Idaho National Laboratory and Argonne National Laboratory have developed and demonstrated a pyroprocessing technology for the U.S. Department of Energy to reprocess spent nuclear fuel. One of the key steps in the pyroprocessing was electrorefining the spent fuel in a metal form in a molten LiCl-KCl-UCl3/liquid cadmium (Cd) system using the Mark-IV, an engineering-scale electrorefiner (ER). This paper summarizes experimental observations and engineering aspects for the roles of the Cd in electrorefining spent fuel in the Mark-IV ER. It was found that the Cd pool acted as an intermediate electrode during the electrorefining process. The Cd level gradually decreased because of its high vapor pressure at the ER operating temperature. The low Cd level caused the anode assembly to electrically short with the ER vessel hardware, which resulted in difficulty determining the endpoint of uranium dissolution from the anode baskets and reduced the current efficiency. A reflux Cd vapor trap successfully prevented the Cd level from decreasing and mitigated Cd vapor deposition on the cold metal surface inside the ER.