Behavior of UO2 in a room-temperature ionic liquid in the presence of AlCl3

Article abstract of DOI:10.1007/s11137-005-0032-7

A study was made of interaction between AlCl₃ and room-temperature ionic liquid (RTIL) [C₈H₁₅N ₂][N(SO₂CF₃)₂], or BuEtIm-Tf ₂N, and of anodic dissolution in RTIL of UO₂ and of a simulated oxide fuel at 297-302 K, depending on the AlCl₃ concentration. It was shown that anodic dissolution of UO₂ pellets and a UO₂-Al mixture in RTIL yields soluble uranium species. Potentiostatic electrolysis of the resulting solutions can yield uranium compounds at the cathode, though with low current efficiencies. The role of AlCl₃ in these processes was suggested. A heterophase reaction between UO₂ and AlCl₃ was studied, depending on the content of AlCl₃ in solution. It was found that the exchange reaction products, soluble uranium species, are accumulated in solution only at the molar ratio AlCl₃/Tf₂N > 1.

Full text of DOI:10.1007/s11137-005-0032-7

Radiochemistry, Vol. 46, No. 6, 2004, pp. 583 586. Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 536 539.
Original Russian Text Copyright 2004 by Smolenskii, Bove, Borodina, Bychkov, Osipenko.
Behavior of UO in a Room-Temperature Ionic Liquid
2
in the Presence of AlCl3
V. V. Smolenskii*, A. L. Bove*, N. P. Borodina*,
A. V. Bychkov**, and A. G. Osipenko**
*
Institute of High-Temperature Electrochemistry, Ural Division, Russian Academy of Sciences,
Yekaterinburg, Russia
*
* Research Institute of Atomic Reactors, Russian Federation Research Center, Dimitrovgrad,
Ul’yanovsk oblast, Russia
Received January 21, 2004
Abstract A study was made of interaction between AlCl3 and room-temperature ionic liquid (RTIL)
C H N ][N(SO CF ) ], or BuEtIm Tf N, and of anodic dissolution in RTIL of UO and of a simulated
[
8
15
2
2
3 2
2
2
oxide fuel at 297 302 K, depending on the AlCl concentration. It was shown that anodic dissolution of UO
3
2
pellets and a UO Al mixture in RTIL yields soluble uranium species. Potentiostatic electrolysis of the
2
resulting solutions can yield uranium compounds at the cathode, though with low current efficiencies. The role
of AlCl in these processes was suggested. A heterophase reaction between UO and AlCl was studied,
3
2
3
depending on the content of AlCl in solution. It was found that the exchange reaction products, soluble
3
uranium species, are accumulated in solution only at the molar ratio AlCl /Tf N > 1.
3
2
Implementation of both pyrochemical and conven-
tional aqueous methods of nuclear material processing
involves certain problems. These include high opera-
tion costs, high corrosivity of structural materials,
abundant waste, risk of accidents, etc.
RTIL by linear and cyclic voltammetry and zero-
current chronopotentiometry. Also, we carried out
electrolysis of ionic liquids of different compositions,
followed by the chemical and X-ray phase analyses of
the solid and liquid phases.
Low-temperature solvents such as room-tempera-
ture ionic liquids (RTILs) eliminate or, at least, signif-
icantly reduce some of the above-mentioned draw-
backs. It is known [1 9] that RTILs are incombustible
and virtually nonvolatile at process temperatures of up
to 573 K; they exceed aqueous media in radiation
resistance and exhibit a high selectivity with respect
to the components being separated.
All manipulations, including assembling the elec-
trochemical cells, were carried out in a dry glove box
in an argon atmosphere. Argon was permanently freed
from oxygen-containing impurities by forced circula-
tion through a column packed with metallic Zr chips
and heated to 1000 K. Argon circulation was effected
by a membrane pump placed directly inside the
glove box.
It was of interest to study separation and purifica-
tion of nuclear materials using such new process
media as room-temperature ionic liquids, in particular,
to study the possibility of utilization of uranium from
the pressed mixture of 90 wt % Al with 10 wt % UO2,
simulating the irradiated oxide fuel. To this end, we
As solvent we used RTIL of the composition
BuEtIm Tf N, synthesized at the Los Alamos Nation-
2
al Laboratory (the United States). Before use, the
RTIL was additionally freed from oxygen-containing
impurities by keeping at a residual pressure of
3
10
atm for 12 h.
studied interaction of AlCl and anodic dissolution of
3
UO and of the pressed Al UO mixture, as well as
Electrochemical studies were carried out in a quartz
2
2
the exchange reaction between UO and AlCl in an
cell with a ground-down cover, equipped with two
pipes connecting the cell to an additional gas-purifica-
tion system. The cell was placed into a stainless-steel
beaker hermetically built into the glove box bottom,
which also constituted the external part of the tem-
perature-control system.
2
3
RTIL of the composition [C H N ][N(SO CF ) ], or
BuEtIm Tf N, at 297 302 K, depending on the AlCl
concentration.
8
15
2
2
3 2
2
3
EXPERIMENTAL
We studied the chemical and electrochemical be-
havior of UO and Al UO mixture in BuEtIm NTf
The electrolyte under study was placed into a glass
container. As working electrode served a face elec-
2
2
2
1
066-3622/04/4606-0583 2004 MAIK Nauka/Interperiodica

