Principal Research Results

Electro-chemical Reduction of UO2 Fuel and MOX FuelBackgroundSpent MOX fuel and spent high burn-up fuel are expected to be generated and, therefore, reprocessing technologies for thesespent fuels are required in the near future. Also, discussion on fuel-cycle technologies to introduce fast reactors will be started after theoperation of Rokkasho reprocessing plant begins. It will take at least several decades to replace all present light water reactors with
fast reactors. Thus, flexibilities to correspond to the change of social needs will be one of the most important factors for the fuel-cycle
technologies during the replacement period. An electro-chemical reduction process has various characteristics suitable for the replacement period. (i) Low capital and running costs even in a small size plant. (ii) High ability to treat various kinds of spent fuels withoutmajor changes of process. (iii) High performance to resist nuclear proliferation.

Objectives
The aim of present study is to show the validity of the electro-chemical reduction process based on the experimental studies
using 100g-scale of UO2-fuel and 200mg-scale of MOX.

Principal Results
1. Electro-chemical reduction of UO2 fuelFig.1 shows a schematic image of test cell and a picture of UO2 fuel set on the Ti-cathode. Following results are obtained: (i)In LiCl solvent system, UO2 fuel was completely reduced to U metal with 53% of current efficiency. Fig.2 shows the cross section ofmetal product. (ii) In CaCl2 solvent system, UO2 fuel was not completely reduced in the present test because the current efficiency wasmaintained at around 25%. However, the morphology of the product as given in Fig.3 showed that UO2 fuel would be completely reduced if the operation continued. (iii) The morphology of the product was affected by the type of molten salt solvent, and the particle
size and density of initial UO2, as shown in Fig. 4. The adjustment of the product morphology could accelerate the process speed to besuitable for the industrial process.
2. Electro-chemical reduction of MOX fuelIn the MOX tests, (i) Both in LiCl and CaCl2 solvent systems, MOX fuel containing Pu was completely reduced to metal.
(ii) At the interface between the reduced and non-reduced regions, both regions formed a layer and a network structure in UO2 fueland in MOX fuel, respectively, as shown in Fig.5. Since the network structure could supply larger reaction area, the reduction of MOX
fuel easily occurred in comparison to UO2 fuel.
Part of this work was sponsored by MEXT (Ministry of Education, Culture, Sports, Science and Technology), Developmentof Innovative Nuclear Technologies Program.

Future Developments
Since the particle size and density of fuel is one of the most important parameters to control the process, optimization will be
carried out. Also, the investigation of fuel-cycle scenario using the electro-chemical reduction process will be materialized.
Main Researchers:
Masaki Kurata, Senior Research Scientist, Ph.D., and Yoshiharu Sakamura, Research Scientist, Advanced Nuclear Fuel Cycle Sector,
Nuclear Technology Research Laboratory
Reference
M. Kurata et al., 2004, “Electrochemical reduction of MOX in LiCl”, J. Nucl. Mater. vol.328, PP.97-102.

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C. Harmonization of energy and environment

100g of UO2 on cathode.

Fig.1 Image of test cell.
Grinded cross section

1cm

100μm

100μm

Recovered product.
Completely reduced.

Fig.2 Reduction product obtained inelectro-chemical reduction test in LiCl.

Reduction ongoing.

Fig.3 Various reduced UO2-particles
obtained in electro-chemical reduction
test in CaCl2.

(Grinded cross section shows metallic-shine.)(White part：U-metal, grey part：U-oxide)

The formation of porous
structure progresses
reduction reaction.

In MOX fuel, a network
structure was formed and it
could increase the reaction
surface.

15μm

50μm

Enlargement

300μm

50μm

300μm

LiCl system
CaCl2system
Fig.4 Dfference in eduction products obtainedin LiCil and CaCl2 systems.
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UO2 fuel
MOX fuel
Fig.5 Interface between reduced (right)/ non-reduced (left) regions