Nuclear Technology / Volume 171 / Number 3 / September 2010 / Pages 292-299
Technical Paper / Pyro 08 Special / Reprocessing
The results of a recently reported series of bench-scale actinide recovery experiments with liquid cadmium cathodes (LCCs) are subjected to a more detailed analysis in this paper. It is suggested that separation efficiency (SE), not separation factor (SF), should be used to assess the effectiveness of an LCC to separate actinides from rare earth (RE) elements. The common definition of SF for any pair of actinide and RE elements in the molten salt/liquid Cd system is the ratio of their distribution coefficients, which are measured under equilibrium conditions. The definition of SE is broader than that of SF. For any pair of actinide and RE elements in the molten salt/liquid Cd system, SE is the ratio of their distribution coefficients, such as SEPu-U = DPu/DU, where DPu and DU are measured at either equilibrium or nonequilibrium conditions. The relationship of SE with SF is linear and can be expressed as SEPu-U = DPu/DU and DPu = SFPu-UDU + b. When DPu and DU are measured under equilibrium conditions, SE is equal to SF.
The physical or chemical meaning of the intercept b is not clear. From a mathematical point of view, the absolute values of b reveal the differences between the measured DPu/DU or SE and SF. The negative values of b indicate that the SE measurement results are smaller than the associated SF. The values of b may be used to evaluate the SE of LCC on electrochemically recovered actinides from fission product elements. An electrochemical model was developed to investigate the mechanism of RE contamination of the actinides collected by the LCC. It was confirmed that REs were electrochemically transported into the Cd phase. A more negative LCC voltage has a stronger impact on the quantities of REs transported into the Cd than those of the actinides.