584
SMOLENSKII et al.
RESULTS AND DISCUSSION
Figure 1 presents cyclic voltammograms of the
BuEtIm NTf RTIL with and without AlCl addition,
2
3
recorded at 302 K in an inert gas atmosphere. The
cyclic voltammogram of the RTIL without AlCl3
(
curve 1) exhibits an increase in the current at the
potential of 2.1 V in the cathodic branch and a weak-
ly manifested wave at +2.3 V in the anodic branch
relative to the glassy-carbon reference electrode. Upon
introduction of AlCl , the run of the voltammogram
3
regularly changed (curve 2). In the anodic branch
of the cyclic voltammogram, a wave appeared at
1
.7 V, probably due to oxidation of the chloride ions.
Fig. 1. Cyclic voltammogram of BuEtIm NTf RTIL with
2
A similar pattern was observed by Store et al. [10].
Increase in the AlCl concentration (curve 3) leads to
a regular increase in the limiting currents correspond-
ing to discharge of the chloride ions. This suggests
that the appearance of the chloride ions in solution is
AlCl addition. Potential sweep range from 4.0 to +4.0 V.
3
1
3
Sweep rate 0.2 V s . Temperature 302 K. Working elec-
trode tungsten; reference and counter electrodes, glassy
carbon. AlCl concentration, mol %: (1) 0, (2) 38, and
3
(
3) 61.
due to AlCl occurring in the RTIL. Gaseous chlorine,
3
trode made of VRN metallic tungsten or of SU-2000
glassy carbon, both with and without fixed surface
a strong oxidant, released at the anode and in the bulk
of RTIL was responsible for a change in the color of
the ionic liquid and its partial degradation.
3+
area. As reference electrode served an Al /Al system
placed into a capsule with a porous diaphragm, filled
with BuEtIm Tf N RTIL with a fixed AlCl concen-
The Al3+ cations exhibit a complex behavior in
2
3
the ionic liquid. Supposedly, AlCl , a strong Lewis
3
tration. An alternative reference electrode was an
SU-2000 glassy carbon rod immersed directly in the
melt under study. As counter electrode served an
SU-2000 glassy carbon rod, a VRN tungsten rod, or a
graphite basket with holes, suspended on a molybde-
num hanger. A UO pellet or an Al UO mixture was
acceptor, reacts with the Tf N anion, forming a
2
heteroligand complex, in which the coordination
sphere of Al, along with the three chloride ions, con-
tains one of the oxygen atoms from the SO group of
2
the Tf N anion. This was the case for GaCl in
2
3
2
2
RTIL, as demonstrated by our spectroscopic and
voltammetric studies [11 14]. Close chemical proper-
ties of gallium and aluminum compounds suggest
their similar behavior in identical solutions.
placed into the basket and used, in a number of cases,
as anode. The atmosphere inside the cell was addi-
tionally subjected to closed-loop treatment, via a zir-
conium gas-purification system, using a Zalimp PP-
1
0-5A peristaltic pump. The AlCl concentration in
Figure 2 (curve 1) shows a cyclic voltammogram
3
the electrolyte was varied by dropping the required
recorded upon dissolution of the UO pellet in the
2
weighed portions of AlCl via a sluice without chang-
BuEtIm NTf + AlCl solution. After anodic dissolu-
3
2
3
ing the gas phase compositions.
tion (uranium concentration in solution after dissolu-
tion 1.7 mol %), peaks were observed in the anodic
branch of the curve at 0.997 and 0.914 V. Formation
of these waves is evidently due to the appearance of
soluble uranium species participating in the electro-
chemical process. It is known [4, 15] that uranium
oxide species are converted into oxygen-free species
AlCl was prepared in a special test tube under a
3
purified Ar atmosphere, by slowly introducing metal-
lic Al into molten PbCl . The AlCl sublimates were
2
3
condensed in the upper, specially cooled, part of the
cell. The resulting product was transferred in a dry
box. Prior to experiments, AlCl was subjected to
3
upon introducing AlCl into an RTIL. Probably, in
double distillation.
3
our case oxygen-free uranium species were also
present in solution.
Electrochemical studies were carried out using a
standard PI-50-1 potentiometer with a PR-8 program-
mer, by recording electrochemical responses of the
system with an S9-8 digital storage oscillograph con-
nected to a PC. The potential sweep rate was varied
The potential vs. time plots recorded after brief
polarization of the cathode in the region of possible
deposition of uranium compounds (E > 1.0 V)
exhibit a plateau in the potential decay curves, which
suggests formation of a solid product containing,
1
from 0.05 to 10 V s . The experiments were run
at 298 308 K.
RADIOCHEMISTRY Vol. 46 No. 6 2004

