Reduction of U(VI) and some fission products in HNO3 media by galvanostatic electrolysis

Article abstract of DOI:10.1016/j.jallcom.2007.04.204

In order to investigate the applicability of the galvanostatic reduction of uranium(VI) to U(IV) in the electrolytic reduction and ion exchange (ERIX) Process, a novel aqueous reprocessing technique for spent mixed uranium and plutonium oxide (MOX) fuels for fast-breeder reactors (FBR), an improved electrolytic cell was developed and fundamental electrolytic properties were examined using U and some fission products (FPs) in HNO₃ media. As the result of the electrolysis for a 200 cm³ solution of 0.5 M (M = mol/dm³) U(VI), 6 M HNO₃, and 0.5 M N₂H₄, it was found that more than 99% of U(VI) was reduced to U(IV) before the shift to the side reaction when the initial constant current supplied was no higher than 10 A. It was also found that the reduction of U(VI) to U(IV) was achieved using the cell under coexistence with the FPs, e.g. noble metals, which are easily deposited by the electrolytic reduction.

Full text of DOI:10.1016/j.jallcom.2007.04.204

Journal of Alloys and Compounds 451 (2008) 358–360
Reduction of U(VI) and some fission products in HNO3
media by galvanostatic electrolysis
M. Nogami∗, Y. Hirose, T. Arai, Y.-Z. Wei, M. Kumagai
Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan
Available online 19 April 2007
Abstract
In order to investigate the applicability of the galvanostatic reduction of uranium(VI) to U(IV) in the electrolytic reduction and ion exchange
(ERIX) Process, a novel aqueous reprocessing technique for spent mixed uranium and plutonium oxide (MOX) fuels for fast-breeder reactors
(FBR), an improved electrolytic cell was developed and fundamental electrolytic properties were examined using U and some fission products
(FPs) in HNO3 media. As the result of the electrolysis for a 200 cm3 solution of 0.5 M (M = mol/dm3) U(VI), 6 M HNO3, and 0.5 M N2H4, it was
found that more than 99% of U(VI) was reduced to U(IV) before the shift to the side reaction when the initial constant current supplied was no
higher than 10 A. It was also found that the reduction of U(VI) to U(IV) was achieved using the cell under coexistence with the FPs, e.g. noble
metals, which are easily deposited by the electrolytic reduction.
© 2007 Elsevier B.V. All rights reserved.
Keywords: ERIX process; Reprocessing; Uranium; Electrolytic reduction
1. Introduction
2. Experimental
2.1. Electrolytic cell
We have been developing a novel aqueous reprocessing tech-
nique for spent mixed uranium and plutonium oxide (MOX)
fuels for fast-breeder reactors (FBR) [1]. It is called the ERIX
Process, which mainly consists of electrolytic reduction and ion
exchange. In this process, uranium(VI) in HNO3 solutions is
reduced to U(IV) by the electrolysis for the efficient separa-
tion of U from other nuclides in the following anion exchanger
column. A potentiostatic control of a flow-type electrolytic cell
using three-dimensional cathode made of glassy carbon fibers
has been investigated for the electrolytic reduction [2]. It has
been revealed, however, that this type of cell had some problems
due to the “closed system”, e.g. blockage of the cathode by the
deposited Pd and Ag, accumulation of the generated gas mainly
resulting from the decomposition of hydrazine added to keep the
reduction state. In this study, therefore, electrolytic reductions
of U(VI) were examined by galvanostatic electrolysis using an
improved cell where the reduction is performed in an “open
system”. The effect of some of the coexisting redox couples,
assuming possible fission products (FPs) was also investigated.
The new electrolytic cell is a batch recirculation type using a three-
dimensional working electrode made of carbon felt in which the current flow
is kept parallel to the electrolyte flow and is open to the atmosphere. The con-
figuration of the cell is shown in Fig. 1. It consists of a Pyrex glass vessel, a
cathode, an anode and a porous ceramic separator. The vessel has a side leg to
which the probe can be inserted for the measurement of the absorption spectra
of the catholyte. The cathode consists of a current feeder made of Ti expanded
mesh and carbon felt (thickness: 2.7 mm, bulk density: 0.13 g/cm3 (theoretical
density: 7.6%)) as the three-dimensional working electrode with 100 cm2 of two-
dimensional surface area. The felt is pressed to the inner of the current feeder
with two pieces of Ti ring over a Ti pressing mesh. The tubular anode is made of
Ti plated with Pt. The porous ceramic separator is made of Ba1.25Zr4P5Si0.5O24
with 1 ␮m of mean pore diameter and 25% of porosity.
The cell can contain ca. 200 cm3 of the catholyte for a normal operation. It is
stirred with a magnetic stirrer at the bottom of the vessel during operation. The
catholyte flows through the working electrode from the outer to the inner forced
by the hydrostatic gap induced by the vortex motion, driven by the stirring.
2.2. Electrolytic reduction
Solutions containing U(VI), HNO3, and N2H4 for keeping the reduction
state were used as the catholyte, and some FPs such as Rh, Pd, Ag, Fe were
added in some cases.
The electrolytic experiments were performed by the galvanostatic system.
The progress in the electrolytic reaction under the galvanostatic control was
monitored by the terminal voltage. It was assumed that an acute increase in
∗
Corresponding author.
E-mail address: nogami@iri.or.jp (M. Nogami).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.04.204

