The development of metal fuel has taken place in conjunction with that of the “integral fast reactor” (IFR) concept, which is based on close integration of a reactor with a reprocessing and fuel fabrication facility. It is not appropriate to give a full description of the IFR here, but it is necessary to describe it in outline if the nature of the fuel it uses is to be understood.

Central to the IFR system is the reprocessing of the irradiated fuel at high temperature in the molten state, a process called “pyropro — cessing”. The earliest version involved “melt-refining”. After a brief cooling period the irradiated fuel was melted in a zirconia crucible. Some of the fission products were driven off by evaporation and oth­ers formed a residue in the crucible. The fuel, containing the remainder of the fission products and with the addition of new fissile or fertile material as required, was then cast into new fuel elements and sent back to the reactor. The main disadvantage of this simple process was the buildup of fission products after multiple recycling, which reduced the reactivity significantly making it hard to reach high burnup. In addition there were unacceptable losses of uranium and plutonium in the crucible residues.

To eliminate these disadvantages the separation efficiency was increased by replacing the melt-refining process by electro-refining, which involves electrolysis after dissolution in a mixture of molten

salts. The fuel elements are chopped into short lengths and placed in a steel basket that is immersed in a steel vessel containing a molten mixture of lithium and potassium chlorides at 500 °C. At the bottom of the vessel is a layer of molten cadmium. Cadmium chloride is added, and this oxidises the actinides so that they produce a sufficient ion concentration to allow the salt mixture to conduct electricity.

Table 2.4 shows the free energies of formation of the chlorides of the various metals. They fall into three groups: the chlorides of the alkaline earths and most of the rare earths (Ba to Y) are stable and tend to remain in the salt phase; those of the transition metals (Cd to Ru) are unstable so the metals are precipitated in the molten cadmium; whereas the actinides and zirconium form chlorides of intermediate stability that can be separated by electrolysis.

A potential of about 1 volt is applied between the basket of fuel element fragments, which becomes the anode, and two cathodes are used in sequence. The first is a steel rod, on which uranium is deposited as the metal. Plutonium, being more stable, cannot be precipitated until its concentration in the molten salt is high, but when the uranium concentration has been reduced sufficiently it can be precipitated at a second cathode which consists of a crucible containing liquid cadmium. At this cathode, plutonium and the higher actinides form inter-metallic compounds such as PuCd6. After electrolysis the deposits from the cathodes are taken to a furnace where the remaining salts and the cadmium are removed by evaporation. They are then blended to obtain the required fuel composition, melted and cast into moulds to form new fuel pellets.

The decontamination factors are low, by design, and as a result the new fuel, containing significant amounts of fission products, particu­larly the rare earths, is highly radioactive. For this reason the entire process has to be conducted remotely, but it confers the advantage of protecting the new fuel, and the plutonium it contains, from illicit diversion.

The main advantages of pyroprocessing with electro-refining are that it is cheap and the out-of-reactor fuel inventory is minimised. Another advantage is that the minor actinides are recycled and do not appear in the waste stream (see section 2.7.4). There is evidence that small additions of americium and neptunium to the U-Pu-Zr fuel alloy do not affect its performance adversely, although the high volatility of americium makes for difficulty in the high-temperature fabrication process.