the niobium compounds reacted exothermically and were volatilized completely by the reaction with F
considered that niobium volatilized as NbF
and 300 °C for niobium metal, NbO, NbO
product NbO
2
5
2
O
2 5
2
O
2 3
2
2
2
. The reaction mechanism for the fluorination of niobium compounds obtained in this study is ap-
plicable to evaluation of the niobium transfer phenomena in the reprocessing process of the fluoride volatility
method.
1
. Introduction
important to understand the fluorination behaviors of FPs. The authors
previously investigated the fluorination behavior of antimony com-
The fluoride volatility method is one of the promising pyro-re-
pounds [12]. In the work reported in this paper, we studied the fluor-
ination behavior of niobium, another of the volatile FP fluorides. In-
cidentally, the fluorinated FPs can be converted to oxide forms by using
a steam oxidation method [6,7] and finally disposed of as vitrified
waste.
processing methods for spent nuclear fuel that have been developed by
several research groups [1–7]. Following the severe accident at the
Fukushima Daiichi NPP, this method was investigated as a means for
treating the fuel debris produced in the accident [8]. In applying the
method, the spent nuclear fuel reacts with a fluorinating agent such as
Niobium is one of the FPs contained in spent nuclear fuel. One re-
ported amount of niobium in the spent fuel is 5 mg/tHM [13], which is
smaller than that of other FPs. However, niobium has radioactivity and
F
2
3
, NF , ClF
3
, and BrF [9,10]. The elements in the spent fuel are se-
5
parated into a number of elemental groups depending on the difference
in vapor pressure of the fluorinated products. For example, uranium
forms volatile fluorides such as NbF
5
for which vapor pressure is
and plutonium are fluorinated respectively to gaseous UF
6
and PuF
6
.
100 kPa at 234 °C [1]. Therefore, niobium may volatilize in the fluoride
However, PuF
poses to non-volatile PuF
products (FPs) become non-volatile fluorides such as SrF
are recovered as solid components and separated from gaseous UF
Other FPs form volatile fluorides and are co-volatilized with UF gas.
For this reason, separation methods such as distillation and adsorption
6
is thermodynamically unstable and gradually decom-
solid [11]. On the other hand, some fission
and LaF and
volatility method if NbF is formed by the fluorination reaction. For this
5
4
reason, niobium is considered an important element for the fluoride
volatility method. Kleykamp [14] reported that niobium was contained
as oxides in spent nuclear fuel. Since the fluoride volatility method may
also be applicable to metal fuel, it has been considered desirable to
clarify the fluorination reactions for niobium metal and oxides.
Experimental investigations have already been carried out on the
fluorination reactions of niobium compounds [15–17]. Junkins et al.
2
3
6
.
6
methods have also been developed for separating FPs from UF [1–7].
6
From this viewpoint, the transfer mechanism of FPs should be clarified
to design the process of the fluoride volatility method. Therefore, it is
[15] reported that NbF was obtained by fluorinating niobium metal
5
⁎
Corresponding author.
E-mail address: daisuke.watanabe.kp@hitachi.com (D. Watanabe).
https://doi.org/10.1016/j.jfluchem.2018.09.010
Received 31 July 2018; Received in revised form 25 September 2018; Accepted 26 September 2018
Available online 26 September 2018
0022-1139/ © 2018 Elsevier B.V. All rights reserved.

D. Watanabe et al.
Journal of Fluorine Chemistry 216 (2018) 1–6
with F in a nickel reactor at 300 °C. Rakov et al. [16] determined the
2
reaction rates of the fluorination reactions of Nb
2
O
5
and NbO F within
2
the range from 330 to 430 °C. However, it has been considered neces-
sary to further investigate fluorination behavior characteristics such as
reaction temperature and reaction path. Additionally, to the best of the
authors’ knowledge, no reports have been published about the fluor-
ination reactions of the other niobium oxides NbO, Nb
2
O
3
, and NbO .
2
These reactions should be investigated to facilitate understanding of the
fluorination behavior of niobium compounds with F .
2
In this research we experimentally investigated fluorination beha-
vior characteristics such as reaction temperature, volatility and reaction
path for several niobium compounds. The target compounds for fluor-
ination were niobium metal, the niobium oxides NbO, Nb
2
O
3
, NbO
2
and
Nb
2
O
5
, and niobium oxyfluoride (NbO
2
F). Thermogravimetric analysis
(
TG) and differential thermal analysis (DTA) using a thermogravimeter-
differential thermal analyzer and X-ray diffraction (XRD) analysis were
used in the investigation.
