A Study of Continuous Electrochemical Processing Operation Feasibility for Spent Nuclear Fuel

BOBOLEA, RUXANDRA. A Study of Continuous Electrochemical Processing Operation Feasibility for Spent Nuclear Fuel. (Under the direction of Dr. Man-Sung Yim.) Several methods of reprocessing are currently available to separate recyclable materials from spent nuclear fuel. Electrochemical processing, also known as pyroprocessing, represents a non-aqueous method of reprocessing that uses high temperature molten-salt based electrochemical technology. This method provides several advantages over conventional aqueous processing with respect to proliferation resistance. With electrochemical processing there is no pure plutonium separation and the presence of large decay heat and high radiation barriers dissuades diversion attempts. As the current electrochemical processing relies on a batch operation, the total throughput of the system is inherently limited and nuclear materials accounting is difficult due to the nonhomogeneous nature of the process. This results in much larger uncertainties in the total amount of material processed compared to the aqueous UREX+ or PUREX processes. Continuous electrochemical processing was considered as a way to address these concerns. The objective of this research was to investigate the feasibility of a continuous electrochemical processing operation to achieve the desired separation performance by using computer based simulation. The conceptual design of the continuous electrochemical processing includes two separate stages in a molten salt medium. First, a pure uranium deposit is collected at a solid cathode during the uranium extraction stage. When the amount of plutonium in electrorefiner becomes comparable or higher than the amount of uranium in the electrorefiner, a liquid cathode is employed to extract both uranium and plutonium in the second stage. In this approach, molten salt, as the material carrier, flows through the electrorefiner while chopped spent fuel is continuously fed into the system. Simulations of electrochemical reactions at the electrode surfaces were based on the kinetic modeling capability of a time-dependent code, REFIN. Based on a screening study performed for the most significant process parameters over a broad range of values, a functional combination of initial uranium and plutonium concentrations at the anode and in the molten salt was determined for continuous operation. This dictated the use of a higher concentration of uranium than plutonium at the anode and a lower concentration of uranium than plutonium in the molten salt. Furthermore, using design of experiment technique for computers, a refinement of initial concentrations was performed to maximize the total throughput and minimize the operational time. The flow velocity profiles and chemical concentration distributions of elements in molten salt have been determined through three dimensional Computational Fluid Dynamics simulations using ANSYS CFX. This approach resulted in the need to evaluate the diffusion layer thickness at the cathode – molten salt interface, an important parameter for the electrochemical process. Computer based simulations of the continuous electrochemical processing concept presented in this study have provided an indication that electrochemical processing could be a viable technology for closing the nuclear fuel cycle. A Study of Continuous Electrochemical Processing Operation Feasibility for Spent Nuclear Fuel