In the electrorefiner, LFP in the molten salt gradually builds up during processing of the spent fuels. In order to limit the decay heat content and the contamination of the cathode products, the molten salts in the electro­refiner are periodically treated in the ‘counter current extraction’ process and ‘zeolite column’ process to decrease LFP content. As shown in Fig. 10.5, feed salt from the electrorefiner comes in contact with liquid cadmium at approximately 450 °C to extract actinides into liquid cadmium (as a

liquid metal solvent). The treated salt (raffinate) is additionally decontami­nated during the ‘zeolite column’ process, and the decontaminated salt comes in contact with the cadmium (extract) from the first stage of the counter current extraction, to strip actinides from the molten salt to be recycled in the electrorefiner. The number (N) of stages necessary depends on the separation requirements and distribution characteristics. Figure 10.6 shows the measured distribution data that rules separation efficiency at each stage (Koyama, 1992). In a liquid chloride salt and cadmium metal system, equilibrium among the elements is achieved by redox reaction between cations in the salt and metal atoms in the cadmium. The chlorine anions remain in the salt and are not oxidized. Thus, the equilibrium among
two cations can be represented as an exchange reaction between the pair of elements. Take, for example

nUCl3 + 3M = nU + 3MCln 10.6

The equilibrium constant, Ke, for the reaction is

where [M], [MCln], AG°, R and T denotes activity of M in the cadmium phase, activity of M in the salt phase, standard free energy of formation, gas constant and temperature in Kelvin, respectively.

Here the distribution coefficient of an element, M, in a molten salt and liquid metal system is defined as

Y

Dm = Y“, 10.8

XM

where YM and XM denotes mole fraction of M in salt, and atom fraction of M in metal, respectively.

The separation factor of element M relative to uranium is defined as

SFm = — 10.9

M Du

According to the thermodynamic relationship described in equation [10.7], the separation factor is described as

SFM = Ke1/3DU(n-3)/3f-^^ YYuCl^ 1 10.10

VY MCln А Y и )

where у denotes the activity coefficient.

As shown in Fig. 10.6, the distribution coefficients of actinides and lan­thanides have a linear relationship with that of uranium. Agreement of the slopes suggest that these chlorides are the same, actually in trivalences. Hence, equation [10.10] can be simplified as

SFm = Ke f^-lfYuCl 1. 10.11

VYMCln A Yи )

This equation clearly shows the nature of the separation factor in this system. It is not just a chemical technological value but a function of ther­modynamic properties that depends only on the temperature. In the counter current extraction process, therefore, each stage has different distribution coefficients but the same separation factor for each pair of elements. As

shown in Table 10.2, the measured separation factors of trivalent lanthanides are more than ten times larger than those of trivalent actinides, suggesting that sufficient separation between actinides and lanthanides can be expected for the counter current extraction of several stages.