2
. Results and discussion
Fluorination experiment results are shown in Section 2.1 for each
niobium compound. The results of thermodynamic consideration on
related reactions are given in Section 2.2. Finally, on the basis of these
results, fluorination behavior is discussed in Section 2.3 for each nio-
bium compound.
2
.1. Results of fluorination of niobium compounds with fluorine
Fig. 2. TG-DTA curves of the Nb
2
O
5
fluorination experiment with F at three
2
heating rates: (a) 10 °C/min, (b) 5 °C/min, and (c) 2 °C/min.
2.1.1. Niobium metal
The TG-DTA curve of the reaction of niobium metal with F is
2
shown in Fig. 1. The temperature was increased from room temperature
to 400 °C at a 10 °C/min heating rate. The mass change ratio (ΔM) of a
sample obtained by TG is shown as solid lines in the figure. The heat
transfer obtained by DTA is shown as dashed lines; an upward signal
means the reaction was exothermic.
volatile product by the reaction with F2.
2.1.2. Nb O
2 5
The TG-DTA curve of the reaction of Nb O with F2 is shown in
2
5
Fig. 2. The temperature was increased from room temperature to
500 °C. The heating rates for (a), (b), and (c) were respectively 10, 5,
and 2 °C/min. In (c), the temperature was increased from room tem-
perature to 250 °C at a 10 °C/min heating rate to shorten the experiment
time. Since the heating rate was changed during the experiment, the
baseline of the DTA curve in (c) changed at 250 °C unintentionally.
As shown in Fig. 2, the ΔM started to decrease with the exothermic
reaction above 300 °C and reached about -100% at the end of the ex-
periment. This indicates that Nb O reacted with F exothermically and
As shown in the figure, the ΔM started to decrease with the exo-
thermic reaction above 180 °C. This indicates that niobium metal re-
acted exothermically with F above 180 °C. The ΔM reached about
2
-
100% at the end of the reaction, showing that niobium metal was
completely volatilized by the fluorination with F . Since the ΔM de-
2
creased simply with an exothermic peak, the fluorination of niobium
metal seemed to proceed by a single-step reaction.
To investigate the reaction path, we carried out XRD analysis of the
residue of the fluorination reaction. The residue was prepared by an
2
5
2
the sample was volatilized completely. At a slower heating rate, the F
2
additional fluorination experiment. The F
2
flow was stopped at around
feed time per unit temperature increase became longer. For this reason,
the temperature at which niobium was completely volatilized shifted to
lower one for a slower heating rate; actually, the volatilization of nio-
bium was completed respectively at 490, 430, and 400 °C for (a), (b),
and (c). When the heating rate was faster than the reaction rate, the
temperature increased before the reaction was finished. Therefore, the
heating rate affected the shift of the TG-DTA curve. Actually, the peak
temperature of the first exothermic reaction shifted to a higher tem-
perature as the heating rate increased. However, the difference in the
heating rate did not seem to affect the reaction mechanism because the
TG-DTA curves in (a), (b), and (c) resembled each other.
2
10 °C during the experiment, the heating rate for which was 10 °C/
min. Only a niobium metal phase was identified in the diffraction
pattern of the residue and no peaks of any other niobium products were
observed. This suggests that niobium metal was directly fluorinated to a
In (a)–(c), two exothermic peaks were observed above 300 °C and
the trend of decreasing ΔM also seemed to change with the first peak.
This indicates that a two-step reaction occurred in the fluorination of
Nb
2
O .