M. Nogami et al. / Journal of Alloys and Compounds 451 (2008) 358–360
359
Fig. 1. Configuration of electrolytic cell.
the voltage would reflect the start of the side reaction of hydrogen evolution.
Basically, the electrolytic current was reduced to one-thirds from the original
when a significant voltage increase was detected, in order to try to achieve
an additional degree of reduction at a mass transport control current, if it was
applicable.
The results are shown in Fig. 2. Our target in this electrolytic
reduction process is to achieve [U(VI)]/[U(IV)] = 5 × 10−3, and
the potential of 0.35 V was found to be equivalent to the condi-
tion. Under the condition, it is possible that the concentrations
of Pd(II), Ag(I), Rh(III), and Rh(II) in the solution are reduced
below 10−6 M, and ca. 10−4 and 10−3 M for Rh(I) and Ru(II),
respectively.
3. Results and discussion
From these calculations, it had been expected that Ru(III)
would not be deposited and that a large part of Rh would be
It was found, from the electrolytic reduction of a solution
consistingof0.5 M(M = mol/dm3)U(VI), 6 MHNO3, and0.5 M
N2H4, that more than 99% of U(VI) was reduced to U(IV) before
the shift to the side reaction when the initial constant current
supplied was no higher than 10 A. It took ca. 66 min at a constant
current of 5 A.
Rh(III) and Ru(III) was added at the concentration of 10 mM,
respectively, to the above U(VI) solution and the prepared solu-
tion was electrolytically reduced at a constant current of 5 A. As
the result, the concentrations of Rh and Ru kept constant until
the reduction of U(VI) was completed, which means that neither
Rh nor Ru was deposited by the electrolysis.
The redox potentials for some redox systems relevant to the
irradiated MOX fuel solution in 6 M HNO3 as functions of ionic
concentrations and of concentration ratios were calculated by
the Nernst equation based on the Standard Redox Potentials.
Fig. 2. Effect of concentrations/concentration ratios of redox systems on redox
potential ([HNO3] = 6 M).

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M. Nogami et al. / Journal of Alloys and Compounds 451 (2008) 358–360
and 0.1 M N2H4, respectively, made the solution turbid, then
curdy solid was accumulated on the carbon felt during further
liquid circulation. The changes in the concentrations of metal
ions along with electrolytic reduction carried at a constant cur-
rent of 0.55 A are shown in Fig. 4. The concentration of Pd just
before the start of the electrolysis is 0.5 mM, which means that
ca. 95% of the added Pd has been precipitated as Pd-hydrazine
compounds. Among the elements examined in this electrolysis,
99% of Ag deposits along with the reduction of Fe(III), and
a half of remaining Pd(II) additionally deposits accompanied
with Ag. Rh is not deposited as in the previous experiment.
During 2 h of electrolysis, the curdy solid on the cathode dis-
appeared and black fine but heavy solid was precipitated on the
surface of the cathode and at the bottom of the vessel. It would be
likely that the Pd-hydrazine compounds underwent the U(IV)-
mediated reductive decomposition and turned into Pd black. The
voltage pattern became so flat that the end-point of U(VI) reduc-
tioncouldhardlybedetected. It wouldbeattributedto thechange
in the characteristics of the cathode by the deposition of the Pd
black.
Fig. 3. Effect of addition of Rh and Ru on end-point detection of electrolysis of
U and Fe.
4. Conclusion
It has been revealed that the improved electrolytic cell devel-
oped in the present study was capable to achieve the electrolytic
reduction of U(VI) to U(IV) under the galvanostatic control. It
has also been shown that the electrolytic operation was not dis-
turbed by the deposition of Ag and Pd, unlike the case of the
conventional flow-type cell. Further investigations might be nec-
essary for the detection of the end-point in the U(VI) reduction
when some specific FPs coexist.
Fig. 4. Change in concentrations of metal ions by electrolytic reduction vs. time
by electrolysis at a constant current of 0.55 A.
Acknowledgement
deposited as well as Pd and Ag under our electrolytic condi-
tion. The reason for the discrepancy for the result of Rh remains
unknown.
The content of this paper is a part of the results from “Devel-
opment of the ERIX Process for Reprocessing Spent FBR-MOX
Fuels for fast-breeder reactors” entrusted by the Ministry of
Education, Culture, Sports, Science and Technology of Japan
(MEXT).
The terminal voltages along with electrolytic reduction of
the U solutions with and without Rh and Ru described above
are shown in Fig. 3. Those for 1 M Fe solutions instead of 0.5 M
U are also shown as the references. It can be seen that the final
pattern of the voltage change for U(VI) reduction is significantly
modified although it is not the case for Fe(III). This fact indicates
that further investigations might be necessary for the detection
of the end-point in the reduction of the MOX solution.
Addition of 2 mmol of Pd(NO3)2 to a 220 cm3 solution con-
taining 45 mM U, 22.5 mM Fe, 9 mM Rh and Ag, 3 M HNO3,
References
[1] Y.-Z. Wei, T. Arai, H. Hoshi, M. Kumagai,;1; Proceedings of Global 2005,
Tsukuba, Japan, October 9–13, 2005, No. 032.
[2] T. Arai, Y.-Z. Wei, M. Kumagai, Proceedings of Global 2005, Tsukuba,
Japan, October 9-13, 2005, No. 072.