Methods of the present invention can be used in the treatment of irradiated fuels for producing actinides in metallic form suitable for use as feeds in subsequent electrorefining processes.

Background to the Invention Two processes have been developed for the treatment of irradiated nuclear fuel making use of molten salts. As used herein, the term"molten salts"is intended to cover salts such as lithium chloride which melts at an elevated temperature and also ionic liquids which typically are liquid at room temperature or which melt at a temperature up to about 100°C.

The Dimitrovgrad SSC-RIAR process makes use of chemical oxidants (chlorine and oxygen gases) to react with powdered uranium dioxide fuel to form higher oxidation state compounds such as UO2C12 which are soluble in the molten salt. In an electrochemical cell the uranium compounds are reduced to U02 at the cathode, forming a dendritic deposit. This process has both technical and environmental limitations.

The second process, developed by the Argonne National Laboratory (ANL) is fundamentally an electrorefining technology which uses current flow to oxidise anodic uranium to form uranium ions in the molten salt electrolyte. At the cathode the uranium is reduced and electrodeposited as uranium metal.

The ANL process requires a metal feed. If oxide fuels are to be treated, it is necessary to reduce the uranium oxide (usually U02 pellets) to the metal. This

reduction process is carried out chemically, using lithium metal in a LiCI or LiCl/KCl molten salt at 500 to 600°C. Alternatively, a salt transport process can be used involving a Cu-Mg-Ca alloy and molten CaCl2 salt. However, in both reduction methods the by-products, Li20 and CaO respectively, need to be recovered from the molten salt phase by an electrolysis step. Effectively this means a two stage process.

A disadvantage of the lithium reduction process for producing a metallic feed from an oxide is the production of Li20 by-product. This requires recycle to make the process economic, and this is done by an electrolytic recovery of lithium metal.

Hence you have a two stage process, a reduction step, followed by a lithium recovery.

Statements of Invention According to the present invention there is provided a process for reducing to metallic form a metal oxide present in spent nuclear fuel, the process comprising cathodically electrolysing the oxide in the presence of a molten salt electrolyte, the potential of the cathode being controlled so as to favour oxygen ionisation over deposition of the metal from the cations present in the molten salt.

Accordingly, the present invention involves the use of a single electrochemical process to reduce the metal oxide fuel to a metallic form, with oxygen produced as the only by-product. It is important that the potential of the cathode is maintained and controlled so that only oxygen ionisation occurs and not the deposition of the cations (eg Ca ions) in the fused salt.

Typically the oxide comprises an actinide oxide, such as uranium oxide, irradiated uranium oxide or mixed uranium/plutonium oxides.

The oxide may be in any physical form, and this is generally dependent on the particular chemical nature of the spent nuclear fuel and the processing to which the material has previously been subjected. For example, the fuel may comprise a powder, an amorphous mass, or a dense solid agglomerate. In any event, the material

may be treated according to the method of the present invention by connection to an electrical circuit such that it serves as the cathode during electrolysis. Connection to the circuit may be effected by any of the standard means well known to those skilled in the art.

Preferably the oxide fuel is in contact with the cathode of an electrochemical cell.

The cathode could be in the form of a mesh basket. The molten salt electrolyte may be any suitable molten salt, for instance, CaCl2 and BaC12.

The anode may be any suitable inert anode, such as carbon. In a process of the present invention the oxide fuel may be first treated mechanically to remove its zircaloy cladding before it is added to the electrolytic cell. Alternatively, the zircaloy cladding is first chopped into segments, and these segments are treated by the process of the invention.

Detailed description of the Invention In order to carry out an embodiment of the present invention, an electrolytic cell is assembled which has a carbon anode and a mesh basket cathode. Irradiated oxide fuel is placed in the mesh basket. The electrolyte consists of a molten salt such as CaCl2 or BaCl2. A voltage is applied between the cathode and the anode. At the cathode the reaction involves the diffusion of oxygen atoms to the surface of the molten salt, followed by ionisation according to the reaction: O + 2e- OZ-.

The oxide ions which are produced dissolve in the electrolyte and are transferred to the anode where they are re-oxidised to produce oxygen gas. The potential at the cathode is controlled, via a third reference electrode, to ensure that the reaction occurring at the cathode is oxygen ionisation and not deposition of the cations in the fused salt. Electrolysis at elevated temperatures results in an increased rate of oxygen diffusion, thereby also encouraging ionisation rather than metal deposition.

After electrolysis the irradiated fuel is left in the form of a metallic solid at the cathode. This metallic solid, which contains fission products, can be removed and used directly as the feed for an electrorefining process. The remaining components of the cell may be re-used immediately without the need for any cleaning.

In an alternative embodiment in accordance with the present invention the electrolytic ionisation of oxygen and the electrorefining processes are carried out in the same cell.

It is to be emphasised that the advantage of process of the present invention is that it is effectively a single stage process. It may be used for the treatment of irradiated oxide nuclear fuel, possibly in the form of pellets, and will be applied to fuels such as uranium oxide, and mixed uranium and plutonium fuels known as MOX fuels.

Changes in the morphology of spent fuel material comprising uranium oxide have been observed following electrolysis, and these changes are indicative of the electrochemical reduction of the oxide to the metal during the course of the process.

Thus, the porous structure associated with the oxide is seen to become much less porous as the metal is formed; this is consistent with the greater mobility of uranium metal, as compared with its oxide.

Detailed examination of the products obtained from the reduction of zirconium dioxide has been carried out by means of energy dispersive X-ray analysis (edax) and has allowed for the identification of a metallic solid containing 0.3 wt% oxygen, thereby confirming the success of the process. Similar studies have also been carried out with cerium dioxide, yielding metallic cerium containing 4 wt% oxygen.