5
To investigate the reaction path, we identified the intermediate
product of the fluorination reaction by XRD analysis. The TG-DTA curve
of the fluorination experiment for preparing the intermediate product is
shown in Fig. 3. In this experiment the heating rate was 10 °C/min, the
Fig. 1. TG-DTA curves of the niobium metal fluorination experiment with F at
2
amount of Nb
2
O (about 10 mg) was larger than the standard experi-
5
a 10 °C/min heating rate.
mental condition indicated in Section 4, and the flow rate of 10% F
2
-N
2
2

D. Watanabe et al.
Journal of Fluorine Chemistry 216 (2018) 1–6
Fig. 3. TG-DTA curves of the Nb
2
O
5
fluorination experiment with F
2
at a 10 °C/
Fig. 5. TG-DTA curves of the NbO
2
fluorination experiment with F at a 5 °C/
2
min heating rate.
min heating rate.
gas (10 mL/min) was half of the condition. For this reason, 85 mass %
were prepared by two additional fluorination experiments. In the first,
the temperature was increased from room temperature to 310 °C at a
10 °C/min heating rate and then kept at 310 °C for 5 min. In the second,
the temperature was increased from room temperature to 360 °C at a
10 °C/min heating rate and then kept at 360 °C for 20 min. The XRD
patterns of the products prepared by the first and second experiments
are shown respectively in Fig. 6(a) and (b). As shown in (a), the product
obtained by the fluorination reaction at 310 °C was identified as mainly
of the starting Nb
2
5
O remained in the pan after the experiment. Since
one exothermic peak followed by another appeared at 450 °C in Fig. 3,
we considered the residue to be the product of the first exothermic
reaction of the two found in Fig. 2. XRD analysis identified this product
as NbO
2
F and no peaks corresponding to other phases such as Nb
2 5
O
and other oxyfluorides were observed in the XRD pattern. This indicates
that Nb
2
O
5
was completely converted to NbO F by the first exothermic
2
reaction and after that all the niobium was volatilized by the second
NbO
2
and contained a small amount of NbO F. No peaks corresponding
2
reaction.
to other phases such as those for other oxyfluorides were observed in
the XRD pattern. On the other hand, as shown in (b), the product ob-
tained by the fluorination reaction at 360 °C was identified as NbO F.
2
2
.1.3. NbO
To further investigate the fluorination reaction of Nb
out a fluorination experiment for NbO F, which was the intermediate
fluorination product of Nb . The TG-DTA curve of the reaction of
NbO F with F at a 5 °C/min heating rate is shown in Fig. 4. The NbO
powder was prepared by the experiment mentioned in Section 2.1.2.
Above 300 °C, the ΔM started to decrease with an exothermic peak.
At the end of the experiment NbO F was completely volatilized. Since
2
F
Since a polyimide tape was used to avoid air-oxidation of the sample
during the XRD measurement, broad peaks due to the tape material
appeared in the 2θ range from 20 to 28 degrees. For comparison, the
XRD pattern of the polyimide tape is shown in (c). These results in-
2
O , we carried
5
2
O
2 5
2
2
2
F
dicated that a part of the NbO
2
was fluorinated to NbO
2
F by the first
F followed
in the second, and all the niobium was volatilized by the third.
exothermic reaction at 300 °C, complete fluorination to NbO
2
2
only one exothermic peak was observed (Fig. 4) and the ΔM simply
decreased, it can be considered that NbO
step reaction.
2
F reacted with F
2
in a single-
2
.1.5. NbO
We next investigated the fluorination reaction for NbO, which has
the lowest niobium valency among the studied niobium oxides. The TG-
2
.1.4. NbO
2
DTA curve of the reaction of NbO with F is shown in Fig. 7. The
2
Next, we carried out a fluorination experiment for NbO
2
, which has
heating rate was 5 °C/min. The ΔM started to decrease with the con-
tinuous exothermic peak above 200 °C. Some exothermic reactions were
observed and the decrease in the ΔM changed step by step, indicating
some species might form in the fluorination reaction of NbO. All of the
NbO was volatilized at the end of the experiment.
a lower niobium valency than Nb
O
2 5
2
and NbO
2
F have. The TG-DTA
curve of the reaction of NbO with F
2
is shown in Fig. 5. The heating
rate was 5 °C/min. A sharp exothermic peak was observed at 300 °C and
the mass of the sample slightly increased by about 1%. A decrease in the
ΔM with two exothermic peaks followed and the entire sample was
volatilized at the end of the experiment.
To investigate the reaction path, we identified the intermediate
products of the fluorination reactions by XRD analysis. The products
To investigate the reaction path, we identified the intermediate
products of the fluorination reactions by XRD analysis. The products
Fig. 6. XRD patterns of the residue for the NbO fluorination experiment. (a)
2
Fig. 4. TG-DTA curves of the NbO
2
F fluorination experiment with F
2
at a 5 °C/
product after fluorination at 310 °C, (b) product after fluorination at 360 °C, and
(c) only polyimide tape for comparison.
min heating rate.
