Contamination of zirconium metal by cadmium dissolved in molten LiCl-KCl salt

Cadmium contamination of the metal waste stream in the pyroprocessing of used nuclear fuel from the Experimental Breeder Reactor-II (EBR-II) has become a significant concern as it creates a mixed hazardous/radioactive high-level waste product. It has been hypothesized that this contamination is a result of a reaction between the zirconium (Zr) in the undissolved fuel segments and metallic cadmium (Cd) dissolved in the molten LiCl-KCl salt inside the Mark-IV electro-refiner. To test this theory, Zr metal pellets were immersed in a molten LiCl-KCl salt bath sitting above a layer of molten Cd and sampled over time. Salt samples were taken at time intervals, and analyzed via inductively coupled plasma mass spectrometer (ICP-MS) to measure the Cd concentration in the salt. The ICP-MS results revealed that the addition of Li metal significantly lowers Cd concentration in the salt. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) of the cross-sections of the Zr pellets was used to measure the relative concentrations of Zr, Cd, and CI along line-scans from the surface to the interior of the pellets. These results indicate that Cd appears to concentrate at the salt/Zr interface and its concentration in the Zr was consistent across samples.