(353a) Recycling Americium from Spent Nuclear Fuel through Molten Salt Electrodeposition

In 2020, nuclear energy surpassed coal in domestic electricity generation. Further, the International Atomic Energy Agency predicts the world nuclear energy capacity to nearly double by 2050. Despite its growing prominence in power generation, the nuclear community does not have a management plan for spent nuclear fuel (SNF). A key challenge for SNF management is the high radiotoxicity from the long-lived actinide fission products (e.g., Am-241 with a half-life of 432 years and its decay product, Np-237, with a two million year half-life). Due to the longevity of the radioactive decay, it takes approximately 300,000 years for SNF to decay to radiation levels of naturally occurring uranium ore. This poses a threat if the integrity of waste storage vessels is compromised. One solution is to: 1) separate the long-lived actinides from the bulk of SNF thereby decreasing the decay time of the waste from 300,000 years to 300 years, and to 2) recycle (reprocess) the actinides into new fuel rods. Pyroprocessing is an electrochemical separation method that performs this separation and recovery of actinides from the bulk SNF through electrodeposition in a molten salt electrolyte. Electrodeposition of actinides is complicated because it involves multi-electron transfer processes that occur at different rates. Therefore, to effectively scale-up this process, it is necessary to understand the fundamental transport and kinetic behavior of the actinides during electrodeposition.

In this work, we aim to understand the fundamental mechanisms that govern americium (Am) electrodeposition from an AmCl₃ containing LiCl-KCl molten salt. The AmCl₃-LiCl-KCl salt was synthesized by chlorinating AmO₂ with ZrCl₄ in LiCl-KCl at 500^oC. All experiments (synthesis and electrochemical) were performed using a three electrode cell at 500^oC in an argon glovebox. Tungsten wire was used as the working and counter electrodes. In some experiments tungsten was used as a quasi-reference electrode and, in others, Ag-AgCl was the reference electrode. Cyclic voltammetry was performed to study the redox reactions of Am³⁺ to Am²⁺, and the deposition of Am²⁺ to Am. Analysis of peak potential separation and the relationship of peak current density with scan rate shows that the Am³⁺/Am²⁺ reactions are electrochemically reversible. This means that this single-electron transfer reaction occurs at the electrode surface without significant kinetic limitations. However, according to the same analysis, the deposition reaction (Am²⁺/Am⁰) is irreversible, implying that the reaction may be kinetically limited. To further investigate this behavior, a transient diffusion-reaction model was developed and simulations were performed using COMSOL Multiphysics®. The exchange current density, reaction rate, and charge transfer coefficient of the Am²⁺/Am⁰ reaction were determined at 500^oC. The model suggests that the deposition reaction of Am is complex due to slow kinetics, which allows time for Am²⁺ to diffuse away from the electrode surface and not fully react to Am⁰. Accounting for this behavior is important for developing accurate electrodeposition models, and characterizing the overall efficiency of the actinide recovery process.

Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract DE-AC02-06CH11357.