10.1.1 The pyrometallurgical process for metal fuels

There are different versions of the pyroprocess depending on the material to be treated. Figure 10.1 shows a flowchart of the pyroprocess for repro­cessing a metal fuel, i. e. U-Pu-10wt%Zr alloy for the core fuel and U-10wt%Zr alloy for the blanket fuel of a fast neutron reactor. In this section, the principles of each step of the pyroprocess will be described.

Disassembly and chopping

In the ‘disassemble and chopping’ process, hexagonal spent fuel assemblies are disassembled, and fuel elements with stainless steel cladding are chopped into small segments. The technologies developed for oxide fuel assemblies can be applied in this process.

Electrorefining

The next step is the electrorefining process, schematically shown in Fig. 10.2, where the chopped spent metal fuels are dissolved in an anode basket,

10.1 Process flowsheet of pyroprocess for metal fuel reprocessing. (Revised by the author according to the latest experimental results.)

whilst actinide metals are recovered at a solid cathode, or a liquid Cd cathode. The main reaction schemes of electrorefining are described as follows:

Anode — Uin spent fuel ^ U + 3e 10.1

Puin spent fuel ^ Pu3+ + 3e — 10.2

Cathode (solid): U3+ + 3e — ^ U 10.3

Cathode(liquid Cd): U3+ + 3e — ^ U in Cd 10.4

Pu3+ + 3e — + 6Cd ^ PuCd6 10.5

This process utilizes the oxidation-reduction potentials of the relevant elements shown in Table 10.1. The constituents of the spent fuel that have lower standard potentials than zirconium, i. e. actinides and less noble FP (LFP) such as the alkali metal FP, the alkaline-earth metal FP and lantha­nide FP, are electrochemically dissolved at the anode. Conversely, elements having higher standard potentials, i. e. zirconium, iron (cladding), cadmium, and noble FP (NFP) such as ruthenium, rhodium, palladium, technetium and molybdenum, remain undissolved in the anode basket. At the solid cathode, uranium is preferentially reduced and collected since it is the most easily reduced element among the dissolved materials. Other actinides, such as plutonium, neptunium, americium and curium are recovered with uranium in the liquid cadmium cathode because the reduction potentials of these elements are very close to that of uranium, as shown in the right — hand column of Table 10.1, due to their strong affinity with cadmium, i. e.

they form intermetallic compounds such as PuCd6. Consequently, this gives a proliferation-resistant nature to the process, since it is almost impossible to separate out pure plutonium. Typical cathode products obtained in a laboratory-scale electrorefiner (Koyama, 2002) are shown in Fig. 10.3, where (a) is a uranium deposit on the solid cathode and (b) is a plutonium — uranium-MA deposit in the cadmium cathode. As shown in this figure, the actual cathode deposits contain adhering salt and/or alloying cadmium in addition to actinide metals, though the actinide cations are reduced into metal form at the cathodes.