3

D. Watanabe et al.
Journal of Fluorine Chemistry 216 (2018) 1–6
Fig. 7. TG-DTA curves of the NbO fluorination experiment with F
2
at a 5 °C/min
Fig. 9. TG-DTA curves of the Nb
2
O
3
fluorination experiment with F at a 10 °C/
2
heating rate.
min heating rate.
Gibbs free energies (ΔG) for the related fluorination reactions by cal-
culation using a thermodynamic database [18] and the results are
shown in Fig. 11. Since data for NbO F are missing, the related reac-
2
tions for it are not given in the figure.
In the temperature range from room temperature to 500 °C, the ΔG
of the fluorination reactions of the niobium compounds were negative.
Therefore, it can be considered that NbF was formed by the reactions
5
at this temperature range. On the other hand, the ΔG of the reactions
took large negative values in the order of niobium metal, NbO, NbO
2
,
and Nb
2
O . This order seemed to relate to the order of the valence state
5
of niobium. For example, the change in the valence state was largest for
the fluorination of niobium metal; the state changed from zero to
pentavalent. Together with the ΔG calculation results, this seemed to
indicate that the niobium compounds with lower valence state were
more reactive for the fluorination reaction. This suggests that the oxi-
dation reaction from lower valence state to higher one occurs first
Fig. 8. XRD patterns of the residue for the NbO fluorination experiment. (a)
product after fluorination at 240 °C and (b) product after fluorination at 310 °C.
were prepared by two additional fluorination experiments. In the first,
the temperature was increased from room temperature to 240 °C at a
followed by the exchange reaction of oxygen by F .
2
1
0 °C/min heating rate and then kept at 240 °C for 30 min. In the
second, the temperature was increased from room temperature to
2
.3. Fluorination reaction of niobium compounds
.3.1. Niobium metal
3
10 °C at a 10 °C/min heating rate and then kept at 310 °C for 40 min.
The XRD patterns of the products prepared by the first and second
experiments are shown respectively in Fig. 8(a) and (b). As shown in
2
As mentioned in Section 2.1.1, niobium metal was directly fluori-
(
a), the product obtained by the fluorination reaction at 240 °C was
nated to a volatile product by a single-step reaction. The thermo-
dynamic consideration shown in Fig. 11 leads to the evaluation that
identified as mainly NbO and contained a small amount of NbO F. No
2
peaks corresponding to other phases such as other oxyfluorides were
observed in the XRD patterns. On the other hand, as shown in (b), the
product obtained by the fluorination reaction at 310 °C was identified
NbF was produced by the fluorination reaction of niobium metal.
5
Junkins et al. [15] reported that NbF was formed by the reaction of
5
niobium metal with F at 300 °C. On the basis of this report and our
2
as NbO F. Since the polyimide tape was used only for the XRD analysis
2
obtained results, we considered the following reaction (1) is the fluor-
of (a), broad peaks for it appeared in the 2θ range from 20 to 28 de-
grees. These results indicated that a part of NbO was fluorinated to
ination reaction of niobium metal.
NbO
2
F with the first exothermic reaction at 200 °C followed by com-
2/5Nb + F
2
→ 2/5NbF
5
(1)
plete fluorination to NbO
2
F in the second reaction. After that, all the
We also confirmed that this reaction started above 180 °C.
niobium was volatilized by the third reaction.