BEHAVIOR OF UO2 IN A ROOM-TEMPERATURE IONIC LIQUID
585
In the case of anodic dissolution of the pellets (con-
taining 90 wt % Al and 10 wt % UO ), simulating the
2
oxide nuclear fuel, in the potentiostatic electrolysis
mode (Fig. 2, curve 2), the voltammogram was vir-
tually identical to that recorded with the UO pellet
2
under identical conditions (Fig. 2, curve 1). Prolonged
(
>24 h) potentiostaic electrolysis at a potential of
6
V vs. glassy carbon reference electrode yielded
a cathodic deposit, the current efficiency being no
greater than 5% either. Chemical analysis of the
deposit revealed 2.75 wt % U, the final concentration
of uranium in solution being 0.013 mol %.
Komarov and Borodina [16] reported on a hetero-
phase reaction between UO and trivalent aluminum in
2
Fig. 2. Cyclic voltammogram of the BuEtIm NTf RTIL
2
a NaCl KCl melt. We studied a chemical reaction bet-
ween UO and AlCl in the RTIL over a wide range
with AlCl and UO additions. Potential sweep range from
3
2
1
2
3
6
.0 to +6.0 V. Sweep rate 0.2 V s . Temperature 302 K.
of AlCl concentrations by voltammetric and spec-
Concentrations, mol %: AlCl 10 and uranium 1.7. Work-
3
3
troscopic methods. We found that, at AlCl /Tf N < 1,
ing electrode tungsten; reference and counter electrodes,
3
2
glassy carbon. (1) Voltammogram recorded after anodic
the voltammograms and spectrograms remained un-
dissolution of the UO pellet (m = 2.00 g); uranium concen-
changed upon keeping the UO powder and pellet in
2
2
tration 1.7 mol %, and (2) voltammogram recorded after
solution for 72 h. This suggests the lack of chemical
reactions between the components, UO and AlCl ,
anodic dissolution of the pressed Al UO mixture (m =
2
2
3
2
.67 g); uranium concentration 1.8 mol %.
under the actual conditions. This can be due to forma-
tion in solution of a heteroligand complex of alumi-
2
num, AlOCl , as mentioned above. The acceptor
3
2
power of AlOCl3 should be much weaker compared
to AlCl . Indeed, more recent studies showed that
4
soluble uranium species appeared in solution only
when the AlCl /Tf N molar ratio exceeded unity,
3
2
i.e., when oxygen-free aluminum species appeared in
solution. In this case, soluble uranium species were
revealed in the concentration of 0.016 mol % upon
6
-h keeping in RTIL.
ACKNOWLEDGMENTS
This work was financially supported by the US
Fig. 3. Potential vs. time plot recorded after brief polariza-
tion of the tungsten cathode in RTIL. Temperature 302 K.
3
+
Working electrode tungsten; reference electrode Al /Al;
and counter electrode glassy carbon. RTIL BuEtIm NTf .
Concentrations, mol %: AlCl 30 and uranium 2.3. Polari-
3
zation potential 4.1 V; polarization time 120 s.
2
Department of Energy according to a joint memoran-
dum with the Russian Academy of Sciences.
The authors are grateful to the Los Alamos Nation-
al Laboratory for financial support and interest in this
work.
evidently, uranium compounds or uranium. In the case
of the RTIL with AlCl without anodic dissolution of
3
UO , there was no plateau. Chemical analysis of
2
the solution after anodic dissolution of UO confirmed
2
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