2
.3.2. Nb
2
O
5
and NbO F
2
2
.1.6. Nb
2 3
O
As mentioned in Section 2.1.2, Nb
2
O
5
was fluorinated by a two-step
Finally, the TG-DTA curve of the reaction of Nb
2
O
3
with F is shown
2
reaction above 300 °C; NbO F was generated in the first step and all the
2
in Fig. 9. The heating rate was 10 °C/min. The exothermic reaction
started above 200 °C and four exothermic peaks were found at 230, 300,
niobium was volatilized in the second. Additionally, as mentioned in
Section 2.1.3, NbO
step reaction above 300 °C because ΔM simply decreased with an exo-
thermic peak (Fig. 4). NbF and NbOF were possible volatile products
of the fluorination reaction of NbO F. However, Rakov et al. [16]
suggested that NbF was formed by the fluorination of NbO F. There-
2
F was fluorinated to a volatile product by a single-
3
50, and 440 °C (Fig. 9). The ΔM started to decrease above about 230 °C
and the ΔM curve changed with the peaks. Finally, the entire sample
was volatilized at the end of the experiment. Nb of 99.9% purity
Kojundo Chemical Laboratory Co., Ltd.) was used as received. The XRD
pattern of the Nb used in this study is shown in Fig. 10. Peaks for
NbO and NbO phases were observed in the pattern, suggesting the
Nb we used was a mixture of NbO and NbO
5
3
2
O
2 3
(
5
2
O
2 3
fore, we consider that NbOF was not formed in the fluorination reac-
3
2
tion and that even if it had been it would have immediately been
fluorinated to NbF5.We therefore consider that the following reactions
2
O
3
2
.
(
2) and (3) describe the fluorination of Nb
2
5
O .
2.2. Thermodynamic consideration of fluorination of niobium compounds
Nb
2
O
5
+ F
2
→ 2NbO
2
F + 1/2O
2
(2)
(3)
This section describes the thermodynamic consideration results of
1
/2NbO
2
F + F
2
→ 1/2NbF
5
+ 1/2O
2
the fluorination reaction of niobium compounds. We evaluated the
4

D. Watanabe et al.
Journal of Fluorine Chemistry 216 (2018) 1–6
mentioned in Section 2.2, the ΔG of the fluorination reaction shown in
Fig. 11 seems to indicate that niobium compounds having a lower
niobium valence state are more reactive for the fluorination reaction.
For this reason, a sharp exothermic peak for the reaction was observed
at 300 °C, which is a lower peak temperature than the 380 °C of the
Nb
2
O fluorination reaction.
5
2
.3.4. NbO
As mentioned in Section 2.1.5, NbO reacted with F above 200 °C
2
Fig. 10. XRD pattern of the Nb
2
O
3
used in this study.
and NbO F was formed from the reaction. Since ΔM started to decrease
2
at 200 °C (Fig. 7), a volatile product was also formed concurrently
above 200 °C. It was necessary to add oxygen to form NbO F from the
2
fluorination of NbO. However, since oxygen gas was not contained in
the feed gas in the experiment, it appears that the following reaction (5)
occurred.
2
/3NbO + F
2
→ 1/3NbF
5
+ 1/3NbO
2
F
(5)
In this reaction, volatile NbF
5
was formed and the ΔM actually
started to decrease above 200 °C (Fig. 7). Then, NbO
using oxygen generated by the NbF generation reaction from NbO. On
the other hand, NbO was completely converted to NbO F at 310 °C as
shown in Fig. 8(b). When reaction (5) was completed, the ΔM was
2
F was formed
5
2
-
34%. Actually, around about 330 °C, at which the ΔM reached -34%,
the exothermic reaction came to an end and the decreasing trend of the
ΔM changed (Fig. 7). This led us to believe that the fluorination reac-
tion observed from 200 to 330 °C in Fig. 7 was reaction (5). On the
other hand, the last exothermic peak at 400 °C (Fig. 7) corresponded to
the fluorination reaction of NbO F and was described as reaction (3).
2
Fig. 11. Gibbs free energies of the fluorination reactions of niobium com-
The starting temperature of the fluorination reaction for NbO
200 °C) was lower than that for NbO (300 °C) and Nb (about
30 °C). As mentioned in Section 2.2, niobium compounds with lower
valence state seemed to be more reactive for the fluorination reaction.
Actually, the starting temperature of the fluorination reaction for NbO
pounds.
(
2
O
2 5
3
The calculated ΔM value for the fluorination from Nb
2
O
5
to NbO F
2
was + 8%. However, in Fig. 2, the decrease in ΔM seemed to stop
temporally at the first exothermic reaction, although the value of ΔM
2
was lower than that for Nb
2
O as mentioned in Section 2.3.3. For this
5
did not increase over + 8%. This is due to the formation of NbO
2
F and
reason, we felt that the starting temperature of the fluorination reaction
the subsequent formation of volatile NbF
In this research, we clarified experimentally that these two reactions
started above 300 °C at the same time, that NbO F was formed in the
first reaction. And it was considered that NbO F was volatilized by the
further fluorination to NbF in the second reaction.
5
.
for NbO would be the lowest (e.g., 200 °C) among the niobium oxides.
The fluorination reaction of NbO started at 300 °C, but stopped
2
during the course of the reaction. It can be considered that the reason
2
for this is that solid NbO F covering the surface prevented the fluor-
2
2
ination reaction. On the other hand, NbO was fluorinated in bulk even
at 300 °C. The fluorination reaction of NbO can be completed in bulk at
5
such a low temperature because, as shown in reaction (5), NbO F and
2
2
.3.3. NbO
As mentioned in Section 2.1.4, NbO
exothermic reactions above 300 °C A part of the NbO
2
volatile NbF
NbO.
5
are concurrently generated in the fluorination reaction of
2
was fluorinated by three step-
was fluorinated
F followed in
2
to NbO
2
F in the first step, complete fluorination to NbO
2
the second, and all of the niobium was volatilized in the third. These
results indicate the first and second exothermic reactions are as follows.
2.3.5. Nb
Finally, we will briefly discuss the fluorination reaction of Nb
because the Nb used in this experiment was identified as a mixture
of NbO and NbO on the basis of XRD analysis. As mentioned in Section
.1.6, four exothermic peaks were found at 230, 300, 350, and 440 °C
2
O
3
O
2 3
2 3
2
O
2
NbO
2
+ F
2
→ 2NbO
2
F
(4)
2
Piennar et al. [17] have reported that NbO
2
F was decomposed
(
Fig. 9). The peaks at 230 and 300 °C seemed to be similar to those
above 655 °C. This means that NbO
2
F remains a solid component at
found in the fluorination reaction of NbO (Fig. 7), while those at 350
and 440 °C seemed to be similar to those found in the fluorination re-
3
00 °C. The calculated ΔM value for the fluorination from NbO
2
to
F is + 15%. Since the increase in the ΔM was only 1% at 300 °C
, the fluorination reaction at 300 °C
F were near the surface it might
from coming into contact with NbO in bulk and stop the
fluorination reaction on the surface. On the other hand, NbO was
NbO
2
action of NbO (Fig. 5). Therefore, we believe that the fluorination
2
and the residue was mainly NbO
2
reaction of Nb
2
O in this research can be simply described as the
3
seemed to be a surface reaction. If NbO
prevent the F
2
summation of the fluorination reactions of NbO and NbO .
2
2
2
2
completely converted to NbO
2
F at 360 °C as shown in Fig. 6(b). This
3. Conclusion
means that reaction (4) proceeded in bulk in the second exothermic
reaction shown in Fig. 5. The third exothermic reaction, for which the
peak temperature was 430 °C (Fig. 5), corresponds to the fluorination
To ascertain the fluorination behavior of niobium in spent nuclear
fuel in the fluoride volatility method, we investigated the reactions of
reaction of NbO
2
F and that reaction is described as reaction (3).
niobium metal and the niobium oxides NbO, Nb
2
O
3
, NbO
2
, and Nb
2 5
O
The fluorination reaction of NbO
at 300 °C that was not found in the fluorination reaction of Nb
valence state of niobium of NbO is smaller than that of Nb
2
showed a sharp exothermic peak
. The
. As
with F
exothermically reacted with F
200 °C, and consider that NbF
2
by TG-DTA and XRD analyses. We found that niobium metal
2
O
5
2
5
and was volatilized completely above
was directly formed by the reaction.
2
2
O
5
5

D. Watanabe et al.
Journal of Fluorine Chemistry 216 (2018) 1–6
NbO
2
2
and Nb
F was formed as the intermediate product of the fluorination re-
F also exothermically reacted with F and was volatilized
was
formed by the fluorination reaction. NbO exothermically reacted with
above 200 °C and formation of NbO F and volatilization of niobium
occurred concurrently; niobium was volatilized as NbF in this reaction.
Nb , which was identified as a mixture of NbO and NbO , reacted
with F as described by the summation of the fluorination reactions of
NbO and NbO
2
O
5
exothermically reacted with F
2
above 300 °C and
was covered as necessary using polyimide film to prevent exposure to
air. The phases observed in the diffraction pattern were analyzed by the
PDXL2 program using the ICDD powder diffraction file.
NbO
actions. NbO
2
2
completely above 300 °C; it can be considered that volatile NbF
5
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2
2
5
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6